Mitsubishi mini split system

Has whomever made the proposal gone through an used a recognized method to calculate heating and cooling demand?

Is there a reason they didn't go with a couple of multihead units?

This appears to be in about 55,000 BTU per hour.....what is your current method of heating and can you determine the approx BTU per hour (delivered) of it? You wont get 55K delivered into the house from the heat pumps all the time, depending of outside temps, snow cover, defrost mode requirements, but estimating 40K BTU would probably be fair........The modern heat pumps can get you up to COP of 3 and sometimes better, but again this is weather and Temp dependent.....If your 18XX home has a fair bit of thermal mass and decent insulation 40K delivered might be enough!

I've forgotten all the calculations needed but I would add a couple of things.

You want to ensure that the outside units are not near windows. They get noisy and with multi splits, you may have a unit on to cool one room but the window near the compressor may be open to let in fresh air.

I would want to know EXACTLY where and how all the electrical and cooling lines are routed. It sounds like your house could be a nightmare. You may not be able to hide lines in the walls, they may need to treat out wall sections and they may run into things that were not predicted, running up your price.

What heats the house now? I agree that the cost is to high. How many years is the payback, if there is one? How many years is the complete warranty? The cost of repairs is just as staggering as the cost of the units. My mini split was great for 2 years, now it has a leak. Out of warranty and it will cost nearly $400. to fix.

Okay that makes a bit more sense. My guess is that the units were sized for cooling load?. I believe Mitsubishi units are sized for full rated output down to 5 deg F. They still put out heat down to -15 but its reduced. Considering that I routinely heat about 1000 square feet with 1 ton unit down to about 25 F, the total heating capacity looks excessive. I don't have a lot of use for AC in my neck of the woods so others will have to comment if the AC sizing is correct but I speculate that the tonnage would be considered low if the system was just single a standard ducted heat pump. You wont have duct loss so you can get away with a smaller total tonnage. My guess is the unit sizing is mostly due to having a cut up house with a need for lots of heads.

Unless the reliability/complexity of the multihead units have improved, most installers I have talked to really prefer the single head units.

Give wood and electric baseboard, I sure wouldn't size for 100% heat in worst conditions. The ASHRAE annual extreme design temp is only around 20 deg F so I would size for this or even warmer and use the wood or electric on rare occasions when you drop below (The ASHRAE temp is the temp that covers the winter low temp 99% of the time). If you shoot for 30 degrees, the splits gain about 20% additional heating capacity.

You should try to work out how many BTUs you are needed now per degree day. Add up your excess annual winter usage kWh *3400 BTU/kWh and cords of wood times some number for output, like 15 million BTUs/cord. Divide by your heating degree days last year, probably about 4000-5000, and you get BTU/HDD for your residence. It should be 5-10X your square footage. Divide that by 24 h/day and multiply by 50 to get your BTU/h heating demand when it is 20° outside.

Of course, the nominal output of the minis is at what outside temp?

So, if you can estimate your BTU demand at 17°F, and the units meet that, I would say your sizing is pretty good. Your baseboards would provide backup when it gets colder than that. If you oversize the minis a little, you will use less backup. You just don't want to way oversize or undersize.

Mv7fd said:

I'm removing the wood stove because it's in the living room and it's a terrible mess with dust and ash and wood and my house is small so it really cuts down on the living room size

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Hey wait. You're getting rid of your wood stove because "...it's a terrible mess with dust and ash and wood...." ?
Is this mostly a "wood" heating site ?
No "dust and ash" in our place. There is however "wood". It is all in the proper management of burning: some minor cleanup, some ash removal, but not a big deal since most of us also enjoy a clean home.
You will miss that stove.

DUMF said:

You will miss that stove.

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I will admit even with a wood boiler the heat quality is a lot better with my baseboard radiators than the minisplits. The baseboard runs along the outer wall so I don't have significant drafts from my windows. With the minisplit, it blows the heat around imperfectly. There is noticeable difference between the temp at the ceiling and temp at the floor. I end up running the minisplit at a higher temp to compensate and play with the outlet damper angle so the heat goes where I am. My wood boiler and storage is in the basement so when I use the boiler, the basement and my first floor is warm, if I switch to the minisplit the basement cools down and my first floor is cold.

With a wood stove in the room, there is a lot of radiant heat. Hard to beat radiant heat when you are cold.

I have a suspicion that you have a very poorly insulated and drafty house with a high heating load. The first thing to do is stop and get an energy audit, then spend money to tighten the place up and even consider having the auditor come back and perform the blower door test again after you have sealed and insulated everything they caught the first time and look for more. Most energy audits rank the cost of the improvements versus payback and you will be surprised on what the recommendations will recommend. Before you insulate much, they are going to suggest air sealing which is chasing after cracks in the outer envelope of the house. Its slow work but basically the first phase is foaming the big gaps and caulking the small ones.

Indeed. Can you estimate the amount of kWh you used last year (either total, or for heat alone), and the number of cords of wood?

Ok. And how many kWh in 12 mos? I would assume 8000 kWh for appliances and AC.

If your house is well-insulated, I would suspect that the sizing quoted is quite a bit more than is necessary. Why not try first try a couple of units in the larger rooms and see how it works? From my own experience in looking at the ratings of the Fujitsu units, the rated heat load is at a fairly low temperature (I recall my 15,000 BTU unit was rated to product 15,000 BTUs at ~10-15 degrees F). Then, at higher temperatures, they will produce 2-3x the rated BTUs.

These units are remarkably quiet - I wouldn't hesitate to put one near a window.