Mini Split in Cold Climate questions / advice

[grail21]

This might get a little long, but please bear with me – I could use the advice.

I’m in the middle of having a 24’x28’ detached 2-story garage and I really need to make a decision on the type of electric heater to install. Slab is in, framing starts today and everything is coming together quickly. The shop will be insulated to the best that I can afford (R15 Roxul in the 2x4 walls, insulation in both the second-floor joists and the rafters). I know many will tell me to use natural gas or propane and I don’t disagree, it’s just not an option for more reasons I can get into. I also don’t want to use a wood or pellet stove, so electric is my only option. While I know that comes along with additional energy cost, I’ll be able to offset them down the road with some solar panels.

The building will be used as a hobby workshop – metalwork primarily, but I’ll also have a portion sectioned off for a little paint room. I’ll use it on the weekends and a few nights during the week after work. While I won’t be in there constantly, I do want to keep it heated throughout the winter – even when I’m not out there working. I don’t want the temperature to dip below 40-45, when I do go out to work, I’ll crank it up.

I’ve been looking at every type of electrical heater – from your typical Fahrenheat / Modine style to infrared and everything in-between. I like the idea of the mini split solution primarily because they seam to be the most energy efficient (please correct me if I'm wrong), plus the AC side of the system is a nice to have. I also (maybe incorrectly) assume that they’re better suited for a constant 'ON' application. I do have a couple of concerns with the mini split which I’d love to advice / perspective on.
1) Being in Massachusetts – cold weather is a reality. I know these units quickly start to loose efficiency as the temperature drops, but if I’m only trying to maintain 40-45F will they be able to keep up? I’m hoping with the insulation it should be able to hold temperature when I’m not out there. I can always use a supplemental heater to warm the place up quicker when I go out there to work on a cold day. I know Mitsubishi has a hyper heating line that remains 100% efficient down to 5F, but they are really pricey. Do I really have to get into that type of unit for this to work?
2) While I won't be shooting saw dust around 24-7, it is a shop, so there will be more dust floating around than a typical home installation – how well do they hold up to that? Am I going to need to clean / replace the filter constantly?
3) Anyone have a link to a good load calculator for determining the size of unit I should get for my specific application? I’ve tried a couple that google has spit out and it ranged from 11,000 to 20,000+ BTUs. I don’t want to oversize it and lose efficiency, but obviously I want it big enough to handle the demands.

If anyone wants to give me a different idea for electric heating options, I’m all ears. One question I did have about a Fahrenheat / King Electric type heater is if it’s OK to keep them on 24-7 during the winter.

Alright, I’ve rambled long enough – any help is appreciated.

 

[pgray007]

Love my Daikin and it’s handled “cold” (in SC that’s probably 20’s at the worst) just fine. It doesn’t noticeably affect our Electric bill. I converted to all 90% LED in the house around the same time I installed the mini and our bill still went down slightly. Our Daikin runs on a 20A circuit with 2 head units (max of 3 on mine) with a shop/upstairs so it really sips the electricity versus the other electric options.

Price of entry was high ($3k or so) but I have no complaints.

I usually try to do woodworking outside, until I get some dust management installed, but the filters seem OK and you just run them through the sink every so often.

I looked at the Mitsu too, but a family member who does high-end commercial HVAC (think museums, retrofits to 500 yo buildings, etc) said Daikin was the way to go.


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[justinjoyal]

I have not seen a mini-split with a lower indoor temp setting under ~62F, so it won’t work if you want to keep your shop around 45F.

Now on to the outside temperature part, there are mini-splits out there with great efficiency and that perform very well at temperatures as low as -25F.

If you don’t want the price tag that comes with the name Mitsubishi, look at the Crown or Sapphire models from Gree.

They are solid performer and I would install one of those before a Mitsu Hyper Heat.

Filtration is gonna be an issue if you don’t plan on cleaning the filters often, but if you take care of it on a regular basis it shouldnt be a problem.

 

[tyme2par4]

For a rarely used garage, I would go with a regular mini split heat pump (not the low temp models), and then for another $100 you can throw in a resistance baseboard to take the chill off on those super cold few days.
In the north shore area, you don't get a ton of days where it's much below freezing do you?

 

[2level]

Won't a single mini-split over-condition one floor of your 2-story garage? Using 2 lower cost separate units may be best / most cost efficient.

