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10W max & 15kW-hr/m^2 for passive house: is this for heat or energy?

Peter_R | Posted in PassivHaus on

My understanding is that a passive house must consume less than 10W/m^2 max and less than 15kW-hr/m^2 per year.  But is this heat consumption or energy consumption?

For example, imagine a 100m^2 house that requires 2000W of heat to maintain 20 degC at the minimum design temperature in winter.  This works out to 2000W / 100m^2 = 20W /m^2 of heat.  

Now image that this heat is produced with a heat pump, which takes only 700W of electrical energy to produce the 2000W of heat energy.  Does this house meet the passive house requirement or not?  In other words, would the house’s consumption be calculated as 20W/m^2 or 7W/m^2?

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Replies

  1. Trevor Lambert | | #1

    I don't know for sure, but I believe it to be the energy required to heat the space. In other words, it doesn't take into account the COP of the heating source. The reason I think this is that our PHPP report doesn't include any place to specify the particular heating equipment being used, or its COP. At least not that I could find.

    Note however that the two requirements you listed (15kW/m^2a and 10W/m^2 peak) are alternative requirements. You only have to meet one of them, not both.

  2. Peter_R | | #2

    Thanks Trevor. My impression is also that the COP of the heating source is not included, but I haven't seen this written explicitly anywhere.

    Thanks for the correction about the OR vs AND for heat loads. For the max heat load, do you know if buffering from thermal mass "counts" towards the 10W/m^2, or if heat consumption must be modelled in steady state? For example, let's say the time constant for your house was ~15 days, and so you can whether through a cold snap consuming less than 10W/m^2, but not if the cold snap continued indefinitely.

  3. Trevor Lambert | | #3

    I don't think thermal mass is factored in anywhere. When you're talking about peak heating load, you're typically looking at a diurnal cycle, not ~15 days. A thermal mass capable of retaining surplus heat ~15 days is going to be huge. I know there are practical problems with dealing with a mass that size, but that's way above my pay grade. I can tell you that my house has a pretty substantial thermal mass (1600ft^2, 8" thick concrete slab), passive house level insulation and glazing, and it can't maintain the temperature overnight in the dead of winter, let alone multiple days.

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