Based on temperature alone, it appears that I'm gaining more heat than losing in my HRV. What I'm seeing is the warm air leaving my house dropping by 10C, and the cold air coming in rising by 12C. It would seem I'm getting a bonus 2C of heat from somewhere. Not being a 'free energy nut', there's something else happening and I'm looking for a moderately simple explanation. In addition to heat (watts?) and temperature (C) being different concepts, I suspect it has to do with humidity...


  • Stale (warm) air leaving the house gives up some heat as it passes through the HRV. This is indicated by the red line below 0.
  • Fresh (cold) air entering the house absorbs some of this heat passing through the HRV. This is the green line above 0.
  • The sum is the black line.

The black line is the interesting 'conclusion'. What it is showing is that Tdelta of the fresh air is larger than the Tdelta of the stale air. Which is not what I expected (certainly not in cooler weather).

With real life inefficiencies, I would expect the black line to be below 0 indicating that not all the heat given up by the stale air is being absorbed by the fresh air.

I'm posting this late fall. Outside temperature is < 10C. Inside is ~20C.

The link is a (near) real time monitor of my HRV unit. I have temperature probes in each of the HRV's 4 ports (stale air in & out, fresh air in & out). The probes have a precision of < 0.1C and an accuracy of +/-0.5C. (raw data: http://flyinglow.ca/furnace/hrvday.png). The HRV unit does not mix the 2 sources of air - they are kept separate by a passive heat exchanger.

  • $\begingroup$ Is your unit inside the house or outside? Is it surrounded by warmer air? $\endgroup$
    – BowlOfRed
    Oct 23, 2015 at 21:19
  • $\begingroup$ The HRV is inside, however, it is extremely well insulated and the probes are all located very close to the core. There may be trace heat gain via ambient temperature, but that doesn't explain the dramatic "gain" when someone is showering. $\endgroup$
    – ehud42
    Oct 24, 2015 at 22:03
  • $\begingroup$ The issue is certainly more dramatic while someone is showering, hence my suspicion that humidity in the stale air is carrying / supplying more heat without as dramatic of a temperature drop. $\endgroup$
    – ehud42
    Oct 24, 2015 at 22:08

1 Answer 1


One possible explanation: you may have a lot of electric appliances, such as a refrigerator, that heat up your house. Furthermore, some HRVs consume electric energy.

  • $\begingroup$ Other appliances would be heating the stale air before it enters the unit. The HRV consumes less than 100W. Might account for the 1-2C (net) rise during normal use, doesn't account for the 10+C net rise when someone is showering. $\endgroup$
    – ehud42
    Oct 24, 2015 at 22:05
  • $\begingroup$ @ehud42: It is not obvious that the mass of outgoing warm air is the same as the mass of incoming cold air. What I wrote can explain why the energy balance can be positive if you don't take into account the appliances. As for showering, there is another reason for positive balance: you should take into account the hot water that warms up the air. $\endgroup$
    – akhmeteli
    Oct 24, 2015 at 23:00

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