Heat pumps raise burning questions If they work so well, why do so many builders opt for gas furnaces?


February 02, 1997|By Karol V. Menzie and Randy Johnson

A FEW YEARS ago, heat pumps were all the rage. Utility companies were promoting them, and builders were installing them in every new house. They seemed to hold out the promise of efficient heating and cooling at rates that were cheaper than other forms of electric heat, such as baseboard heaters.

They're still being promoted in some places, but builders,

especially in the mid-Atlantic, are touting "gas heat" in their developments.

What's the truth about heat pumps?

Do they work?

Do they fulfill the promise?

The simple answer is, yes, they do work. They do exactly what they are touted to do, which is provide safe, clean, efficient heating and cooling in an all-electric dwelling.

But people who don't understand how they operate, or who don't know how to maintain their units, might be disappointed with the performance of their heat pumps.

Here's a primer of how they work, to help you decide if they're right for you (or to help you figure out why you're unhappy with your unit).

Heat pumps are used mainly in two situations: Where electricity is the only source of power, and where people who could have an oil-burning furnace are leery of running out of oil during a cold snap, or of paying for oil they don't use, or of adding carbon dioxide, sulphur dioxide and nitrogen oxides to the atmosphere in the burning process.

Without getting into anything too complicated in the way of physics, we can say that heat pumps work because they exploit the fact that heat flows from a higher to a lower temperature.

Normally, systems have two components. The outdoor unit has a coil, a fan, a liquid refrigerant that absorbs heat and a compressor that converts the liquid to gas. The indoor unit has another coil connected to the outside one, a blower, a filter and, usually, a backup heating source.

During the heating season, outdoor air containing heat (thermal energy, from the sun) is circulated across a coil on the outdoor unit that is filled with a liquid refrigerant cooler than the air. The heat in the air moves to the cooler liquid. The liquid then evaporates into a low-pressure gas.

Next, the compressor in the outdoor unit compresses the gas into a hot high-pressure state. The hot gas is pumped to a coil in the indoor unit where cooler indoor air is blown across the hot coils. The coils warm the air, and it is distributed by the duct system.

The natural action of hot air rising to the top of the house and cooled air falling to the lowest point distributes heat throughout the house. The air-return system guides the cooled air back to the coils for reheating.

After the gas has flowed through the indoor coil and given up its heat, it condenses back to a liquid state. It's then piped back to the outdoor coil where the process starts over.

In the summer, the refrigerant changes direction, and the outdoor and indoor coils exchange functions. The coils in the interior unit remove heat from the interior air. The refrigerant carries the heat to the outside coil where the fan cools it, blowing the heat into the outside air.

Once the refrigerant is cool, it circulates back inside to collect more heat from interior air.

Pressurizing the refrigerant is what manipulates the system to collect heat from the appropriate area, depending on the season.

So far, so good. But the catch is that heat pumps work best when it is not real cold outside. When it is extremely cold, the heat pump will require backup heat to keep the interior comfortable -- and, in most cases, the backup is expensive electric-resistance heat.

When temperatures approach freezing, water vapor from the outside air will condense on the outdoor coil and freeze. The ice interferes with the coil's ability to pick up warmth from the outside air.

The heat pump, which is actually quite an intelligent device with a thorough understanding of thermodynamics, will switch to the cooling mode, temporarily pretending it's summer, and blow warm air across the coil to melt the ice.

As it's defrosting, the system is switched over to backup heat to maintain the temperature inside. The colder it is outside, the more you will spend on backup heat.

In fact, areas where the outdoor temperature routinely falls to about 15 degrees are not good candidates for heat pumps. Even at 30 degrees outside, you might need sweaters and blankets to keep warm inside.

You can save money by lowering the thermostat (3 percent for every degree), but if you have a heat pump, it requires a specially designed thermostat that turns the heat back up slowly so you don't go into auxiliary heat mode and spend a lot of money bringing the temperature up to normal.

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