Zones, Zoning, Zoned.

 (This posting is not specific to GSHP systems; just an example of what can go wrong with a two zone HVAC system.)

Our old system had 2 furnaces. One for each floor of my home. With the new system we were given either the option of having two GSHP systems or one system with zones. The two GSHP system would have been much more expensive so we went with the two-zone design. Unfortunately, that design did not work out so well. Here's that story of what happened with the zones and how we fixed it.

The Plan

The plan was to have one 4 ton GSHP with two zones. There is a zone damper on each of the two supply trunks. We have two thermostats. When one thermostat calls for heating/cooling or air circulation, that zone damper opens.  If they both call, then both zone dampers open. There are were several problems with this design.  

Problem #1 is that the air blower on the GSHP was too big for one trunk when only one zone was calling for air.

Problem #2 is that to handle problem #1, they installed a bypass damper to shunt conditioned air from the supply back to the return. In heating season, the bypass also sends heated air back in to the heat exchanger which reduces the system efficiency. In cooling season, the bypass sends cooled air back to the heat exchanger increasing the probability of freezing up the heat exchanger.

Problem #3 is that the bypass damper did not really work properly. It was handling a huge volume of air and bouncing around as the variable blower changed intensity. We were also getting too much air at the vents around the house when the GSHP was in stage 2 (or aux heat, stage 3) and it tells the blower to blow faster (more CFM).

Basically, it was not a good plan.


The fix: disable the zone dampers 

I talked to a lot of people. Posted to various message boards. This is the solution that seems to work for us.  I really did not like the idea of sending conditioned air around the bypass as we were.  So my goal was to avoid that.  Some people said to just cut out the bypass and dump the air in to the basement. But my basement is pretty small and we felt the volume of air was too great.

The solution we chose was to disable the zone dampers. The zone dampers we have are power-open/power-close. That means if you remove the power from the dampers, they stay in whatever position they were in when you removed the power.   So that's what we did. We made sure both zones were open, and then disconnected the two power wires for the zone dampers.

So instead of sending the "extra" conditioned air around the bypass damper and through the heat exchanger, we are sending the "extra" air to the other zone.  If either thermostat calls for heating or cooling then they both get heating/cooling.   I may get a manual damper installed to balance the air flow (or partially close one of the zones) to balance the air flow but for now the system seems to be working very well.

Plan C
 
My installer had another clever idea that we were going to try if the above did not work out. He designed a little circuit to have the system operate as originally intended in stage 1 with two zones and the zone dampers. But if either thermostat was calling for stage 2 heating, which entails more air movement (higher CFM), then his circuit would open both zones. Then we would avoid sending a huge volume of air around the bypass damper only when using stage 2 heating or cooling.