Options for heating a hobby workshop
I’m in climate zone 6 (Midwestern Ontario Canada) and have a 24x40x12 hobby shop that I also park in. Currently I just use a woodstove when I’m working out there, but it’s slow to heat up, takes up valuable floor space and is a hazard when my young boys are tearing around in there on their bikes. When I built the shop, I put 2″ insulation under the concrete and 4 runs of 1/2″ pex in the floor in case I ever wanted to heat the floor.
I’m looking for an alternative to the woodstove. No natural gas here but we have propane for the house. Based on my last bills, propane was $0.72/L and electricity was $0.17/kWh. I have done some rough calculations with surface areas, R values and average temperatures for my area. To maintain 50F inside takes about 23 million Btu annually and at -4F (pretty well the coldest we see) the heat loss is around 14000 Btu/hr.
I’m not looking for the absolute cheapest system but I probably only work out there one or two part days a week so it’s likely most economical to maintain a baseline temperature and then turn it up as needed. Something I can install and maintain myself is a definite plus, I don’t have gas or refrigeration licenses but can handle pretty much anything else.
I have a couple options in mind, but want to hear what others have to say first.
Thanks in advance for your opinions!
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Replies
So I have been doing a similar thing in a similar sized space. We were supposed to tear it down and build a beautiful new garage, but life intervened.
What I have: a 24x36x14[peak] garage with 3 1/2 inches of fiberglass and no foundation insulation. UGH
What I have found: It takes very little energy to keep the place at 40 degrees F .
Why 40 degrees? it stops the condensation cycle that turns every steel surface dull brown. It makes it so all the tools are not painful to touch. It makes it so that, in my case, the torpedo heater will have it workable in 15 minutes.
I am doing it this year with a small 220v electric heater, like this:
https://www.amazon.com/Infrared-Heater-DR-988-Garage-5600W/dp/B003XOZN7A/ref=pd_lpo_vtph_201_lp_t_3?_encoding=UTF8&psc=1&refRID=0YGS58T7N9MW9XXNG5XJ
but long term a mini split is in the cards.
I think this will work for you also.
While you are in a colder climate[ I am in Eastern Ma] you have a much better insulated building. You have a wood stove to provide 'acceleration' so you do not need to oversize you heating system. You are not sleeping there, or required to spend time when it is 20 below zero, so the risks of undersizing are minimized. Keep the woodstove. Put a little fence around it and deal with the lost space[or find a more space efficient model] By the time you get everything perfect, the kids will be driving cars anyway.....
I am pretty convinced that a 18k btu minsplit will do what I need, but as a practical matter, there is no difference in price between a 2 ton and 1.5 ton, so that is most likely what I will do.
Electric heat is not cheap, but for less then a 100 bucks you and do your own science project, and I am pretty confident it will work for you. I found the internal thermostat on the listed style heater only reliably goes down to 50F, so I wired in a Honeywell 220v electric heat thermostat
I'm planning to use a minisplit in the workshop section of my garage in Ottawa, something small with decent low temperature performance. I think it will work well, and even in the coldest weather will be about half the cost to operate compared to a conventional electric heater. It also offers air conditioning and dehumidification for the summer months, not necessary but a nice to have for those few hot sticky weeks we get in Ontario. Nothing worse than leaving the door open in the evening to cool the place off and ending up with a garage full of mosquitos!
The only real concern I have, being a workshop and all, will be keeping airborne contaminants out of the wall unit. My plan is to build a shroud around the top that holds a couple of furnace air filters. Not only will these be easy to change, but if I get it to seal well it may reduce or eliminate the need to disassemble the head and do thorough cleaning of the inside. As far as I know, the included filters in minisplit heads only filter out large dust, not fine particles, and the heads require regular cleaning if used for air conditioning since debris builds up on the cold wet coils.
Having it mounted near the ceiling should help keep it in cleaner air.
Do you have foam or another thermal break between the edge of your slab and the perimeter of the foundation? If not, heating the slab could take more energy than you think as there's a direct thermal bridge to the outside.
A mini split is probably going to be your cheapest option in terms of operation, and it will give you air conditioning too.
