High Electricity Costs for New Mini Split System
Hi,
I recent completed a 24×28 addition to my home in Massachusetts. We decided to go with a Mitsubishi heat pump to condition the new space instead of a fossil fuel system. My electricity bills for November and December are much higher than I expected. I believe I have narrowed this down to the heat pump, as I have turned off all other items that might be big electricity consumers (backup heat strip for heat pump, whole house steam humidifier, heated floors for bathrooms, etc). With all of those things off I am still averaging usage around 40-45 kWh per day.
My electric bill for December 2020 was nearly $385 and consumption was 1789 kWh. In contrast my electric usage for December 2019 was only 645 kWh.
For reference November 2020 consumption was 1231 kWh with November 2019 being 528 kWh.
My HVAC contractor told me I should be able to heat and cool this space for $50-60 a month.
I searched several previous questions where folks are trying to figure out why their heat pumps are using so much energy. I think I’ve got some of it pieced together but would love some help to make sure I’m calculating this correctly.
Addition and HVAC Details
The addition itself is two floors, 24×28, with a garage on bottom and a master suite on top. Thermostat is set to 69 degrees F.
The HVAC system is a Mitsubishi MXZ-3C30NAHZ2-U1 heat pump, with a SVZ-KP18NA air handler (installed in the attic) for the master suite and a MSZ-FH12NA head in the garage. The head in the garage is rarely used and was not on in November or December.
The Mitsubishi system was also ducted to heat one room in the original house which is 12′ by 11′. Total square footage being heated between the new addition and the one room is 804.
Insulation in all walls (including the garage) is 5 1/2″ rockwool comfort batts which are R-23.
Insulation in the attic is 15″ of blown in rockwool.
Insulation in the garage ceiling/living space floor is 2″ closed cell spray foam with 5 1/2″ rockwool comfort batts.
I’m trying to determine if this kind of energy consumption is what I should expect for heating this kind of space during a New England winter.
The submittal for the heat pump (https://nonul.mylinkdrive.com/files/MXZ-3C30NAHZ2_ProductDataSheet.pdf) shows heating at 17 degrees to consume 1993 watts. I was trying to calculate daily electricity consumption doing something like this:
1993 watts / 1000 (convert into kW) * 12 (number of hours the system runs) = 23.9 kWh used a day.
That calculation seems fairly rudimentary to me and I think there are a number of factors I am leaving out:
– I am not sure how many hours a day the system is running.
– How does COP factor into this? The submittal says COP is 2.58 at 17 degrees but I’m not sure how that factors into the equation.
– I am assuming that wattage is when it is calling for full heat load, which may not be the case all the time.
I am definitely out of my depth here and would love some feedback given the above information.
Beyond the energy consumption, the other question I have is should I leave the thermostat configured to be circulating air all the time. There is an advanced thermostat setting under option 125 to circulate air all the time or only circulate when heating. I know the air handlers are relatively efficient so I didn’t think that would contribute too much to the energy consumption but wanted to know if there was benefit to leaving it on to circulate air all the time.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You might want to try putting a sensor on the system to monitor how it is using power.
My understanding, based on previous posts on this site, is the minisplit use should be level, without spikes. You want to monitor actual use, not a system that estimates based on monitoring current. If the minisplit has spikes, that tells you it may not be set up properly (either programming or the way it is charged)
I research power monitors and reported on my findings in this thread. I have not yet bought one...
https://www.greenbuildingadvisor.com/question/energy-monitoring-systems-which-is-best-combo-of-affordability-ease-of-operation-and-accuracy
Multi-splits are a lot less efficient than single-heat mini-splits. Just a guess, but I suspect your 2.5 ton system is vastly oversized for the space it serves and is short-cycling like crazy. Does your system tend to run for a few minutes and then stop?
On another issue, I would start monitoring your indoor humidity levels. Humidifiers can be risky if not managed correctly.
Does seem like a crazy amount of extra energy consumption for this room. Rooms over garages are tricky as they are cold on all sides. Abundant insulation and meticulous air sealing are needed for comfort and performance. I added an art studio for a friend (22'x22') over an existing garage in 2013. Ceiling R-60, walls R-30 and floor R-50 and very airtight, quite a bit of glass at R-3 . This room is very comfortable winter and summer and is heated and cooled by the existing forced air furnace supplied by an 8" x 10" trunk line. The thermostat for this multi story house is on the main level so this room gets heating and cooling when the rest of the house calls for it. I would estimate the ACH50 for this art studio is 1 or less. Extra attention was paid to the rim joist area with 3 layers, R-30 polyiso. The T & G plywood subfloor is glued to the 2x4 web trusses and serves as the air barrier in that location. I also add construction adhesive between the tongue and groove to eliminate squeaks and to increase air tightness.