Minisplits and wood

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Max W

Feeling the Heat
Feb 4, 2021
Hope this is OK to repost this way.

A recent thread on selling firewood drifted for a bit into minsplit vs wood efficiency and break even cost. . It would be sad to see one post, Peakbagger’s informative delve into minisplit efficiency get lost among those piles of firewood. For me it provided a better understanding of mini split heat pump characteristics. I also got a link from that which led to finding the actual and very hard to find specs on my mini split. Additionally it became clear to me that something I have considered, adding a second mini split to help with more capacity at low temps, makes less economic sense. While engineering buffs may have fun with the math you don’t have to be one to get that better understanding. This post started me going back to my electric bills and looking the mini split, our main source of heat to see how much our minisplit and wood is costing. Thought I’d share that and what I am heating.

Peakbagger’s post
Dec 14, 2023

BEWARE major thread drift (since others have already started it )

It's not incredibly hard in theory to come up with justification for mini split versus wood, it is just very tedious to build up the spreadsheets. And depending on your spreadsheet skills you may need to spend some time relearning various built in functions, included the dreaded array functions (that take me hours to get right if I havent used them lately)

First thing is do a manual J calculation heat load calculation with outdoor temp as a variable and if the space is not heated 24/7 then consider adding indoor temp as a variable. There are multiple sites on the internet where you can download local temperature data usually from a nearby airport. You can get really detailed but it comes down to how many columns of data you want to generate. So lets assume you use a daily average ambient and disregard direct heating from the sun, its indirectly in there based on average ambient but its a stretch if a house is optimized for solar gain.

Now knowing the daily ambient you can do a look up table for a particular heat pump which is KWhr input for heat output. The best source of info is NEEP They are big enough that they have
enough clout to get low temperature performance data that is not normally published.!/ NEEP has an advanced calculator that does some of the heavy lifting based on accessing local weather data and it generates some real nice graphs but as far as I can see, there still is a need to upload the data to a spreadsheet and do the KWh calculations. Note that some companies do not participate in all Energy Star ratings, I was surprised that Mitsubishi does not appear to participate in the Energy Star V6.1 Cold Climate category (while Fujitsu and others do) so I skipped that category and grabbed an arbitrary Mitsubishi model and pasted it below.


For now, lets skip the cooling. Take a look at the COP which is the amount of electric power required to put out the heat load you need. You will notice it varies depending on load and outdoor temp. Keep in mind the COP for an electric heating is effectively 1.0 so the extra efficiency gained by heat pump is COP -1.0. A wood stoves COP is at best 0.8 and an old smoke dragon is probably 0.4. Luckily wood is cheap per BTU compared to electric power so COP is less important for wood at least initially. Keep in mind 1 kW= 3412 btu. These are also best case numbers, many cold climate units have electric pan heaters to keep from freezing up the outdoor unit and that is not counted in the COP.

Notice the kW line. It is showing you how many kW are required to put out the heat load you need. It is related to COP, so in the -13 F case to get 2,830 Btu you are putting in 0.54 kW. For an hour of heat that is 0.54kW/hr. Notice that if you need more heat, the COP goes up, so the unit looks to be more efficient. I (could really confuse things at this point on how the efficiency is calculated but lets not and save that for the advanced course) ;). Ultimately you need to plug in a KW demand for your load that is accurate for the ambient temperature.

With this variable efficiency you need to figure how you are going to deal with what happens when your unit runs out of capacity. So now your spreadsheet needs to have some "IF" statements on how you want to split the heating loads between multiple units or do what most smart designers do, write off really cold temperature operation and use a backup heating source.

I know I am pissing into the wind on this compared to the PR being pumped out to the public but not many folks are willing to pay an extra $5 to $6K each for a one or two extra mini splits to cover the coldest temps. Even if they do, they are probably paying a premium for power in the winter (obviously depends on who you buy your power from). Down at those low temps, those higher electric costs may offset any savings running multiple heat pumps. Far better to crank up the backup fuel paid for by the savings of not buying the extra units. In a cold area like where I live, I would need four 17,000 Btu units to heat my house in the coldest temps and that is lot of power My 5 to 6 K was for a 12,000 BTU unit so lets use 7K for swag at cost per unit. I can heat my house a long time on the $21,000 needed to buy 3 additional units to cover my worst case load especially if I am burning wood. I need backup as my ambient temps go below -13 F on occassion but most folks live farther south so its not a consideration.

