Why is cold fusion considered bogus? Cold fusion is being mentioned a lot lately because of some new setup that apparently works. This is an unverified claim.
See for example:


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*http://hardware.slashdot.org/story/11/01/24/1550205/Italian-Scientists-Demonstrate-Cold-Fusion

*http://www.physorg.com/news/2011-01-italian-scientists-cold-fusion-video.html

*http://www.wipo.int/pctdb/en/wo.jsp?WO=2009125444&IA=IT2008000532&DISPLAY=DOCS

*http://www.journal-of-nuclear-physics.com/files/Rossi-Focardi_paper.pdf 
(Archived copy of that last link in the Wayback Machine, given frequent http 403 errors from that page.)
While we should give the scientific community time to evaluate the set up and eventually replicate the results, there is undoubtedly some skepticism that cold fusion would work at all, because the claim is quite extraordinary.
In the past, after Fleischmann and Pons announced their cold fusion results, in perfectly good faith, they were proven wrong by subsequent experiments.
What are the experimental realities that make Fleischmann and Pons style cold fusions experiments easy to get wrong?
Would the same risks apply to this new set up?
 A: This new "cold fusion" reported in what is really a blog is a commercial enterprise to all intents and purposes. Their claims are so large, that either their  constructs will be successful or they will eat their hat. We do not have long to wait.
If they are successful, the theory will be found. 
One note about crystals ( they are using Ni crystals) and large energies: Along the crystal axis high energy muon beams go through intact, without interacting with the coulomb barrier. Hard to find references, I know Tom Ypsilantis was working on this in the '80s. Here is a proposal for a muon collider that uses the concept.
So if the announcement is not snake water, an appropriate solid state/nuclear theory will emerge.
I am editing this to include a recent video by NASA that may be saying that the Rossi claims may not be after all totally bogus. Note the Ni28 in the table of possible elements to be used for low energy nuclear fusion ( LENR). 
BTW Rossi et al are now planning small heaters! lap top size for the home market!! If it is a scam it will be the scam to end all scams !.
A: The treatment of Pons/Fleischmann is, by far, the worst scandal in modern science. It is probably worse than Galileo. Their claims were true, were reproduced immediately in several labs (sporadically, many labs failed too), and good research continues to this day, without funding and without theory.
Experimental History
Their claim is not completely new. It goes back to the 1920's, when Paneth and Peters reported Helium production in Pd heavy water electrolysis. Paneth and Peters' claims were dismissed for essentially the same reason that Pons and Fleischmann's were, the theorists' imaginations were too stunted to think up a mechanism that could bridge the gap between chemical energies and nuclear energies. In the 1950s, there was a Soviet scientist who also claimed that Pd heavy water electrolysis leads to nuclear anomalies. That claim was also dismissed by the Soviet scientific establishment, but his career was rehabilitated somewhat after Americans reproduced the effect in 1989.
Many people (i.e. graduate students) who worked with Pd/deuterium system noticed anomalies in the system for decades, and it was folklore in the chemistry community that deuterated Palladium acts up, while Hydrogenated Pd does not. Pons and Fleischmann decided to get serious about the anomalies, and did extremely careful calorimetry on the system for many years, until they were certain they had a reproducible effect for which chemistry could be safely excluded. Then they held their press conference, and chaos.
Part of the problem is that once they claimed fusion, people insisted that the fusion should emit neutrons, just as hot fusion does. This is impossible, because, considering the energy released, the number of neutrons would have cooked Pons and Fleischmann. Then nuclear physicists demanded that they measure nuclear effects, and they tried to do this, but their nuclear measurements were riddled with errors, and it is possible that they fudged a plot that they showed at a conference (although considering Fleischmann's impeccable scientific integrity, I find it more plausible that they made an honest mistake). It is important to note that their published paper contains only calorimetry data, and no nuclear data of which they were unsure.
Some people speculated that the effect can be explained by chemistry, or by insufficient stirring, or by storing electrical energy for later release. These claims are all idiotic. The effect is not small, the only reason it requires instruments to detect is because Pons and Fleischmann deliberately used a tiny Pd wire as a cathode. When they used a bigger Pd plate, the thing melted the table, and blew a hole in the concrete floor below. There just is no source of chemical energy, nor a battery, which can store energy chemically at more than about 1eV per atom. Other people noticed similar runaway explosions too.
