HVAC system type for detached garage

[BrianS]

I have a 24x32 detached garage with 14' ceiling and an attic bonus room above. I want to install a system that can keep both spaces around 45-50* in the winter and 80* or so in the summer. This is only a storage space, no need for 70* year round. It will be fully insulated, location is SW Ohio.

I have propane available and 50 amps of electric service.

One contractor spec'd a traditional 70k BTU propane high efficiency furnace with 2 ton A/C. Another spec'd a dual fuel 2 ton heat pump with 60k BTU propane as backup. I've asked about a 2 head mini split but they've all said that would cost even more.

The heat pump with propane backup seems like it would be most economical to run, but one contractor said they are designed to maintain temps around 70* and it wouldn't work as well at the lower set temps.

Anyone have thoughts on the best type of system to go with, both from an up-front cost and running costs perspective?

 

[jlv03]

The problem with heat pumps is you can't yo-yo the temps that well; there is no quick recovery. However by doing dual fuel you can side step that problem. Use the heat pump to maintain, use the propane to quickly bring the temps up.

 

[BrianS]

Yea, understood. I don't plan to swing the temp up and down, except for the fact that there is a large overhead door (18x12) on one end, so if it gets opened it is going to lose a bunch of conditioned air.

It didn't make sense to me why the heat pump wouldn't like a set temp of 50*. What does it care? It produces the same air temp no matter the set point, it just shuts off sooner, no?

 

[jabelding]

Hi Brian
I had a similar situation on my 28x36. I found a used med efficiency hanging modine 70K btu unit for $100. Then I purchased a 2 ton Mr cool diy heat pump. When it is really cold out 30 and below (most of the winter) I run the propane averages about $3 per day then when the temp is around 30 i run the heat pump but keep it on freeze protection mode runs around 46-48 deg. I normally keep my temp at 48-50 deg unless I am out working then I bump it up to 65 and the propane unit can bring it up very fast about 20-30 min. I have r23 mineral wool in the walls and r53 in the ceiling. 2- 2" insulated garage doors 10x10 and 9x8

 

[BrianS]

Thanks for the info. I'm just wondering if the heat pump only heats above 30* and your set temp is only 50*, how much is that really saving you? I'm guessing it wouldn't take much propane to maintain 50* if it is above 30* out. It makes more sense if you are trying to maintain 70* inside.

Any other thoughts?

 

[theoldwizard1]

The heat pump with propane backup seems like it would be most economical to run, ...

This is a good case for a mini-split heat pump. The large open area down stairs may require 2 air handlers. Obviously you need a separate air handler for the bonus roo,.

... but one contractor said they are designed to maintain temps around 70* and it wouldn't work as well at the lower set temps.

That person does not know what they are talking about ! A proper mini-split heat pump system will provide heat down to BELOW ZERO !! A back up of some kind, (not a full system, maybe just a wall furnace) is not a bad idea, especially is there is pluming in the building, in case there is an extended power failure. Of course a furnace stills need electricity to run ! A couple of tank top propane heaters would hold the temp above freezing, but you had better have spare tanks on hand !

Anyone have thoughts on the best type of system to go with, both from an up-front cost and running costs perspective?

This is the problem. The solution I suggested is expensive to install, but long term, it would be the least expensive to operate. "Pay back" could be 5-10 years, and is very difficult to calculate.

 

[mobetta]

This is my first winter with a MR cool hyper heat series 2 ton(24K) minisplit for my 24x26x9+ R15 mineral wool walls, R40 ish lid, well sealed

I set it to freeze protection mode and it has no problem maintaining a 45F shop. Coldest exterior temps so far only -6 and no issues yet.

It gets very much colder than that here so I did put in a 7500w electric unit heater for backup.

I got the wifi option so I can bump it up from anywhere.

It warms to 65 inside an hour.

I'm thinking a 12k-18k would do it for me ASIDE for the make up of pulling in a 8000lb block of ice when I bring my service van home from being outsdie for 10 hours.

 

[theoldwizard1]

It gets very much colder than that here so I did put in a 7500w electric unit heater for backup.

This would NOT be my choice for backup ! Something propane fired that required little/no electric power.

 

[mobetta]

That's great for your garage.

But i think you and i are thinking "backup" differently.

For me I think there's been 3 power outages in 15+ years living here that lasted up to 4 hours so its great for me and the 6 days a year this unit may not heat due to ambient temps. I'm sure not spending $2500+ to get gas service out there when I can hang a $150 unit.

Now if I was in an area that had frequent outages or more rural than I may have went another way.

 

[Chateau Slate 66]

This is my first winter with a MR cool hyper heat series 2 ton(24K) minisplit for my 24x26x9+ R15 mineral wool walls, R40 ish lid, well sealed

I set it to freeze protection mode and it has no problem maintaining a 45F shop. Coldest exterior temps so far only -6 and no issues yet.

It gets very much colder than that here so I did put in a 7500w electric unit heater for backup.

I got the wifi option so I can bump it up from anywhere.

It warms to 65 inside an hour.

I'm thinking a 12k-18k would do it for me ASIDE for the make up of pulling in a 8000lb block of ice when I bring my service van home from being outsdie for 10 hours.

I'm considering something very similar to yours. Do you have a feel yet for the monthly cost to maintain 45-50 degrees in our winters? PM me if you prefer.

 

[BrianS]

After some thought, I think I'm going to do a Mr Cool DIY multi zone mini split with a propane Mr Heater as backup. I think this will be the most efficient to run and much cheaper up front than the full up system.

I've got 24x32 with 14' ceiling main area, and a 12x32 with 8' cathedral ceiling upstairs. A 9k head will be fine upstairs. Trying to decide between 18k and 24k downstairs. If I go 18k, I can use a 27k outdoor unit, otherwise I'll have to bump the outdoor unit to 36k. I've heard bigger isn't always better for these units, they are most efficient when just big enough. Thoughts?


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