Heater Size Up or Down?

[Tuhlmann]

This will be my last unheated winter. Thanks to this site, I learned how to do heat loss calculations and much, much more. Here's what I crunched:

∆40F = 77,913 BTU/hr heat loss

∆30F = 58, 435 BTU/hr heat loss

I'm kinda stuck on whether I should error on the side of a larger or smaller unit since I'm right at that 60k/75k point. I don't want to be under gunned, but I don't want short-cycling either. Better to have a smaller unit working more for the averages, or a larger unit to better handle the extremes? Or is there even that much practical difference?

*This is a 33x33x10' attached 4 car with a single 9x18' garage door that is opened several times per day. Common construction. Wisconsin winters. Keep cars warm and wood/metals/putzing shop used occasionally.

 

[finn]

Most of the heater ratings are based on btu input. Heat loss has to take heater efficiency, usually ~80%, into account.

That’s my understanding, anyway.

 

[Mr onetwo]

Due to the size of your door you should go to the high side.I question your heat loss calc because of the difficulty of accurately calculating the loss around and thru that huge door.Have a pro do a calculation to double check if possible.

 

[Tuhlmann]

Due to the size of your door you should go to the high side.I question your heat loss calc because of the difficulty of accurately calculating the loss around and thru that huge door.Have a pro do a calculation to double check if possible.

I took 1/(R-value) of the garage door x the area of the door. All other numbers were on the low side of the efficiency scale to account for age of error & additional latent losses in addition to those built in to the factors. Is there something else I should be considering? I am confident in my calculations, unless there are variables I’m not aware of.

ETA: Independent of whether or not my numbers are legit, my bigger question is “Is it better to have a smaller unit working more for the averages, or a larger unit to better handle the extremes? Or is there even that much practical difference?”

 

[D45]

I would go with a 75k

Larger fan and longer heat throw also

I have a 75k in my shop that's roughly the same square foot. My old 45k wasn't enough

 

[SeanH]

I have an 80k in my 1250sqft garage, with half the walls shared with the house. It does a fine job, even if I open one of my two stall doors (one is 18x9 and the other is 16x9). I would definitely go larger.

 

[Sweetcorn]

I have a 1,000 sq. ft. shop, very well insulated, and a 75K unit.

Very happy with what I have. No issues with short cycling.

 

[yeldogt]

You are looking at ..... let's call it 1000sf.

Are you saying the door has an R of 1?

The key with doors is leaking ... it's very hard to determine needed BTU's with a leaking space because of the huge variables. The warmer the inside the more heat is lost to the outside with leaks. Also, anyone who has a partially insulated space can't be used for comparison -- or poorly insulated.

I have a well insulated 1700sf studio -- two connected buildings of about equal size .. both with cathedral ceilings. One is 28'. Due to construction -- my 40k sealed combustion wall/cabinet unit is non operational. It's propane and I'm waiting for the new underground tank to be installed on the property -- the unit is 80%. So it makes 32kBTU.

For the past two years it been heated with a 4kw electric heater -- so I'm getting around 13.5k BTU from the heater.

It has no problem keeping the space at around 58 -- I turn it up when I get there and have another small 110 v electric to help by my paint mixing area when I'm standing there and I just arrive. It will make the whole space warm in a few hours.

So I can do the whole thing with 15k BTU -- but, I like the cabinet heater because it can throw a bit more heat into the area when needed -- w/o it being so oversized ..that it over heats before items warm.

So you have to look at what you want to do. If you have a space full of items and you let them all cool down .. and then one day want to heat the space .... you have to heat all those items. It's much easier to heat an empty space ... that's just easy to heat air. Bringing a full shop up to temp requires more BTU's -- the problem is the air heats before the things in the shop heat.

I always tell people to seal and insulate -- with natural gas you will find that maintaining a higher temp in the shop will be very affordable .... it much easier to get to 65 from 55 than it is from 45 .. a lot ... especially when it's cold out.

Most people oversize .. that does have an on and off penalty. It's much more comfortable when a smaller heater is running longer vs the on and off of a big.

So if your building leaks and you want to keep it cold and then work in a warmer space -- you need a larger heater than what the calculation says. Also -- some-of the calculators are assuming a set maintained temp .. make sure you understand that.