Aside from the explosion, there is possible heat from recombination, which has often been emphasized by critics. The electrodes separate H2 from O2, and if the two mix together and the hydrogen burns, you will see excess heat. To control this, groups used infrared cameras to locate the heat source at the cathode, rather than the water where bubbles of gas can mix. They also separted the anode and cathode. But most definitively, in 1994, Pons and Fleischman demonstrated heat after death in 1994, where they cycle the temperature in the anomalously heating cell up, then shut down the current entirely. The cell continues to produce heat for hours with no current, no oxygen, no hydrogen, and many times more heat than you can store in the cathode by any chemical means.
The effect is very sensitive to the metallurgy of the Palladium, and Pons and Fleischmann couldn't reproduce the finicky effect on demand once they ran out of the good palladium. The experiment sometimes takes weeks, and many people just didn't have patience. Still, the effect was reproduced immediately in a handful of places. MIT ran an infamous reproduction that noticed excess heat production, and were going to press with a reproduction. Then they realized that this effect was going to be labelled bogus, and they cut off their graph to show no excess heat. One of the graduate students who was involved in this experiment, Eugene Mallove, was so outraged that he quit his position and became a cold fusion promoter.
Several groups published reproductions. These groups were attacked in the most unscientific of ways. Several groups, Bocris at texas, but also reputable researchers at Bhabha institute and Los Alamos, reported low levels of tritium production in the system. Since tritium is radioactive, it has a clear signature, it can't be mistaken for anything else. Since it is very expensive and is ordinarily produced in nuclear reactors, the only way such a signal could be seen is if it was deliberately faked by spiking the heavy water with tritium. Bocris was accused of doing just this--- spiking his cells with tritium, so confident were the deniers that he had committed fraud. Despite the intense pressure, he never retracted his claim. Another colleague at Texas who claimed tritium, Wolf, did retract the claim when he saw what was happening to Bocris, and never spoke in support of cold fusion again. Bocris was investigated for scientific misconduct and exonerated. No plausible way he could obtain tritium (other than cold fusion) was ever found. The tritium observations would require all the researchers who observed tritium to be engaged in deliberate fraud. It is impossible to misidentify tritium.
Two extermely well respected theorists, Julian Schwinger (emeritus UCLA) and Peter Hagelstein at MIT, were convinced that the effect was real. Schwinger was not allowed to publish in the field, and Hagelstein, who was tenured, had all his funding cut and was moved into a closet.
In the early 90's, without any official funding, McKubre quantitated Helium production to correlate with the excess heat. Low levels of ordinary fusion are seen to happen in the system. The SPAWAR group in the U.S. Navy reproduced the effect in co-deposition experiments, where they plate Pd onto a surface in the presence of heavy water. Their experiments mostly detect nuclear products, because the plated surface is so small, but the effects are 100% reproducible. More recently, the navy presented evidence of sporadic high-energy neutrons coming from the co-deposition system.
In Japan, Mizuno noticed new elements being produced in the Pd system with abnormal isotope ratios and atomic number near Pd (this was also detected by Wolf, as reported by Eugene Mallove, but Wolf would not publish after the tritium fiasco). Abnormal isotope ratios cannot be fraud, because such materials are so difficult to make. In Japan, Arata reproduced the effect by using gas-loading of deuterium into Pd, which has no heat source, so there is no calorimetry error to blame things on. This was a foolproof version of Pons' and Fleischmann's "heat after death" experiments, and it also rules out calorimetry/recombination error entirely. The effect has been reproduced many hundreds of times, in far away labs with no mutual interests, and everybody should be certain by now that it is real. I am ashamed of myself, in that I couldn't bring myself to trust the experimental data until I came up with a reasonable theoretical story to explain it.
Focardi/Rossi claims are more dubious. Their effect is in Nickel/Hydrogen, which has some reported energy anomalies too, but not with the same level of confidence. In terms of theoretical craziness, Nickel Hydrogen fusion is to Palladium Deuterium fusion as Palladium Deuterium fusion is to standard hot fusion. The cold fusion community is taking a wait-and-see attitude, but I think consensus is that the device is not likely to work. In his demonstrations, Rossi measured heat production using steam, not water, and by understating the water-content of the steam, you can inflate the energy output by the latent energy of vaporization, which is huge. For me, it is most suspicious that his claimed transmutation products were analyzed and have natural isotope ratios. It is possible that his machine works, and it is possible that it does not produce any excess energy at all, we will know soon enough. What is impossible is that there are no nuclear effects in Pd/deuterium.
Here is an update regarding the e-cat, which, as people expected, is a sophisticated scam: Is the E-cat by Andrea Rossi et al. for real?.
Theoretical work
One major difficulty for acceptance of the effect is that theoretical work in this field is not sound. There are several theories, each of which are more or less preposterous. The central difficulties are overcoming the Coulomb barrier somehow, and making energy without nuclear reaction byproducts:


