Heat pumps: hot air in from the cold

Heat pumps: hot air in from the cold
Last Updated: Friday, November 13, 2009 | 9:19 AM ET Comments7Recommend2
Grant Buckler, Special to CBC News

It sounds like it shouldn't work. To warm your home on a cold day, you bring in heat from outside.

"What heat?" you ask. "This is Canada in winter. The heat went south in the fall with the migratory birds."
Is a heat pump really environmentally friendly? It depends on your location and how your electricity is generated. (iStock)Is a heat pump really environmentally friendly? It depends on your location and how your electricity is generated. (iStock)

Yet, in fact, heat is out there. Anything at a temperature above absolute zero (273 below zero Celsius) contains heat. Just as emptying a glass of water into a bathtub raises the level in the tub — though only slightly — transferring some heat from outside to inside will warm the house.

The problem is how. You can't just open the window, because of the second law of thermodynamics, which says heat won't move on its own from a colder place to a hotter one. But the law doesn't say you can't transfer heat from cold to hot — just that you have to work at it.
Like a refrigerator

That is what heat pumps do. A heat pump doesn't create heat as a furnace does, but rather moves existing heat around. Most of the time it takes less energy to move heat than to create it, and that's why heat pumps are more efficient.

A heat pump isn't the only machine that moves heat. You have at least one appliance that does it: your refrigerator. A refrigerator cools by removing heat from inside the fridge to outside — from a colder place to a hotter one, just like a heat pump.

In fact, a heat pump does almost exactly what a refrigerator does, only on a larger scale and with an opposite goal: to warm the place the heat goes to rather than to cool the place it comes from. Unless, of course, it's summer and the heat pump is operating in cooling mode, when it reverses and removes heat from the house to outdoors. Sounds like an air conditioner? That's just what it is — heat pumps, air conditioners and refrigerators all work on the same principle.

The key is that compressing a gas heats it, says Brian Killins, a senior standards engineer in Natural Resources Canada's Office of Energy Efficiency. "That's kind of the basis for why this works."
How it works

There are other kinds of heat pumps, but for this explanation let's assume we're talking about one that extracts heat from outdoor air — an air-source heat pump.

You start with a liquid that is colder than the outside air. You circulate this through pipes outside, blowing outdoor air over it. Because it's colder than the outside air, it absorbs heat from the air (even though that air might be at below-zero temperatures).

Then you compress the liquid, causing it to heat up and turn into a gas. Once it is warmer than the indoor air, it passes through another series of pipes, called a heat exchanger. A fan blows air over the heat exchanger, heating the air and blowing it through ducts to heat your house, while cooling the gas in the pipes, which condenses back into liquid.

Then the liquid passes through an expansion valve, which decompresses it and thus cools it to below the outdoor temperature so the cycle can begin again.

If the heat pump is ground-source instead of air-source, then instead of blowing outdoor air over a coil you have pipes buried in the ground and liquid circulates through these to pick up heat from the earth. Most designs use a separate antifreeze solution in the underground pipes that transfers its heat to the refrigerant that goes inside the building, Killins says, but in some designs the same liquid circulates through the whole system.

In a water-source heat pump, water is pumped from a well, lake or pond, heat is extracted from it using the same process as with other heat pumps, and then the water is discharged into a second well or body or water.
Costs and considerations

Ground-source heat pumps are most efficient because the ground stays warmer in winter than the air does. Some of the heat in the ground comes from within the earth, so these types of heat pumps are often called geothermal systems. But ground-source heat pumps are also expensive, because they require a well or trench to bury pipes.

The cost depends partly on location. Killins says quite a few are being installed around Winnipeg, where drilling is fairly easy.

An air-source heat pump costs less, because the outdoor portion is just a metal box containing a compressor and some piping. But if the outdoor temperature drops too low, it won't heat a house. Thus air-source heat pumps are popular in the warmer climate of southern British Columbia, says Jeff Zimmerman, technical co-ordinator for the Heating, Refrigeration and Air Conditioning Institute of Canada.

They can also be combined with other heat sources. A typical installation might have an air-source heat pump and a conventional furnace, with a programmable thermostat that chooses which to activate based on the outdoor temperature and the amount of heat being called for inside.

Most air-source heat pumps work at outdoor temperatures down to around –15 Celsius, but air-source heat pump technology is improving. Killins says some newer units are designed to be effective at outdoor temperatures as low as –30 Celsius.
Ask about the refrigerant

Is a heat pump really environmentally friendly? It depends on your location and how your electricity is generated.

A heat pump using electricity generated from fossil fuels may not ultimately be more efficient than a high-efficiency gas furnace, Killins says, but electricity from hydro plants is better.

Another possible environmental concern is the refrigerant in the heat pump. Like older air conditioners, early heat pumps used chlorofluorocarbons, or CFCs, that have been linked to depletion of the ozone layer and global warming. Other substances have since replaced these. One more recent refrigerant, a hydrochlorofluorcarbon (H-CFC) called R-22, has been widely used but will be banned as of next year.

Zimmerman says the market is largely moving to the safer hydrofluorocarbon R-410A. He recommends asking about the refrigerant when buying a heat pump.