I will say this as well -- a very well insulated space w/ little leaking -- is forgiving. My 40k 80% is ideal .... but I'm sure if I had a 75K 80% -- making 60k ... it would work nearly as well. Especially if I was keeping it at 70 degrees. If I'm there full time for a few weeks I set it for 68 ... so the whole space is warm anyway. When a space is cool/cold it's more comfortable when the heater is running ... that's why modulation units make for nice spaces.

 

[wes73]

I have a 75K in my 30X40X10 with two insulated OHD (9X20 and 9X10), 1 man door, and 3 windows. I maintain 50 degrees all winter and kick it up to 70 when working. I'd go onthe higher end if your OHD gets opened alot or is not insulated.

 

[Bert_]

People saying they have a 75,000 btu in a 1000sqft well insulated building and it doesn't short cycle are full of it.

My 24x40 garage has no insulation in the walls and maybe an inch of old vermiculite in the ceiling. It's sealed up ok, but there are a couple drafty spots. I only heat when I'm out there and with a 70,000btu it can be 0* outside and I can maintain 60* inside. Was out there one evening when it was about -10* and the heater hardly shut off but it held inside at about 55*. That is with almost no insulation.

Once I get some insulation I will get a permanent heater, I won't be going more than ~45,000

 

[klassenl]

I have 936 Sq ft with 40 000 btu. Well insulated. I had no trouble keeping it at 9*c (say 48f). The longest cold snap we had was about a week. No problems. The big door get opened 1 to 2 times a day.

 

[Sweetcorn]

People saying they have a 75,000 btu in a 1000sqft well insulated building and it doesn't short cycle are full of it.

My 24x40 garage has no insulation in the walls and maybe an inch of old vermiculite in the ceiling. It's sealed up ok, but there are a couple drafty spots. I only heat when I'm out there and with a 70,000btu it can be 0* outside and I can maintain 60* inside. Was out there one evening when it was about -10* and the heater hardly shut off but it held inside at about 55*. That is with almost no insulation.

Once I get some insulation I will get a permanent heater, I won't be going more than ~45,000

This is the internet, so I personally don't care what your opinion is, but my heater kicks on about every 45 minutes or so in the really cold weather. It doesn't run long, but it certainly doesn't short cycle.

The shop is always heated and never turned off. Always set at 65 through the whole winter. Everything in there is always warm (enough).

 

[ripperd]

This will be my last unheated winter. Thanks to this site, I learned how to do heat loss calculations and much, much more. Here's what I crunched:

∆40F = 77,913 BTU/hr heat loss

∆30F = 58, 435 BTU/hr heat loss

I'm kinda stuck on whether I should error on the side of a larger or smaller unit since I'm right at that 60k/75k point. I don't want to be under gunned, but I don't want short-cycling either. Better to have a smaller unit working more for the averages, or a larger unit to better handle the extremes? Or is there even that much practical difference?

*This is a 33x33x10' attached 4 car with a single 9x18' garage door that is opened several times per day. Common construction. Wisconsin winters. Keep cars warm and wood/metals/putzing shop used occasionally.

Fully insulated? What r values in walls and ceiling? Those BTU usages seem way high if so. I think you are discounting the fact there is zero heat loss in the attached walls.

Honestly 40k will be fine for you, unless you really need really quick recovery after cycling the doors.

My attached is 25x40x10' and 45k btu works great. Even at -20F it will maintain 50F inside without running continuously. Thats 70 degree delta. It is new construction, 6" walls, good insulation. About 50% of wall area and 50% of ceiling area shared with the house though.

 

[yeldogt]

This is the internet, so I personally don't care what your opinion is, but my heater kicks on about every 45 minutes or so in the really cold weather. It doesn't run long, but it certainly doesn't short cycle.

The shop is always heated and never turned off. Always set at 65 through the whole winter. Everything in there is always warm (enough).

Sweetcorn .. that's short cycling.

A properly sized heater at design temp .. and design temp is the coldest day and the highest temp you will ever want. So if the design temp for your area in the winter is 5F -- and you want the shop at 70. That's design temp ... a properly sized heater should be running almost continuously.