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*Hydrinos/little hydrogen: this theory states that the electron in Hydrogen can find a closer orbit than the ground state, and spends some times close to the nucleus. This requires that quantum mechanics is wrong, or that there is some new electron/proton force which has been missed, and somehow does not alter the ground state energy, but is capable of sucking the electron into the proton every once in a while.

*Bose-Einstein condensed deuterons/alphas: this idea is that the cross section for fusion is enhanced by identical-particle effects, since deuterons and alphas are both bosons. In theory, you can enhance reactions by having a coherent source of bosons all go through the same reaction to a coherent superposition. This theory fails both because the temperature is too high for coherence between deuterons, and because when it is implemented in specific cold fusion papers, the deuterons are treated as non-interacting particles in a product state, so that the amplitude for being at the same point is big. But this is ignoring the whole difficulty, because the electrostatic repulsion leads the wavefunction to be entangled, with little probability of any two deuterons getting to the same point.

*Lattice enhancement mechanisms: This was the focus of Schwinger and Hagelstein, neither of whom claimed to have solved the problem. The problem with such theories is only that the effects have to be collective over thousands of atoms to explain taking eV energies into KeV energies, and it is thermodynamically difficult to imagine how you can take such entropic energy into such an entropically unfavorable place as a single particle.

*Weak Force Neutron production: The Widom Larson theory claims that it is possible for a proton and an electron to do inverse beta decay on the surface of a metal, where there are large local electric fields. This is preposterous, because of the MeV difference in proton and neutron mass. It requires millions of volts to accelerate an electron to enough energy to be able to do an inverse beta-decay, and such energies are not available on the surface of a metal. Further, this theory will predict transmutations of plus/minus one mass unit predominanatly, which is not observed, and does not explain how a deuteron can absorb an electron.


The following lists are bogus theories I speculated would work, other people come up with these too every once in a while:


*

*sporadic atmospheric muon capture: The idea there is that muons are captured by the metal, and lead to fusion. This doesn't work, just because there are not enough muons, the deuterons are separated from each other in the lattice, and if the muon is captured by a Pd nucleus, it's wasted.

*tunneling with weird many-body enhancement: the idea is that the tunneling amplitude is always estimated, not calculated, and this is an impossible-to-solve many electron/many nucleon system, so perhaps the tunneling amplitude is just off by many orders of magnitude. This doesn't work, because there is a way of giving a lower bound to tunneling amplitudes which excludes any appreciable fusion reaction by tunneling. To do this, you exploit the fact that tunneling is a ground state property, and the deuterons which are imagined to tunnel are bosons, and their imaginary time ground state has no nodes. The electrons have nodes, since they are fermions, and at high energies, but when the electron states are all fully occupied, they might as well be a vacuum, with structure only near the Fermi surface (this follows from the particle-hole symmetric approximate description of the Fermi liquid). There were rigorous upper bounds on the tunneling probability for deuterons in a metal that claimed to prove cold fusion is impossible.