The easiest way to heat your slab is to use a water heater as a boiler (some are rated for this, so check their info before buying) and a small circulation pump. This will probably cost around the same as the minisplit, but will give you radiant heat.
I’d probably go with the minisplit myself.
Bill
I think a propane space heater is the best option here. You actually want something capable of putting out a ton of heat in a short amount of time.
If you wanted to always keep it at 65F out there, a minisplit would be a good option - although that electric price still makes me cringe. What HVAC charges to install mini splits is almost criminal too, especially when it's just a garage.
Running hot water through the floor would also be really bad at quickly heating the space.
Propane is 56% of the cost of electricity in this scenario, based on the prices he quoted. It only takes a COP of 2:1 to make the mini split a better option. This is assuming that he keeps the building at least 50F. Adding some PV panels will only tip the scales more towards the mini split. Propane is a very un-green option, and I don't see justification for it here.
If he was maintaining 65-70F 24/7, sure. If you only need to heat the space 1-2 days a week and you don't want to use a woodstove, a 50k BTU ceiling mounting propane heater will get the job done.
It's a garage, not a house. Take the $5000 in savings insulate the heck out of the house, which is heated all the time.
One DIY friendly option would be a through the wall heat pump PTAC that also has electric backup heat.
These don't normally go bellow -20C but you can still have the stove for the cold days.
Exactly. For intermittent use spaces an oversized PTHP (oversized to be able to heat up the space more rapidly) is a more cost effective solution than a mini-split, even though most of them shut down the heat pump and switch over to resistance heating at about -4C to -7C. During the dead of an Ontario winter it's not more efficient than resistance heating, but at 0C and above it's more than half as efficient as a mini-split.
14000 BTU/hr is about 4kw. A 3/4 ton or 1 ton PTHP with 3.5KW or more of resistance backup would be appropriate. A 1 ton Amana PTH123G50AXXX switches to resistance heating (3.5KW or 5KW optional) at -7C, and at 5KW would fully cover the heat load with the heat pump down to that temp, with the resistance heat sufficient to cover the load at -20C.
But bumping up a size to the 1.2 ton PTH153G50AXXX would bring the place up to temperature more quickly. When it's +7C outside the thing delivers about 17,000 BTUY/hr. With the wall sleeve, breaker etc all-in it would come in under USD$1500 as a DIY project. That series has options for both wired and wireless a wall thermostat kits. For reference:
https://www.alpinehomeair.com/related/Amana%20PTAC%20Owners%20Manual%20and%20Installation%20Instructions.pdf
LG and Gree and a few others have similar PTHP product lines. The Gree 15HP230V30A-A is very comparable to the 1.2 ton Amana.
Thanks Dana,
How would you rate those products compared to an off-brand mini-split such as this:
https://www.sogoodtobuy.ca/12000-btu-mini-split-air-conditioner-sena-12hf/
Their claims are fairly impressive and even if they only come close to them, they are beating the ptac by quite a bit. The only disadvantage is needing an hvac tech to do the final hookup.
Since you already have the hydronic tubes, I'm going to suggest something unconventional. Whether this makes sense depends on a number of factors, some of which you'll have to work out.
Install a series of PV panels, either on the roof or beside the shop (depending on roof exposure, how much land you have). Wire the panels directly to a water boiler, which feeds in the in-floor heat. If you DIY this, the total cost will be very low. The operating cost will be literally zero. The idea is that this is what will maintain your 50F temp, and you would boost it up some other way. Now, if this idea appeals to you, you will have to do the math to see if it is actually practical. Figure out your average insolation in the winter time, the cost of how many panels you need, etc. You might find out that you can maintain higher than 50F most of the time, and then maybe electric heat to boost up makes sense (also dirt cheap to buy). It might make sense to have a storage tank, so that some of the heat can be delivered at night.
Again, I don't know that the numbers will work out, it's just an idea. I did run numbers for my house a while back, and determined that two 300W panels connected to one element of my water heater made financial sense, even if all it did was offset the standby losses of the water tank.
Thanks everyone for the excellent responses, I'll try to respond to them all.
