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It's all about the C.O.P.
The average residential utility costs in Grand Rapids, MI. are $1.21 per Therm (100,000BTU) for natural gas and $0.157 per KWh (3,413BTU)
You need to find the temperature for the heat pump installed where the C.O.P. calculates to a lower cost per 100,000BTU's of heat.
Given that a Therm of natural gas is $1.21, We can take the $1.21 and divide it by $0.157 (cost per KWh) which equals 7.70 So, (100,000BTU divided by 7.70) X furnace efficiency ( .96) = 12,467
12,467 Divided by 3,413 = 3.65 So, I'm looking for a C.O.P. of 3.65 in my manual.
A Goodman GSZB4 has a SEER2 rating of 14.3 and a HSPF2 of 7.5, and, depending on the size, a C.O.P. of 3.65 at about 45 degrees F.
A more efficient heat pump, like the GSZC7 with a SEER2 of 17 and a HSPF of 8.2 would be set at a change over of 40 degrees F.
Propane cost is about $2.47 per gallon which will render about 91,500BTU. So you need 1.09 gallons for 100,000BTU's at a cost of $2.69
Same math. $2.69 divided by $0.157, equals 17.14 100,000BTU's divided by 17.14 = 5,834 5,834 divided by 3,413 = 1.70
The same GSZB4 will run down to 5 degrees before the LP becomes economically viable.
Mark Buelow
Johnstone Supply Technical Support
See attached charts for a visual of savings.
