If I replace all my lights with LEDs will my heating costs increase? A number of nations are passing bills to phase out incandescent light bulbs. The thinking is that the tungsten filament is an inefficient method of turning electricity into light, the rest of the energy is converted to heat, so LED will save power and lower energy usage, electricity costs.
In the UK where the atmosphere is generally cooler than room temperature, except for peak summer weeks when the evenings are light, will the use of LED lights result in my central heating having to work harder, burning gas, to make up the difference?
 A: In theory, in an over-simplified building physics model of your home's thermodynamics, yes, all of the wasted energy from inefficient devices goes to providing useful heat to the home, so increasing their efficiency will increase the heating you need from the space-heating system.
But in practice, no, your heating costs will not increase. Not unless you choose to turn up your heating.
At least, in general, that's how it goes.
The thing is, that incandescent lights tend to add heat to places where it's not particularly useful: the centre of the ceiling of a room.
For a combination of reasons, that's rarely useful heat.


*

*Particularly in older UK housing stock, rooms have high ceilings with thermal stratification, which means a layer of warm air stays stuck at the top of the room. Tha layer is higher than thermostats and radiatior TRVs, so changing its temperature won't change the behaviour of the automatic heating controls.

*What will happen is that the void between floors, or the loft, will get very slightly warmer. The void between floors is typically ventilated to the outside with air bricks, so your incandescent lights will warm up the outside air slightly.

*The temperature field in most rooms in UK housing is pretty steep, with floors being a few Kelvin cooler than ceilings. The bigger the difference between the temperature at your feet and at your head, the less comfortable you are. The heat from lighting tends to make that temperature difference bigger, thus decreasing your thermal comfort.

*and although you referred to peak summer weeks, I'm guessing your heating is turned off for a lot more of the year than that: May to September or so would be fairly typical in the UK.
To bear the theory out, see Brunner et al, who looked at heating bills in dwellings which had incandescent light bulbs, and those that had more efficient compact fluorescents (CFL). They found no support for the hypothesis that CFLs cause increased heating costs.
A: Yes.
Of course, an efficient gas boiler to hot-water radiators is probably a cheaper way of heating your house than electric incandescent light bulbs. So the LED conversion is still probably worthwhile. It also saves filling landfill sites with broken glass I suppose.
Perhaps you should swap your bulbs back and forth in spring and autumn :-)
A: Yes, heating costs will increase. However, gas heating is more cost-effective than electrical heating. In other words, you get more "heat for your buck" with gas than with electrical heating.
To confirm this, let's say that the light bulb's heat corresponds to a conversion of 100% from electricity to heat. And that gas is 90% efficient.
Here in US, a you might spend around 11$ per 1,000,000Btu with gas. And electricity, anywhere between 7-33 cents per kW*hr. Doing the math, we get that with gas we would spend around 0.001 cents/kJ, while with electricity between 0.002 - 0.009 cents/kJ, i.e., with gas you would be paying, at worst, 52% of what you would be paying with electricity.
All was back of the envelope calculations, you are free to look for your prices and crunch in the numbers.
Cheers!
A: Phasing out incandescent light bulbs in countries with a cold climate boils down to reducing electric heating in favor of gas heating. This makes a lot of sense when you look at the energy efficiency from a total systems perspective. 
In order to heat your house, should you bring the gas molecules to a distant power plant, burn these molecules and convert some 60% of the chemical energy into electric energy, 95% of which you transport to your home via power lines, to finally convert to heat in incandescent light bulbs? 
Or should you bring the gas molecules to your home and burn them, thereby heating your house?
If the infrastructure is in place to do the latter, it makes little sense to opt for the former.
A: If electric heating is the most cost effective where you are, then the answer is yes, as the light bulb is essentially an electric heater.
If gas heating is more cost effective than electric, as it often is, then you will still come out ahead with the LED bulbs.