If you only need to run every 45 min ... with typical thermostat fluctuation -- it's got good insulation. You need about 10-15k BTU's

 

[Showkey]

Another data point:

1000 sqft stand alone shop in north central Wisconsin is considerably colder than southern Wisconsin. Well insulated and sealed.

45k NG unit does just fine. Had a 60k unit that rusted out after 20 years of unknown use. 3 of the 20 was my use. While I never bothered to check or track on off times of the 60k unit, it seemed to “short cycle” ( run time was short).

I would definitely NOT recommended a 80k unit. Especially in an attached garage. My attached 1000 sqft attached well insulated garage with no added never really freezes. Even at -25* actual temperatures the water on the floor never freezes. That’s with three 9’ doors and 2 man doors.

 

[greenskeeper]

This will be my last unheated winter. Thanks to this site, I learned how to do heat loss calculations and much, much more. Here's what I crunched:

∆40F = 77,913 BTU/hr heat loss

∆30F = 58, 435 BTU/hr heat loss

I'm kinda stuck on whether I should error on the side of a larger or smaller unit since I'm right at that 60k/75k point. I don't want to be under gunned, but I don't want short-cycling either. Better to have a smaller unit working more for the averages, or a larger unit to better handle the extremes? Or is there even that much practical difference?

*This is a 33x33x10' attached 4 car with a single 9x18' garage door that is opened several times per day. Common construction. Wisconsin winters. Keep cars warm and wood/metals/putzing shop used occasionally.

Are you planning on constantly heating the space or only when you are out in the garage....if you are only heating when you are out there I would go for a larger than spec unit.

My detached garage is heated with an old (free) forced air oil furnace about 3x oversized and it works great since I only heat when I'm in the garage....it will go from 40F to 70F in about 5 minutes and it doesn't short cycle once up to temp.

 

[Shop Specialties]

I would say about 50,000 BTU should be more than adequate. 60-75,000 BTU would be unnecessary overkill.

 

[Bert_]

This is the internet, so I personally don't care what your opinion is, but my heater kicks on about every 45 minutes or so in the really cold weather. It doesn't run long, but it certainly doesn't short cycle.

The shop is always heated and never turned off. Always set at 65 through the whole winter. Everything in there is always warm (enough).

Ideally on the coldest day the heater should NEVER shut off at your design temperature. Now it is a good idea to oversize a little bit in a garage so it can recover if you open a garage door.

Running for a few minutes an hour on a very cold day is serious short cycling. This is not opinion, it is an industry accepted fact.

 

[jvitez]

Running for a few minutes an hour on a very cold day is serious short cycling. This is not opinion, it is an industry accepted fact.

Well said, and correct.

 

[Sweetcorn]

Well I'm the idiot then. I understood short cycling to mean frequently turning itself on and off for very short running cycles, sometimes to the point of being unable to get to the thermostat's set temperature.
I've never timed how long mine runs when it kicks on, but it "feels like" a few minutes. I'll clock it sometime.

 

[Showkey]

The other way to look at short cycle:

Heat OFF time is based on insulation, building air exchange rate, outside temperature, inside temperature over shoot.

Heat ON is based on size of the heater. In theory 50k unit will run twice as long as 100k unit. If a 50k unit runs 5 minutes at 10* outside temp then w100k unit would run for 2.5 minutes.

If you have leaky building and BIG heater you have the ultimate short cycle machine.......short off time and short run time.

 

[Jackfre]

My 960 sq ft heater fires from 13,200-36,500 on LP. I turn it on when I go into the shop and off in the evening. Now, the foothills of the Sierra are not SE Wisc. Once I turn my unit on it runs continuously all day long as it modulates and once it gets the temp up it rides low all day on the burner and blower. The temp is consistent throughout the space because the blower doesn't turn off. It delivers excellent comfort and ultimately we are talking comfort, not heat. My heater does nothing all day but ride the temp set-point with very little variation. An oversized single stage unit will bring the temp up very quickly, but its temp curve is a very peaky sine-curve. Yes it will heat, but it is not comfortable. Making the decision to size the unit for a -20* design condition makes the heater oversized 95+% of the time. Design condition occurs only about 1.5% of your heating hrs. A 50kbtu heater of good quality will heat your space nicely.

 

[Steve W.]