Keith, I have a 4800W heater which is what I used before I put the woodstove in. It's fairly slow at bringing the temp up, but there's no reason I couldn't use a second one occasionally. If I make any changes, the woodstove is going. My time in the shop is usually limited so I hate spending it on the woodstove. The space it takes up is a nuisance and the risk of a fire is always there. Lastly, my chimney leaks so I'd be happy to see it gone! The leak is not where it's sealed to the roof, but water must be getting in the vertical seam and making it's way down as it comes out at the first joint below the roof line.
Lance, you, Keith and Zephyr7 all mentioned mini-splits which I have strongly considered as well. The downsides for me are cost and long term reliability and servicing. I was quoted $5k installed for a Mitsubishi 15k or 18k (I can't remember anymore). I can hang the units but the refrigeration part would have to be hired. If it breaks down I likely can't fix it myself and who knows about parts availability down the road. My shop is just a slab on grade; about 2' of the perimeter is 16" thick with several runs of rebar tapering to a 5" thick floor. The 2" xps is under the entire slab (it was a real pain fitting it to the taper!) but the weak point insulation wise is the edge of the slab. I didn't want to overhang to floor with the wall or use a step flashing so I just have 1" xps covered with 1/2" pt plywood to match the thickness of the 2x4 strapping. This allowed the steel wall sheets to overlap the plywood. There is also exposed slab at the overhead door so heat would certainly be lost there.
Zephyr7, I have considered using a tank type water heater to heat the floor for my baseline. With a tank full of hot water, a fan-coil could be used to bring the air temp up when I'm out there. Obviously it's not as efficient as a mini-split, but is very diy friendly and the cost difference from a mini-split could be put towards solar pv or household insulation as Nathan mentioned.
Nathan and Trevor, I have a fairly detailed spreadsheet looking at propane vs electric costs and options (I guess that makes me a nerd but might be accepted on this site?). Based on the prices I quoted and 80% efficiency for a cheap unit heater and 100% for electric resistance, propane is good for 25k btu/$ and electric is good for 20k btu/$. Even a cheap knockoff minisplit would beat the propane. The propane is best for bringing up the temp quickly of course so would likely be cheaper to operate the way I plan to. Legally it's not a diy install though and as Trevor mentioned, not very green. It was only 2.5 years ago that our electric prices were $0.28/kWh (all in, approx. 700 kWh per month usage) but then it got political and the government kicked the can down the road to appease the public.
Akos, I have looked at PTAC heat pumps, but the only ones I've seen have very marginal performance as a heat pump. Any suggestions for a good one?
Trevor, your last idea of directly connecting solar pv is very interesting! I definitely plan to install a net-metered system in the future (the shop trusses were spec'd for it when I built it) but I still have to look into whether I'd be able to connect as Hydo One (our utility) has some pretty strict requirements. The panels themselves have gotten pretty cheap but the rest of the system can add up quickly. I could even mount the direct-connect ones on the wall and save the roof for the grid-tie. Unfortunately the shop faces SE, but that was due to property limitations. I have to play around with different scenarios on pvwatts; I wonder how it would compare to evacuated tubes? Is there a good site to predict evacuated tube performance?
Thanks again for all the ideas!
Paul, give Hydro One a call. I spoke to them last year and they gave me the information on how to check our local grid for % renewables, though I didn't look into it at the time as it was well before our build. As far as I remember, they allow up to 7% renewable generation on any leg of the grid, at least out here near Ottawa anyway. If you don't have any large solar "fields" in your area you're probably OK.
Another ding against propane is that you can't run an unvented propane heater indoors for any length of time. A properly installed vented propane heater isn't cheap, though a propane powered water heater could be an option for heating the floor.
Paul,
When I was looking for heat for a studio in Toronto, I went through the same search. Narrowed my search down to an LG PTAC (LP123HDUC) and a EMI mini split (WALH12A SZ1H12A). Low temp is marginal on both, nowhere near as good as a proper low temp heat pump.
Ended up going with the mini split as it was an inverter unit (much quieter and proper modulation) and was a bit cheaper. Install is more. The unit puts out good heat down to -10C here, bellow that it keeps running but doesn't put out a whole lot of heat.
