Install ductless mini-split

Money goes in, heat leaks out
kevm14
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Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

I made a video exploring the cost difference of a mini split in heat pump mode vs my oil furnace and it is dependent upon the price of oil, price of electricity, temperature outside and the inside setpoint. I run through some scenarios and the conclusions are interesting.

https://youtu.be/elQUIYOF6tM
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

So with some nights down to 32F and the room like 57 or even colder in the morning I've had an opportunity to observe additional behavior. Basically, this thing does a much better job keeping a room comfortable when it maintains a consistent temperature. It doesn't seem to work as well for warming a cold room 10 degrees. I don't think this is a capacity issue as much as it is a temperature sensing issue. I think if I had a proper remote thermostat it would work better. But as it is, with the sensing happening in the unit (because I haven't figured out a way to repeat the IR signal from the hand held remote on the wall), it seems to have difficulty warming the room quickly. It just seems a little confused, often satisfying itself when the room is still 2-3 degrees below the setpoint. But, once the room and all the stuff in it has reached my setpoint, it does a great job maintaining it as I have said before.

So I need to take this into consideration. I know it's less efficient running at full capacity but that has to be offset by the entire overnight period with zero heat in the room. We aren't in the room. Why should we heat it?

I wonder if the ideal scenario would be to use my oil furnace to heat the entire house first thing in the morning when it is sitting at 58 or whatever. Once it initially gets to the 65 setpoint, I would close down the dampers and let the mini-split do all the temperature maintenance. This could work well. Problem is, that requires moving the dampers daily. I'd have to open them at night to the great room. Then once the house was warm by 7am or so, close down the dampers and run the mini-split as necessary.

What I really need is another thermostat in the great room and electronic zone dampers for the great room. That way I could set the thermostat in the great room for 58 like the main one and the heat would kick on as needed to maintain that. Then I'd raise that to 65 to match the other thermostat. Then I guess I'd set it back again and switch to the mini-split. That way the zone dampers would stay closed for the rest of the day and the furnace would do the rest of the house. This is somewhat complicated but seems like the idea way to run things based on what I know right now.

What I don't have is any idea on actual costs (oil down vs electricity up). On the other hand, the efficiency calculations I ran before were based on full capacity efficiency. Assuming it is the same or more efficient when run at a decreased capacity, I guess technically what I am doing is still OK...

Maybe what I really should do is figure out that IR extender solution and see how it works with a thermostat on the other side of the room. Then go from there.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

Last weekend I opened the dampers to the great room all the way. I just don't think the minisplit is efficient at bringing the room from like 54 all the way to 68 or whatever. It was an excellent job maintaining a temp but it has to run hard to increase the temp substantially (though it can do it which is good). So what I've settled into now is letting the furnace warm the whole house to 65 or whatever the set point is. As before, the great room is a solid 3-4 degrees colder so I just run the minisplit at 68 and then it gets to do its job of maintaining temp but the furnace does the initial warm up in the morning. I think this is probably good but it's just my intuitive guess. Still don't have a numbers way to validate any of this other than my initial calculations.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

The other cool thing that happens is the great room warms the house a bit. So with that on 68 and the house on 65 it will keep the furnace from running as frequently. As long as the actual energy cost is cheaper this is obviously a win win.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

This is impressive. Up until around 12/4 or 12/5 I had closed off the heating ducts to the great room and was heating it solely with the mini-split. At that time, I opened the ducts and used the mini-split only as a supplemental source for the room (it's still needed - great room runs 2-4 degrees colder without it, as before).

Here is my electricity use that spans this time period. I am pleasantly surprised.
energy 11-9 to 12-10.PNG

I am actually quite a bit below the "similar homes" despite what I would assume is much heavier electricity use with the mini-split. I should look up the degree days during this period to get a sense of what kind of heating demand there was but this is impressive if you ask me. I was thinking that my oil use would go down but it would spike my electricity use. I don't see much evidence of that from this.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

How does it do in a blizzard where temps are way colder than normal for snow?

https://youtu.be/bqwSrWIJ1r0
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

I ran it all winter for the most part. On more mild days I started running it harder, like up to 75, and made sure the furnace didn't turn on. Definitely saved on oil.

As for electricity, here are my home energy reports. Not sure how useful this is because the insulation work came in ~May 2021.
energy Dec 21.PNG
Nov to Dec 2020: 712 kWh
energy Jan 22.PNG
Jan to Feb 2021: 625 kWh
energy Feb 22.PNG
Feb to Mar 2021: 592 kWh
energy Mar 22.PNG
Mar to Apr 2021: 600 kWh
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

Based on the little heat wave last weekend (upper 80s though I saw over 90 on my outside thermometers) I would say that 18k BTU of great room A/C plus 9k of mini-split in each bedroom (totals 3.75 ton) is probably the perfect amount of A/C. Probably even more than necessary. As it was, I had the great room, an 8k in the master bed and a ~6k in the other upstairs bed (2.66 ton). The peak temp downstairs reached 75 but it was quite comfortable. So adding another A/C in the downstairs bed would basically fix that. 3.75 ton would give a little headroom even for unreasonably high temps, like mid 90s or a bit more. I would have to drive the rooms with A/C a little colder than I might otherwise though, because there would be a fair amount of unconditioned space (living room, dining room, kitchen, downstairs bath; on the order of 900 sq ft). But it's a pretty open space so the great room and 3 bedrooms should all contribute just fine as long as the doors are open.

I have also been putting a box fan in the great room doorway to help spread the A/C into the living room and that seems to help, too. Definitely getting tired of window A/C units though.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

Continuing the discussion on moving away from window units in the rest of my house for A/C, I have mostly come to a decision on at least general capacity if I were to switch to mini-splits.

