This is another often misunderstood topic. Some people insist that all geothermal heat pumps will need back up heat during extreme cold.
First lets separate back up heat from supplemental heat. Back up heat refers to a way of heating if the GSHP is down and out. It means that there is a second conventional source of heat that is there to take over if the geothermal heat pump is totally out of commission for whatever reason. Now that may seem ridiculous but I can assure you there are a lot of custom homes in the rocky mountain west that have both GSHP systems and a boiler that is sized to handle the full load.
Supplemental heat refers to a secondary heat source that can assist the operation of a heat pump during peak loads. It could be an electric plenum heater on a water to air heat pump or it could be a small boiler or even a water heater tied into a radiant distribution system.
The real answer as to if GSHP’s need back up heat has less to do with the geothermal heating system and much more to do with the heat loss of the structure room by room and the type(s) of distribution. Rooms that have garage door sized windows and 20 foot c eilings have very high heat loss. We need to meet this heat loss by delivering BTU’s at a rate as fast as they leave the room If the room has radiant floors we will need to determine how many BTU’s we can deliver through the floor. This delivery will depend on type of radiant floor (slab, thin slab, staple up, Warmboard etc), floor coverings, and temperature of heat transfer fluid (120 F max with GSHP)
If we are unable to deliver enough heat through the floor to meet the demand we could think about adding forced air distribution. This forced air distribution could have a fan coil that is getting 120 degree water from the GSHP and this combined with the radiant might satisfy the demand. The other solution would have been to raise the temp of the heat transfer fluid over 120 F with a conventional energy source but this has it’s own problems and limitations.
We are limited to making the floor surface temp no hotter than 85 degrees. Hotter than that can cause problems with the floor coverings and is uncomfortable to the occupants of the space. Because of this limitation the “problem” is not that the heat pump can only produce 120 F it’s that even if it could be hotter we would not want to send hotter than that to the floor. In these situations a boiler would not help the situation. The solution is adding distribution capabilities.
In summary, a geothermal heat pump can be designed to deliver more than enough BTU’s to match the pe ak load of the project. The challenge is in delivery of that heat energy to where it is needed and for that there are basically two options. One is to add more distribution capability that is compatible with low temp 120 F max. The second way is to increase the temperature of the delivery keeping in mind the other limitations that exist with regard to max temp of floor surfaces.
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