Mitsubisi Heat Pump- heat and cool
flymee
| Posted in General Questions on
I am building a 1 1/2 story barn/garage 36 X 34 with an open upstair 2nd floor. It will have 2″ form in the walls and 3″ sprayed form in the ceiling. Cement floor. not too many windows and 2 garage doors.
I don’t need it heated like a home but just confortable when I work in the barn. I would like to keep it about 40 degrees in the winter when not in use and will rarely need it cool in the summer. I live in southern CT.
I’ve been looking at the high efficienty units around 24,000 BTU. I also need to be able to set the temperature low (about 40) when not in use.
Can you give me some advice.
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In 2009 and climate zone 4C, I built a 24'x28' pole building with 13' interior ceiling and 14' eave height as a machine shop. The building is fully insulated on all six sides (though to a far lower standard than what you propose), but has a 12'x12' (insulated) overhead door with attendant air leakage.
I heat and cool this building with an 18,000BTU Mitsubishi Mr. Slim heat pump and a single overhead air handler cassette. This unit keeps the building interior at a steady 68F without difficulty, although the compressor does run heavily when the exterior air temperature dips to freezing. My specific location spends a lot of time in the 40'sF, rarely drops below freezing and typically gets one week in the high 90'sF each year.
I have been quite happy with the results. Initially I would have said "ecstatic" but I have had two fan failures (motor bearings, specifically) in the compressor. This seems to be a very rare occurrence, and the service manager at my HVAC contractor says he's never heard of even one failure elsewhere. Searching the web myself, I found only one other report of this problem. Sadly, the fan motor is a totally custom unit, and even with the full resources of a machine shop I cannot repair them or replace bearings.
No ductless heat pumps have a setpoint as low as 40F.
There isn't sufficient information about the window sizing or the R values or type of foam to make a reasonable estimate of the heat load. If it's closed cell foam, you can get twice the thermal performance going with thicker open cell foam for the same money.
If it's only going to be used intermittently, you may be better off using a couple of point-terminal heat pumps such as those used in hotels, rather than a more expensive mini-split.
If I understand right, your plan is to keep it at 50 most of the time, but heat it up to much higher (65?) on some days. That might mean that your sizing is determined by speed of heating it up, rather than by maintaining steady-state temperature, which would mean you'd probably want to overisze it. But we might need to know more about how you plan to use the space--how long, how localized is the use, etc.
Yes, that's correct. I just want to keep the Barn above freezing most of the time. If I plan to do some work, I'd like to be able to warm it up to about 60 in 20-30 minutes. I found the optional wireless thermostat that has a set point range of 40-90 but I'm not sure if the unit is efficient at maintaining those lower temperatures.
I do plan to use 3" "closed-cell" foam in all the roof rafters. . I will have some significant heat loss with my 2 large wooden swing out garage doors and there is on fairly large window. The second floor will also only cover 2/3 with a balcony so heat will drift upstairs.
I just want to be comfortable, not toasty warm. I'm thinking around 24,000-30,000 BTU.
3" of closed cell foam is about R20, max, and it would have to be installed in 2 passes with a cooling off period as it cures. This is both a fire-safety and installation quality issue. With open cell foam you could install R20 in a single pass at about half to 2/3 the cost.
At the same cost of 3" of closed cell you'd get R30+ out of open cell foam and it would be a lot nicer to the planet. Unless you have a major source of indoor humidity (say, a dozen horses or a few hundred chickens) the indoor dew points are going to track the outdoor dew points due to the high infiltration factor of the barn doors, which means that the roof deck would not accumulate wintertime moisture through the open cell foam.
Ramping it up from 40F to 60F in 20-30 minutes just isn't realistic with a 2 or 2.5 ton heat pump, due to the thermal mass of the barn and it's contents. A 1 to 1.5 ton PTHP could heat up a decent sized doored-off office-loft room that quickly, but not the whole barn. You might be able to get there with a 5-6 ton old-school heat pump (with the resistance heaters disabled), but it would be ridiculously oversized for the low loads you would have at 40F indoor temps.
