I have a heating system with both a heat pump and a boiler. The rule for the system is that if the temperature in a zone is 5° F less than called for, then the boiler will come on, but otherwise the heat pump will try to heat the zone by itself.

The problem with this is that I can't get one of the zones to 72° F. What happens is that it is cold outside, so the heat pump can only get the temperature to 70° F and it stays there. If I increase the called for temperature in the zone to be 75° F then of course the boiler will come on, but it would raise the temperature to 75° F which is too hot.

How do I solve this problem?

  • you need to change the rule that calls for the boiler so that the 5F is reduced if the pump is running continuously. – Jasen Dec 1 '18 at 3:56
  • Is your heat pump system VAV zoned (with dampers and a control board attached to a standard split system heat pump, or multi-split zoned with multiple indoor "head" units driven by a single outdoor unit? – ThreePhaseEel Dec 1 '18 at 5:14
  • @ThreePhaseEel The heat pump is outdoors on a concrete pad. It has fluid-carrying lines that go into the house and into an air handler. There is one heat pump and one air handler. There are 3 zones. Each duct that goes to a zone has an electronically controlled damper that can close or open the duct depending on whether the zone needs heat or not. – Tyler Durden Dec 1 '18 at 5:31
  • Any heat pump I’ve had experience with has an outdoor temperature sensor that shuts down the heat pump and switches to auxiliary when it’s too cold outdoors to be efficient. Does yours have that? Is the low outdoor temp threshold adjustable? – Tyson Dec 1 '18 at 15:32
  • @TylerDurden -- so, VAV then, with your hydronic system standing in for the reheat function on a typical commercial VAV system. What are the other zones set for? – ThreePhaseEel Dec 1 '18 at 16:16

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