An explanation to assist motorhome owners in deciding which equipment best meets their overall needs.
By Bill Hendrix, F761S
June 2002
When selecting cooling equipment for your new motorhome, or if you’re replacing an existing roof unit, several choices exist. You can opt for a conventional air conditioner, a conventional air conditioner with an optional heat strip, or a heat pump air conditioner. Unfortunately, it may be difficult to find the information you need to make an informed decision. The price difference is significant, so following is a plain-language comparison to assist you in making the decision.
If you want your air conditioner to provide some heat when marginal temperatures don’t justify running the furnace, you should choose either the optional heat strip or the heat pump model. Dollar-wise, the heat strip option will raise the price about $70; a heat pump will increase the price about $100 more than the heat strip.
The heat strip is an electrical resistance heater that will produce about 5,000 Btus of heat at about 15 amps of current, which is comparable to one of those little portable electric heaters commonly sold at hardware stores. In addition, the heat strip consumes about the same amount of energy as the cooling mode of the air conditioner. When the room is cool, the air from the heat strip may not feel very warm, since the blower is a high-volume unit and the 5,000 Btus are distributed among many more cubic feet of air volume as compared to a portable electric heater with a low-volume fan.
When an air conditioner is operated in the heat-pump mode, the amount of heat produced will vary according to the outside temperature. At cool temperatures, a heat pump may produce two to three times the amount of heat as a strip heater. In very cold weather, especially below freezing, a heat pump may not produce as much heat as a strip heater. The reason is rather simple. A heat pump is an air conditioner circulating its refrigerant in a backward flow by using a reversing valve. The evaporator that used to get cold now gets warm; the condenser that previously got warm now gets cold; and the blower circulates the warm air into your living compartment.
In order to understand heating and air conditioning, you must recognize the fact that cold is actually the absence of heat. This sounds strange, but it is true. In order to make air conditioning work, heat must be absorbed from the air and displaced to another area. When the refrigerant evaporates, it causes a temperature reduction, making the evaporator coil colder, and heat is absorbed into the cold evaporator and discharged out of the condenser as warm air.
Therefore, air conditioning is just a means of transporting heat from one place to another. When the air conditioner is operating in the cooling mode, the blower circulates the living compartment’s air across the cool evaporator, and the heat is absorbed into the refrigerant and circulated to the condenser, where the heat is expelled to the outside atmosphere. Just the opposite occurs when the air conditioner is operating as a heat pump. Outside air is circulated across a (now) cool condenser, heat is absorbed out of the outside air and transported to the (now) warm evaporator, and the blower circulates the interior air across the evaporator, where the transfer of heat takes place, thus warming the cooler air.
The reason the heat pump can produce more heat at warmer ambient temperatures is because more heat is available in the outside air. In very cold weather, there is less relative heat to extract. When a heat pump is operating in temperatures near freezing, not only is the efficiency poor but the condenser (now an evaporator) will start freezing if too much humidity is present. Many heat pumps have a temperature sensor on the condenser to tell the control board to reverse the cycle and melt the frost. This automatic feature is great, but it is on the negative side of efficiency.
The choice between a heat strip and a heat pump, therefore, is influenced by how you use the equipment. If you’re a winter sports enthusiast and expect to be traveling in very cold climates, a heat strip would be a better choice. Conversely, if you seldom travel in colder weather, a heat pump would be prudent.
Another factor to consider is who receives the electric bill. The heat pump produces much more heat in moderate climates (see the chart), so this would be my choice if I were paying the utilities, as I don’t plan to be in colder temperatures. Another minor point is that, compared to the heat strip, the heat pump produces more noise since the compressor is running.
Of course, if you really can’t decide, you could always opt for the best of both worlds — one air conditioner heat pump and one unit with a heat strip.