A mini-split running full tilt during the recovery ramp won't do more than a COP of 2 when it's cold outside, or 2.5 even when it's in the 40s. And if it's oversized to the point that it can ramp up quickly, it will lose efficiency to on/off cycling when you're maintaining it at 40F. Your usage patterns are contra-indicative of modulating mini-splits being a good solution.
The dust & dirt factor is also an issue when installing heat pumps in barns and workshops. What is the intended use of this space?
I completely agree with the others that a heat pump suitable for maintaining a steady temperature is no way capable of rapidly warming the building (and its contents) from 40F to 60F.
On the other hand, despite doing lots of metal grinding and welding (generating dust and fine fumes), I've had no dust/dirt issues with my Mitsubishi overhead air handler cassette. The system wired controller reports when the filter should be cleaned; I get out the ladder to remove the filter; and blow it clean with a compressed air gun. Done.
The Garage/Barn will be used primarily for car, Lawn mower, snowblower and tool storage. I'll also set up a work bench and woodworking area. Upstairs might be an office so heavy dust will be at a minimum.
I really just want to keep it somewhat above freezing most of the time and have the ability to warm it up to normal temperatures if I'm doing a project for a week or two.
The walls of the Barn will be wrapped with 2" sheet foam insulation and sided a second time with shiplap siding. I assume the sheets are open-cell. They claim R-13. The spray foam in the roof rafters could be either open or closed. I think the closed eliminated the need for a roof ridge vent and better for moisture.
All you guys are great. Thanks for the help.
Overhead reflective radiant heaters would provide comfort for the human occupants during a warm up ramp, even at 40F, and may be the "right" solution here.
Start by insulating to R20 at the walls and R40 on the roof with open cell foam, install R10 slab-edge insulation, and use better weatherstripping on the doors. Those measures alone would probably keep the place at 40F or higher all winter even without the heaters:
The binned-hourly mean mid winter temp in southern CT is about 30-32F. With a 40F indoor temp and an R20 wall, R40 roof the average heat load runs about 0.5 BTU/hr per square foot of wall, 0.25 BTU per square foot out the roof.
Assuming an average wall height of 18' in a 34 x 36' barn that's less than 1500 BTU/hr of wall losses, and about 300 BTU/hr out the roof. The doors and windows probably add another 1000 BTU/hr. With infiltration call it a 3000 BTU/hr average load.
The deep subsoil in southern CT is ~50F. Every square foot of slab emits about 2 BTU/hr for every degree F it is above room temp. So with 1200' of slab even when the slab temp drops to 42F in a 40F room the slab would be emitting about 4800 BTU/hr- it's really heating the space. The deep subsoil & slab itself would be heating the space to 40F or higher at least most of the season assuming decent slab-edge insulation.
It won't take a lot of solar gain to cover the rest at 40F, which means it wouldn't take a lot of resistance heating to get it through occasional cold snaps either. Any low-E U0.37 or lower windows gain more heat than they lose, even if facing north (though other directions would be better.) That's going to keep the slab from losing much (or any) heat during the daylight hours.
If you only insulated to R13 wall & R20 ceiling with the 2" & 3" closed cell approach the average load will be twice as high, and it's less likely that 50F dirt plus the modest solar gain will cover it.
If it's air-leaky like a sieve it might have issues in a high wind, but 6-8 kw (20-27,000 BTU/hr) of radiant resistance heater should have no problem keeping it above 40F, even with some leaky doors (as long as they're closed). You can probably get there for about $1000 in up-front hardware cost, and you don't actually have to bring the full thermal mass of the building or the indoor temp up to 60F to be comfortable under radiant heaters. Just be sure to turn them off/down when you leave.
We did some cross-posting- didn't see reply #7 until I'd posted it.
All sheet foam is closed cell, independent of foam type. At 2" most foil-faced polyiso is rated R13 these days, and that's definitely a closed cell product, and I'm assuming that's what you're talking about? If so, it's performance will be more like R10 in mid-winter, but that would be enough for dew-point control for any 2x4 batt insulation on the interior side to beef it up. Air-tightness wise it won't be as tight as 5.5"/R20 of open cell unless you detail it well.