There are not a lot of options out there for PTAC heatpumps.
Ontario’s grid tie program is very generous. I have family in Hamilton, and since I’m an EE they ask me about these systems all the time. I strongly recommend you go with a grid tied solar system since you have that option. Grid tie eliminates the need for batteries, and batteries are probably the largest headache, and certainly the largest ongoing operational cost, of the typical residential solar systems.
Going non-grid tie for heat only is an option, but without batteries you would only have heat when the sun was out. You could potentially make this work though and I’ll suggest an option for you:
Volrage for AC power is defined as the RMS value, which means that the same voltage in DC will deliver the same wattage of energy to the load. Resistive heating elements don’t care if they are fed with AC or DC. Why is that interesting? Because it means you can operate a regular ol’ electric water heater on DC power without and trouble EXCEPT for
The control system part. The problem is regular contractors (fancy relays) are rated to interrupt AC power and not DC. This means you would need to modify the water heater controls to have a DC rated contactor, and the thermostat would have to operate that contactor with a low voltage supply. If you’re comfortable doing that, you can string regular solar panels together to get a 240v string and then use it to feed the electric water heater whenever the sun was out. The water heater would store the energy as hot water until it reached its setpoint, and if you had the circulation pump running, energy would also be stored in your slab and the building itself. You could use your regular line voltage to run the circulation pump and to provide the control voltages to run the water heaters thermostat.
Note that if you try this, you also need either a DC rated circuit breaker, or better yet, DC rated fuses. You need breakers/fuses rated for the full 240v DC too — many are rated 600v AC but only 32v DC. Please be sure you know what you’re doing if you want to try making a regular water heater run on DC power.
Your other option is to get an an inverter and use it to run everything when there was enough sun. This is a more conventional route to take, but the inverters are an expansive component of a solar system.
Bill
Most electric water heaters use bimetallic switches, not relays (fancy or otherwise) for thermal regulation. However, you bring up a very important point I hadn't considered. While the bimetallic switch in a water heater will certainly work with DC, the voltage it can accommodate will be much lower (as much as ten times lower), due to higher arcing potential and disinclination to arc interruption. This is a non-trivial problem, and I'm not sure there exists a plug-and-play solution, not involving installing a separate temperature sensor and external switching.
The simplest solution is to use a DC rated contactor to handle the high-current feed to the heating element, and run the low voltage control circuit for the contactor through the existing thermostat contacts (which is what I was recommending). This way the original thermostat still functions, but it only has to interrupt the low power control circuit for the contactor (usually 24v or lower), which shouldn’t be a problem.
There is no way around some electrical modifications to make this work, but the required modifications aren’t very complicated.
Bill
Help me out here, as my understanding of a DC contactor is that it functions the same as a relay. It would need a DC voltage applied across a coil to activate the contacts, no? Where is that voltage coming from?
If there was a DC rated contactor with a 240Vac coil, that would be something I would call fairly uncomplicated to install. I'm sure that exists, but I suspect it's a specialty item with a specialty price.
Coil and contact ratings are completely separate things. Many large contractors even have replaceable coils with different ratings. For an application where the contactor has to interrupt the DC voltage to turn something off, it’s important that the contacts be rated to interrupt DC (DC is more difficult to interrupt because there are no zero current points like there are 120 times per second with 60Hz AC). The contacts need to be rated for DC in this case at the voltage and current levels required.
The could could be anything. 12v DC, 24v AC, whatever you need. If the solar power was only for heating, then control power could be derived from the utility. The only reason would be to keep the system cost down by avoiding the need for an inverter. It’s not necessary for the entire system to run only on solar power in this case.
This would be a pretty advanced DIY project and I wouldn’t recommend undertaking it without be used to working with these systems.
Bill
Folks,
I feel the need to jump in here and add a bit of clarity/sanity from someone that works with PV.
Please don't start wiring up high voltage DC unless you know what you are doing. As Bill said, high voltage DC requires special components, contactors and fuses.