On the other hand, I have a 12,000 Btu rated mini split and that covers a lot of my shoulder season heating load down to about 20 F overnight temps ( I have a small house and set the temps way back on my second floor at night), it is an older cold climate unit and I do not even have a rating sheet as NEEP was not around. My wood boiler with storage is not very efficient during shoulder seasons, as I need to heat up a lot of mass and water to heat up the storage. The storage has pretty good insulation but the 1000 pound plus steel boiler and 40 gallons of water (320) pounds inside it is effectively not insulated. The boiler is in a basement so everytime I heat up the storage, I am effectively throwing away a portion of that 1320 pounds for warming up the ground and floor outside the basement. The other aspect is my house is somewhat well optimized for day time solar heat gain, I have some terrain blocking my house at low winter sun angles so I do not get full sun until 9 AM this time of year and I lose it around 3:30 PM but I do get 6 hours of free heat where I really do not need auxiliary heat if the temps are over 30 F and the sun is out. I do have R5 double cellular blinds with side seals on almost all the double pane windows which I close at night but around 5 PM its time for auxiliary heat, so on those days I will probably run the minisplit during the day and keep the storage warmer until the evening when I am normally recharging it.

Going back to the table, take a look at COP at load heating load versus high between 47 and 17 F. Sure the COP is great if its idling but when cranked up it has gone way down. Still twice as efficient at electric baseboard but keep in mind the colder it gets the more time the units going to run at high output.

It is important to note that woodstoves are going to have similar variations in efficiency. At steady state operation they do have some limited turn down range where the efficiency stays up but during start up and shut down they are far less efficient. If the stove can idle along during long periods of no heat demand like my 6 hours a day, that is great but I expect the efficiency at idle is zero during that time if there is no need for the heat.

So feel free to run the calcs, the data is available and for a spreadsheet guru its probably an interesting challenge. as far as I am concerned, I will keep my minisplit on my main floor and am considering replacing a 15 year old cooling only minisplit on my second floor to a cold climate unit and running my wood boiler when the heating demand goes up. The strange thing is, I have surplus solar credit so I could probably heat full time down to -13F with minisplits but I hate how they run down at low temps. Defrost cycles happen more often and the heat coming out the register is cooler than forced hot air. Yes its warm but not hot and the air flow and noise is higher. I also notice far more room stratification with warm ceilings and cold floors. I know I personally have to keep the temperature setting higher with the minisplit on than if I am running baseboard.
My post
First I looked at the difference in electricity used between summer and major heating months. (chart) Then as a conservative guesstimate I attributed 7/8th of that difference to running the minisplit. Where Maine’s still high electricity rate dropped 6 cents January 1st the cost should be going down to about $99/ month average. With this very mild winter we will most often but not always fire up the cookstove in the morning, again in the evening and into the night if dropping into the teens or lower. On colder days it will run all day and maybe add wood if up during the night in the low teens or single numbers. Yesterday was mild and I ran it just for cooking pizza. With the older stove, our good draft and lots of kindling on hand starting fires is easy.

So far we have burned less than 5/8 cord. This year our wood took a big jump to $290. (I guess that also raises the value of the three years worth we have in the shed.) At that rate we have used $180 worth so far or a little less than 90/ month ( Dec - now). That’s not too far from the $99 cost of the running the minisplit, the main heat source. I am surprised that they are so close.