I would do a 9k unit in each bedroom, so that's three. They don't come any smaller than 9k so that is a pretty easy decision. Master bed has an 8k window unit and it is more than adequate on even the hottest day (particularly after the insulation work) so 9k is even more headroom. Then I have also decided that I think the downstairs needs something. Yes my house is open and 3 bedrooms all cooling should theoretically cool the main floor but I don't quite buy it. I have seem the main floor get to 74F (which is totally comfortable) while the rooms with A/C are quite a bit cooler. So I think it would be better to not have the other units try to overcool their respective rooms. Therefore, I think I'd put a 12k mini-split head in the living room area, blowing toward the dining room. This would mean the kitchen kind of gets the short end of the stick both for heating and cooling from a mini-split perspective. A unit in the dining room pointed toward the kitchen might be better but has two main issues: 1) it would blow cold air at the dining room table and that would likely result in my family wanting to turn the set point up and not cooling the overall space enough just to stop the draft and 2) it would be considerable effort to run the lines on an interior wall.

Anyway, at least as far as total DIY options, I think I have it narrowed down to two:

Option 1
4 zone Mr Cool. Current 4th gen cost $4,939. Plus electrical (8 gauge, disconnect, whip) and one condenser mounting.
https://iwae.com/shop/36k-btu-21-5-seer ... 22072.html
Same inside unit placement as listed for Option 2, but with a single outside unit, placed probably where the System B describes (east wall).

Option 2
Two 2 zone Mr Cool units. $5,576 plus electrical (two runs of 12 gauge, plus two disconnects, plus two whips) and mounting two condensers.

System A would do the master bed and living room. Was thinking this unit would work. Condenser would be located basically near the front door somewhere to the right (in the mulch area, north/front side). $2,854 is the current 4th gen cost, plus electrical and condenser mounting.
https://iwae.com/shop/18k-btu-21-seer-m ... 22046.html

9k head in bedroom and 12k head in living room, but still 18k total condenser capacity. Master bed unit mounted on front wall. Living room unit probably also front wall but maybe not). I think the bedroom demand would almost always be under 9k so the downstairs unit should have access to the full 12k without issue. They do also make a 21k unit (true 9+12) but I don't think I need that.

System B would do the other two bedrooms and I would choose the 9k + 9k system for that. Condenser would be located on the east side of the house. Downstairs bedroom unit would be mounted on front wall, pointed toward door and dining room. Upstairs bedroom unit maybe mounted on east wall just to avoid having an extra set of PVC lines on the front of the house. $2,722 is the current 4th gen cost, plus electrical and condenser mounting.
https://iwae.com/shop/18k-btu-21-seer-m ... 22045.html

Splitting into two systems (with Option 2) does increase cost at least $650 and probably more. But it should simplify the installation process by not having to run everything to the same outside unit. And, I can break the project into chunks.

I think I need to continue thinking about this though and make sure I am not making a mistake with inside unit placement. At least for the living room/dining room unit, maybe it does make sense to attempt to run inside the house and pop the lines/wires back outside.
kevm14
Posts: 15200
Joined: Wed Oct 23, 2013 10:28 pm

Re: Install ductless mini-split

Post by kevm14 »

kevm14 wrote: Tue Aug 09, 2022 10:03 am Option 2
Two 2 zone Mr Cool units. $5,576 plus electrical (two runs of 12 gauge, plus two disconnects, plus two whips) and mounting two condensers.

System A would do the master bed and living room. Was thinking this unit would work. Condenser would be located basically near the front door somewhere to the right (in the mulch area, north/front side). $2,854 is the current 4th gen cost, plus electrical and condenser mounting.
https://iwae.com/shop/18k-btu-21-seer-m ... 22046.html

9k head in bedroom and 12k head in living room, but still 18k total condenser capacity. Master bed unit mounted on front wall. Living room unit probably also front wall but maybe not). I think the bedroom demand would almost always be under 9k so the downstairs unit should have access to the full 12k without issue. They do also make a 21k unit (true 9+12) but I don't think I need that.
I need to update this. The first floor main area equates to somewhere in the neighborhood of 820-856 sq ft (depends on whether I consider the bathroom to be cooled by Ian's room, which it is directly adjacent to). Considering the whole back of the house is vaulted AND southern facing, that is 100% in the 18k BTU category, not 12k. So System A would need 18k + 9k. Even more expensive. $3,332 up from the $2,854. Adds almost $500 but that is capacity I think I need/want, and it would increase the 4 zone cost, anyway. But changes the new total to $6,054 (plus misc stuff) which is eye watering. It would also put total household cooling at 5.25 ton which I assume is way more than I actually need. But remember these units throttle down, the air handlers are not necessarily perfectly scaled (i.e. 9k is actually too high for the bedrooms but that's as small as they go), but it means more heating capacity, which means less reliance on oil.

And for placement of the first floor unit, I looked more critically. I think the best I can do is use the front wall and change the hole orientation to be more in the middle of the unit rather than the edge. I need to do this because of one of the porch lights. Even with 16 feet of line that will be too much, which is another problem. Really didn't want extra line just coiled up as the condenser alone will be an eyesore. A messy looking condenser install is probably a bridge too far so I need to think about this. I COULD try running the lines down inside the wall and pop out above the sill plate but honestly that probably doesn't waste that much line.

The other option for first floor placement of air handler would require something over 25 feet of line and a run through an interior wall, across the basement, and out above the sill plate. Maybe better performance having it in the vaulted section of the room, pointed toward the dining room/kitchen, rather than sort of stuck in the living room with the low ceiling, attempting to cool around the corner.
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