Goodman GMVC dual fuel: the heat pump lockout temperature

What this guide covers

• Gas heating runs in mild weather when the heat pump could have covered the load.
• The house feels short of heat in cold weather and the heat pump seems to be doing all of the work.
• A dual fuel system was installed and nobody knows at what temperature the changeover happens.

What changes the answer

• The furnace control holds a heat pump compressor lockout temperature that decides which appliance is the primary heat source.
• The default is a factory value rather than a value chosen for the property, the equipment pairing, or the local fuel prices.

Why the number is a decision rather than a specification

A heat pump and a gas furnace both heat the house, and which one should do it at a given outdoor temperature depends on things the factory cannot know. The heat pump's output and efficiency at that temperature, the price of gas against the price of electricity, and the load the house presents all bear on it, and all three vary by property and by year.

So 45 degrees Fahrenheit is a starting point rather than an answer. Set too high, the furnace takes over while the heat pump would still have been the cheaper source, and gas is burned in weather where it did not need to be. Set too low, the heat pump is asked to carry conditions where its output has fallen, and the house recovers slowly.

The setting is also the thing to check when a dual fuel system behaves unexpectedly. A homeowner who reports that the gas furnace runs on mild days has described the lockout temperature doing exactly what it was set to do.

What happens during defrost

A heat pump in cold weather has to defrost its outdoor coil, and while it does so it is not heating the house. The GMVC control carries a setting for gas heat operation during defrost, with two options, low heat and high heat, and the documented default is high heat.

That choice is a comfort and consumption trade. High heat covers the defrost period more decisively and burns more gas doing it; low heat is gentler and may allow a noticeable dip. Neither is wrong, and which suits a property depends on how long defrosts last there and how sensitive the occupants are to the supply air temperature.

The control also holds a compressor off delay at the beginning and end of a defrost cycle, documented at 30 seconds. Its documented maximum compressor run time between defrost cycles is 120 minutes. Those values define defrost timing and how the handover is sequenced.

Who should change it, and what to ask

These settings live in the installer menu on the furnace control, inside the cabinet. Reaching them means opening a furnace that contains gas piping and line voltage connections, which puts the adjustment with a technician rather than with an owner.

The useful thing an owner can do is ask. What is the lockout temperature set to now, what was it set to at commissioning, and what would the installer recommend given the heat pump fitted and the way the house behaves. Those three questions turn an invisible default into a decision somebody has taken deliberately.

It is worth asking again if the fuel picture changes. A setting chosen when electricity was expensive relative to gas is not automatically the right setting later, and nothing in the equipment revisits it.

Questions people ask about this