Warm up to your heat pump's personality

Home Work

September 20, 1998|By Karol V. Menzie and Ron Nodine

YOU KNOW FALL is approaching, in most parts of the country, when you can't decide from one day to the next whether you need to turn on the furnace or the air conditioner. That's typically the case if you use a traditional HVAC system (heating, ventilation and air conditioning), with a furnace and separate air conditioner.

In the future, it's likely that the HVAC system will do the monitoring, checking the temperature outdoors and inside your house and turning on whatever devices will make the appropriate adjustments.

A lot of people thought a step in the direction of simplicity had been taken some time ago when heat pumps were developed about the middle of this century. Initially, they were used mostly in the southern part of the country. But when the energy crisis hit in the 1970s, heat pumps came into vogue in other areas. They were popular because they promised energy efficiency in a single-unit system that could provide both warm and cool air. And all it took to run them was electricity, which made them especially welcome in new housing developments. Millions were installed.

Then the complaints began.

What is it that people don't like about heat pumps? There are two main problems. One, heat pumps don't seem to get air hot enough or cool enough, and two, when the temperature drops below a certain point outside, the heat pump can't keep up and the backup electric resistance heat kicks in -- making the electrical meter spin like a top.

The heat pump itself is not a villain -- when it's working properly, it's a model of environmental engineering, using the natural tendency of heat to flow from a higher to a lower temperature. The problem is that there's a limit to how much the heat pump can heat or cool indoor air relative to outside temperature -- and it's a fairly narrow range. That's why it doesn't seem to work very well in harsher northern climes.

When a heat pump is cooling, it is circulating coolant through coils in the air handler (inside the house) and coils in the condenser (outside the house). Heat is removed from the house by the coils in the air handler, then discharged to the outside by the coils in the condenser. The air temperature inside is lowered by about 15 degrees as it passes across the coils in the air handler.

If it's 90 degrees outdoors, and you set the indoor temperature at 70 degrees, that's a difference of 20 degrees, which the system can handle. But if it's 100 outside and you want it to be 65 inside, that's 35 degrees, which exceeds the system's capacity.

Systems in the Baltimore area are designed for temperatures of 95 degrees outside and 75 degrees inside for cooling, and 0 degrees outside and 70 degrees inside for heating.

The size of a particular unit is based on air-conditioning needs. Oversizing that part of the system will slightly increase the capacity of your heat pump. However, if the air conditioning is too oversized, it will not dehumidify the house properly -- meaning the air would feel cold and sticky in the summer.

When a heat pump heats, it works in reverse, and the heat is absorbed from the coils outside and released in the coils at the air handler. And that's another problem with heat pumps: The air coming out of the ducts is usually about 85 degrees. While that's a lot warmer than the 70 degrees you would like, it arrives as a draft, so it feels cold. And that causes people to turn up the thermostat.

All heat pumps in the mid-Atlantic climate should have a backup source of heat. Every house is different in its capacity to hold heat, depending on construction, age, insulation and so on. When the outdoor temperature exceeds the capacity of the heat pump, the backup heat comes on.

There are some alternatives to expensive electric backup heat, such as a conventional oil or gas furnace, used only when needed, or a heat-pump helper, a device that uses hot water from the water heater to boost air temperature.

There's some interest these days in geothermal heat pumps, which are similar to electric except that there is no condenser. You still need electricity to run the fans, but the coils in the condenser are replaced with coils in the ground. Water or antifreeze solution is circulated through buried plastic pipes.

In winter the fluid collects heat from the earth and carries it to the coil in the air handler. In summer the process is reversed.

The system works more efficiently than an air-based system because, below a certain level, the temperature of the earth is a constant 52 degrees. That means in winter, instead of trying to extract heat from air that is zero degrees, the system is working with temperatures no lower than 52. And in summer, instead of trying to cool 100-degree air, it is cooling from a base of 52 degrees.

While the efficiency of the geothermal system is spectacular, the installation can cost up to $8,000 more than the other type of system. And it requires a lot of land for the coil system. The bigger the house, the bigger the coil system.

There is something to look forward to -- and it's probably in the near future. The technology exists for a self-monitoring and self-operating heating and air-conditioning system. It's part of the so-called Smart House technology, which uses a housewide computer system to control environmental conditions. It's been tried in some areas and, as the costs of technology come down, more builders are likely to use it.

Pub Date: 9/20/98

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