Well I'm the idiot then. I understood short cycling to mean frequently turning itself on and off for very short running cycles, sometimes to the point of being unable to get to the thermostat's set temperature.
I've never timed how long mine runs when it kicks on, but it "feels like" a few minutes. I'll clock it sometime.

You are not the only "idiot" out there.

I have also believed that to be the definition of "short-cycling". Usually caused by a heater that is WAY too large for the space, coupled with some drastic heat loss. The result is that the (large) heater does not take long to heat the space, then shuts off. The drastic heat loss cools the space, triggering another heat cycle.

I know I'm gonna catch some flak for what I do, but I don't really care. My shop is 24 x 30, attic trusses so there is a 10 x 30 room upstairs. R-13 insulation in all walls, roof (with baffles for eave-to-ridge vent air flow) and the floor between the two levels. Four windows (about 2' x 4' awning units), 7 x 10 OHD and a man door. All reasonably tight. Until I get around to installing my mini-split, my TEMPORARY heating solution is a kerosene-fired torpedo that has a built-in thermostat. Yes, I know about all the hazards involved. I have been using kerosene to heat my garage(s), shop and parts of my house for 35 years. Even with outside temps in the 20s, the torpedo fires up for about 3 minutes every hour to maintain 60° inside.

In my opinion, "operating for a short time, rather infrequently" is not the same as "short-cycling".

.

 

[Showkey]

You are not the only "idiot" out there.

I have also believed that to be the definition of "short-cycling".
In my opinion, "operating for a short time, rather infrequently" is not the same as "short-cycling".

.

Short run or short cycle no matter how you get there........

The concern With both is RUST and CORROSION.

 

[Steve W.]

As long as it gets to full operating temperature to 'burn off' any condensation,
how much does it matter?

A small torpedo like mine gets plenty hot in those three minutes.

.

 

[Showkey]

As long as it gets to full operating temperature to 'burn off' any condensation,
how much does it matter?

A small torpedo like mine gets plenty hot in those three minutes.

.

Just guessing, the OP is not thinking torpedo heater

 

[Tuhlmann]

Good discussion. Just some added details:

1.) I'm looking to maintain a constant temperature of 55-60° F throughout the cold season. I may or may not increase temporarily to suit my projects/comfort level on evenings/weekends, but primarily as a comfortable buffer from vehicles to home, pre-warmed vehicles, & a controlled environment to reduce condensation on tools & equipment in shop space.

2.) Recovery time from large overhead door use isn't my biggest concern, but it is a factor.

3.) The most efficient means to provide the most comfort for cost to operate this type of unit.

Many good points, but this one is an excellent point on heat vs comfort:

My 960 sq ft heater fires from 13,200-36,500 on LP. I turn it on when I go into the shop and off in the evening. Now, the foothills of the Sierra are not SE Wisc. Once I turn my unit on it runs continuously all day long as it modulates and once it gets the temp up it rides low all day on the burner and blower. The temp is consistent throughout the space because the blower doesn't turn off. It delivers excellent comfort and ultimately we are talking comfort, not heat. My heater does nothing all day but ride the temp set-point with very little variation. An oversized single stage unit will bring the temp up very quickly, but its temp curve is a very peaky sine-curve. Yes it will heat, but it is not comfortable. Making the decision to size the unit for a -20* design condition makes the heater oversized 95+% of the time. Design condition occurs only about 1.5% of your heating hrs. A 50kbtu heater of good quality will heat your space nicely.

It looks like I should opt for a smaller unit that runs a bit more. My concern about that is that a 60K @ 82% efficiency is 49.2K in output. A 50K unit running @ 82% efficiency is 41K in output. The lower of my delta temp HLCs is 58.4K, which makes a 75K unit (61.5K @ 82% efficiency) the recommended fit according to the manufacturer numbers. What I'm learning here is there are too many variables of climate, construction, insulation efficiency, & use involved for all of the recommendations given here to use to make anything other that a scientific wild-*** guess. I'm going to re-validate my calcs, verify & trust them, then size accordingly rounding to the low end to error on the side of more run time.

 

[SeanH]

So, using this logic (which is good logic since it takes into account the full volume of the garage), my heater almost sounds undersized. https://www.newair.com/blogs/learn/how-to-size-garage-heater

I have a 1,250sqft garage with an average of about 11ft ceilings, so 13,750cuft.

