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SPACE HEATING : Direct Hot Air Heating, Heat Pump Preheat, Air Radiant Floor Heating
WATER HEATING: Water Preheat
EQUIPMENT HEATING: Clothes Dryer Air Preheat, Heat Pump Heating
COMMERCIAL: Industrial, Commercial, Agricultural
How to Use Your Heater Boards for Heat Pump Preheating
Heat pumps can provide efficient heating when the outdoor temperature is above 50 degrees F. However, when the temperatures begin to drop into the 40s and 30s and below, the heat pump doesn’t work very efficiently. If fact, it becomes one of the most inefficient heating systems, not much better than electric resistance heat in an electric furnace or baseboard heater.
However, if Heater Boards are used to preheat the air flowing across the outdoor coil of the heat pump, the performance of the heat pump can be dramatically improved. The warmer air moving across the coil means that the heat pump can operate without electric resistance heat, saving electricity and money. The biggest benefit of this approach is that the solar heated air from the Heater Boards can be used to heat the heat pump, even when it is colder than comfort temperatures. So you get more hours of use and energy savings from your Heater Boards than if you just used them when they were delivering air at 85 F and above. They will save energy at the heat pump even if they are delivering 50 F air on a 10 F degree day.
Heat Pump Solar Pre-heating download .pdf
To preheat outdoor air for a heat pump you need to send the warm air stream across the outdoor coil of the heat pump. This is done by bringing the outdoor solar duct up to the heat pump and flooding the air around the heat pump with warm solar air. A section of outdoor duct that is perforated to allow solar air to be released is installed on the ground around the base of the heat pump. This section is installed about 8 inches away for the base of the unit to allow unimpeded airflow across all of the coil area during summer and winter.
The fan inside the outdoor unit will draw air across the heat pump coil. As it pulls air across the coil it will move both the solar heated air and any additional cold outside air it needs across the coil. The more solar heated air you can provide, the better the heat pump performance will be.
Typical airflow across the outside coil of a heat pump is about 800 cubic feet per minute for every “ton” (12,000 BTU/hr) of heating capacity. A 3 ton unit is typical for 2,500 square foot home. So, outside air flow can be around 2,400 cubic feet per minute for an average home.
There is no need to solar heat this entire volume of air. Instead, preheating up to a maximum of about ¼ of this air is a better strategy. The preheated air will blend with the outside air to boost the temperature seen by the heat pump. This elevated temperature will result in higher efficiency.
During the spring and fall when the weather is cool, in the 40s and 50s, the solar heated air will bring the blended air temperature up to about 60 degrees F. Most heat pump manufacturers set a maximum operating temperature for their heat pumps of about 60 degrees F. For that reason, you must use a thermostat that shuts down the solar air flow when the blended air temperature reaches above about 60 degrees F. If you know the manufacturers maximum operating value, you can adjust the thermostat to shut down when the blended air temperature reaches about 5 degrees F, below the maximum. A few heat pumps, e.g. many pool heating heat pumps, have higher operating temperatures. In those cases, you can set the solar air flow cut off thermostat to higher temperatures, in accordance with the manufacturers stated values.
The strategy that delivers the most solar heating benefit is to use the solar heated air directly for space heating, whenever it is warmer that the indoor air, and to run an exhaust fan that exhausts the excess household air across the heat pump coil. Whenever the solar air temperature is below comfort temperatures for direct space heating, the solar air can be sent across the outdoor coil. This approach is more complicated and requires coordination of two fans, but it will deliver more hours of heat at higher efficiency than just heat pump preheating alone.
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