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Kaplan ITA's Monthly Inspection Tips - Free Electronic Newsletter

April, 2007
Dual-Fuel Heat Pumps
Issue #60

This month I would like to discuss dual-fuel heat pumps.

Dual-fuel heat pumps are a standard electric heat pump and gas (typically) furnace as backup in one system. When outdoor temperatures are above 35 degrees (the balance point of most heat pumps) the most efficient way to heat a home in most areas is with the electric heat pump. This, of course, is dependant upon the cost of electricity. However, with the rising cost of fuel gas and the fairly stable cost of electricity combined with the efficiency of new heat pumps, their use makes sense in most areas today.

There are two settings for heat pumps and modulating operation between electric heat and the fuel gas backup:

Unrestricted Mode; During the call for first stage heat (heat pump only), the thermostat is satisfied during a normal run time and the heat pump turns off. If outdoor temperature is below the system balance point and the heat pump is not handling the load (thermostat senses temperature drop during heat pump operation) the second stage is activated by turning off the heat pump and turning on the furnace. The furnace will satisfy the second stage call only then turn off and the heat pump will turn back on after a few moments to resume satisfying the first stage call. If the room temperature continues to rise, the thermostat will be satisfied and the system will turn off. If room temperature drops, another second stage call will be initiated and the process will repeat. Should the user transfer the thermostat to emergency heat, the heat pump is bypassed and the gas furnace only will run for all heat calls.

Restricted mode; This setting uses the Outdoor Thermostat (located in the outdoor coil cabinet) to modulate which system operates. The outdoor thermostat is set at a temperature at or above the system balance point. At any temperature above the Out Door Temperature (ODT) setting only the heat pump will satisfy first stage heat calls. When the temperature is below the ODT setting the furnace satisfies calls for heat. Should the user transfer the thermostat to emergency heat, the heat pump is bypassed and the gas furnace will run for all heat calls.

Inspecting complicated systems requires advanced knowledge. Check out Kaplan ITA’s Advanced Continuing Ed courses to experience more in-depth education about HVAC and other systems.

We’ll talk next month,

Mike Casey
Kaplan Professional Schools
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