The roof should not be vented, and the foam should be continuous, whether open cell or closed cell. You're not cooking & showering in there, nor are you growing a weed-farm, so there's no wintertime moisture drive to speak of. If that extreme remote possiblity is going to keep you up at night 3" of rigid polyiso above the roof deck with 8" of open cell foam sprayed to the under side (or R25-R30 batts tight to the underside ) works from a dew point control perspective, even in an occupied humid house.
If you're going to be working there regularly for a week or more on end, keeping it at temp (even for the hours that week while you're away) with a ~1-ton PTHP will be worth it, using suspended radiant heaters only as the Hail Mary back-stop. Most PTHPs switch over to resistance heat when it's below 25F outside (COP=1, with out the comfort benefit of radiant heaters), but would deliver a COP of 2.0- 2.5 at 30F outdoors, and about 3.0-3.5 at 45F outdoors. During weeks of regular activity it's cheaper & better to keep it up to temp even for the hours your out using the PTHP than to let it cool off and have to bring it up to temp with resistance heaters.
A PTHP like the 1.2 ton Amana DigiSmart PTH153G35AXXX can deliver ~13-14,000 BTU/hr in heat-pump mode in a 70F room, probably 15K+ in a 60F room. It would cost about a grand to install (including the wall-sleeve and panel breaker), and would just about cover your average wintertime load at 60F (but not the design day peak load a 60F) If you back-stopped it with some overhead radiant heaters to 45-50F the recovery rate would be OK in the AM even on fairly cold days. During the shoulder seasons it could cover the whole load at 60F.
The Barn we are building is Post and Beam so normal wall insulation is not an option. The barn will be sided twice with a sandwich 2" form and Tyvek. The roof rafters are 2 x 8's so I can spray foam and cover with wood or sheetrock. That being said, I can't quite reach your R values. The Amana unit you describe looks interesting but I'm not sure it would look appropriate in an antique Post and Beam barn.
I thought the Mitsubishi heat pumps was a good clean compromise but perhaps they are not the right solution. I don't see needing the cooling side very much but it could be nice at times.I will definately go with your slap edge insulation. Its interesting that you feel the insulated barn tend to hold a fair amount of heat. Friends that have built uninsulated post and beam complain about the bitter cold.
Having a rough idea of my barn size and insulation R values, do you think the 24,000 btu would be sufficient on 20-30 degree days. I also like your Hail Mary radient heaters. I can always add later.
Again, I can't thank you enough for your help.
I'm not sure how a mini-split compressor hanging on the side of the barn (or next to it) would be dramatically different from a PTHP. You could certainly use a 1-ton FH12NA or FE12NA instead of the 1.2 ton PTHP, but it'll be 3-4x the upfront cost and not sufficiently more efficient to make up the difference in an intermittent-use situation. The lowest you can set the indoor temperature controls on those is ~60F, and at about 16,000 BTU/hr out @ +5F in a 70F room, and much much more in a 40F room, it would be enough to maintain 40F+ on the coldest nights without resorting to resistance heating. The key would be to turn it off until/unless it got down to 40F inside on nights when it was forecast to be well below freezing, since you don't have the option of a 40F setpoint on the controls. At +5F running full-tilt it would be using about half the power of the radiant heaters at any given room temp, but that room temp when it's 5-10F outside will likely be higher than 40F. It would almost certainly cover the load when it's 25F outside, 60F inside.
The 1.5 ton FE18NA or FH18NA might cover the entire load at +5F with a 68F room temp, so unless you really need to keep it above 60F at 5AM on the coldest night of the year a 2-tonner would be overkill. The minimum modulation of a 2-ton unit would also be too high- it would likely be cycling on/off when it's 35F or warmer outside, whereas the 1-ton units would be modulating at higher efficiency & comfort.