You can add air filtration pretty cost effectively by using free-standing / ceiling-hung filter boxes. And you can get around the ~62F min. temperature settings by using timers to cycle the heat pump power on / off.

I use PTAC heat pumps (PTHP), 240v timers, and dust-filter boxes in my detached 2-story garage.

 

[rsanter]

Are you looking at heating only or both heating and cooling.
For heating a hydronic system in the floor may be best.
I think you may be able to combine a hydronic system into the works. If you had hydronic maintaining the tempature and then crank it up when you work out there that could be a good combo

Bob

 

[grail21]

Thanks for the recommendations on different brands – most of these I hadn’t heard of before or I wasn’t sure if they were reputable.

I didn’t realize that these type of units wouldn’t go down to the lower temperatures. Using the timers is an interesting idea – I’d probably do that because I really don’t want to keep the heat at 62F when I’m not in there.

I may have overplayed the filtration issue, I’ll mostly be doing metalwork in here (no saw dust) and I’ll spray paint in a room that’s separate from where the heater will be. I just wasn’t sure how sensitive these machines were to dust. I also don’t have a problem putting in a ceiling mounted filtration unit – in the end, that’s good for my lungs as well. I would expect to routinely clean the filter, I just didn’t want to have to do it every other time I walked into the shop.

Tyme2par4: I was thinking along the same lines – well at least when I was going to be working in there. As for days below freezing – this is a really great question that I didn’t know the factual answer for (besides – dude, it’s Boston, it gets cold). So I fired up the google to see what I could find. Looking at the data from 2009 – 2014, we had between 7 and 38 days of temps below 32F and between 1-4 days of temps below 20F. That’s a pretty large range for days below freezing, but that’s Boston. 20 days below 32F seems to be the statistical average – that seems like less than what it feels like ¬– that’s why we have data I guess. So not a huge amount, but I feel like we get a full week of temps below freezing at least once a year, and plenty of other weeks where the temp just hovers around 32 and then dips way below at night. We’ve already had a few overnights last week where the temps dipped into the high 20s. I’m hoping plenty of insulation will make a big difference.

2level – I’m not sure on the over-condition part – why would that be? A 1500BTU unit would seem to be the average size for a space of my size (672 main floor square footage). But that doesn’t account for the open stairwell to the second floor. I actually want some of that heat to rise because I don’t want it to freeze up there either, but the first floor is where all the important stuff will be. Maybe I’m thinking about this wrong – this is mostly new to me, so I’d love to hear more.

Rsanter: heating is my primary thing, getting the AC with the mini split is a bonus. If I could get a mini split that was just heat and it would save me a bunch of money, I would. I have a couple extra window ACs and for the number of times I would need them, I’d probably be fine using those. But – at least in my research so far – most mini splits that do heat also do AC. As for putting hydro in the floor – the slab is already poured so not really an option. It actually wasn’t an option from the start, my foundation is actually built on helical piles and atypical from normal slab construction. I’m sure there’s a way to do it, but I was already paying through the nose for the helical piles and doing a complicated in floor heating system wasn’t in the cards.

Thanks for chiming in on this, it’s helpful to hear others experience. Hopefully a few more folks who’ve faced similar will chime in.

 

[justinjoyal]

Even if you were to keep the place around 62 degrees the cost difference would be minimal unless electricity is reeeaaaaaally expensive where you’re at!

I would definitely not bother with timers!

Plus going by your weather data, pretty much any heat pump will work for you. I don’t know of any new one that does not provide efficient heating from 0F and up.

Just make sure you put in some kind of backup heat just in case the heatpump breaks or something.

If heating is your main goal you can oversize the system a little bit to have more heat output to start with.

 

[2level]

I didn’t realize that these type of units wouldn’t go down to the lower temperatures. Using the timers is an interesting idea – I’d probably do that because I really don’t want to keep the heat at 62F when I’m not in there.


2level – I’m not sure on the over-condition part – why would that be? A 1500BTU unit would seem to be the average size for a space of my size (672 main floor square footage). But that doesn’t account for the open stairwell to the second floor. I actually want some of that heat to rise because I don’t want it to freeze up there either, but the first floor is where all the important stuff will be. Maybe I’m thinking about this wrong – this is mostly new to me, so I’d love to hear more.

