View Single Post
Old 02-12-12, 09:34 PM   #1
abogart
Efficiency Afficionado
 
abogart's Avatar
 
Join Date: Jan 2012
Location: Michigan, US
Posts: 68
Thanks: 21
Thanked 6 Times in 4 Posts
Default Running cost: ASHP vs. Nat Gas Furnace

I have been crunching some numbers lately, considering if it would be worth the investment to replace my outdated, undersized central AC unit with an ASHP. The idea is to use the ASHP to provide primary heating, with the gas furnace as fossil fuel backup on the really cold days. I have to find out how much I would actually save in energy costs using the ASHP to determine if it's really worth the investment. I have found that our electric company has a special rate for ASHP's, which is the winter half of the rate schedule for interruptable air conditioning during the summer months.

The attached spreadsheet details my findings. On the upper left are the electric and gas rates. The electric rate is the combined rate per kWh for interruptable air contitioning/ASHP, and the gas is February's current combined rate per therm. The reference HP is a Rheem Prestige series model 048JEC 2-stage 4 ton split system. The specs can be found here.

Basically I broke each unit (or stage) down into cost per running hour. Then, since the units and stages produce different outputs, I determined a duty cycle (% run time per hour) for different BTU/h heating requirements. Next, I converted that into a cost per hour for each unit and BTU/h. Finally, to determine at which outdoor temperature the gas furnace becomes cheaper, I figured a cost per 100,000 BTUh for each unit at each temperature.

It looks to me like at any outdoor temp below about 40°F, the gas furnace is cheaper to run. That is quite disappointing, since most winter days here stay below that. Did I just pick a poor performing unit here or is it really just cheaper to heat with gas through the winter? Did I do my calculations wrong somewhere along the way?

This could be the deciding factor as to whether I upgrade to an ASHP or a high-efficiency gas furnace. Hopefully both someday.

Attached Files
File Type: xls Heating Costs.xls (14.0 KB, 566 views)
abogart is offline   Reply With Quote