With 2x8 rafters you'd have an R27-R28 roof if filled with 7.25" of open cell foam . It would have to be installed in two lifts. If you paint the ceiling gypsum with "vapor barrier latex" it will further moderate the peak moisture content of the roof deck without blocking inward drying. Going one better would be to install a sheet of Certainteed MemBrain smart vapor retarder behind the ceiling gypsum, since it would reduce moisture uptake about as well as vapor barrier latex, but would let it dry 10x as fast if needed. (It's a couple hundred bucks in material cost to go with MemBrain, $50-100 for vapor barrier latex. Some box stores in New England are starting to carry MemBrain.)
Two more questions:
The foundation face sticks up about 10 inches above grade. Is there a slap-edge insulation with a surface that can be visible from the outside or do I have to cover it with something. I imagine it would not be ideal to leave that 10 inch uninsulated.
Do the PTHP units have to be shielded from rain and snow on the outside. Are they designed to be used in my southern CT weather and temperature. I imagine they also have a minimum set point of 60 degrees. Can they be mounted 3-4 feet above floor level.
HD sells the membrain.
You can cover up EPS slab-edge insulation with a cementicious EIFS material such as QuiKrete Foam Coating to protect it from deterioration by sunlight. You can texture it like stucco, trowel it smooth for a concrete-type look, or even mold it to a brick look if you're really into that sort of thing.
The slab edge insulation should extend to at least 2 feet below grade- or all the way down to the footing if you can.) Code min for residential would be R10, but in your situation R8.4 (2" of Type-II EPS) would be fine. Don't use polyiso for slab-edge insulation- only EPS or XPS. Be sure to install flashing to direct bulk water to the exterior side of the EPS.
PTHPs are used in motels & hotels in all sorts of climates. They're not too fussy- they work in Winnipeg & Anchorage just fine and they'll work just fine anywhere in CT. Unlike mini-splits, the heat pump inside turns off when the outdoor temps are below ~25F or so, turning on internal electric resistance heaters instead.
Dana,
If I get the walls up to R-15, the roof rafters up to R28 and add the rigid slap R10 insulation, do you think the Amana PTH153G35AXXX 14,200 BTU unit will be able to heat my barn to about 60 degrees on a typical 20-30 degree winter day in southern CT.
Its such a simple and inexpensive instillation, I must say I'm leaning in that direction. I fiqure I can always add your radiant heat panels or maybe even an attractive fan driven propane stove.
I like that you unit is also safer for wood dust from a table saw than any open flame heater. I would not use the propane stove when making dust.
Fred,
Consider covering your exposed foam insulation with a fiber cement panel (hardie or certainteed). The panel gets ripped in half and glued to the foam. Looks good and is more durable than EIFS. I'm in Zone 6A and placed the panels on the building 8 years ago. They still look fantastic! (***note that the manufacturers don't recommend this application) There has been no delamination or other damage regardless of what they recommend though.
Fred: Do the math, at least the napkin math. The U-factor of a continuous R15 wall with some siding and interior wallboard comes in at about U0.06. The U-factor of R28 between rafters is about U0.04. With 20F outdoors and 60F indoors you have a temperature difference of 40F.
Measure up the total wall area in square feet then do some simple arithmetic:
Wall Area x U0.06 x 40F= BTU/hr losses through the walls
Measure up the total roof area, then:
Roof Area x U0.04 x 40F= BTU/hr losses through the roof
If the garage doors, windows and other doors have a published U-factor (sometimes called U-value), run the same calculation for those elements.
Add them all up, and come up with a number. With air leakage or forced ventilation the number is going to go up (by quite a bit on a 20F day if it's a screaming wind), but use a 1.2x or 1.3x multiplier as a fudge-factor to account for slab losses and infiltration losses.
If with fudge factors the napkin-math load is much over 15,000 BTU/hr by very much it probably won't quite cut it at 20F, but you'll be able to figure out at what temperature it probably would carry the whole load.
Note, at +20F the Amana PTHPs would actually be running resistance heating strips, but at 25F or higher it would be all heat-pump, using half the power or less. So maybe it's better to just run the load numbers at +25F, which would be a temperature difference of 35F, which goes something like this:
A 34' x 36' barn with a serious 12:12 pitch to the roof would have about 1700' of roof area.