I asked about the possibility of over-conditioning an area because the top floor usually requires more heating and cooling capacity than the main floor. If using a single unit, the upstairs heating / cooling may be inadequate, depending on your goals, ect. Are you thinking about using a 15,000 btu mini-split, with a single indoor air handler? Second air handler upstairs?

The timers work well for me, on thru-the-wall units, because having 2 to 3 two-hour run times in a 24 hour period meets my winter heating needs. And they help keep my garage heating bills below $200/year.

 

[Chris Stapley]

The biggest downfall I see with ductless split units is ,yes,they are very sensitive to dust and will require a lot of cleaning to keep them functioning well. Unsure if you have any kickback incentives in your area but for much of Ontario there are programs in place to give back a good portion of the initial cost due to the fact that so many people have shied away from electric heat in the past few years.. May be wise to look in to that and if there is,do not sell yourself short on quality,my suggestion is a Mitsubishi unit with two heads ,one for each floor. The guys are right too, left on low,the difference in cost will be minimal enough to leave running as opposed to trying to make up the difference of a cold shop every time you wish to work.

 

[grail21]

Great points - I honestly hadn't even thought about putting a second head upstairs. The second floor will a storage space for foreseeable future - stuff I need to sell, future projects I pick up off the side of the road and can't pass up, maybe a little bit of material storage as well. From a construction standpoint I can see why adding a second head now would make things easier, but would it be that difficult to add one down the road should the need arise?

Last time I looked I think my electric rate was about $.11/KWh - so not high, but not as low as some get - I think that's a hair short of the national average is memory serves me. But I think the point about leaving it on low is a good one - I can see how the additional cost would be minimal and not worth the hassle of timers. I'm guessing I could always get a wireless thermostat as well and just turn it off with my phone for short periods of time so that it keep cycling... just got to remember to turn it back on.

I also hadn't thought about rebates - Massachusetts is usually pretty good about that, I'll have to look into it. I wonder if it would apply to accessory buildings versus homes.

 

[pgray007]

Most of the higher end minis have variable speed (that might not be the right term) so they can ramp capacity based on demand, so it’s harder to “wrong size” them. Also 90% if the ones I looked at have Remote controls that have timers and all sorts of antics built in. I think mine even has some sensor that supposedly tracks your body and moves the vents to blow towards you.

Also not sure about the filter comment. I’ve seen some mini splits in Asia that had about 4” of dust and funk hanging off them and were running like champs (it was 100f+ outside and cool inside).


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[theoldwizard1]

The shop will be insulated to the best that I can afford (R15 Roxul in the 2x4 walls, insulation in both the second-floor joists and the rafters).

Roxul is good, but expensive. Its big win is noise reduction. Do you need that ?

I like the idea of the mini split solution primarily because they seam to be the most energy efficient (please correct me if I'm wrong), plus the AC side of the system is a nice to have.

A more correct way of saying that is, mini-split have the "lowest cost of operation".

I also (maybe incorrectly) assume that they’re better suited for a constant 'ON' application.

In the end, they are as good as any "forced air" solution. Cool air is blown across a hot surface.

...if I’m only trying to maintain 40-45F will they be able to keep up?

Properly sized (like any other solution) this will not be a problem

I can always use a supplemental heater to warm the place up quicker when I go out there to work on a cold day.

Supplemental is good to quickly bring temp up to your normal "work" temp. It should not be "required" to maintain temp unless you are below 0F. If you have items that can NOT survive a hard freeze during an extended power outage, then you really need a backup heat source.

I know Mitsubishi has a hyper heating line that remains 100% efficient down to 5F, but they are really pricey. Do I really have to get into that type of unit for this to work?

YES, unless you want to use your supplemental heat a lot ! Some other mini split start to lose efficiency at 20F.

While I won't be shooting saw dust around 24-7, it is a shop, so there will be more dust floating around than a typical home installation – how well do they hold up to that? Am I going to need to clean / replace the filter constantly?

Mini-split are not the best at filtration, so they won't clog up a lot.

 

[theoldwizard1]

I didn’t realize that these type of units wouldn’t go down to the lower temperatures.

That is a thermostat issue. Nothing to do with the actual heating/cooling part of the system.