The previous failures leads one to expect that the effect is out of equilibrium, and involves highly excited atoms.
My Personal Theory
To bridge the gap between the scale of chemistry at eVs and of the nuclei at MeVs, one should take note of the fact that there are K-shell electrons orbiting very close to the nucleus at KeV energies. The K-shell electron of Pd has a 20KeV ionization energy, and if you have a K-shell hole in one Pd atom, it stores an amount of energy non-entropically in an amount sufficient to lead to deuteron fusion. While this energy is large, it is not large enough to knock a Palladium atom out of its lattice position, so it cannot dump its energy by locally breaking the lattice. The reason is that the Pd nucleus requires more than the 20KeV ionization energy to be knocked out of position without it's core, and you can't conspiratorially transfer the hole energy to the entire core in one step, it's phase-space impossible.
Such K-shell holes usually decay by X-rays, but this is an electromagnetic process which is suppressed by powers of v/c when the electron is nonrelativistic, as it is even in the K-shell. This is a well known effect--- it's the same reason that atomic spectral lines are narrow. Emitting a photon takes many orbits because of the mismatch in scale between the photon's wavelength and the size of the orbit. This is ultimately because the orbit is nonrelativistic. Because the emission takes so long, the spectral lines are sharply defined and narrow, and the emission is dominated by the matrix elements of the dipole moment of the atomic state between stationary states.
Other observed ways for K-shells to lose their energy is to kick out an outer shell electron from a neighboring atom. This process is electrostatic, and nonrelativistic, so it is not suppressed by 1/c factors. It is only suppressed by the smallness of the charge on the electron and the distance between electrons on neighboring atoms. There is a significant fraction of decays in K-holes in Pd in this channel.
In a metal with protons or deuterons, a K-shell hole should be able to also kick its energy into a proton or deutrons by electrostatic forces. The matrix element is exactly the same as for kicking an electron, but the density of states is 30-50 times bigger (depending on whether it's a proton or a deuteron) due to the heavier mass. The proton, unlike a Pd nucleus, will leave its lattice site under such a transfer. So, considering that the cross section for a K-shell hole to kick an electron is not small, I feel safe to conclude that the proton-kicking process is the dominant decay mechanism for K-holes.
These deuterons have exactly the same energy as the K-shell hole, which means that their classical turning point when approaching a Pd nucleus is exactly the same distance from the nucleus electrostatically as the K-shell is wide, about 100 fermis. These holes can then excite another electron coherently, and travel many steps in the lattice before decaying by X-ray to the ground state. These hole-deuteron states make bands of several KeV width at energies around 20KeV, and these bands are full of classical turning points at 100fermis from a Pd nucleus.
Now suppose that two of these accelerated deuterons happen to come close to the same Pd nucleus. This can easily produce a fusion event at the turning point, the deuterons have around 20KeV after all, and the fusion rates at 20 KeV in beams is not that small, let alone in cases where the wavefunction is concentrated near a nucleus with a classical turning point (where the wavefunction is enhanced).
This fusion does not necessarily happen in the usual hot-fusion way, since it is very close to a Pd nucleus. Let us suppose that the fusion transfers the excess energy/momentum to a nearby charged particle electrostatically, the obvious candidate being one of the protons Pd nucleus. Then the alpha particle and whatever it transferred its energy to are moving with 24MeV of energy together, and they go through the metal, ionizing Pd atoms. Energetically, they can make up to 1000 K-shell holes, all within a millimeter, since the penetration depth is so tiny. The true number is more likely a hundred or a few hundred, since all levels are excited during the Bethe process of charged particle ionization. These holes are then banded with deuterons, so they accelerate new deuterons, and this can easily lead to a chain reaction. I believe this explains the cold-fusion.
There are two problems with this idea:


*

*The cross section for fusion at 20 KeV is not that huge, and it does not lead to a chain reaction by itself through the usual hot-fusion channels. The multiplication factor is around .001 from beam fusion on deuterated Pd, which has a 1 in 100,000 success rate, not 1 in 100, at 20KeV.

*The actual observed reaction produces an alpha particle without an emitted neutron or proton nearly all of the time. This is a 1-in-a-million event in hot fusion.


I think that both problems are related to the fact that the reaction is happening inside a dense metal. The first problem is not present if two deuterons are banded and both turning around near a nucleus, the result is like a directed collision of two 20KeV beams with a very good focusing device (the nucleus) to concentrate the scattering wavefunction.
The fusion of deuterons always happens through unstable intermediate states, and the cross section to alpha particle is only small because of the same non-relativistic issue. To get an alpha, you need to emit a gamma-ray photon, and emissions of photons are suppressed by 1/c factors. When there is a nucleus nearby, it can be kicked electrostatically, and this process is easier than kicking out a photon, because it is nonrelativistic (the same holds for an electron, but with much smaller cross section due to the smaller charge, and there is no reason to suspect concentration of wavefunction around electron density, as there is for a nucleus).
The time-scale for kicking a nucleus is the lifetime of the two-deuteron resonance, which is not very long, in terms of distance, it is about 100 fermis, this is about the same size as the inner shell. If the deuterons are kicking about at random, this coincidence is not significant, but if the deuteron-hole excitations are banded, it is plausible that nearly all the energetic deuteron-deuteron collisions take place very close to a nucleus, as explained above.
There are conservation laws broken when a nucleus is nearby. The nucleus breaks parity, so it might open up a fusion channel, by allowing deuteron pairs to decay to an alpha from a parity odd state. Such a transition would never be observed in a dilute beam fusion, because these fusions happen far away from anything else. This hypothesis is not excluded by alpha particle spectroscopy (there are a lot of relevant levels of different parities), but it is not predicted either.
But since something has to explain the experimental data, and this idea is the only story that isn't completely far fetched, I believe this is what is going on.
This theory predicts the following


*

*when undergoing cold fusion, the material should emit copious X-rays at the KeV range (from those K-shell holes which happen to decay electromagnetically anyway).

