A GHX is an integral component of a GCHP system (hence the name "ground coupled). But in many commercial buildings there are times when there is cooling and heating going on at the same time. The center core of a building without any external walls or roof can require cooling because of internal gains (lighting, electrical equipment, occupancy) while the perimeter of the building may need heating. At times the amount of heat rejected to the GHX by one heat pump balances the amount of heat extracted from the GHX. The net effect on the GHX is zero. So why circulate fluid through the entire GHX?
Decoupling the GHX from the building piping system ensures you can have enough circulation through the heat pumps to allow them to operate properly, while only circulating enough fluid through the GHX to ensure the temperature of the fluid to the heat pumps is within their operating parameters.
When all heat pumps are operating in heating mode, the GHX pump is activated with the building pump and all the fluid circulates through the GHX to provide the energy needed by the heat pumps. The same happens when all the heat pumps are cooling and need to reject heat to the GHX...both pumps are fully engaged.
When building loads are reasonably balanced the GHX pump can be shut down completely. All of the flow bypasses the GHX...the GHX is not needed if the temperature remains the same as it picks up heat from one heat pump and provides it to the next. The pressure drop through the entire GHX is eliminated and pumping becomes much more efficient.
When the building requires only a little energy from the ground, most of the flow bypasses the GHX through the common pipe. The GHX pump is controlled by the temperature of the fluid circulating through the building. As the temperature drops or increases to near the minimum or maximum system setpoints, more and more of the flow is diverted through the GHX to extract or reject heat to the GHX.
Most of the time, the GHX is flowing at much lower flow rates than the building. This provides the flow needed by the heat pumps that are operating, while reducing pump power through the GHX.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.