New construction: multiple mini-splits or single central unit?

I have a new 40x80 metal building. It has 12 foot tall walls and a roof peak of 15 ft. I plan to keep the ceiling open to the top in all the rooms except the closets and bathrooms which are in the middle of the building. It has spray foam insulation on all of the walls and ceiling. I have divided it into a shop and living area. The living area is approx 1,400 sqft. I plan to heat/cool the entire building.

The framing for the interior walls will begin in July. I have been trying to decide the HVAC issue for more than 2 years. When we moved to the property full time, we placed a small 12x30 tiny house here and installed an EG4 AC/DC 12k mini split. I have 4 385 watt panels running it and I am happy with the performance (for the most part).

My question: disregarding initial installation costs, which would be more efficient in terms of operating costs over a long period of time for a hybrid inverter or even off grid situation?

Thanks

Jeff

Individual minisplits have higher efficiency than a central variable zoned heatpump system. Don't know about lifetime costs when replacement is included.

DIYrich said:

Individual minisplits have higher efficiency than a central variable zoned heatpump system. Don't know about lifetime costs when replacement is included.

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A mini split unit have a run time for 10/15 years if you use it for cooling and heating
And really only thing to replace is out side unit

You can use one out side unit with multi inside unit.
So you have more heating options.
In side unit do not use a lot of power.
And a bigger out side unit is cheaper to run than 2 smaller outside unit .

And go for a model that use r32gas .
More efficient than the old a410 gas

If you were closing off the individual rooms, I would say the mini-splits. I'm not really sure how those mini-splits are going to do when they are just blasting into a giant 40x80 metal building with 12-15 tall ceilings. They might still be more efficient, but I'm not sure by how much vs an appropriately sized central unit with the ductwork inside the cooling envelope.

It used to be true that minisplits were generally more efficient than central units. But that's no longer the case. You can buy variable speed compressor central heat pumps that are just as efficient and are often more efficient as you move into the 3 ton and larger size range.

A good example is the Trane XV20i


I installed a 3 and a 4-ton XV20i units in my main house. My Lake house has 6 mini-splits for the bedrooms and then central units for the common living areas. That makes a lot of sense in the case of my lake house since a lot of the bedrooms are unoccupied most of time except when it's just my wife and I.

Both houses are essentially the same square footage. When family is at the lake house, and all the minisplits are in use, the combined energy usage is more than the XV20i units at the main house. So overall I think the XV20i are more efficient than the minisplits when you have to heat or cool the entire space and don't get the advantage of not heating or cooling unoccupied rooms.

Because all the rooms are open to the roof area in your building, you don't really have the option to not heat or cool unoccupied rooms. Based on that, if I were doing this, I would go with a very high efficiency variable speed central system such as the Trane XV20i or XV19.

The only part I'm not sure about is total costs of the systems. Especially if you're not installing them yourself. I install all my own HVAC equipment, so I don't have a good feel for total installed cost of a central Trane XV20i or XV19 vs several minisplits.

jpwhit said:

The only part I'm not sure about is total costs of the systems. Especially if you're not installing them yourself. I install all my own HVAC equipment, so I don't have a good feel for total installed cost of a central Trane XV20i or XV19 vs several minisplits.

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Maybe they would give him a discount because they don't have to be in a hot attic doing any ductwork during the summer?

cs1234: The whole building is in the cooling/warming envelope. The rooms on the outside of the space will be open to the ceiling. The rooms in the middle (bathroom & closets) will have ceilings. I was wanting to do DIY mini splits but the guy that did the spray foam "knew a guy" that did new solar friendly heat pumps. I am still exploring options.

jpwht: One of the concerns about a 4 or 5 ton unit is the start up current as I may be off grid with this building. In the rooms that are open to the ceilings, the walls go all the way up to the roof, so in essence they are separate spaces.

I find both setups work well together. Central at one temp and the mini's adjusting certain areas for comfort. If it all works out right your central unit won't come on that often.

Ruff said:

jpwht: One of the concerns about a 4 or 5 ton unit is the start up current as I may be off grid with this building. In the rooms that are open to the ceilings, the walls go all the way up to the roof, so in essence they are separate spaces.

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Variable speed compressor units, such as the XV20i and XV19 don't have large start up currents. It's only the fixed speed compressor units where that's a problem.

jpwhit said:

It used to be true that minisplits were generally more efficient than central units. But that's no longer the case. You can buy variable speed compressor central heat pumps that are just as efficient and are often more efficient as you move into the 3 ton and larger size range.

A good example is the Trane XV20i

21.5 SEER2 Air Conditioner - XV20i TruComfort™ Variable Speed - Trane®

Learn how the Trane XV20i TruComfort™ Variable Speed 21.5 SEER2 air conditioner automatically maintains speeds to avoid temperature swings.

www.trane.com

I installed a 3 and a 4-ton XV20i units in my main house. My Lake house has 6 mini-splits for the bedrooms and then central units for the common living areas. That makes a lot of sense in the case of my lake house since a lot of the bedrooms are unoccupied most of time except when it's just my wife and I.