*the material should emit KeV deuterons in a mm skin around it.

*the material should emit MeV-range alphas and Pd nuclear fragments, protons, deterons, tritium, and Pd fusion products, as appropriate for ~10MeV electron scattering. The alphas should go up to 20MeV, which is the maximum energy when the entire nucleus is scattered. The Pd fragments should be MeV energy.

*There should be a small amount of hot fusion happening, with the associated fast neutrons and tritium, just from the occasional accidental hot-fusion collisions of 20KeV deuterons far away from a nucleus. If the bands become incoherent, you can get a burst of neutrons, as the incoherent fast deuterons fuse randomly.


These were "predictions" only in that I did not know about them when I made up the theory. I found out on lenr-canr.org that 1,3,4 are observed by Mosier-Boss using CR-39 plastic detectors and X-ray film, along with others, although it is hard to see a KeV deuteron.
The theory also predicts the following


*

*Proton based cold fusion doesn't work (although there might be a way of storing KeV scale energies in a Nickel hydrogen system for a long time in K-shell bands, releasing it in bursts, although it seems unlikely to me). This requires that all Ni-H cold fusion excess heat reports is due to chemical recombination, none of it should show any nuclear products. This is not inconsistent with any data I have seen.

*Transmutation products in cold fusion are due to Pd fragmentation during fusion and fast alpha absorption/scattering or fast Pd fragment absorption/scattering.


