Reznor garage heater sizing

[jjscott]

I am getting ready to install a Reznor UDAP heater in my garage and I’m debating between the 45k & 60k units. I have a slightly oversized 3 car attached garage that measures 33x24x11. The walls, ceiling and garage doors are insulated, but I don’t know the R values. I can say that one of the 24ft walls border the house and half of the 33ft wall also borders the house. The remainder are exterior walls. There are two insulated garage doors, one is 16x7 and the other is 9x7. These doors are from the 90’s and are pretty thin. Nothing like some of the insulated doors I saw at a garage door company yesterday. There are two windows in the garage along with one standard maintenance door.

I live in Northeast Ohio and want to keep the garage heated through the winter to 50-55 degrees, but also want to bring it up to 65-70 degrees when working out there. I don’t want to wait for an hour for it to come up to temp. The calculations call for about 33k, so a 45k BTU, 83% efficient unit should work, but I’m debating putting in a 60k BTU unit. What are your thoughts. Do you think the 45k unit would be fine? Do you think the 60k unit would be overkill? I don’t want to get into an issue with short cycling the 60k unit and cause issues with it. I also don’t want the 45k unit to run all the time.

What do you think?

 

[zmotorsports]

I think the 45k would be sufficient, especially where you have two walls that share a heated space of the house. Although the 60k would also do a fine job and not need to run as long. Personally I wouldn't go any larger though, you don't want to go oversize too much and end up short cycling the heater.

In my last shop I had a 75k btu Modine Hot Dawg but it was detached and a bit larger @ 34x34x14. It did a very good job of keeping it heated here in northern Utah. I kept the overnight temperatures around 48-50 and when I would get home from work kick it up to 64* and it would only take about 10-15 minutes to get to that temp even with outside temps in the teens.

I think you will like that Reznor UDAT heater. I just installed a Reznor UDAT 150k btu in my new shop and it works great. It is also much, much quieter than my previous Hot Dawg.

 

[jjscott]

Thanks for the feedback. One of the contractors that bid on the job calculated the heat loss to be about 36k and that includes a slight outside to inside draft. That number assumes maintaining 70 degrees, which I wouldn't do. I'd be maintaining about 55 degrees and bumping it up to 70 when working on the garage. The 45k unit has an output of 37,350 with it being 83% efficient so I would be just a bit over the heat loss.

Anybody have a similar sized garage (33x24x11) running either a 45k or a 60k heater? how does it work for you?

 

[D45]

Go with the 60K

Natural gas?

 

[jjscott]

Yes, it's natural gas. Several have stated concern about oversizing and the potential of short cycling the heater. They say that the higher efficiency heaters can generate condensation which will corrode the heat exchanger when short cycled. This is my main concern.

There was another post on this forum which I need to review on this topic. I believe the poster got a great deal on his heater but it was grossly oversized and he was looking for a thermostat that could be configured to run for a preset amount of time each time it was calling for heat. This would allow the heater to warm up long enough to burn off any condensation it generated. Not sure if he found one to do that or not, but it's an interesting idea. Just want to make sure I'm covered if I go with the 60k.

 

[D45]

How well insulated and sealed is the shop?
I had a 45k last winter and felt it ran too much and took too long to heat the garage.......1,000 sq feet

I now have a 75k

 

[thammel]

my 32 x 28 x 12 with a full walk up attic is heated with a reznor udas 60k. Walls are r19 insulated and ceiling is r38. It's a perfect size and has no problems heating the place.

Tom

 

[jjscott]

How well insulated and sealed is the shop?
I had a 45k last winter and felt it ran too much and took too long to heat the garage.......1,000 sq feet

I now have a 75k

No sure about the R value insulation. The insulation in the ceiling is blown in and is about 8-10 inches. All of the walls are insulated, but again, I don't know the R value. One of the 24 ft walls borders my family room. About half of one of the 33 ft walls borders my office. The remainder of the walls border the outside world. There are two metal garage doors. One 16x7 & one 9x7. They are insulated, but it's pretty cheap stuff. The thickness of the door is probably 3/4". There are two windows and one metal maintenance door. I would say the garage has an average seal.