Assuming an average wall height of 18' you'd be at about 2500' of wall area, less about 200' of door & window area comes to 2300'.
Wall losses:
2300' x U0.06 x 35F= 4830 BTU/hr
Roof losses:
1700' x U0.04 x 35F= 2380 BTU/hr
Assuming crummy uninsulated U0.5 doors & windows, the door & window losses would be
200' x U0.5 x 35F= 3500 BTU/hr
Add it all up and you'r at 10,710 BTU/hr.
Throw in a 30% fudge factor to cover infiltration & slab losses and you're at 1.3 x 10.710= 13,923 BTU/hr
Call it 14K. The Amana would cover that much load in heat pump mode in a 60F room.
That's the right ball park at 25F.
At outdoor temps of about 24F or lower it kicks into resistance heating mode, where it has 14,400 BTU/hr of output.
Only you have the real measurements, so run the numbers on the real roof & wall areas, and look up the real U-factors & sizes for the doors & windows to fine tune it, but it looks like your winter average load would most likely be covered with a 1.2 ton Amana PTHP, especially if the garage doors are insulated and your windows are code-min, not clear-glass double-panes.
Note, it won't be able to raise the temp from 40F to 60F very quickly when it's 25F outside (it could take several hours), but it should be able to keep up.
On my Fujitsu, the thermostat normally cannot be set lower than 60 degrees. However, it also has a setting called "minimum heat" that sets the temperature at 50 degrees. Posters are correct that it would be inefficient to regularly set the thermostat down to 50 then increase it, because the losses from rapidly increasing the temperature are significant. But I use that setting when the house will be vacant for a week or more. That approach could make sense for Fred if he can leave the temperature at 50 for an extended period.
Dana,
In a similar situation (large heated garage/workshop), what would you recommend for equipment if you wanted to maintain an average temp of 60F rather than intermittent heat (Zone 6A- 7900 HDD) - PTHP or MSHP? I hadn't considered the PTHP, but after this discussion, and the substantial upfront cost difference it seems to make sense. I'm just not sure about energy efficiency and PTHP at our cold temps. The house is Zero Energy, and the detached garage will be super insulated too, because of an in-law apartment above (which is heated by MSHP).
Yours is NOT a similar situation if you're planning to keep it at 60F all winter (rather than 40F except when using the place, as has been discussed here.)
PTHPs go into resistance heating mode (COP=1) in the mid-20s, which is probably above your average mid-winter average temp in zone 6A.
A decent mini-split would be delivering COPs north of 2 down to 0F, using less than half the power than a PTHP in mid-winter.
The much lower efficiency may not be all that important if the heat load is low enough. But without the load numbers there's no way really estimate it. Best guess: For your application you'll probably be better off with a mini-split.
Dana....thanks
Dana....I have a question on the Rigid slab insulation for the outside of the foundation. Should I use the 2" pink Foamular 250 2' X 8' panels doubled up to cover 4 feet of wall or should I use the Aluminum faced 2" 4' X 8' panel. Are they both OK and do they get glued to the cement walls. Are they both OK underground.
With aluminum faced EPS the facers would degrade over time, but it would still work. Foil faced polyiso should NOT be used in contact with the ground, since polyiso will wick moisture over time, but on the interior side of foundation walls (the interior of a basement or conditioned crawl space) would be fine.
2.5lb XPS will be fine on the exterior, but needs to be protected from sunlight degradation where it's above-grade. (Cementicious EIFS goods like QuiKrete Foam Coating is fine.) It's cheaper and greener to use 1.5lb density EPS though (unless it's reclaimed XPS & EPS, in which case it doesn't matter.)
Fred,
Here is a link to an article that discusses the advantages and disadvantages of different types of rigid foam: Choosing Rigid Foam.
Here is a link to an article that discusses all the different ways to protect exterior rigid foam from sunlight and physical abuse: How to Insulate a Basement Wall.