The simplest fragmentation and alpha absorption predictions mean that you should observe Pd transition to Au (+1, from alpha absorption and proton ejection, or alpha absorption, gamma emission and beta decay) and +2 Cd (from alpha absorption gamma emission), but no higher, and transitions down are due to fragmentation, so you should see Pd fission products, Rh (from ejected protons) and Ru (ejected alphas). These are precisely consistent with Wolf's transmutation data, which was from the gamma spectrum of the radioisotopes present in the cathode after a successful run, as leaked by Eugene Mallove.
But these predictions are not compatible with all the experimental data presented at lenr-canr.org regarding tranmutations. I believe that the inasmuch that the transmutation data disagrees with this theory, it is mistaken.
Transmutations
The transmutation products in deuterium forced through Pd presented by show peaks at mass +8,+12. Bombardment by alphas cannot produce this, as there is absolutely no chance of the same atom being hit twice by two different alphas.
This requires that a fusion fragmented Pd ejected a Be8 as it's fusion fragmentation product, and that this Be8 then was absorbed by another nucleus in transit, giving 8 units of mass to another nucleus by absorption. If this is so, there are sum rules for the transmutation elements: the amount of light element X produced is equal to the difference of (Pd+X) and (Pd-X), where Pd+X means add all the protons and neutrons in X to Pd, and Pd-X means subtract all the neutrons and protons in X from Pd. This sum rule is a stringent test of the theory.
Further, if you assume the absorption probability is roughly geometric, it must cut off precisely when the Coulomb barrier exceeds 10MeV, which means transmutations of  (this is reasonable for absorption of a small nuclear projectile by a large nucleus), you can conclude that you will detect peaks at Pd+X systematically shifted by the same factors as Pd-X.
But one would expect the heavy transmutation products to fall off as the coulomb barrier suppression. This is only qualitatively plausible given Iwamura's observations. You can see Iwamura's mass spec results on lenr-canr.org
This theory is more or less a bulk theory, so it is hard to understand why surface is more important. Bands at 20 KeV are predicted to occur in deuterated metals, and this would give a whole zoo of useful effects, regardless of the nuclear stuff. The bandwidth shifts the X-ray spectrum of the deuterated metal from 20KeV to a broad range, which is also an easily tested prediction of the theory--- the K-shell resonance frequencies are altered by deutration .
I am telling this theoretical story because I think it is plausible and consistent with the data with no new fundamental physics, so that people should not dismiss cold fusion. Since no other explanation even comes close to working, I suspect this explanation is correct.
Problems with the theory
The major problem with the theory is the incompleteness, it's a sketch. But the major points of the experimental data are not in conflict with the theory anymore. This is a major edit to the original post, which I put up a while ago. At the time of original posting, it didn't occur to me that transferring 10MeV's of energy to a nucleus electrostatically can lead to nuclear fragmentation, and without this, the theory is incompatible with the transmutation data.
A: This was beautifully answered theoretically right away at the 1989 APS session in NY, I think by Koonin.    Theoretically, for any sort of fusion one needs to overcome the Coulomb repulsion of the relevant nuclei, on the order of MeV in order to allow the nuclei to get close enough for their wave functions to overlap and fuse.  Because of the phenomenom of quantum mechanical tunnelling, this can be reduced to tens to hundreds of kev.    So temperatures of >> 10^5 K, or cold muons (which outweigh electrons by 200x) are required to reduce the internuclear distance (as in muon catalyzed cold fusion, a real phenomenon), or some other special mechanism is required to allow this close approach. 
However, for any sort of chemically catalyzed fusion, i.e. via the valence electrons, to take place, the binding energy of the two H atoms to the catalyst would have to be so high, that the particular configuration of the low energy valence electrons, etc. would necessarily be entirely irrelevant to the problem, i.e. whatever their arrangement they could not possibly catalyze the fusionable nuclei to approach close enough to fuse.   So no clever packing arrangement, quasiparticles, special adsorbtion, special crystal lattice structures, etc. could ever alter this conclusion.   Whatever was happening at such low energy scales would appear as a kind of irrelevant fluff compared to the energy scale of the internuclear distance necessary for fusion.    
Therefore valence electron catalyzed cold fusion would violate the fundamental laws of quantum mechanics, nuclear physics, etc.   Leggett and Baym also published an argument like this around the same time (summarized for free here).  Koonin and Nauenberg published an accurate calculation here, showing that if the mass of the electron were 5-10 times larger than it really is, chemically calalyzed fusion could work.   Note however, that the reaction rate depends on the electron mass very, very strongly, so that this remains impossible in our universe. 
A: Very simple! They can never repeat the results in a scientific way to demonstrate to others that it works. What's the point of science if we simply ignore the scientific method? If they did truly come up with something then they wouldn't have a need to be secretive and not show exactly what they did. If somehow they got it through accident then that is not science. Once they are able to reproduce the results and show others and others can reproduce them then it becomes useful and becomes science. (even if cold fusion is possible it is useless if we can't ever reproduce it)
The reason why cold fusion is considered bogus is mainly because many people have tried and all have failed... and those that have claimed to succeed never proved they did.
No one knows if cold fusion is possible but given all the schemes and cons that have happened in the past it's easier to be skeptical than not. If someone does discover some way then it will be VERY easy to prove and if they are true scientists they won't have any issues doing so.
A: Why was cold fusion considered bogus?  Because it was not easily reproduced when initially announced, because the original suggested mechanism was inconsistent with known physics at the time, and because the evidence presented at the time purporting to show it was nuclear fusion (specifically D-D fusion) was flawed.
Perhaps the better question is:  Should it be considered bogus today?  No, not completely.  There are many instances of similar results available, yet the effect is not under control yet.  The best reason for that is that the researchers are attempting to control the wrong things when they do their experiments, and that ends up with a lot of different results depending on what values the real controlling factors took during the experiment.  
There are conventional explanations for those semi-reproduced results that suggest other things to control, but which the cold fusion researchers fail to employ becasue they refuse to consider non-nuclear solutions.  For more details see the URL below and read it and references therein:
https://docs.google.com/open?id=0B3d7yWtb1doPc3otVGFUNDZKUDQ
(ref'd in http://www.networkworld.com/columnists/2012/102612-backspin.html?page=1)
A: The Fleischmann and Pons device relied on calorimetry (measuring the energy balance in terms of heat) maintained over multiple day time spans to ascertain that something unexpected was happening in the cell. 
This is experimentally tricky, as it requires high precision temperature measurements to be maintained against a consistent reference, and relies on an understanding of how the calorimeter may or may not be losing heat though unmonitored channels.
In short a large heat loss is being subtracted from a large heat input, and both measurements have some uncertainty. That's a warning sign in any experiment, but not a death knell if the uncertainty can be quantified with sufficient accuracy.
The situation is complicated by the time dependent heat loss that Fleischmann and Pons reported. If real, that would indicate some unexpected process at work, though we have no way of know a priori if that is fusion or some energy storage-and-release mechanism.
It seem to have been lack of consistency in reports of energy gain or neutrons that swung opinion against any fusion actually happening in the F&P cell.