I'd say my insulation is above average, so we'll go with 0.6 for the ratio. And, I usually keep the garage at 40 degrees in the winter when I am not using it (just so everything stays above freezing). So, if you assume I want it to be 65 degrees, you would say a temp difference of 25 degrees.

So, (0.6 * 13750 * 25)/1.6 = 128k BTU

Is this poor logic, or what?

 

[Showkey]

A few more points, those size charts by garage heater manufacturers are surprising close to reality. Insulation is likely the largest variable. They take the efficiency factor of the heater when recommend a certain size.

http://www.ultimategarageheater.com/natural-gas-garage-heater/search.php



Chart area enlarged
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I have single story ranch home half with 15’ ceiling witha full basement about 5000 sqft. total. It’s heated witha 70k multistage furnace that rarely needs to operate at high stage. So it operates at 35k low heat low fan mode the vast majority of the time. Home heating is more about comfort, no drafts, even heat swing range, even zone control, quiet and efficiency.

Bottom line the 40-80k unit will heat your 1000 sqft garage with no problem. If your ceiling insulation is just marginal lets say R19. Then increasing to R38 would be effective in controlling heat loss and low operations costs over time.

 

[BoostAddiction]

Insulation is second to sealing- and you have at least one large door with sealing that, even if it is good for a garage door- is terrible compared to a "normal" door, as well as having lots more area to seal. It's hard to account for that in a manual J, but most users are too conservative in their defaults if the building is well insulated and sealed.

Also, I hope you don't really believe the R value that garage door companies advertise- it's almost never even close to what the door assembly actually does.

I agree with the rest of the advice: better to have a smaller unit that is right-sized so you don't short-cycle. On the other hand, in your climate, recovery time is important as well, so a larger capacity unit is something to consider.

Finally, don't assume 80% efficiency- there are many new furnaces with 96% efficiency ratings or better, so validate what you are selecting in your calculations.

 

[yeldogt]

Good discussion. Just some added details:

1.) I'm looking to maintain a constant temperature of 55-60° F throughout the cold season. I may or may not increase temporarily to suit my projects/comfort level on evenings/weekends, but primarily as a comfortable buffer from vehicles to home, pre-warmed vehicles, & a controlled environment to reduce condensation on tools & equipment in shop space.

2.) Recovery time from large overhead door use isn't my biggest concern, but it is a factor.

3.) The most efficient means to provide the most comfort for cost to operate this type of unit.

Many good points, but this one is an excellent point on heat vs comfort:



It looks like I should opt for a smaller unit that runs a bit more. My concern about that is that a 60K @ 82% efficiency is 49.2K in output. A 50K unit running @ 82% efficiency is 41K in output. The lower of my delta temp HLCs is 58.4K, which makes a 75K unit (61.5K @ 82% efficiency) the recommended fit according to the manufacturer numbers. What I'm learning here is there are too many variables of climate, construction, insulation efficiency, & use involved for all of the recommendations given here to use to make anything other that a scientific wild-*** guess. I'm going to re-validate my calcs, verify & trust them, then size accordingly rounding to the low end to error on the side of more run time.

What Jack is talking about is a modulating furnace -- a modulating furnace may or not be a condensing furnace. Don't get all the terms mixed up .... most hanging garage gas heaters are not condensing -- so they are 80% efficient.

You first started seeing modulating in home units -- two stage .. over time some manufactures got to 10 stages .. and then fully modulating. What's all this and why? You have to size a heater so it can maintain the temp you want on the coldest day ... how often does that happen? In some places maybe only once every 5 years ... but you need it when you need it. So the heater is sized for 2 degrees and putting out BTU's for 2 degrees when it's 40 degrees out.

Heaters with the ability to adjust output -- "modulating" match what is needed at the time and temp.

Another long standing problem -- limited sizes. This is less of a problem now as manufacturers have been making smaller sizes. But -- it was not uncommon for HVAC installers to find themselves not able to match the needed load properly because there was not one small enough. Another problem is people using "general rules" . It's this many SF so it needs "X" unit and then they install one bigger .. just in case. It's why many small houses have 120k BTU furnaces and 160k BTU boilers.