What's the efficiency of your heater? The Reznor is an 83%.

 

[D45]

Sounds like your area is well insulated, much better than my shop

I have a Hot Dawg 75K (power vented), which I believe is an 80% efficiency

I am in NW Indiana, close to Chicago........COLD winters

I only use my heater when I am out in the shop or turn it on to help protect things from freezing......an hour in the morning and then an hour at night

Usually the garage stays warmer than the outside, I think, because it is attached to the main 2 car garage

Usually temps are 15-20 degrees warmer than outside temps

 

[jjscott]

my 32 x 28 x 12 with a full walk up attic is heated with a reznor udas 60k. Walls are r19 insulated and ceiling is r38. It's a perfect size and has no problems heating the place.

Tom

Thanks, Tom. I'm sure a 60k will heat my garage fine. I'm just wondering if it will be too much...like way over the top too much. Several contractors that bid on the job sized it for a 45k. They seem to think it will be fine, but I'd rather be safe than sorry.

What temp to you normally keep your garage at?
What state do you live in?
If you had to guess, how often does it kick on and for how long?

 

[zmotorsports]

Thanks, Tom. I'm sure a 60k will heat my garage fine. I'm just wondering if it will be too much...like way over the top too much. Several contractors that bid on the job sized it for a 45k. They seem to think it will be fine, but I'd rather be safe than sorry.

What temp to you normally keep your garage at?
What state do you live in?
If you had to guess, how often does it kick on and for how long?

A 60k may be a bit oversized but I wouldn't say it is grossly oversized or "way over the top" to be as concerned about short cycling. Now it you were in the 45k-60k range and you installed a 100k or more I would say it was grossly oversized.

My last shop was 1150 square feet with 14' tall ceilings so just over 16k square feet and my calculations put it around 60k~ish depending on the ambient temp I used in the formula and I installed a 75k btu but I wanted it a bit on the larger size for speed of heating more than anything.

I had, and have my new shop, programmed for 50 degrees overnight and then bump it to 64* when I am working in it. I am in northern Utah so we have temps as low as single digits on occasion but usually teens for overnight temps and high 20's to mid 30's daytime. Temperature rise was usually quite fast from that 50 degree to 64 degree, less than 15-minutes.

For comparison, my new shop calculates out in the 113k-123k and I went with a 150k btu heater. I did it for similar reasons as my last shop but also due to the fact that from the 100k to the 125k was a pretty good jump in price and I my calculations showed I needed a little more than the 100k could provide. However, to go from the 125k to the 150k there was not much price difference so I just went ahead with the 150k and so far I am very pleased even though it is a bit oversized. With our ambient overnight temperatures barely in the high 20's and my working late in the shop I have not experienced any short cycling at all. Very nice and quiet heat thus far.

That's just my experience based on my two shops.

 

[TTA89]

I called Beacon Morris and asked them what they thought based on my garage info. I'll bet they know best.

 

[thammel]

I'm in Maryland so moderate temps most of the time but in the winter it gets below freezing a lot. I keep my garage at about 55F but raise it to 65 when I'm working in it. It's tough to say how often it kicks in....probably every 15 to 30 minutes when it's really cold. You do realize that the key is how well insulated you are, including all doors and windows. And you need to be very careful about air leaks around the garage door(s). I got the 60k unit because I found a good deal on a new unit, but 60k is what I'd go for. Recall that the "furnace" is not concerned with moisture removal as an AC unit. is. If you oversize an AC unit in the summer you may not get adequate humidity control.

Tom

 

[jjscott]

Thanks for the information. I spoke directly with the Reznor technical rep and he went through all of the calculations and came up with a 45k heater. His calculations were taking into account an outside temp of 0 degrees and maintaining 70 degrees in the garage. I got my last estimate in today and they also came up with a 45k unit, so that's what I'm going with. Hope it works!

 

[Gerry347]

JJ.... Who are you getting your estimates from?? My setup is pretty close to yours and I too live in NE Ohio.

I have a 24x32x10 free standing shop with a full attic upstairs. This summer I installed R19 insulation in the walls and R30 on the ceiling. I plan to keep the shop between 40-45 degrees most of the time and bump it up to 65-70 when I'm working in it.

