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AC-83-07-2 -- Operating Cost of Gas-Fired Furnace Heating Systems with Add-On Heat Pumps
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AC-83-07-2 -- Operating Cost of Gas-Fired Furnace Heating Systems with Add-On Heat Pumps
Conference Proceeding by ASHRAE, 1983
A. Patani, Ph.D.; U. Bonne, Ph.D.
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The recent shifts in energy prices have reduced the electric/natural gas cost ratio, RC, from a previous U.S. average of over 5 in 1973 down to 4 in 1981. This continuing trend and lower off-peak electric rates are making electric heat pumps to be economically viable retrofits in more U.S. locations than before. This paper presents findings on conditions and limits for possible operating and life-cycle cost savings. We analyzed the achievable savings with add-on heat pumps by taking into account the furnace/boiler losses during prolonged periods of stand-by and other losses analyzed previously by the HFROST and HFLAME simulation models. Analysis on the basis of Operating Cost Efficiency showed that:
--Add-on heat pump operating cost savings may accrue if RC, the dimensionless electric to natural gas cost ratio, is below 2.5 to 3.5, depending on heat pump and furnace efficiencies and system sizing.
--The RC value for which add-on heat pumps showed positive operating cost savings was independent of the furnace draft control air flow or the heat pump size.
--Milder climates accrued savings at a higher (seasonal) percentage rate than colder ones, although savings started (= switch-over point) at very similar RC values.
--Optimum switch-over temperature was found to be strongly dependent on RC; the slope was determined to be about 44C (80F) for each RC unit.
--Increasing furnace system efficiency (e.g., by adding as tack damper or a power burner) shifted the RC value at which savings accrue to a lower value. Increasing heat pump efficiency shifted this RC value to higher values. For buildings with good internal communication with the furnace room, stack damper savings were large compared to those achievable with an add-on heat pump system.
RC value and climate are the major regional factors that, together with the heat pump cost and reliability determine the potential benefits of add-on heat pumps. Our analysis showed that installation of add-on systems generates life-cycle cost savings for locations with RG=2. Local and off-peak rates need to be incorporated into the switch-over control for economic operation.
Citation: ASHRAE Transactions, 1983, vol. 89, pt. 1B, Atlantic City, NJ