Buildings

Heat Pump Options

Q: Our old heat pump is on its last legs and we want to decide what to replace it with before it actually dies. We're not sure if we should get another heat pump, since we're not always comfortable, especially in the winter, but we don't know how to evaluate the options. Any suggestions?

A: First of all, congratulations for being among the rare few who actually think ahead enough to look for a new boat before you're standing knee deep in water, bailing frantically. Normally, people don't deal with this question until the thing dies in the middle of a 20 o December night, and they buy whatever the contractor can install NOW.

Fortunately, you have the luxury to think about it a bit. Since comfort issues are important, the first thing to understand is that how tight and well-insulated your house is is much more important in terms of comfort than what kind of equipment you have.

So the first thing to do is have a skilled person evaluate your house. They should use a device called a blower door to identify the spots where outside air is entering the house, and conditioned air is leaving. Often, some relatively inexpensive strategic sealing can dramatically improve comfort.

Check the insulation levels in the floor and attic, and have them give you an estimate of what it would cost to bring them up to current building code levels. If there's no insulation in the walls, get an estimate for blowing insulation into the walls. Dense-packed cellulose insulation is often an excellent choice in older houses.

The second thing to check is the duct system. Have the ducts pressure tested to see how tight they are, where the leaks are, and how much it will cost to seal them. At the same time, have the technician check to see whether the return ducts are adequately sized, whether the ducts are delivering enough air, and whether the registers are placed correctly. We typically find very leaky duct systems with inadequate returns and poor register placement. Let's pull cold air into the system from the attic and blow it right on you as you step out of the shower, wet and naked. Talk about comfort problems!

Once you've dealt with the "thermal envelope" of your house and the duct system, it's time to turn your attention to the piece of equipment you'll hook up to it. The most common problem we find with equipment is that it is over-sized. Typically, the contractor will replace your equipment with a new unit of the same size. If you tell him you haven't been comfortable, he'll probably make it a little bigger.

In your case, however, since you've improved the house and the duct system, the new unit should be considerably smaller than the old one. Make sure the contractor does a heat loss/heat gain analysis of your house and sizes the new system accordingly, and not based on some "rule of thumb". Over-sized equipment costs you more to buy. It costs you more to operate, since it doesn't run enough to reach its most efficient level. And it will cost you more because it will wear out sooner due to cycling on and off a lot more. In the case of a heat pump or air conditioner, over-sized equipment will also lead to less comfort in the summer, since it won't run enough to dehumidify the house.

As a bonus, the money you save by using smaller equipment will help pay for the other improvements you've made to the house. As in the case of the leaky boat, you can invest your money in a bigger pump to try to get rid of the leaks so you can stay dry, or you can seal the leaks and buy a smaller pump.

Other important issues have to do with health and safety. If you decide to get a gas or oil furnace, make sure the products of combustion can't get into the house. We often measure carbon monoxide in houses as a result of negative pressures causing the gasses to come down the chimney instead of up and out. These negative pressures are typically caused by duct leaks, exhaust fans, including dryers, and even by closing doors in some houses. The best solution is to make sure the furnace is a direct vent or power-vented model and is a sealed combustion unit.

Heat pumps, of course, don't have carbon monoxide risks, since nothing is burning in the house. I've been in hundreds of homes with heat pumps that were comfortable and economical. The key is to get the house and the ducts reasonably tight and well-insulated