I bought a Big Maxx 50k heater because everyone I talked too said it's the perfect size for my space given the insulation I have. I'll be running propane because we don't have gas in my area but everyone said I'll be fine.


Thanks,
Gerry

 

[6768rogues]

I have two heated buildings, one is 36'x48' and the other 40'x60', both with 14' ceilings. They each have one Reznor 100K BTU heater and they heat easily, come up to temperature quickly and are low cost to operate. I would recommend the 45K unit.

 

[jjscott]

JJ.... Who are you getting your estimates from?? My setup is pretty close to yours and I too live in NE Ohio.

I have a 24x32x10 free standing shop with a full attic upstairs. This summer I installed R19 insulation in the walls and R30 on the ceiling. I plan to keep the shop between 40-45 degrees most of the time and bump it up to 65-70 when I'm working in it.

I bought a Big Maxx 50k heater because everyone I talked too said it's the perfect size for my space given the insulation I have. I'll be running propane because we don't have gas in my area but everyone said I'll be fine.


Thanks,
Gerry

Just sent you a PM with some information. Note, I am only having the contractor install the vent pipe, tie in the gas line and electrical at the unit and perform a startup test. I'm hanging the unit, running the electrical, running the gas pipe and installing the thermostat.

Based on my research, the 50k sounds fine. I would have gone with that size just as added insurance, but the next size up from Reznor is 60k.

Let me know how it works out for you. Mine should be done on the 27th.

 

[Gerry347]

Just sent you a PM with some information. Note, I am only having the contractor install the vent pipe, tie in the gas line and electrical at the unit and perform a startup test.

Let me know how it works out for you. Mine should be done on the 27th.

That's pretty much what I want too but mine won't be done that soon. I still need to hang mine, run the gas line an have the tank installed. I'll PM you back in a few.

Thanks,

Gerry

 

[Keith_MN]

If you are worried about short cycling, you could install an industrial temperature controller that allows you to set the differential temp between on and off. I had to do this when I was running a 48kbtu output furnace in my one car garage. I normally ran the differential temperature at 10 degrees, which elimated short cycling. Normal house stats run about 1 degree.

 

[Showkey]

1000 sqft well insulated stand alone shop in cold climate . Replaced a failed 75k BTU NG with 45K BTU. BM 45 Heats fine with no issues. The old unit rusted out the exhaust B vent and exhaust blower box severe rust before my purchase of the property. Prior use unknown but it would not be hard to guess the larger unit was short cycling ??????

BM chart says 45K will be OK.

 

[jjscott]

The contractor finished up yesterday and ,so far so good. I had it set for 70 degrees last night when I was working the garage. I think the outside temp was in the 40's. It will be interesting to see how it performs when we are in the single digits.

One part of the install that I didn't like was that they had to install two 45 degree elbows in the vent pipe run in order get the exit point through the roof away from the second story of the house, but it was needed based on the location I wanted the heater in the garage. Here are some pics...hopefully they come through ok.

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[D45]

Is there 1" clearance around the entire vent, where it goes through the roof? Looks like it was a few inches off

 

[jjscott]

Is there 1" clearance around the entire vent, where it goes through the roof? Looks like it was a few inches off

Yeah, I caught that too. It's tough to tell, especially since I can't stand on a ladder up there, but it looks close. I did call the HVAC company to come back out and take a look at it.

 

[92nsx]

Personally I like the 75K's. When I want heat I want heat. I use Menards beacon Morris units. Only $50 difference between 45 and 75k.

I have them in both attached garage (22x22 with 9 foot ceiling) Insulation is "so so". Ceiling and only 2 walls insulated, and detached garage (22x45 with 12.5 foot ceiling) insulated to the max.

2017-11-29_10-00-05 by Rob Miller, on Flickr

Neighbor across the street has 60k Reznor in his 22x36 attached garage. Also works good, just cost $300 more.

