REPRISE: Do You Have a Heat Pump?

January 3, 2010

In response to last night’s 12-degree low, we are reprinting a story that originally appeared in the Falls Church Times on Jan. 17, 2009.

By GEORGE SOUTHERN
Falls Church Times Staff

“These are the times that try men’s heat pumps.”
(Apologies to Thomas Paine)

It’s 10 degrees outside: do you know where your heat pump is? Our first experience with such a machine came when we moved into our Falls Church townhouse six years ago. The first cold snap, I was horrified to look outside and see our heat pump encased in frosty, white ice. A frantic call to Eric at SCS Heating & Air Conditioning ensued:

Eric: “Have you ever had a heat pump before?”

Me: “No.”

Eric: “You’re not going to like it.”

It’s normal for heat pumps to ice over, Eric explained, but they have a defrost cycle that should melt it every so often. Our tired and rusty unit was low on refrigerant and couldn’t muster enough heat to properly defrost. Eric gave it a shot of Freon, and another shot the next summer, but ultimately we replaced it with a shiny unit that hums away and keeps us warm.

Well, sort of. I took its temperature yesterday, dangling a thermometer down a heat vent. The best it could do was 84.4 degrees. Since body temperature is 98.6, the “heated” air feels cool to the skin, even though it “warms” the house. For the last few days our heat pump has run continually but never has managed to raise the room temperature above 66 degrees. That’s actually due to my own frugality – last year I tampered with the thermostat and managed to partially disable the emergency heat strip function. Now the heat strips only turn on if the room temperature is at least 6 degrees lower than the thermostat setting.

If you have a heat pump you probably know about heat strips – little electric wires in your furnace that glow red hot, just like a toaster except on a massively larger scale, causing your electric meter to spin so fast that looking at it makes you dizzy, and looking at the bill makes you faint. Heat strips are the “backup” heat source when it’s too cold for a heat pump to warm the house to your liking. They actually have little to do with a heat pump, being an entirely separate source of heat.

So how does a heat pump work? Where’s the heat come from? Astoundingly, that 84.4 degree heat I’m getting is being extracted from the 10 degree “cold” outside. It turns out that 10 degrees is a lot of heat compared to no heat at all, which is defined as “absolute zero,” or minus 459.67 degrees Fahrenheit.

The unit sitting outside your house that makes so much noise is actually pretty simple – just a giant radiator wrapped around a compressor. In the heating cycle, very cold, liquid refrigerant circulating in the radiator actually absorbs heat from the 10 degree outside air, causing the refrigerant to “boil,” or evaporate into gas. The compressor then compresses this gas (I tried to think of a word other than “compresses,” but that’s what compressors do) and forces it into the house to the furnace section of the heat pump. Here, the gas enters another radiator where it condenses into a liquid. The condensation process throws off heat, which a fan then blows through the heat vents of your house. The now-liquid refrigerant then gets pumped outside to “warm up” again. (I hope I got this right!)

Eric actually was wrong: I have learned to like my heat pump. Not love it, but like it. It’s the most efficient source of heat available, since it uses the heat that’s right outside my window. And were I willing to spend thousands more $$$ I could make it a lot more efficient by burying the refrigerant tubing deep underground where the temperature is about 60 degrees year ’round. Once compressed, the refrigerant could throw off a LOT of heat.

The other big advantage of a heat pump is that with the flick of a switch the whole operation can be reversed such that heat is instead pumped OUT of your house – but that’s a different story for a different time, say ’round about August.