Let’s start with a quick overview of the differences between Air to Air heat pumps and ‘Water to Air’ or ‘Water to Water’ heat pumps.

Air to Air heat pumps

These units work very much like an air conditioner but can also run in reverse to heat your house or building. The way they work is by transferring heat from the air outside into the air inside your home. Even when the air outside is cold (let’s say 10°C / 50°F) you can still warm your house to 25°C / 77°F.  It would cost four times (4x) as much to heat the same space using an electrical heater.

Unfortunately, when the outside temperature drops to below zero these units can no longer pull as much heat out of the outside air.  That’s when an electrical back up kicks in and heats up the air inside your house.  This decreases their overall efficiency, but they’re nevertheless efficient the rest of the year (fall, spring, and summer).

Water to Air or Water to Water heat pumps

These are a whole lot different because their source of heat is not the outside air temperature, but instead, the ground temperature underneath or near your house. The ground temperature remains relatively constant even during a Canadian winter.

If the water going into a heat pump is above freezing*, the heat pump can suck degrees out of the water and transfer that energy into hot air or hot water.  This hot air or hot water can then be used to keep your house warm.  These units work by adding a small amount of heat over a longer period of time compared to a gas or oil furnace.  So, if you opened your front door long enough in the middle of winter and the house temperature dropped suddenly, a back up electrical heater kicks in to quickly raise the temperature.  Of course, this costs more money to run and you want to avoid that sort of thing.  In my experience, the electric backup heater only kicked on once or twice for about half an hour during a winter season.  Overall, I’d say the heat pump furnace probably reduced my electricity bill by more than half (probably close to a quarter).  More details about the residential geothermal heat pump experience.

The absolute best choice for keeping your house an even temperature is thermal mass (with or without a heat pump). Your house can act like a passive solar collector. The thermal mass around and at the center of the house will radiate energy back into the home during the night after it has ‘charged’ up from solar exposure during the day. If building from scratch is not an option there are small modifications that you can make to reduce the amount of energy and the amount of money you spend to keep your home warm and lit.

*note: a closed looped system can also be filled with anti-freeze.

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So the way it works is really quite simple, you have a source of constant temperature water year round. The water might be cold (8 to 10 degrees C) however, it still has some warmth in it before it will freeze. So you run it through your heat pump and out comes water that’s a few degrees cooler. So where does the heat come from? The water. For every 1 unit of electricity put into running the heat pump, you get 3 units of heat out of the ground, producing 4 units total by the system. I was often asked, “How efficient is it?” The answer is usually unexpected. It’s not 100% efficient, it’s like 400% efficient. This usually starts some argument that I don’t know what I’m talking about, but the proof is in the dollars spent running the system. Instead of costing me $1400 to $1800 a year to heat and cool the house, it became more like $400 to $600. Less energy is being consumed from hydro Quebec, more is being used from the warm ground around your house. If I were to get another one, I’d go for a closed loop system. Preferably the loop would run into a pond or river instead of the ground (cheaper to install).

WaterFurnace has a nice pamphlet describing the way it works.

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