I am sure there is a solution out there.

 

[terabitdan]

For calculators to determine heat load, I used an archived post for instructions and built a spreadsheet:

https://www.garagejournal.com/forum/archive/index.php/t-47200.html

You can also search for HVAC-Calc, which can be purchased inexpensively for a homeowner. I’ve used it in the past and it was pretty easy.

Some mini-splits have a freeze protection mode to maintain about 42 deg F


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[trashmanssd]

I am on the south shore and looking to do the same thing as you. I just want to keep temps bearable and dont mind adding resistance heat when I first get out there to help with bringing the temp up. I am in the process of putting in a full duct heat pump system (been a **** show so far, manufacturer put wrong unit in the box they have to swap out the inside coil portion as its only a A/C unit). I went with a duct system for higher BTU's and more air flow so it should heat and cool more evenly and quickly, Also I have room to spare in the system and can add supplies and a return up stairs later if I want to but for now upstairs is behind a door and un conditioned.

I am going to set mine to 60 degrees and see what it costs me this year if it seems high I can drop it back to 55 or 50 but thats as low as I will go what it cost for 50 is what it cost. I want to try and keep the bill under a grand for the year for heat and A/C. I am hoping to be pleasantly surprised and maybe spend 700-800 for the year to keep it 60 in the winter and 80 in the summer. With no A/C this summer it only got into the low 80's a few days in there when it was 90 plus out in the late afternoon. So far this fall it has got down to 53 inside this weekend when it was in the 20's over night.

I am also not opposed to putting in a pellet stove for auxiliary heat if its allowed in mass or in Plymouth. Who knows what the psycho local politicians will allow here.

 

[justinjoyal]

Most of the higher end minis have variable speed (that might not be the right term)

Actually I believe all mini-splits produced nowadays are inverter-type (variable capacity).

All of the brands I sell are inverter-type. I couldnt get a non-inverter even if I wanted to.

 

[grail21]

Thanks for the additional comments - it's helpful as I wrestle through all the details of a new build and wanting to get it right from the start. A new question down below, but let me answer some comments first:

theoldwizard1: All the comments were super helpful, thanks for chiming in. I'm going with Roxul less for the noise reduction and more for the fire safety. Call me paranoid - know plenty of people throw sparks in buildings that use fiberglass insulation or weld above wood floors. It's just peace of mind knowing that I've got a bit of extra protection - I like other attributes of it, but it's certainly possible I've drank a bit too much kool aid. You’re points about supplemental heat are well made – I certainly want to size it correctly so that it’s not necessary, but not opposed to using it to get warm fast.

Trashmanssd: Pellet stoves are certainly allowed in MA (I have a couple friends who have them) but I know each town approaches them slightly differently in their town regs. Not sure about Plymouth, but they might have something in their town ordinances that SHOULD be available on the town website.

Question time: So I’ve been pricing out Mitsubishi and Daiken units that are rated for low operating temps. Man they get pricey fast when you add on things like the wall mount, pan heater and cost for the installer to come out and get it running. Since I’m looking at units that meet rebate efficiency requirements I’d get $300 back, but still pricey. So I went back to looking at typical electric garage heaters and came across the King Electric KB ECO2S series - https://www.king-electric.com/pdfs/KBECO2S_Garage_WEB.pdf
At under $500 for the 7500W unit and under $700 for the 10K unit (quick Amazon prices) these are vastly less expensive than any of the Mini Split options I was looking at. I get that these will likely have a higher cost of operation – but by how much? An extra $10 a month? $20? Rough math says I’m looking at a price difference between the two of about $2,000. Even if it cost me an extra $20 a month, it would take me 20 years to make my money back (assuming 5 months of heating per year). That assumes the mini split would last 20 years. I don’t want to be penny wise and pound foolish, but that’s a long time to wait for return on investment. I realize I lose the AC capability, but I have a couple of window units laying around and don’t mind using them for the handful of times I might need them during the summer. So am I totally off here? Is my perception on the cost of operation totally off – would the mini split save way more than I’m estimating? Even if it was an extra $40 a month it would take 10 years to make my money back. Please poke holes in my logic.

 

[justinjoyal]

A load calculation will provide you with a baseline to start your calculations on approximate runtime per hour, or per day, of the King heater versus the mini-split.

