This particular geothermal system experienced heat pump faults and unreliable operation due to an undersized horizontal closed loop geothermal field. The field was not large enough for the heating load of the house – the heating system and the heat pumps would work well at the beginning of winter but by January the Sun’s heat had been extracted from the Earth and the geothermal field temperature started to plummet.
This was bad news for the heat pumps as they would not operate with geothermal water below 35 F, and the return water temperature was dropping daily and soon got into the 30s. This meant that the geothermal heating system would not be able to provide heating until the Earth warmed up in the spring OR we found a way to make the water warmer.
To make the geothermal water warmer we connected the inside boiler pipes (the boiler was also used for the domestic hot water system) to a flat plate heat exchanger (the rectangular object suspended below the strut framework) and then to the geothermal pipes. It is able to transmit heat very effectively while isolating the two fluids from each other.
Because we only wanted to heat up the geothermal water inside the building that went to the heat pumps and not use the boiler to heat up the entire outside geothermal field (we were not even sure that would be possible!), we installed motorized valves to close off the outside field from the inside heat pumps.
Then another motorized valve opened to connect the inside heat pumps with the heat exchanger to that heat could warm up the inside heat pumps.
The entire installation was controlled automatically by an aquastat (small rectangular gray box on the upper strut below the ductwork) on the geothermal return line going out to the outside geothermal field. When it sensed that the geothermal water was below 40 F, it commanded the motorized valves to change position.
Once the motorized valves had fully moved, two circulators were started. One circulator connected boiler water to the boiler side of the flat plat heat exchanger and the other connected inside geothermal heat pump water to the heat pump side of the flat plate heat exchanger. The hot 160 F boiler water heated up the heat exchanger which heated up the inside geothermal heat pump water up to 55 F. As soon as the inside water temperature came up to 55 F, the aquastat would be satisfied, turn off the circulators, and activate the motorized valves.
The geothermal field would have had some time to collect more heat from the Earth and would be able to operate for several more hours. The boost from the heat exchanger system permitted the outside geothermal field to recover and gradually take over the heating load.
The end result was no more fault codes for any of the heat pumps and reliable heating all winter long.
Our goal is to be Martha’s Vineyard’s premier plumbing and HVAC shop – we appreciate the opportunity to show you some of our work.