We live in upper midcoastal Maine and heat the redone 900 sq.ft. main floor of our 1 1/2 story 1850’s house with an 18,000 btu (heating) Fujitsu heat pump and our cookstove. The minisplit has a COP ratings of 2.34 @ 17deg and. 2.17 @ 5deg. The cookstove has a firebox a tad over 1cu.ft. We do have a boiler with panel radiators which may get used a few days a year mostlyaround the holidays. Our walls are very well insulated and we have six inches of roxul insulation in the ceiling between the main and second floors. The cellar is unheated and uninsulated as is the floor above it. A wintertime custom quilt is spot velcro’d over the open stairway. It intentionally allows some heat to the poorly insulated upstairs. All of the windows are newer. We have three 40 year old skylights, once four, soon to be two and better zero. Our house suits our needs well. Looking down the road when wood could be a challenge to one or both of us I wondered about a second minsplit which to pick up the slack. I think as long as our boiler runs well it be a cheaper although less attractive back up.

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Wow I was on a "tear" one day, thanks for the compliment. GIven your coastal Maine location and this "warm" winter so far I think a lot of folks who bet on minisplits only are patting themselves on the back and the folks down the coast a few states are probably wondering what the fuss is about a mini split only.

One addition piece of info is the upcoming federal IRA rebates for energy savings.
Here is snip from a NH website

Rebate amounts are as follows:

  • Heat pump hot water heater: $1,750
  • Heat pump for space heating and cooling: $8,000
  • Electric stove, cooktop, range, or oven: $840
  • Heat pump clothes dryer: $840
  • Electric load service center upgrade: $4,000
  • Insulation, air sealing, and ventilation: $1,600
  • Electrical wiring: $2,500
  • Installation: $500 and is commensurate with scale of upgrades installed
Combined rebate amounts are capped at $14,000 total.

Rebates are only available to those making up to 150% of area median income.

Rebates are capped 50% of the project cost for those earning between 80% and 150% of area median income subject to the caps listed above.

Those making less than 80% of area median income are eligible for the rebates up to 100% of the project cost, subject to the caps listed above.

Sounds great but there is big BUT

Unlike past programs, this one is targeted to lower income folks. The problem is that the federal law handed off the disbursement to the states hoping that the money will go to the people who really need the help instead of the well off. Every state has to create or expand a new large department of government and new rules to hand out this money and before they can do so, the federal DOE has to approve the program and the rules. The additional problem is that the federal government included some terms in the deal that they have not defined so the states cant really write the rules until they get clarification. Until that gets sorted out in states who want to participate in the program (Florida's governor decided he didnt want to go to the trouble), this program is on hold and is not retroactive. So, there is a nice potential check out there that may cause folks to decide to hold off on minisplit install. The bummer is the AMI is one of those undefined terms in in that case the devil may be in the details on how the household income is calculated to compare to the AMI.

I am not sure about Maine but NH claims that the money may be available sometime this upcomign summer. I know that I am holding off installing a second minisplit to replace an AC unit.

In the meantime I was too lazy to start the boiler yesterday as it was quite a warm day so my minisplit is heating the house. Temps are dropping tonight so it will be back to the wood boiler tonight and for the next 5 or 6 weeks with the minisplit running on warm days.
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We have some friends in the SE corner of VT. Last summer they converted to mini split only. It’s been a warm winter, but they had a single digit cold snap and their house got down to 59. They are adding an additional source of heat.
The "experts" who are pushing minisplits only, would just say they needed additional minisplits. A vented fuel oil heater, like a Toyostove would be a nice easy backup of they do not have a wood option.
If one really cared to be really accurate they would install an emporia Vue.

And if you wants to maximize savings you could edit the specifics of the Maine energy cost calculator. My edits say once you figure in 20% moisture content and a 70% efficient stove and a cop of 2.15 and 17 cents per KWH mini split and stove are with in 10 % of costs.

What this tells me is that the only way I choose wood heat is if I get wood for free and enjoy processing it. And paying a little more for ease of use of the heatpump is fine for me.
We have some friends in the SE corner of VT. Last summer they converted to mini split only. It’s been a warm winter, but they had a single digit cold snap and their house got down to 59. They are adding an additional source of heat.

A properly designed HVAC system only can meeting the homes heating needs 99% of the time. That leaves on average 42 hours a year it’s not going to keep up. Your choices are to over size not a big deal with variable speed equipment (it’s a big deal with single speed) or have backup heat. Resistance heat is the cheapest backup on the long run due to the low install cost.
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