Only then will you be able to calculate operating costs.

Mini-splits don’t always make financial sense when electricity is cheap and when heating needs are not high and constant.If you don’t need the cooling part on top of that, well, the decision becomes easier to make.

If I was in your situation, I would get one of these http://www.stelpro.com/en-US/dragon-ceiling-fan-heater for the main area and a properly sized wall heater for upstairs. Simple and cheap.

The King one you posted will work, but it is kinda bulky and much less discreet so it would not be my choice. Plus it is wall hung so you cannot mount it in the center of your workspace (on the ceiling) so I figure there will be more temperature differences across your shop.

 

[trashmanssd]

Had similar thoughts as you on just resistance heat, Its very hard to calculate. Lots variables, how well insulated are you, how often and long will the overhead doors be open, what inside temp do you want to hold, is that the temp you will keep it all the time or do you want to keep it low and raise it when you go out there one day a week?, will 1 resistance unit do the job or do you need 2 of them to get even heat, how much is the A/C side worth to you, how much temperature differential you are looking for compared to avg outside temps, and some I cant think of.
I came to the conclusions that I cant figure it out and went with heat pump system for the detached garage and a resistance heat system for the attached garage (also attached garage has no good place to put the outside coil unit as it in front the house facing sideways away from the front door and has a walkway going all the way around it to the front door also has a door just to the side of the garage going into kitchen). I leave the door to the house open for an hour or so early in the evening to let it warm up some in the attached garage also it shares 1 and 1/2 walls with the house so it never gets to cold in there anyways so resistance heat should not be as bad.

 

[meathooker]

That is a thermostat issue. Nothing to do with the actual heating/cooling part of the system.

I am sure there is a solution out there.

Some units (Daikin) have proprietary thermostats so it's not that simple

A more correct way of saying that is, mini-split have the "lowest cost of operation".

In the end, they are as good as any "forced air" solution. Cool air is blown across a hot surface.

YES, unless you want to use your supplemental heat a lot ! Some other mini split start to lose efficiency at 20F.

Minisplits are often associated with heat pumps which have a higher COP than traditional systems so a common thought s they're more efficient.

With regard to heating they are not as good as a forced air furnace. The recovery with them is horrible in comparison. You'll need to run the mini split constantly - don't expect to leave it at 40deg them walk in and turn it up to 60deg have it be comfortable anytime soon.

Any mini split (heat pump) loses efficiency as the temp gets colder. Some have a low temp option to help in the close to zero climates.

 

[yeldogt]

Getting a heat load on the building will give you the answers -- unless you insulated the ceilings and door off the upstairs it's going to affect the heat needed in the lower level. You can't heat the upper with the door open unless you really heat the lower.

That said: Electric heat is great -- it's got everything going for it ..... except cost relative to other fuels. At .11 -- you are almost 1/2 of mine and getting close to what I pay for propane. The possibilities are endless with electric. My old studio was electric: I had typical strip heat on open walls and used fan forced in other locations. I placed them low -- so the heat would flow over the floor. Two or three smaller output heaters is nice because you can use one to maintain and have the others for fast warm ups and/or uniform heat. Two 4k's are nicer vs one 10k ... many times the 4k will be all you need. It's more comfortable have the 4 running loner vs the 10k going on and off. Like anything else -- there are quality levels.

A mini-split will work -- the multi head units cost as much as installing separate units -- the reason for a multi is not really about cost. I could never get a two head of any make installed for 3k as one of the posters above. The choice of a mini all comes down to the specifications and who is doing the installation.

I did one of the floor/wall mitsubishi hyper heats when they first came out -- worked great .. Have also used Fujitsu and LG. The mitsubishi hyper is my preference for multi head setup -- less important on single .... although the floor unit Mitsubishi is really amazing.

Basic strip heating is cheap to install --- if you have the power and the panel space. Run the wires and try it out. If you think you may want to do a mini split in the future -- run the wire to the logical spot (or conduit) before the walls are finished. I would run to the second floor also -- think you will need something.

 

[yeldogt]

Some units (Daikin) have proprietary thermostats so it's not that simple





Minisplits are often associated with heat pumps which have a higher COP than traditional systems so a common thought s they're more efficient.