For example if you put 300V DC through a standard circuit breaker, when that breaker trips, it will open but the contacts will continue to arc until either the breaker melts or the contacts burn away (ask me how I know..).
Unlike AC, if you get buzzed by high voltage DC, you generally can't let go, even if you do, the burn creates some nasty health side effects. I would say, I'm pretty risk tolerant, but I would never under any circumstance work on DC above 100V without proper protection.
Safety warning aside, lets look at the practicality of the solution.
In Ontario, in the winter months (Jan/Feb) you solar production is about 1/4 the summer time. For example a 10kw 45deg south facing array produces about 320kWh in Jan VS 1300kWh in Jun. To be able to heat off solar PV in the winter you would need to ridiculously over-sized array for anything over passive-house level of energy use.
Connecting an array to a simple restive heater will produce fairly low average power. Even if you match the array to the load at full sun, as soon as you move away from that, the average power you deliver decreases significantly. For example at 60% insolation, this setup delivers only 30% of the power at full sun. Partial shading, clouds make it even worse.
Solar and hot water is a very good combination. It by far the cheapest energy storage you can install. Solar PV is a good way to heat your DHW in the summer time when there is plenty of excess sun.
You need to make sure that the system is set up properly ( PV-->grid tie inverter-->diversion contorller-->standard tank or PV-->Mppt-->battery-->diversion controller -->low voltage tank).
You make some important points. I've worked with high voltage before (40kV DC once, pretty regularly with 1-5kV DC). That probably bred a bit of complacency when it comes to thinking about <250V. I've received a 2.5kVdc shock, which I can report was more enjoyable than a 750Vac shock, however I've never been in the position of having my actual hands on DC high voltage, so letting go was not a factor. My own system that I was planning was meant to be in the 70-80V range, so that's where my head was, not really considering how much it would change just by going up to 230V. Mine was also for DHW, so payback would include all seasons. That's why I suggested he would have to do the math to work out whether it would make any sense for space heating in his case, as the idea was entirely hypothetical.
You’re lucky to have survived a 2.5KV shock regardless of if it was AC or DC. the highest I’ve ever been zapped with was one leg of a 480vAC system which was a 277v lighting circuit. It was quite a bit more exciting than the usual 120v zap. I did, however, tap the ballast at 277v after I discovered that that one ballast wasn’t fed by the same 120v circuit as all the others. I don’t recommend using your hand as a volt meter. I’ve been zapped by 170v DC which does feel different from AC. I’ve also gotten tingles from the 48vDC plant at work, which can only give you a tingle if you get no-ox paste on you. Dry hands don’t conduct well enough.
I know one guy who survived a 13.2kV hit. He showed me where a skin graft didn’t take on his hand. He got hit in Kentucky I think it was, and woke up in the burn center in the university of Michigan hospital (one of the two major burn centers in the country). He did not remember anything about what happened and is very lucky to have survived. Almost no one survives even a brief hit at those levels.
Some basic electrical safety for anyone reading:
1- NEVER use both hands at the same time. You don’t want the current path to cross your heart. The classic way to get really hurt is to hold the grounded metal box with one hand while working with the other. Don’t do that. Make sure you never have two sides of a circuit in both hands, ever.
2- try to use fiberglass ladders. Wood is second choice. If you only have an aluminum ladder, do the job after you’ve gone and bought a fiberglass ladder.
3- if you don’t feel completely comfortable working with electrical systems, then don’t.
4- don’t get careless if you are comfortable (that’s usually how us electrical pros end up getting hurt)
I tell my crews that the electricity is ALWAYS waiting and ALWAYS ready to nail them the second they make a mistake. You don’t get second chances so always be careful.
Bill
I guess I left out some important context. The 2.5kV (might have been 2.15kV actually, its' been a long time since I recalled the info) was the potential to ground. It's entirely possible no part of my body was touching ground. So I don't know what the real voltage difference was. The point of contact was a solder joint on the back of a circuit board to my wrist. It burned a tiny hole in my skin, maybe 1mm wide and deep. I don't remember feeling it anywhere else, so it must have been an extremely short duration before the skin burnt away and effectively opened the circuit.