The unit Jack has is a modulating cabinet heater. I have the same type in one of my studios that is not modulating. They look like a small cabinet sitting on the floor -- closed combustion (they get air from outside for the flame) -- simple vent through the wall. They cost more but, IMO are nicer. Quiet and the heat comes out at the floor. Mine was bought many years ago before the modulating -- I would buy the type Jack has today.

Most people have never lived in a house with modulating equipment --- it's the same with spray foam ... or radiant heat. What's the big deal? Well you can't explain -- have to experience.

Now -- this is a garage. Some people don't use often ... some people like to wear lot of clothes. Other just like or are used to colder spaces ....it's impossible to give one answer.

Air is easy to heat .. opening and closing a garage door a few times a day in a shop where the temp is maintained is not even a factor. The unit will come on and quickly heat the air. This is what is happening anyway -- the air is heating everything in the space. Most people never measure the heat at the ceiling -- if you have a big heater up on the wall blowing lots of warm air ..where do you thing it's all going ... it's up at the ceiling? You are heating the space from the top down --bigger heater ...more heat to the ceiling.

It's why radiant is more comfortable and can be more efficient --- the hottest spot is the floor and as you go up the air cools ... the ceiling will be coldest if the system is running correctly.

The only reason to oversize a heater is for quicker space warm ups -- and based on your desired temps a properly sized heater should be fine except on the coldest day. IE -- I turn the unit down more than normal and it's now super cold out and I want the space to heat instantly.

My old studio space was propane as well -- when I outfitted it propane was $1 gallon. I kept the space at 68 ... even though I was only there on the weekends. Lots of stuff in the building -- tools/ equipment/ paint/ wood products. When propane got to almost $5 .... I started turning it down. If I got there on the coldest day and wanted to work in the space .. I happened to have an electric heater 4kw that I would turn on with the gas. It only needed to run an hour or so .. in that way I created a modulating furnace.

 

[Jackfre]

I would get the smaller unit and push it. It will be happier and i think you will too. Should you find it a bit to small then you go after the load. Find the leaks, seal it up and bring the load back to the capacity of the smaller heater. Given that you are going to maintain temp bigger is a real waste. The only reason to really have the larger unit is to flash fry the place. Once you have the mass of the place up to temp that point is moot. If you have a clipper come blowing through and the temps really dive for a few days or weeks and the temp drops 5 or 10* does it even matter? You might contact your utility and see if they have a program for heat mapping a place. Infrared pics tell where the sealing/insulating is needed.

 

[D45]

My 45k Hot Dawg was just running way too long and took way to long to heat up my shop

My shop is long and narrow and the BTU output wasn't enough and the fan size/heat throw also wasn't enough.

BTU input vs BTU output is also something to think about and consider

I know I am oversized but it works well for 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

 

[yeldogt]

My 45k Hot Dawg was just running way too long and took way to long to heat up my shop

My shop is long and narrow and the BTU output wasn't enough and the fan size/heat throw also wasn't enough.

BTU input vs BTU output is also something to think about and consider

I know I am oversized but it works well for 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

Start temp and desired temp -- shop filled or empty

It's all about insulation and leaks .... leaks are especially problematic when doing a load calculation.

Trying to heat a garage full of tools and a cold slab from 45 degrees to 60 is going to require much more BTU's than taking one from 60 to 65.

Add in any leaks from doors or weird construction -- sub par insulation and the BTU's increase further.

Another things is the stack effect ... when heating a cold shop with an oversized heater. Much of the heat rises to the ceiling -- works it's way down. Hot air has force -- and that force will exit the building through those leaks.

As I said above -- I have to heat my space with electric until my project is far enough along for the new propane tank. Had I known all the problems with the project I would have never removed the twin 100's. I heat my 1700sf space with around 14k BTU. The propane is a net of 32k BTU

It's all in the insulation and the leaks.

 

[toyotadriver]

I would always size a garage heater a bit larger. You have garage doors opening (and loosing lots of heat) and if you heat it only when you are working out there, you will need to heat up both the air as well as the tools. Your concrete floor will also absorb some heat. Realistically, there aren't a lot of options for garage heater sizes. So, you pick the one that's closest to your needs. Most garages should not use normal heater sizing because you aren't typically heating it like you would heat a house. Size it on the larger size and you'll be happy with its performance. Under size it and you'll be unhappy with it.