With regard to heating they are not as good as a forced air furnace. The recovery with them is horrible in comparison. You'll need to run the mini split constantly - don't expect to leave it at 40deg them walk in and turn it up to 60deg have it be comfortable anytime soon.

Any mini split (heat pump) loses efficiency as the temp gets colder. Some have a low temp option to help in the close to zero climates.

It's all a question of capacity. 40k is 40k ... but yes some want a 150k heater to get the place from 40-60 quickly. But the 150k heater is not a comfortable way to heat a space.

I heat my 1700sf studio with about 35k -- but I can maintain it with a unit much smaller.

 

[pgray007]

In terms of installation costs of a mini (I mentioned the $3K I believe) you can buy the units online, do 90% of the install yourself and then get an HVAC guy to pull vacuum and all that. If you can run a drill and screwdriver you can “install” a mini, and I paid $150 for an HVAC Pro to connect the line set. He also gave me an invoice so when I had a warranty claim (blade motor was DOA) it went through without hassle. Just throwing out the option...


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[yeldogt]

In terms of installation costs of a mini (I mentioned the $3K I believe) you can buy the units online, do 90% of the install yourself and then get an HVAC guy to pull vacuum and all that. If you can run a drill and screwdriver you can “install” a mini, and I paid $150 for an HVAC Pro to connect the line set. He also gave me an invoice so when I had a warranty claim (blade motor was DOA) it went through without hassle. Just throwing out the option...


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That's a great option ... not available to all. In some areas it's difficult to find someone willing to do it .... and it cost me $150 to get them into my driveway.

 

[theoldwizard1]

Any mini split (heat pump) loses efficiency as the temp gets colder. Some have a low temp option to help in the close to zero climates.

Old school thinking ! While it is common for central heat pumps and some mini-split to have a resistance heater, the newer, top quality mini-split (like Mitsubishi HyperHeat) make heat WITHOUT a resistance strip down to below 0F !

View media item 43975

 

[theoldwizard1]

The choice of a mini all comes down to the specifications and who is doing the installation.

Why do you emphasize installation ? High wall or low wall installation is very simple, definitely DIY level (except for the vacuum testing). I have heard that ceiling cassettes don't work as well and there is always the concern about the condensate pump/drain.

 

[aunsafe2015]

I get that these will likely have a higher cost of operation – but by how much? An extra $10 a month? $20? Rough math says I’m looking at a price difference between the two of about $2,000. Even if it cost me an extra $20 a month, it would take me 20 years to make my money back (assuming 5 months of heating per year).

If the mini split you get has, for example, a COP of 3 at a given temperature, that means it will use 1/3 the electricity of an electric resistance heater to produce the same number of BTU of heat.

Efficient mini splits are generally above a COP of 3 at higher temperatures like 40 F, but are closer to 2-2.5 at temperatures closer to 10 F.

So depending on your climate, maybe you could estimate what the average COP of your mini split would be. Say it would be 3, for example. That should theoretically mean you would spend 3x more for the same BTU produced by resistive heating than by heat pump. So if your winter heating bill would be $30 per month with a heat pump with average COP of 3.0, then your heating bill per month with resistive heating would be about $90.

So the colder your climate, and the lower your average COP, the less money you save with a heat pump versus resistive.

Does that sound about right to folks?

 

[tab2]

I put in a non-hyper heat Mitsubishi about a year ago. I was nervous all winter but I kept an eye on it and I never noticed it putting out less heat. It wasn't a bad winter here but I do have electric baseboard as back up; I never used it. If you want some other info about send me a PM.

 

[theoldwizard1]

I put in a non-hyper heat Mitsubishi about a year ago. I was nervous all winter but I kept an eye on it and I never noticed it putting out less heat. It wasn't a bad winter here but I do have electric baseboard as back up; I never used it. If you want some other info about send me a PM.

A very good compromise for people who have mild winter, or at least only a few days of really cold weather ! You probably saved yourself a lot. Electric baseboard is cheap and easy to install.

 

[Notgrownup]

If they can use them in New Brunswick Canada then you can use it in Mass. just get the right one.
If it were me I would get a better heater. And a decent A/C unit...What are you going to use more and want instant on...probably heat...
All depends how much time you are going to spend in there atwhat Time of the year...