The 750Vac was truly a near miss, across the chest. That was a stupid mistake that scared the hell out of me. That may have been the last real shock I got, about 18 years ago.
Make sure to verify you can grid tie before devoting any thought to it. I'm in Ontario too, and ineligible, so it's not guaranteed.
Hydro One's station capacity calculator is available for download here as an excel spreadsheet:
https://www.hydroone.com/business-services/generators/station-capacity-calculator
You have to know your station and feeder, but I had that information from when I moved my transformer pole to accommodate the shop. According to the calculator, the test passes at 2MW but not 3MW, so my shop roof is probably ok!
Akos, I found the same thing while researching PTAC heat pumps.
Bill, very good points about running a water heater on DC.
I guess my next step is to look at pv system pricing and expected output on pv watts to see if direct heating is worthwhile pursuing.
So I ran some numbers and pvwatts simulations; direct connect pv doesn't make sense for winter heating if grid-tie is an option. I used a 7 panel 2.1 kW system as that's what would fit on my wall, panel pricing is what I can find for sale online in Canada, the rest of the system is just a semi-educated guess.
Edit, for some reason, my attachment doesn't want to show up.
7 panels was $1400, I guessed $500 for the rest of the direct system and $2000 for the rest of the grid tie system.
The average contribution of the pv over the heating season was 11% of the heat loss, in January it was only 4%. Simple payback using $0.17/kWh was 14.5 years, that same system tied to the grid was 10.5 years. Mounting that grid-tie system on the roof lowered it to 8.1 years. Using that system directly for domestic water (like Trevor is doing and assuming all the power can be used) makes the payback only 4.3 years.
Smart analysis! Well done. I agree with your conclusions.
Thanks for doing that, it's a sobering reality check.
When you calculated the heating loss, what interior temperature did you use, 50F?
Yes, 50F.
Paul,
Your conclusion is abosultely correct. To get realistic energy numbers for a direct to hot water system, you also need to look at performance at partial sun.
For example I've taken a PV insolation curve and sketched on the load placed by a simple heater element. You can see that when the element is matched for peak power at full sun, the load and power at partial sun is well away from the MPPT point.
Without a proper MPPT (or something similar) you will not get the energy that PV watts predicts into a DHW tank. This is not a small difference, you need upsize the array by a significant amount to get the same energy as with an MPPT.
I was doing some thinking of how to get around this (ignoring all the issues with working with DC I mentioned earlier).
One solution is to go with different element sizes in the tank. Say your upper element is 1kW, lower 500W. With some simple control you now have a load of 500, 1000 or 1500.
This would still need some sort of PLC to do the control or you might be able to do it with an insolation sensor and some relay logic. You would get way more energy out of the panels over a day, don't know how it would compare with a proper MPPT.
Micro inverters are cheap. If you want solar, connect it to the grid and use a standard electric tank. If you want to get fancy add in controls to run the tank when the sun shines. This way if you don't use the energy for the hot water the energy of the panels still can go somewhere else.
>”One solution is to go with different element sizes in the tank. Say your upper element is 1kW, lower 500W. “
That’s the usual 1/3 / 2/3 sizing rule for big systems that are staged. It’s handy for all kinds of mechanical systems since it lets two devices give three power levels.
For sensing/control, maybe you could get by with some simple voltage sensitive relays (settable to trigger at specific voltage points)? If you have a controller that can do load shedding, you could use that load shed output to control a contactor on the higher-wattage element. It’s simple and cheap enough to implement to be worth thinking about, all you need to do is have a control output from something smart enough to know what to do when.
Bill
From an overall reliability point of view, having the refrigerant loops charged and sealed at the factor makes a PTHP solution more rugged. Some of the DIY pre-charged mini-splits have lost some refrigerant (sometimes all) in storage & shipment, and every site-made refrigerant line connection has a risk of leaking. There is also potential contamination of the refrigerant with water etc unless the system is pumped down to a very low pressure and kept there for a couple of days (something rarely done with low-end mini-splits.)