 

[jjscott]

The contractor came back out and fixed the clearance issue. The foreman didn’t seem too happy about it, but hey, do it right the first time and you won’t be called back. I had them do the startup test on the unit they were supposed to do when installing it. I actually forgot to remind them when they were on site. The Reznor manual states the manifold pressure needs to be set to 3.5" of water column. Anything higher can cause an over firing condition that will lead to issues with the heat exchanger. The technical rep at Reznor was adamant that this be checked and set properly. The installer laughed when I told him this. He said they never check it and it's correctly set by the factory. After some prodding, he checked it and found it to be at 4.5". So much for the factory setting. They also used the yellow flexible gas tubing to connect the heater to the black gas pipe. As you can see from the pics above, the thing looks like it's been laying around in the shop for 10 years. It's not a big deal, but I will probably replace it to make it look nice. I guess I'm **** like that. BTW, that stuff is really expensive!

Now I just need to get some extra insulation up in the attic. The existing blown-in stuff really got matted down from working up there. After that, lighting.

Thanks for everyone's feedback. This really is a great site.

 

[MotecM]

Great looking setup JJ. I'm in a similar spot you were and trying to decide on a unit heater size.

I was planning on a 75k Reznor unit for my 1,200sq garage with 12' ceilings, but after reading your thread feedback I'm considering stepping down a size.

Now that you have your heater running, are you happy with your 45k unit or would you have gone larger?

 

[D45]

I was planning on a 75k Reznor unit for my 1,200sq garage with 12' ceilings, but after reading your thread feedback I'm considering stepping down a size.

Now that you have your heater running, are you happy with your 45k unit or would you have gone larger?

Depends on your location and your shops insulation

I have a 1,000 sq foot shop.......which is long and narrow

I installed a 45k and it worked, but I felt it was running a lot and took too long to bring the shop up to comfortable working temps. My shop loses heat through the ceiling/roof.....only the walls are insulated

This summer, I replaced the 45k with a 75k and even though temps are not that cold right now, it seems to be working much better for my shop

The heat throw and the fan size I believe is also a factor, to better move the warm air down and around my shop.

Some comparison specs for the 45K vs the 75K

45K FAN SIZE: 10"
75K FAN SIZE: 14"

45K INPUT: 45,000
45K OUTPUT: 36,000

75K INPUT: 75,000
75K OUTPUT: 60,000

45K HEAT THROW: 27 feet
75K HEAT THROW: 38 feet

45K AMP DRAW: 3.7
75K AMP DRAW: 2.5

 

[MotecM]

Good point D45 - I didn't think to factoring in the throw distance... My garage is an "L" shape that shares two walls with the house (picture a 2 car + a double deep single). The longest wall that's 42' is what the heater will be blowing parallel to. Sticking with a slightly larger unit may be helpful to reduce cold spots in the rest of the garage.

 

[jjscott]

Great looking setup JJ. I'm in a similar spot you were and trying to decide on a unit heater size.

I was planning on a 75k Reznor unit for my 1,200sq garage with 12' ceilings, but after reading your thread feedback I'm considering stepping down a size.

Now that you have your heater running, are you happy with your 45k unit or would you have gone larger?

As of right now, I am very happy with the 45k unit. Doesn't take long at all to get up to 55 degrees in the garage nor going from 55 to 65 when I'm working out there. However, it hasn't been all that cold here, so I'm mildly optimistic. We are just now getting some cold weather so it will be interesting to see how it performs.

I will say that my garage is completely insulated, including the garage doors and that really helps maintain the heat. I also have my thermostat run just the fan for five minutes every hour if there hasn't been a call for heat during the previous hour. That will actually raise the temp by a degree or so.

If you are going with an oversized unit, make sure to install a cleanout on the 90 degree elbow going into the unit. That will act as a trap for any moisture build up rather than allowing it to drain back into the unit. There are also considerations for the functionality of the thermostat to help reduce moisture as well.

Good luck with your project. A lot of good information on this board to help you along the way.

 

[kimper007]

I live in northern Minnesota and just built a 30x40x10 with R21 in the walls and R 60 in the attic. I ran tubes in the slab for in floor heat (which is not hooked up yet) and 2” pink foam under the slab. I just installed a reznor 60btu heater and it works great especially with the 5 degree temp we had this morning.