BTW--People who kept at it for a few years after the consensus turned against were engaged in good science. There was always some small possibility that the process was dependent on some un-measured and un-controlled factor. To find out, one would need to accumulate a non-trivial set of working cells and then investigate how they were different from non-working cells.
But as the man said "If at first you don't succeed, try, try again. Then quit. There's no use being a damn fool about it." Sooner or later you just have to give up.
A: There are a few reasons.  


*

*There was never any clear reason why electrifying palladium should create pressures sufficient to ignite a fusion reaction.  Without a mechanism, this seems to be the most ridiculously radical and sensational conclusion possible, even if the calorimetry says that electrified palladium creates net energy somehow.  Why not start with simpler explanations than saying that it's fusion?

*the only evidence anyone has ever offered was calorimetry of a small-scale experiment.  But calorimetry is a complicated thing--you have to model the insulation of the system just right, and you have to measure the heat inputs for the system correctly, and so on.

*No fusion products have ever been observed--fusion would be plausible if they were able to show that the reactions produced some helium-3 or lithium or whatever.  

*The setup they describe in these experiments seems quite simple--just submerge some palladium underwater, and run electricity through it.  If this really produces a fusion reaction, why not just scale it up and build a reactor?  I'm sure if they could power a building with a palladium fusion reactor (or even just a light bulb), every critic would instantly shut up and get behind them.  And they never seem to explain why they don't just do this.  I'm never going to believe in your revolutionary new form of cheap energy unless you just start producing cheap energy with it (or at least explain why you can't).  
A: Pons and Fleischmann originally reported in 1989 that their chemical cells had produced excess heat, neutrons, and tritium. Their interpretation was that deuterium nuclei were fusing to produce 4He. The branching ratios in this process are known: 50% n+3He, 50% p+3H, and 10^-6 4He+gamma. If the claimed excess heat had been produced by fusion, then the experimenters would have been killed by the neutrons coming from the 50% of the decays that proceeded by neutron emission. Neutron detectors are infamous among nuclear physicists for being difficult to use, and for having a tendency to produce spurious signals. Even if all the neutron counts claimed by Pons and Fleischmann had been real, the observed flux of neutrons would have been many orders of magnitude too small in relation to the amount of excess heat claimed. Subsequent measurements by nuclear physicists using state-of-the-art neutron detection techniques demonstrated that no neutrons above background level are produced by chemical cells of the type used by Pons and Fleischmann.[Gai 1989] There has also been no plausible evidence for production of 3He, 3H, or gamma rays, all of which would have been copiously produced in d-d fusion reactions that produced measurable amounts of energy through nuclear reactions.
As of 2010, the consensus among scientists is that cold fusion was an example of pathological science. However, a few true believers continue to do experiments and make claims of positive results. A 2010 review by a prominent believer[Storms 2010] says that "Many people feel that the correlation between heat and helium is the strongest evidence for cold fusion." The trouble is that heat is produced by chemical reactions in any case, and the levels of helium claimed are not high enough to show a convincing excess relative to background. If these claims were correct, they would also require a fundamental rewriting of the laws of physics. They would require that the branching ratio in d-d fusion be drastically altered by the chemical environment, but this is impossible because in a nuclear reaction, the electrons are mere spectators. In order to conserve energy and momentum, d-d fusion also requires the emission of two particles in the final state. To get around the nonexistence of the second particle, cold fusion enthusiasts suppose that energy from the reaction is transmitted to the electronic lattice. No known mechanism exists by which such transmission could occur. As the experimental and theoretical constraints have gotten tighter, the believers have responded by coming up with crazier pseudoscience, insluding the production of nuclei with atomic number 126 and transmutation of the elements by plants and bacteria.[Storms 2010]
In summary, claims of cold fusion cannot be correct unless they overturn firmly established knowledge of nuclear physics. This would be an extraordinary claim, and it would require extraordinary proof. After two decades, no such extraordinary proof has emerged.
Gai et al., "Upper limits on neutron and gamma-ray emission from cold fusion", Nature 340 (1989) 29–34.
Storms, "Status of cold fusion (2010)", Naturwissenschaften (online) 97 (10): 861–881
A: If there had been a fusion process there would have also been neutron production.  There were some claims of neutrons, but they were not verified.  There is another problem with neutrons, if there was fusion Pons and Fleischmann would have been irradiated by neutrons.  They might in fact have gotten sick or died.  There was something amiss with the whole thing from the start.  If they were intending to get cold fusion, why did they not shield themselves?  If they did not intend to get cold fusion, but later suspected they had it, why did they not then shield against neutrons?  They played around with this set up for some time, and if they really thought they were getting fusion, why did they not set the apparatus behind lead bricks?  Either they were stupid or they were scamming.
The idea that interionic lattice potential could force nuclei together is ridiculous.  There is a prospect for a bosonization or condensate physics.  Here the D or T enter into the same quantum state and might transition into H^4.  This is implausible though.
A: Actually to my knowledge scientists have been able reproduce some of the experiments successfully, they just don't fully understand the process as of yet.  Research is being done to come up with a working LENR theory (what it's known as in the physics community) to accurately model what is happening (unexplained excess heat generation).  As some have said yes it's a very complex process and it's still possible that the energy is liberated from some other means (non-nuclear) but I would hardly say the it's completely been proven on way or another.  A good website for learning more is http://lenr-canr.org/
A: Update : I guess it is the same reason that multi universe theory is considered bogus by some : "fantastical speculation, disconnected from the reality that we can access empirically"
http://www.scientificamerican.com/blog/post.cfm?id=is-speculation-in-multiverses-as-im-2011-01-28 
Old content down here :
I wanted to send this as a comment to your question but I don't have enough rep to do that.
In the book "Forbidden Science", Cold fusion is discussed, and the attitude of scientific community mentioned. The book "The trouble with physics" also gives a glimpse into the physics community.
Maybe instead of wondering why it is considered bogus, should change it to considered bogus by some. I haven't been able to find a verification to the story that before the planes were invented there were "mathematical" proofs floating around that why it was impossible for machines made of metal to fly. ( any one has references for this? )
A: There was recently a perspective paper on Nature:
https://www.nature.com/articles/s41586-019-1256-6
http://www.lenr.com.cn/uploadfile/2019/0531/20190531071312608.pdf
They claim the results are still not completely reproducible, but the experiments nonetheless shed some insights on the behaviour of highly hydrided materials.
A: They want to be rich and try to catch funding. ;)
Two ways: 


