Economical Winter Heating: Window Heat Pumps

[air to air heatpumps are not well suited for very cold climates. if you can make snowmen in your backyard, this probably won't help you]


Last year I went about making summer cooling more efficient: http://www.candlepowerforums.com/vb/showthread.php?340427

Its working well and as long as its under 60F at night, the AC rarely needs to go on the next day. We're more comfortable and energy bills are smaller. So this year, I wanted to do the same thing for winter heating. But its tricky to efficiently make heat and if you're not burning something, the costs add up in a hurry. Even a badly designed space heater is near 100% efficient at converting electricity into heat. So how do you get more? By moving heat, instead of making it.

Technically speaking, a standard AC is also a heat pump. It moves heat from the inside to the outside. This is just that, in reverse: moving heat from the outside to the inside. So what does that look like in numbers? Here is BTU per watt for three different parts of the same window heat pump (Frigidaire FRA08PZU1):



So for every 3000 BTU generated by a traditional (resistance) heater, a heat pump will generate 9000 BTU - with the same amount of energy. Or put another way, if half your winter electric bill goes to heating, this can drop your bill by a more than 25%.

This is amazing, so whats the catch? The catches are three: 1) To bring heat in from the outside, there has to be some degree of heat outside. The general limit is 45F, below which the heat pump looses much of its advantage. But this is perfectly suitable for much of the country (southeast/south/southwest/northwest). 2) Units that can do this are hard to find. Less than 10% of all window units listed at ajappliances even have heat pump capability and of those, only 2 will operate on 110 volts (the rest needing 220). 3) Units with the same cool rating that only do cool, cost less. And heat pumps large enough for a an entire house (mini splits) cost as much as central AC and while they can do everything a central system can, if your central AC is still working fine, that a lot of extra cash for one service (heat).

Rather than bump that other thread on cooling, I'm a little confused on how you prevent allergens and generally dust/particulates from getting in.
It seems like you're blowing a lot of outside air in and I realize it's positive pressure and nothing will be pulled in from the pressure difference, but how do you ensure the air coming in is clean?

That's a bit of a mystery. It wasn't planned but is a happy side effect of the main use/goal - cooling.

There are standard screens installed behind the fans, but that's not fine enough to filter allergens. My best guess is that the allergens in our area that affect us, don't get high enough outside to reach the point on the second floor where the fans are installed and air is pulled in.

We regularly have guests who comment on the temp/air quality, including self reported allergy sufferers. But, I've not seen or heard of anyone trying it here or anywhere else, so this is a single data point.