Understanding a Heat Pump

Of all heating components, a heat pump is the most complex. It is best understood as a reversible air-conditioner. Like an air-conditioner, it can lower indoor temperatures by removing heat from the air and expelling it outside; it’s also capable of extracting heat from outside air and pumping it indoors. To do its job, a heat pump— like all refrigeration devices—takes advantage of liquid’s tendency to absorb heat as it expands and turns into a gas, giving off heat as it’s compressed into a liquid (see anatomy diagrams at right).

A split-system heat pump (shown at right in the heating mode) uses two units, one outdoors and one indoors. In the outdoor unit, a fan moves air through a coil that absorbs heat. A compressor then superheats the vapor and sends it through refrigerant lines to a second coil in the furnace. There a blower pushes return air through the coil, warming the air and forcing it into the ducts. Meanwhile refrigerant travels back to the outdoor unit to begin another full cycle through the pump.

An automatic reversing valve reverses these flows. It has to be automatic because when outside air temperatures approach the freezing mark, heat pumps tend to freeze up. When this happens, a sensor activates the reversing valve and the unit defrosts itself. In some heat pumps, the compressor, fan, coils, reversing valve, and blower are enclosed in a single outdoor cabinet, as shown at right. Only the system’s main supply and return ducts penetrate exterior walls; there is no separate furnace.

Operating a Heat Pump

Heat pumps work well down to temperatures of about 15°F. If temps go below that, most units require a backup heating source, usually electric-resistance elements installed in the furnace, the ducts, or the pump cabinet. These units also take over for the pump while it’s defrosting. When a heat pump’s defrost cycle runs continuously—or not at all—the backup system takes over, a shift you might not notice until your electric bill arrives. That’s why it pays to familiarize yourself with what happens during a normal defrost cycle.

When the temperature hovers around freezing, frost forms on the outdoor coil. When this occurs, the reversing valve should activate to melt the ice. You may hear gurgling or even see steam rising from the outdoor unit. Heavy ice accumulation means the unit isn’t defrosting. No ice, or defrost cycles that last longer than 15 minutes, indicate that the pump is stuck in cooling mode. For either condition, check the outdoor coil. Leaves, snow, or other matter may be cutting off airflow through the coil. Clear the obstruction and the system should return to normal operation.

If it doesn’t return to normal and the coil remains coated with ice, the reversing switch may be stuck. Try freeing it by switching your house thermostat to the cooling mode. If the ice remains on the coil after an hour, flip the system selector switch to the “emergency heat” setting and call a heating contractor for service. If all electrical power has been off for more than an hour at temperatures lower than 50° F, because of either a power outage or a tripped circuit breaker, do not attempt to restart a heat pump for at least 6 hours after the power has been restored. Instead, turn the system selector switch to “emergency heat,” wait 6 hours, then return to the normal heat setting; turning the system switch to “off’ doesn’t shut off the heater. This time-delay gives the heating element in the compressor’s oil crankcase time to warm up the system’s lubricant and prevent valve damage. The chart below identifies the most common maladies of heat pumps and what to do about them. As with other heating units, it’s good to have a heat pump serviced annually.

Troubleshooting a Heat Pump System




Pump does not run

No power to the unit or the thermostat is not calling for heat

Check the thermostat setting, the electrical disconnect switch, and the fuses or breakers in the circuit panel. Most pumps have a “reset” switch in the outdoor cabinet.

Short cycles

An obstruction blocking the outdoor coil; malfunctioning blower unit; clogged filter

Clear the outdoor coil (see above). Check the filter and blower unit.

Long or frequent defrost cycles

A blocked outdoor coil could cause defrosting that lasts longer than 15 minutes or that occurs more than twice an hour.

See text above for symptoms and what to do.

Uneven heating

Heat pumps deliver a cooler flow of air than you may be used to. Also, indoor temperatures normally will drop 2° to 3°F when the outside temperature reaches the system’s balance point differential. This is the point at which the backup heating kicks in.

Minimize airflow discomfort by carefully balancing the duct system. To offset the balance point differential, you may have to raise the thermostat setting in colder weather.

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