Heating with Heat Pumps
Heat Pumps can be a very economical way of heating and cooling your dwelling depending on geographical location and the cost of electricity in your area. The volatile costs of natural gas, propane, and oil has enabled these conditions where it can be cheaper to heat with a heat pump than with fossil fuels. There is a common conception that a heat pump blows cold air and while this can be true in some cases it is not true to all heat pumps. If a heat pump is blowing cold air when in the heating mode then it needs to be looked at because there exists a problem that needs to be corrected. Even when the outside unit kicks into the defrost mode the unit should provide adequate heat to heat the zones which it serves. Again, if it is not providing adequate heat then it needs to looked at for a technical problem that needs correcting. A well designed heat pump system, working properly and efficiently, can provide competitive and economical heat for your dwelling.
How a Heat Pump Works
A heat pump works much the same way an air conditioning unit works with a few differences. In the cooling mode it transfers heat outside from the inside and in the heating mode it transfer heat from the outside to the inside*. That is, in the heating mode it transfers heat from the outside to the inside with a minimum outside ambient air temperature affecting its efficiency and the amount of usable heat transferred. That outside air ambient temperature is different from one unit to the next and entirely depends on the systems design, refrigerant, and size. The lower the temperature outside goes the lower the ability of any air to air heat pump to transfer usable heat inside. The temperature ranges for the end of effective heating for a majority of heat pumps averages approximately 35 to 40 degrees Fahrenheit.
Heat Pump Geographical Considerations
For this reason air to air heat pumps, which make up a vast majority of the heat pumps installed, are primarily installed in the Southern states regions of the US. It is also necessary to have some sort of back-up heat for heat pumps for two reasons. Most heat pumps come standard with back-up electric heat (some have gas or oil back-up heat called dual-fuel) and this back-up electric heat is used for the heat pumps defrost cycle and when the temperature outside drops low enough for the heat pump unit to lose its effective heating ability.
Heat Pump Defrost Cycle
The heat pump defrost cycle is necessary because the outside heat pump unit will operate below the dew point. It is necessary for the heat pump unit to operate below the outside temperature so that it may absorb heat from the outside air. Remember, even though the temperature outside is 40 degrees Fahrenheit there is heat contained in the air and the heat pump is working to absorb this heat so that it may transfer it into the dwelling for use. Because the heat pump operates below the dew point it will frost over. This frost on the heat pump needs to be removed. Most heat pumps automatically defrost themselves based using a timer control. This timer is usually set by the technician who starts the heat pump unit. The times offered by a majority of heat pump manufacturers range from 30 minutes to 150 minutes. Depending on how this defrost timer is set can depend on the efficiency factors for the heat pump unit. For example, if the defrost timer is set to 30 minutes, every thirty minutes the outside heat pump unit will kick into defrost. The defrost cycle for a heat pump causes a reversing valve to shift in the heat pump unit and it effectively becomes an air conditioner. That means the evaporator coil for the heat pump will begin to introduce cold air into the home. To counter balance this cold air, when the outside heat pump kicks into defrost it simultaneously sends a signal to the indoor air handler to engage the back-up heat. The heat pump defrost cycle will continue based on a set time and then it will return to normal heating operations with the outside unit absorbing heat for transfer to the inside and the back-up heat disengaging. If the unit does not frost up every thirty minutes it is inefficient to have the heat pump defrost timer set to thirty minute intervals. The start-up technician should know the best time intervals based on his experience with your geographical location.
Heat Pump Back-up Heat
Unless you live in a geographical where the temperature never drops below 40 degrees Fahrenheit you need to have some sort of back-up heat with an air to air heat pump. Most heat pumps sold, unless otherwise specified, come standard with back-up electric heating elements for back-up heat. When the outside air temperature drops below the effective heating temperature of the heat pump unit the back-up heat will take over and provide heat for the rare times the temperature drops low enough for the heat pump unit to lose its heating effectiveness. A heat pump should have a temperature control installed to turn the outside unit off when the temperature drops below its effective heating point and allow only the air handler and back-up heat to provide heat. Many heat pumps do not have this type of control so the outside heat pump continues to run even though it is no longer useful for providing heat. This temperature control can be set according to your geographical location and the type of heat pump you have by the start-up technician.
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