Both houses are essentially the same square footage. When family is at the lake house, and all the minisplits are in use, the combined energy usage is more than the XV20i units at the main house. So overall I think the XV20i are more efficient than the minisplits when you have to heat or cool the entire space and don't get the advantage of not heating or cooling unoccupied rooms.

Because all the rooms are open to the roof area in your building, you don't really have the option to not heat or cool unoccupied rooms. Based on that, if I were doing this, I would go with a very high efficiency variable speed central system such as the Trane XV20i or XV19.

The only part I'm not sure about is total costs of the systems. Especially if you're not installing them yourself. I install all my own HVAC equipment, so I don't have a good feel for total installed cost of a central Trane XV20i or XV19 vs several minisplits.

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I just went to trane's website, and looked at the reviews for the XV20i unit. Clicked on the bad reviews, and it's one horror story after another. How much did you get your unit for?

Multiple single zone mini splits is likely technically more efficient but I would rather have central air for the circulation and filtration and possibly better dehumidification.

I have a "multi zone" mini split with one outdoor unit and 3 heads and it's basically the worst option I'd say, all the downsides off mini splits and less efficient than single zones.

Edit

For me, I have a 100% off-grid goal coloring my decisions and I don't have enough solar to run the whole-house heat-pump in the 4 months of winter. Normally this is no problem as I'm OK to use grid-assist but... I'm planning for a backup individual mini-split so I could keep one of our larger rooms warm that can fit within the limited winter PV and the rest of the house can stay cold. This meets "two is one" rule for off-grid.

If this is you're situation, and/or you have multiple zones then perhaps multiple units for different areas would work for you as well.

A 40x80 with 12 walls and 15 peak works out to about 39,840 cubic feet if my 5th grade mathing skills are correct. Attempting to heat/cool that much area is totally insane. HVAC for the living area of 1400 sqft would be a little more realistic. Am I missing something here?

Also, the pitch on that 40 foot span seems to be a bit on the flat side. I assume the living quarters will be 35x40. Then maybe having the front wall mostly of glass. Or not.

For heating you really should consider radiant heat in your slab. If you want to keep it electric, look at something like the SanCO2 heat pump water heater. Individual mini-splits are great for efficiency, but they do not do proper dehumidification so be sure to add that into your equation; spray foam is a great vapor barrier but you still have to address internal humidity loads.

I'd also recommend making sure you have some dedicated source of outside air and exhaust; you can cut your energy losses there with an ERV (not HRV).

So, for Shimmy's dream house, I would have mini-splits in each bedroom or similar space, and the main open space I would have a small air handler that addresses outside air, filtration, and dehumidification.

Cabin Rising said:

A 40x80 with 12 walls and 15 peak works out to about 39,840 cubic feet if my 5th grade mathing skills are correct. Attempting to heat/cool that much area is totally insane. HVAC for the living area of 1400 sqft would be a little more realistic. Am I missing something here?

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If you have 3" of spray foam everywhere and with good windows and doors it is about 24,000 BTU/h in the winter, likely less in the summer. Ventilation air is significant, but an ERV will address that.

I have my 850 sqft cabin that is two story and full 3 inch spray foam on the outside of steel panel walls roof and under floor. Then with 3/4" air gap and another layer of steel panels for exterior finish. There is a 1-1/2" air gap between the inner steel panels and drywall, including the roof.

Even with this it will absorb the outside temps if not controlled all day long. Fortunately it does not take much of an A/C unit to cool the place once you get it under control. During the summer months when it is always above 90 degrees still way after dark, the A/C is running most of the day when sun is baking overhead.

I am in the south and summers are hot. Below 90 degrees it does not work so hard. But I am only using a small 8000btu window unit upstairs. It is rated for 325 sqft. But with it upstairs and blowing at the roof peak, the air is cooled and pushed down. There is only a few degrees difference between up and down stairs. It stays that way so long as the door is not opened and closed all day.

The lower level is shaded from the sun with a wide porch roof and it is also insulated. The upstairs window unit cools the air and upper floor or downstairs ceiling. This way all of the sides to the downstairs are more or less isolated from the outside temps. So long as it does not stay above 100 degrees all day, this small window unit can just keep everything comfortable and dry.

In the next week I will finally be tied to the grid. Working on installing a slightly larger window unit on the north side or a mini split there in the stair opening. It will blow towards the roof peak and the return will be pulled from downstairs.

Right now the small 8000btu window unit is in the south facing wall. It gets sun all day making the unit work hard and run very hot. I need to shade over the unit or move it to the north wall. The mini split is a smaller 1 ton model. It should freeze the cabin and be inexpensive to run. That is what I am hoping for.