Per the AHRI sheet the heating capacity of the 1-ton SENA is was tested at 11,800 BTU/hr @ +47F, 7800 BTU/hr @ +17F, which isn't exactly a useful amount of heat for a building with a heat load of 14K @ -20C/-4F, but those are just be the modulation levels at which they tested it's efficiency to come up with better efficiency numbers. The actual maximum capacity (and efficiency at max capacity) don't appear to be readily available.
http://senville.com/content/certificates/SENA-12HF-AHRI.pdf
Couldn't find an extended temperature capacity chart in a quick web search to see what it's actual capacity is at -30C or any other temperature. It's great that it's still running at -30C, but is it putting out any useful heat there (at any efficiency)?
With no internal resistance heat to cover the shortfall you'd likely have to use space heaters or some other auxiliary heat. Maybe there is better capacity data out there on Senville mini-splits that would give an idea as to it's actual capacity at -20C would be, but I'm not finding it.
The only 1-ton mini-splits likely to cover your 14K load @ -20C are Fujitsu's 12RLS3 & -RLS3H, which are good for 16KBTU/hr @-5F. ( Mitsubishi's FH12NA puts out only 13.6K @ +5F, less at -20C.) The value-priced Gree Crown+ series 1-ton ( 12HP230V1B) is only good for 10.5-11K @ -4F. The 1.5 ton Crown+ or Sapphire series are good for over 15K & 16K respectively, and would cover it, but at an internet pricing slightly higher than a Fujitsu 12RLS3, and almost twice the cost of a 1.25 ton PTHP.
Of the lower priced stuff, the 1.5 ton Midea Premier Hyper (MCHSU-18PHH2) is probably going to cover it too, delivering over 18.5K @ -15C, still under USD$1500 at internet pricing. (I seem to remember they use Toshiba compressors in this series).
https://us.midea.com/dam/jcr:46dc6a93-ff69-45a0-8ae6-546bc7456f5d/US_PP-RAC_PROD_PREMIERHYPER-HIGHWALL_specsheet.pdf
For intermittent use it's hard to say if the additional hardware and installation cost of even the 1.5 ton Premier Hyper will ever pay off, but it looks more like the real deal than any Senville Sena- even in the short-sheet they spec a capacity at -15C/+5F. Whether it will last as long as a 1.25 ton PTHP in your application is a crap shoot. (I've recently been talking to someone with a 1.5 ton Mitsubishi FE18 that crapped out in just over 7 years, though that unit had arguably been abused.)
"If" you can do all the install other than refrigerant
"If" you can get best available pricing on internet mini splits
"If" you can get someone to commission the unit in an hour for 100 bucks
PTHP make no sense
In my experience they are not cheap, don't tend to be low temperature functional [electric strip heat]
You can get 2 ton mini splits for 12oo bucks with -4 degree performance.
PTHP 'seem' to stop at 14k btu and cost several hundred more. For much poorer performance
IF you are paying contractors for every bit of work, perhaps the PTHP comes out ahead.
Compared to a house where we assume a level of constant heat, it is very difficult to measure 'payback' for shop heat. My 5kw electric heater barely runs[I haven't caught it yet] to keep my ratty garage up to 40F. I haven't burned 2 gallons of K1 in my torpedo this winter pulling the temp up
>" "If" you can get someone to commission the unit in an hour for 100 bucks"
In our colllective dreams!
In my area it costs $200 for a refer tech to just show up for a diagnostic. Actually walking through a full & competent commissioning would take around $400. YMMV, but I doubt anywhere in the first-world that work can be done for $100.
>"PTHP 'seem' to stop at 14k btu and cost several hundred more. "
Really? Several hundred more? At 1.2 ton PTHP can be had for well under a grand.