*

*They have 'a secret device' that works -> funding, patents, RICH,RICH,RICH ... (I do not believe they have this secret)

*they have 'a secret lie' - funding,  RICH (it was a risk operation, etc,.. contratual details,...ruined reputation... but RICH)


I will not make a direct judgement on bogus or no bogus. I will wait and see. May be some day, someone, do unexpected experiences like the recent 'anti-laser' that 'destroys' energy.
To justify my 'operational' position I can tell one story of academic arrogance: The national TV broadcast started here in 1955, in 1957 the russian Sputnic was sent to orbit and, conveniently, an academic appeared in TV and said 'It's impossible, russians are liers'. 
*added : *
found:
How to Transmute Elements with Laser Light 

Coherent photonuclear isotope transmutation (CPIT) produces
  exclusively radioactive isotopes (RIs) by coherent photonuclear (γ,n)
  and (γ,2n) reactions via E1 giant resonances.

EDL - Proton-21 (Adamenko)   

The primary focus of EDL's research is based on a newly developed
  and self sustainable    process which leads, through a controlled
  stimulation, to the collapse of condensed matter. In this
  collapsed state thus created, the effect of the Coulomb barrier
  becomes insignificant, and a rapid transmutation of elements and
  isotopes occurs and can be observed.

arxiv 2013/05 update  - experiment 1 - theory 0
SEVEN investigators from Universities of Italy and Sweden, namely Hanno Essén recently reported an
Indication of anomalous heat energy production in a reactor device:  

An experimental investigation of possible anomalous heat production
  in a special type of reactor …  An anomalous heat production was
  indicated in both experiments. .. , the result is still one order of
  magnitude greater than conventional energy sources.

Apparently the theory is in trouble.  
A: Here's the publication where the roots of Rossi et al's studies are: Noninski V. C., Fusion Technology, 21, 163-167 (1992). Apparently the important power imbalance found in this paper (published in a legitimate peer-reviewed journal) is to be sought in some hitherto unknown aspects of classical physics.
