Geothermal Rescue – Closed Loop Too Small and System Won’t Heat – Chilmark


This geothermal rescue involved a closed loop horizontal geothermal field that was too small for the heating load.  Halfway through the winter heating season, the loop temperature would drop and the heat pumps would experience continued fault codes and would stop working.  This issue had not been a problem in the past due to the seasonal nature of house occupancy – the home had been only used in the summer so how the geothermal system handled the winter had never been tested.

The solution was to raise the temperature of the geothermal water going through the heat pumps.  Unfortunately, because of critical distances outside between the geothermal field and wetlands, it was not possible to increase the size of the field.  Our solution was to add a boiler and bypass valves to heat up the geothermal water inside and to give the outside geothermal field a chance to rest and recover.  We had seen that the amount of heat extraction by the heat pumps was out of balance with how much heat the geothermal field could provide.

We automated this process so that the boiler would only run when the loop temperature was below a set temperature.  Motorized valves would send water through the boiler instead of to the outside geothermal field so that we did not use the boiler to heat up the outside earth.


Here is the aquastat (gray box with the dial) that controls the operation of this system.  The large thermometer allows us to correctly calibrate the optimal balance between boiler operation and outside geothermal field operation.





The aquastat controls several things at once – when the loop temperature is too low, the aquastat signals one motorized valve to close off water to the outside loop and the other motorized valve to open up to the boiler.






This lower motorized valve closes off the outside geothermal field.






The aquastat also signals the boiler to turn on via the relay control panel – but only if there is an active call for heating from one of the heat pumps.  Otherwise, the boiler might turn on and mindlessly heat the geothermal piping.





Once the boiler got the signal to turn on and the motorized valves changed the direction of geothermal water, a glycol water mixture flowed from the mechanical room via PEX tubing to another mechanical room with the boiler.  This simplified the installation of this boiler geothermal rescue because there was very limited space in the existing mechanical room.




The PEX tubing connects the room with the motorized valves and the circulators for the heat pumps and the remote room with the boiler.






The boiler is a Buderus condensing boiler chosen for its efficient operation at the temperatures needed by the geothermal heat pumps.  It has a circulator that mixes the boiler water with the geothermal loop shown here.





The boiler vents to the outside with 3″ PVC vent pipe and no clearance necessary to combustibles.  This makes siting of the boiler much easier than the old fashioned non-condensing high temperature boilers.



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.