Low temps of heat pump for hydronic use

Bill Stocky

I guess some unique refrigerant might have the necessary properties to attain the needed temperature extremes for this to work, but it hasn't been found yet.

What did you mean by heat pump radiant heating?

Bill Stocky

Low temps of heat pumps for hydronic

What is the reason heat pump condensor temps are so low (110 - 130 max) which makes it a challenge to use a heat pump system with hydronic radiators?

Eugene Silberstein 3

Heat Pumps for Hi Tecmp Hydronics

Heat pump systems are reverse cycle refrigeration systems that, in essemce, switch the operation of the indoor and outdoor coils. In the summer cooling season, the outdoor coil functions as the condenser, while the indoor coil operates as the evaporator. In the winter months, the indoor coil operates as the condenser and the outdoor coil as the evaporator.

The purpose of the evaporator is to absorb heat into the system. If the air surrounding the evaporator is warm, the amount of heat available for the refrigerant to absorb is high. In the heating mode of operation, when the evaporator is located outside, the amount of heat available for the refrigerant in the evaporator to absorb gets lower and lower as the outside temperature gets colder and colder.

This creates a negative spiral so to speak for, as the outside temperature drops, the amount of heat in the heat pump system is reduced.

Now, during the colder months, the indoor coil operates as the condenser, which is the heat transfer surface that rejects system heat. The heat that is rejected comes from 2 main (although there are others) places: the outside and the compressor itself. Since the amount of available heat from outside is reduced, the amount of heat that can be added to the medium to be heated. The medium being heated can be air or water.

Here's the deal. Since heat pump systems operate as a vapor-compression system, the compressor must increase and decrease the pressure of the refrigerant in order to facilitate refrigerant flow through the system. In order to generate enough heat in the system to effectively heat water to a temperature that could be used for high temperature hydronic heating systems, the compression ratio of the system would need to be about 15:1 with hgih side pressures (for R-22) of over 600 psig. And that is only at about a 40 degree outside air temperature! You can only imagine what would happen if the temperature dropped to 10 degrees.

For standard efficiency systems, the condenser saturation temperature is about 30 to 35 degrees higher than the ambient temperature. So, to get a water temperature of 180 degrees, the refrigerant needs to condense at about 230 degrees. The evaporator (outdoor coil) woud have to be at a temperature below the outside ambient temperature in order to absrorb heat into the system.

As you can see, this concept is definitely not economically or operationally sound.

Now... Using heat pumps for radiant systems is an entirely different story!

Eugene Silberstein 3

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