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Ground source heat pump design questions
Four years ago I purchased a house and inherited a forced air heat pump with resistance backup. I don't have to tell you guys how hard it is to extract heat from winter outdoor air in Philadelphia, so I have been designing a replacement HVAC system since we moved in. I have decided on a hydronic system using as much radiant floor as possible. I have subfloor access to the entire house and plan to install heat transfer plates with PEX tubing. I also built a two story garage/workshop which is attached to the house by a mudroom, all of which have poured concrete floors that are insulated under the slab and have loops of imbedded PEX.
As much as I hate the air-air heat pump, I am fascinated with the idea of using the earth as a "thermal flywheel", and want to install a ground source water-water heat pump to run the hydronic system. It seems like an ideal match since the lower water temperature of the heat pump would be nicely suited to the lower temperature requirements of the radiant floors. I also plan on using the heat pump in the summer to provide chilled water to separate air handlers that will air condition the house in three zones. The air handlers will also be used in the winter as second stage heating to help the radiant floor on cold days. I will run all of the hydronic zones using primary-secondary pumping and design a control system to supply water to the zones/stages that call as required.
A heat loss/gain analysis program based on ACCA Manual J puts the heating load at 64000 BTUH and the cooling load at 45000. If I size the individual air handlers to the cooling load of each zone will that ensure that they run long enough to provide adequate dehumidification? I'm a little concerned because it seems like the heat pump will be oversized for cooling, especially if only one zone is in use. I have read about the problems with oversized heaters and air conditioners short cycling and want to avoid that before it starts. I plan to have a storage tank of heated or chilled water that the heat pump will maintain as a reservoir for the system as needed. Do you think that will be an acceptable way to load the heat pump, or should I consider a two stage system or perhaps separate heat pumps for heating and cooling? Also, are there any unusual piping requirements involved in a system that is used for heating and cooling?
I realize that this is a rather complicated residential design but I think I can make it work. I'm an electrical engineer so the control system shouldn't be too hard for me, but my practical experience in mechanical systems is somewhat limited.
Thanks in advance for your thoughts,
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