High December electric bill with Mitsubishi heat pumps
First, thanks to all who have helped build this forum knowledgebase through questions and particularly answers to those questions. Particular big shout-outs to Dana and Martin for sharing their expertise and helping inform members.
I was shocked when I received my electric bill ($700) yesterday and am trying to make sense of it and am looking for some guidance as, based on this electric bill, the cost of heating my house with ASHPs (plus 50 gal electric water heater) appears to be more expensive than it was to heat my house with a 50 year old oil boiler (plus oil indirect heating of domestic hot water).
For background on my setup, I have two outdoor units (MXZ-3C30NAHZ2) connected to three indoor units. These were installed in March 2019. One outdoor unit is connected to (1) a 9k basement head (2) 12k 1st floor den/kitchen head and (3) 9k 2nd fl master bedroom head. The other outdoor unit is connected to (1) 15k 1st floor living room head (2) 6k 2nd fl bedroom head and (3) a 6k 2nd fl guest bedroom head. I really don’t turn on the 6k bedroom units much for heat since they get heated a lot from the first floor heat traveling up through the house. The manual J on my home in Sherborn MA has the heat loss at 56k BTU/hr at 6F. My home is a two floor colonial built in 1968. It is 3400 sqft with a finished basement that sadly is not insulated or airsealed. I had the attic rafters and gable ends sprayfoamed to R38 a year ago. The 1st and 2nd fl walls have R13 batts in them. Windows are original single pane w/ storm windows. The outdoor unit connected to the head in the basement runs a lot; the other outdoor unit runs intermittently, even at 30F. Please also note that the oil boiler indirectly used to heat my domestic hot water using an electric hot water tank as a storage tank; I disconnected this and wired it up to now heat with electricity a couple months ago to be completely oil-free.
Everything has been great until this last electric bill that hit $700. In summertime the ASHPs performed beautifully and saved me quite a bit of money over the 15 SEER ducted central air system I used to use. The shoulder heating months (March, April, October) were fine too with electric bills that were about 1600 kWhr or less (please see attachments for monthly breakdown of electric use). Based on history, I’d say our base electric bill without heat or AC is 900 kWh/month. Add in electric domestic hot water and we should go to about 1350 kWh/month.
I went back to my oil record from last year before the ASHPs were installed and during one of the coldest periods of the year (08 to 28 January 2019), we used 112 gallons of heating oil over the course of those 20 days (810 HDD with a base temp of 65F, for a daily average of 40.5 HDD). Backing out let’s say 20 gallons of oil for domestic hot water, that leaves me with 92 gallons / 810 HDD = 0.11 gallons per HDD. Assuming 75% efficiency of the oil boiler, that gives me 11,517 BTU per HDD: (139,600 BTU/gal)(0.11 gal/HDD)(0.75 efficiency).
This last bill of $700 would be even more had it not been for my 14.4kW solar array that produced about 500 kWh during that billing period. Adding this to my bill would have me at about 3500 kWh of consumption from 11/6/2019 to 12/6/2019 (was billed for 3000 kWh of use). This period of time represented 969 HDD for a daily average of 31.3 HDD (assuming 65F base temp). FWIW, both units went through a number of defrost cycles over this period. Backing out the above baseline of 1350 kWh, this has me using 2.2 kWh per HDD [(3500-1350) / 969]. So, to hit this 11,517 BTU per HDD heat loss mark means I assume a COP of about 1.5… (3,412 BTU/kWh)(2.2 kWh/HDD)(COP) = 11,517.
A 1.5 COP is totally unacceptable for a 35F average and cold climate Mitsubishi equipment. I have set my house temp a few degrees higher with the ASHPs than I did when we were burning oil, but this still seems very wrong that the electric bill is so high. Does anyone have a sense of what is going on? I believe my math is correct. How can I spend about $500 on ASHP heating costs in Nov and early December??
Thanks for listening to the rant, any offered perspective and answers, all!
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