Mini split plus gas heat?

[INTMD8]

Trying to figure out heating/ac for my 36x47 pole barn. Plan is to do the building with closed cell spray foam.

I keep reading that the mini split heat pump is not sufficient at lower temps.

What are you guys using for backup heat? Would it make sense to have a hanging gas heater as well?

 

[pseudorealityx]

Certain models of mini-split are now capable of 100% of their heating capacity even at 0 degrees F. The biggest issue for you would be that's a large space to only have 1 evaporator on the end of the space. It'll be nice and cozy close to the unit, but you'll end up with cold spots.

 

[theoldwizard1]

^^^^ What he said !

Mitsubishi specifically advertises that their mini-split works at cold temps.

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Do you have anything "critical" that can't freeze (like when the power is out for more than a few hours or when the temps are well below zero) ? A couple of kerosene or LP "torpedo" heaters should be all you need for those emergencies.

 

[theoldwizard1]

The biggest issue for you would be that's a large space to only have 1 evaporator on the end of the space. It'll be nice and cozy close to the unit, but you'll end up with cold spots.

So buy a system that supports 2-4 indoor units (they aren't truly "evaporators" because when the are heating for building they are actually condensers !)

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These can operate independently (separate rooms) or be "slaved" together for a large room.

 

[INTMD8]

Nice but looks expensive! I will do some research on those. I don't have anything that can't freeze but if it gets less efficient at lower temps and can't heat the building to a comfortable temp I won't be inclined to work out there.

 

[INTMD8]

I'm guessing this system would be borderline in terms of btu. Even still, quite pricey.

https://www.younits.com/3-zone-mxz-4b36na-2-msz-ge09na-1-msz-ge24na-42-000-btu-3-5-ton.html

I'll have to think this over. A couple of ptac's would be much cheaper initially

 

[theoldwizard1]

I'm guessing this system would be borderline in terms of btu. Even still, quite pricey.

https://www.younits.com/3-zone-mxz-4b36na-2-msz-ge09na-1-msz-ge24na-42-000-btu-3-5-ton.html

I'll have to think this over. A couple of ptac's would be much cheaper initially

You can pay for it up front, or you can pay for it every time you turn the thing on ! The operating cost would be MUCH less than a couple of PTACs. PTACs have an EER of 10-13. A good mini-split is close to 20.

How much are you going to use the building ? Even if it is only a few hours everyday, you can not leave your HVAC turned off and expect to walk in and have it warm/cool in a short period of time. PTACs don't heat below about 40 degrees unless they have a resitance heater strip built in.

You said you were going to spray foam the building. Have you seen how much that costs ? And if you can only afford 1-2" on the roof/ceiling it will be 10-11 PM before that building is cool it you turn either system on at 3 PM.


Analyzing installation costs versus operating cost is very difficult. Not a job for amateurs like me.

 

[INTMD8]

I plan to use the building often. Working on a project car on the evenings and weekends. Maybe ptac was the wrong term. What I was looking at was a Magic Pak with gas heat.

The foam will be $7200 for 3" roof, 2" walls. I would like to keep the building heated/cooled at all times so I can work in there without waiting for temps to draw up or down.

 

[Jackfre]

Go to Fujitsugeneral.com. Go to the contractor tab and you can download their 2014 catalog. They give very complete specs by model for outputs. Last I heard it still gets kinda cold out your way. There are units by a number of manuf that make the rated output below -15. Those units are in the 9, 12 & 15 kbtu ranges. The right way to start is with a heat loss number, or if you like you can shoot from the hip. In the very cold weather gas will be hard to beat. Once you get above 20* or so the heat pumps begin to sing. Last I heard summers can be kinda moist both rain and humidity wise out your way. The mshp(s) will be very good there. I would not use a multi in your unit. I would probably look, in shooting from the hip, at 2- 24kbtu units placed in opposite corners for air circ purposes. Unless you are looking to divide the space.

For gas equipment I would want a Rinnai ES-38.

 

[bullnerd]

I'm almost in the same situation as the op, so hopefully he wont mind me asking this question.

This was mentioned earlier- "1 evaporator on the end of the space"

Would the cassette style evaporator(?) mounted in the center of the ceiling make any difference in a minisplits performance?

And also, The Rinnai ES-38, That's enough for the space he's talking about?

 

[mikec35]

So buy a system that supports 2-4 indoor units (they aren't truly "evaporators" because when the are heating for building they are actually condensers !)

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These can operate independently (separate rooms) or be "slaved" together for a large room.

The mini split with multiple wall units would be good if you are going to keep the barn near the same temp you will be working in all the time. If you plan to set the temp back in the winter then try to warm the space quickly everytime you come into the shop then you should get a gas heater. Nothing will bring the space temp up quickly like gas. Another really good alternative could be a gas tube radiant heater above the space you will be working. They are super fast at heating.

 

[theoldwizard1]

I plan to use the building often. Working on a project car on the evenings and weekends. Maybe ptac was the wrong term. What I was looking at was a Magic Pak with gas heat.

Those are even WORSE than the PTACs is looked up ! Only 10.0 EER.

You really need to do more research.

The foam will be $7200 for 3" roof, 2" walls. I would like to keep the building heated/cooled at all times so I can work in there without waiting for temps to draw up or down.

That will give you about an R20 in the ceiling. You really need R30, and R40 would be better in the ceiling. R20 for the walls would be good.

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You also need to install at least 2" of foam board with vapor barrier under the floor.


I'm not trying to "bust your chops", I'm truly trying to educate you. Building an efficient to heat/cool building is not easy.

 

[theoldwizard1]

Nothing will bring the space temp up quickly like gas. Another really good alternative could be a gas tube radiant heater above the space you will be working. They are super fast at heating.

In general, natural gas is a very cost effective fuel for heating. If you are only going to heat a structure, the only thing that could beat natural gas operating cost is a geothermal heat pump and it would be difficult to recover that addition cost.

The big "win" on any heat pump system is that you get A/C for "free".

Also, be careful when researching heat pump systems. Only certain models of Mitsubishi "Mr. Slim" heat pumps (ones with "Hyper Heat") can extract heat at really cold temperatures. I don't know if any other brands do



Do Snap-On tools cost more than Craftsman ? You get what you pay for !

 

[INTMD8]

While I do appreciate the education I am aware I need to do more research otherwise this thread would not exist.

So I need about 50% more insulation than what was suggested to me.

As for the floor, the slab is already there as the building existed when I purchased the house. Only thing I was told about it is its supposed to be 6" thick.

Mike- I would prefer to leave it within 5-10 degrees of working temp. I don't like the idea of shutting it down and heating it only when I will be out there. Tools and cars remain cold for too long.

Thanks guys for the input thus far

 

[theoldwizard1]

So I need about 50% more insulation than what was suggested to me.

Insulation, especially in the ceiling, pays back during all seasons. You should have as much insulation as you can possibly afford ! The fastest way to decrease heating and cooling cost is to increase your insulation.

R30 is a minimum. R40 would be better. That is 100% more. Folks in WI (not that far north of you) commonly have 12" of fiberglass in the ceiling for new construction. In MN, 18-24" is not unheard of for new construction.