Picking just one model it took less than 30 seconds to find these:
https://www.thefurnaceoutlet.com/Amana-14000-BTU-PTHP-Heat-Pump-with-5-KW-Heat-Kit--PTH153G50AXXX_p_3078.html?gclid=EAIaIQobChMIu72RxYyW4QIVwkoNCh1jcwqEEAYYASABEgJuHPD_BwE
https://www.h-mac.com/amana-pth153g50axxx.html?gdffi=9f0856ca9fe841edaab22d427404fe06&gdfms=22A5089EC27149F584E0ED84735D8143&gclid=EAIaIQobChMIu72RxYyW4QIVwkoNCh1jcwqEEAYYAiABEgIMifD_BwE
https://www.alpinehomeair.com/viewproduct.cfm?productID=453062722&linkfrom=froogle&gclid=EAIaIQobChMIu72RxYyW4QIVwkoNCh1jcwqEEAYYAyABEgKW_PD_BwE
With wall sleeve & grille, breaker & wiring maybe $1100-1200 all-in as a DIY project.
A PTHP doesn't need commissioning, they're fairly long-lived, and dead-easy to swap out when it croaks.
A low end 2 ton mini-split with 14K of output @ -4F might cost about a grand, before adding in the refrigerant lines, wall bracket or condenser pad & stand, and commissioning costs. It'll be at least 1.5x the installed cost of a 1.2ton PTHP with on board resistance backup, and it probably won't last as long. Whether it ever "pays off" is a matter of just how often the space is heated, and your electricity rates.
Thanks for the reality check Keith!
I have noticed that it takes an extended cold snap for my shop to even get below freezing. Unless it brings a lot of snow in with it, I think even the car (which is in and out once a day on average) brings the temperature up.
Just for interest's sake, I ran a spreadsheet comparing costs of several different options with the following initial costs:
1. Propane unit heater, $1500 installed, 80% efficiency
2. Electric resistance, $200
3. Higher end mini-split, $4000, average COP of 3
4. Low end mini-split, $1500, average COP of 2
I brought all costs to today using an interest rate of 2% but I also assumed propane and electricity would increase by 2% per year, so this effectively negates the interest rate. Using an annual heat load of 2.3 mBTU, I found the following results:
-The low end mini-split beats electric resistance a little over 3 years out
-The high end mini-split beats electric resistance just under 6 years out
-Propane beats electric resistance at about 7 years out
-The high end minisplit doesn't beat the low end one until about 14 years out
Next I lowered the heat requirement to 1.5 mBTU for maintaining a lower temperature and just turning it up when I'm out there.
-The low end mini-split beats electric resistance at 4.5 years out
-The high end mini-split beats electric resistance at 8.5 years out
-Propane beats electric resistance at about 10 years out
-The high end minisplit doesn't beat the low end one at all (my spreadsheet went 20 years)
Simplicity and diy install and repairs is worth something to me too, looks like electric resistance is the answer. If I do go that route, it will definitely be on it's own kWh meter so I can keep track!
>"Using an annual heat load of 2.3 mBTU..."
>"I probably only work out there one or two part days a week so it’s likely most economical to maintain a baseline temperature and then turn it up as needed."
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That's a LOT of annual BTUs for such intermittent use, even if you were maintaining it at 10C between full-heat hobby-working periods.
Also, its not "...most economical to maintain a baseline temperature...". That would be more convenient, but it uses more energy. It doesn't take all day for a 14-15K heat source to bring a building with a design load of 14K from 0C indoors to 20C indoors unless it is approaching design condition outside. With resistance heaters it's cheap to just install 25-30K of heater, but with heat pump solutions some amount of planning, say turning it on the night before the planned work session.
For maintaining a baseline temp, most mini-splits don't have setpoints below 15C (I'm not sure if PTHPs can be set that low either), so some means of turning it on/off would be necessary to run baseline temperatures cooler than that. There are line voltage thermostats out there designed for freeze-control applications that can go lower than 10C which might have to be part of the equation no matter what solution you come up with.
I would consider propane to me a much more price-volatile energy source than electricity, which would be another point in favor of any of the three electrically powered options. Propane roughly tracks oil prices a lot of the time, so a bump in oil prices can bump up propane too. Electric prices tend to be heavily regulated, which usually makes them more stable over longer periods of time, so any increase is likely to be a gradual year over year increase and not a big spike based on market conditions.
Electric options also have zero need for venting and zero carbon monoxide risk. Electric resistance can probably be installed in a few hours too if you already have big enough electric service in your garage.
Bill