I run a 80k BTU in a 30x40x10 shop with decent insulation. I tried a 45K before and it would heat it but took forever to bring the shop up to temp. The 80k does it much quicker. Love it.

 

[InsanePyro]

If its worth doing its worth over doing

 

[yeldogt]

I would always size a garage heater a bit larger. You have garage doors opening (and loosing lots of heat) and if you heat it only when you are working out there, you will need to heat up both the air as well as the tools. Your concrete floor will also absorb some heat. Realistically, there aren't a lot of options for garage heater sizes. So, you pick the one that's closest to your needs. Most garages should not use normal heater sizing because you aren't typically heating it like you would heat a house. Size it on the larger size and you'll be happy with its performance. Under size it and you'll be unhappy with it.

I run a 80k BTU in a 30x40x10 shop with decent insulation. I tried a 45K before and it would heat it but took forever to bring the shop up to temp. The 80k does it much quicker. Love it.

I agree with all you say .. except maybe the door opening .. I find it does not make much different when opening and closing -- but, I'm not leaving it open for 1/2 hour. I can see how at 1200sf you would need above 40k on a cold day with a full cold shop.

What I always trying to get across to people: If you insulate and air seal when building --- the cost to maintain a decent temp if you have NG is lower than people think.

I have close to 1700 sf and it's being maintained with around 15k BTU -- In PA.

This would never work if i did not keep it in the 60's

 

[cadunkle]

Maybe consider a two stage heater? They have valves that will run at some fraction of BTU or full BTU depending on requirements. You just need a thermostat that can run a two stage heater.

For my 720 sq/ft with 10' ceiling (R13 walls, R22 ceiling, insulated 7'x16' door) I went with a Sterling 45k unit and overall am happy with it. After install I initially kept the garage at 50* and it would take about an hour to warm up to 70*. I eventually decided to keep it at 60* so I was comfortable just coming out and getting to work rather than having to preheat before it was tolerable. It doesn't seem to cycle significantly more at 60* vs 50*. The 45k unit works great keeping it at 60*, fairly quick warmup to 70*, wifi thermostat is nice when I remember to turn it on before going out.

I usually don't open my door much or for long though, but what ***** is in winter when I have to do work on my truck which doesn't fit inside so I have to leave the door open. It would be nice to have more output when I have to work with the door open for a couple hours. A 75k two stage unit may have worked out better keeping the garage at 50* for a fast warmup and lower output to maintain temp, but higher output for those occasions I need to leave the door at least partially open for a few hours.

In any event, if you're unsure or between sizes maybe consider a two stage heater depending on space and use. For me it may have been worth a few hundred more upfront cost, though I won't change it now.

 

[outlander800]

I just purchased a Reznor 45k for my 24x40x10. R-13 in the walls and R-19 in the ceiling and a 7x16 insulated garage door as well as 2 man doors. I went back and forth between the 45k model and the 60k but ultimately decided on the 45k even though my brain was telling me to go bigger. I plan to heat all winter around 55 and bump it to 60-62 while im out there. I think the 45k will do the job.

 

[TwoToque]

Interesting read for sure. I will not offer up any heavy wisdom without doing more of my own research outside of this site. I will mention a few things though. For those who want the heater to run continuously you must be rich. That would bankrupt a working man. A few members have clued in to modulating vs non modulating. A newer house uses the technology of a modulating gas valve and variable speed blower motor to try and create a uniform feel of heat. This is rarely installed in garages in my opinion. The parts of these modulating furnaces can be expensive to replace when breakdowns occur. A unit heater is common to have a two stage gas valve which is not the same as a modulating valve. Some more food for thought: radiant heat is more efficient than forced air as it heats objects first then the air. It is also much slower at raising the temperature as they are more for maintaining a constant temperature. This is true of in floor hydronic radiant heat as well as a radiant tube heater. Another thing to consider is what you are planning to do in the space. Woodworking produces fine dust that is highly flammable. Metal dust is also highly flammable if it is a fine enough dust (too hard to get it fine enough from normal grinding). Painting produces highly flammable vapours. Unit heaters usually have an open burner but are availble as a sealed unit for twice as much.