On houses and hairdryers

Temps in the 20s today. House seems to be operating pretty much as expected. Needs only the equivalent of 1-2 hairdryers to maintain inside temps according to the readings I am seeing from our eMonitor device which is attached to the heating circuit on our house. As our current president would say, “Now let’s be clear about something…” A Passive House in New England still needs heat. Somehow. So beware the wood stove behind the curtain. Or the air-source heat pump. Or the electric floor heater in the bathroom. An enthusiastic owner might claim they have no furnace. And that the “mini split heat pump” is just for back up. Or the pellet stove. But they need something!

The more ambitious will have elaborate active solar thermal systems which truly do heat their homes, even if there is a string of 5 days of clouds let’s say. But this is rare to have a 100% solar heated house. I have only read about them! (Norman Saunders, MIT Solar1, etc.) More common are “passive solar” or “solar tempering” where there are bigger windows on the South with high SHGC. This does help. But it’s probably not going to get you all the way.

So when I say that our house needs a hairdryer to keep warm, run some quick numbers! Let’s say one used two 1500W hairdryers (same as a typical electric room heater from the hardware store) running straight for 7 months let’s say… Oct/Nov/Dec/Jan/Feb/Mar/Apr. Then that’s:

7*30*24h*2hairdryers*1.5KW = 15,120KWh. What do you pay for a KWh? Let’s say it is $0.20/KWh. So that hairdryer just cost you $3024! That’s why people do not like to heat with electric baseboard. It’s expensive!

Now, in our case, we also get to divide by approximately 2.5 because our air source heat pumps actually pump out about 2.5x more heat than they use. (Same idea as a geothermal heat pump setup). And reduce it by a bit more since certainly they won’t be running full capacity for 7 months. Let’s say 50%

So 15,120 / 2.5 * 0.50 = 3024 KWh times $0.20/KWh = $604.80 for heat for the entire winter season 2010/11.

BTW, you’d probably get approximately the same numbers if you heat with propane or pellets vs a heat pump. That was my conclusion last time I ran the numbers. The main point is the KWh. It’d still be better to use solar or wood/pellets though from a carbon footprint standpoint. And this is regardless of whether you have PV panels on the roof or not. Don’t let that confuse your choice of how to heat your house!

BTW2, I don’t recall exactly, but that’s probably about the same ballpark that the PHPP software for Passivhaus predicts this house will use “on average” per year for heating.

I’ll let you know how it turns out.


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