 

[grail21]

Again, great info from all you guys - it's really appreciated. In some ways I want to approach it simply - "I just need a damn heater". At the same time - especially when looking at some of the higher costs of a good mini split - I want to make sure I'm spending my money wisely. I just walked in from a long day, so still haven't had time to process all the new info, but I'll close with this tidbit.

I reached out to King Electric to ask about keeping their heater always on. They got back to me very quickly, here's their response:

"You certainly can leave it ON, and let it be all winter. But in our instructions we say to not leave the unit unattended, because we’re not responsible if something’s were happen. But these units have been very reliable, so nothings should happen. �� "

*** properly covered - I do appreciate their honesty. I'm guessing this would be the line from any manufacturer of resistance heaters.

 

[terabitdan]

Modern mini-splits with low ambient heating, and really any heap pump is ALWAYS more efficient, and therefore has a lower cost of operation, than electric resistance heat unless you live in an area with consistent temps less than -22 deg F.

The COP (coefficient of Performance) is the ratio of output to input. A COP of 1 is the same as electric resistance heat, 100% efficient. Low ambient heat pumps range from a COP of 1.29 ( or 29% less energy used to produce the same BTU output) at -22 to 3.67 or higher at 50 deg F. For the OP who wants to maintain a constant temp a mini-split will be less to operate.

If you have natural gas available, it's definitely cheaper to operate. No other fuel (unless you can get free wood) has lower operating costs.

You can maintain whatever internal temp by using a relay and shutting off the power to the mini-split above your preferred temp. Use an air compressor relay with a thermostat instead of the pressure sensor for example. Mini-split typically restore to the previous settings after power loss.

Boston has a 12 deg design heat spec, but I'll bet you have occasional days below that. If you designed any system to maintain 45 degrees or a 33 degree heat rise a -5 day will put the garage below freezing. Better to size based on 60 degrees, ensuring on the coldest day of the year the garage is above freezing and it provides quicker recovery times.

If you need quick heat up, buy a propane bullet heater run it just long enough to raise the temp.

Payback is much harder to figure; I calculated an estimated annual cost for my 2 story garage, which is close to your size with similar insulation. Using the heating degree day data over the last 3 years in SE Michigan came up with about $1,000 annually to heat the garage to 50 with electric, $600 with mini-split and a 78% natural gas furnace would be $400. Assuming those numbers are close to real, you would be at about 70% of mine.

Not bad, but certainly not a slam dunk. However there is efficient A/C as well, and year round comfort.



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[2level]

Payback is much harder to figure; I calculated an estimated annual cost for my 2 story garage, which is close to your size with similar insulation. Using the heating degree day data over the last 3 years in SE Michigan came up with about $1,000 annually to heat the garage to 50 with electric, $600 with mini-split and a 78% natural gas furnace would be $400. Assuming those numbers are close to real, you would be at about 70% of mine.


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Using 70% of your $1,000 / $600 comparison, he'd pay about $280 extra per year using electric resistance heat. And, assuming $4,000 for a mini-split system vs. $600 for an electric resistance setup, it would take ~12 years to 'break even'. The 'break even' point on a $900 PTAC heat pump would be closer to 2 years.

 

[theoldwizard1]

The 'break even' point on a $900 PTAC heat pump would be closer to 2 years.

Whoa, whoa, WHOA !

No PTAC that I know of has anywhere near the COP of a mini-split. Their resistance heat strip usually kicks in ABOVE freezing.

Yes, they are inexpensive. That is why they are used in many apartments and hotels. That and if it fails, they are easy to swap. Most facilities have several spares on hand. When the supply of spares gets low, they send the broken ones to a repair facility.

 

[2level]

Whoa, whoa, WHOA !

No PTAC that I know of has anywhere near the COP of a mini-split. Their resistance heat strip usually kicks in ABOVE freezing.

Yes, they are inexpensive. That is why they are used in many apartments and hotels. That and if it fails, they are easy to swap. Most facilities have several spares on hand. When the supply of spares gets low, they send the broken ones to a repair facility.

No, no, NO !

I didn't say anything about a Coefficient of Performance being equivalent to a mini-split. My example assumed a COP of ~1.5 (even though some PTAC's are rated around 3.3).

Have you done the estimated math on an all-electric resistance setup vs. thru-the-wall heat pump cost to own and operate ?

 

[2level]

Well......Wizard?