Cabin Rising said:

A 40x80 with 12 walls and 15 peak works out to about 39,840 cubic feet if my 5th grade mathing skills are correct. Attempting to heat/cool that much area is totally insane. HVAC for the living area of 1400 sqft would be a little more realistic.

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How about over 75000 cu feet?

Cabin Rising said:

Am I missing something here?

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Yes. My shop is 60x72 with 18 foot walls. 3 overhead doors, two are brown and face south, one is 26x16, the other is 12 x 16. Door on north end is 12 x 14. All 3 are 3 inch insulated but those brown doors do transfer some heat.

Also have 3 walk in doors, 10 windows and some are bay windows.

Walls are R38 and ceiling is R60. Floor is radiant heat, 7 inches concrete with 2" of foam under it.

I've been just running the 2 ton mini split to cool it, I have the option to turn on the 2 ton central unit if I bring in something hot and want to cool it down quicker.

For years I ran the 2 ton central unit and a 2 ton window unit. Wife got tired of installing the window unit. I usually set the thermostat for 68°F, drives her insane but I'm the one working and she is in the office.

Not really a problem cooling this. I do have another 2 ton mini split to install later this year, other priorities now that I have the one in to replace the window unit. I'm leaving the 2 ton central unit in place as backup or if it's 105°F with high humidity and I need to pull something in.

Electrical consumption:

last June was 1800 Kwh for the whole shop

this year so far I've been running about 40 Kwh per day- I just finally fixed the Emporia about as week ago by adding a 2.4Ghz access point 5 feet from it. I expect this to be about the new normal as lights are a big part of daily load.

Is it insane? Not really, the insulation is key. Walls are 11 inches thick. I did 3.5 inches of foam, the rest is cellulose. I'd forget the foam next time. Ceiling is 2 feet of cellulose. Building is tight, interior was completely lined with vapor barrier and all seams taped. Building is so tight that in the winter if opening the large overhead door, then closing it, it causes the walk in doors to have positive pressure as the interior warms up. As for summer, last week I pulled in a hot semi tractor and the mini split water condensate was running out the side instead of down the drain. This was due to negative pressure inside from cooling off the hot vehicle. I have to install a drain tube with an air break to fix the negative pressure from preventing condensate drainage.

It will be in mid 90's today with dew point around 75°F so it will be a real nice day to work inside the shop.

The new construction ~2000SF townhomes nextdoor use the ComfortStar CPR48CD. Nice looking unit, quiet and has a small footprint. Am considering for my next project.

Delmar said:

The new construction ~2000SF townhomes nextdoor use the ComfortStar CPR48CD. Nice looking unit, quiet and has a small footprint. Am considering for my next project.

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That's a Midea unit. Midea doesn't sell under their own brand name in the US, at least not yet. They have a strong re-branding program in the US where they will let folks put their own brand name on the units and sell them.

This is why I say "net yet" on them selling in the US under their own brand name.

Midea open New York Showroom

Shimmy: The slab is in, the building is up, the doors and windows are in.- We considered floor heating. It was heating only and it requires routine maintenance that may be beyond my capabilities as I get older. One of the concepts of a metal building is not having to repair roof shingles or ever painting siding again. The steel doors have R13 insulation value and we went with low E double panel side sliding windows, so it is a tigh building. It has 1 inch closed cell spray foam everywhere, 4 inches open cell foam on top of that on the walls and 5 inches of open cell foam on top of that in the roof. This is a commercial red iron, sheet metal type building with a 120 mph rating and the spray foam may make it even more rigid. I can add a heat exchanger without a full HVAC unit and plan to do so once we see how the house breaths after a few weeks of living in it.

Cabin Rising: I has 5 3 x 3 windows, 2 32 inch doors and 2 overhead doors in the shop. I do not want to see the outdoors from my living room. If I want to see the forest, I will get dressed and sit on the porch. Windows are a weak link in the insulated walls so I limited them as much as possible. No seriously, we had a house in Houston that had big floor to ceiling windows and we kept the blinds pulled on them 90% of the time. Considering the heat loss in windows, why put one in and cover it up to keep the sun out. we just elected to not have the windows.

Zwy: The shop will be heated and cooled, but not at the same level as the living area. In the winter, I just need it to not freeze and warm enough to work on things, Keeping it in the lower 40' or 50's and not about 85 in summer is the plan and we know what a plan is...... "A list of things that don't happen. The living section will have a 2x4 stud wall attached to the z purlins creating about 4 inch dead space between the spray foam and sheep rock. I may fill the studs with fiberglass batting. Not sure, we will see how the budget is at that time.

I am waiting on one more bid for the framing. Once that is determined I will contact a local HVAC company and get their opinion. I am still leading toward DIY mini splits, but I am still open to a better way.

Thanks for the input. I will come back in a few weeks and let you know what I decided.

The building / property / location may not be as I originally envisioned it. Reality gobbled up my dreams and spit out something that meets my needs but may not match my dreams.

Thanks for the help.

Jeff