Picking btu

[Bowman85merc]

Hey there,

I've read all I could trying to find my answer, trying to avoid being "that guy" that asks a questions that's been asked a million times, but there seems to be many variables to this, and that's where my setup differs a bit from all the others I have read thus far. My garage is not built yet, but I am trying to buy all I can while everything is on sale right now.

So my situation,

Location: Central Iowa, can get very cold (low teens) for several weeks during the winter.

Garage: 18x28 with 12' walls, one 12x10 overhead door (R-6.5) and a smaller 9x7 overhead door (re-using old non-insulated door from old garage for now) on the back wall, then x6 36"x36" windows, and one man door.

Insulation: 2x6 walls=R-19, Ceiling=R-30

Ideal temp: 50-55* when not in use, 60-65* when being used. Will be used a few hours a day or two during the week, but mostly on the weekends for several hours.


My dilemma is between the Mr.heater Big Maxx 50k and 80k. Northern Tool has both on sale right now. The 50k is $350 and the 80k is only $377 right now, so not much of a price difference between the two.

Any help is appreciated!

 

[The Cobbler]

30 k works great in my 17x24 x 10 with a 10x7 overhead, 2- 32" man doors with 24x36 thermo glass , and 72x36 slider window. r-20 in walls and I forget but about r45 in ceiling
you probably want to go up a size. but the proper way is to do a heat loss calc & size from that

 

[Jim greengo]

I'd go with the 50k

 

[pbon]

Not more than the 50k. Short cycling is not good for the unit. You could buy another if it fails but why go that route. My big maxx 50 is more than enough for my insulated 750SF with some 18’ ceiling, some 13’ ceiling, some 9.5’ ceiling, and some 8’ ceiling. I heat to 55-60 full time and 65 when working. Ambient outside can range from 35 to -20F.

 

[Bowman85merc]

Thank you for posting the link to that site!
This is what it calculated for me, not exactly sure how to read all of what it says just yet, but wanted to post my results.

 

[Bowman85merc]

Not more than the 50k. Short cycling is not good for the unit. You could buy another if it fails but why go that route. My big maxx 50 is more than enough for my insulated 750SF with some 18’ ceiling, some 13’ ceiling, some 9.5’ ceiling, and some 8’ ceiling. I heat to 55-60 full time and 65 when working. Ambient outside can range from 35 to -20F.

That's what I've been reading/thinking, regarding to the short cycling with a oversized unit. But I wanted to make sure with the height of my space, number of doors/windows, etc. Also with how close the price is between the 50k vs 80k.

So plus side, if I get the 50k I can put that extra $28 towards thet vent kit they also sell ($170 @ Northern, $100 @ Menards & Fleet Farm)

 

[D45]

Make sure it's power vented

I think a 50k would be at the high end, it will get warm fast

Do they sell a smaller heater that will save you some money, like a 45k or 30k-ish?

I had a 45k on my 1,000sq foot shop

You seem pretty well insulated too, 80k is way too big

 

[Ghost Rider]

Heating well-insulated 720 sqft, 10 ft ceilings, with a 45K Sterling on propane in mid Ohio. The 30K unit would have been more than enough.

 

[HoosierBuddy]

I like being a bit oversized on these as in a garage, we often use large setbacks when we aren't using them (say 20 degrees) and want enough extra capacity to heat the space up quickly (say within 30 minutes or so) even on a cold day. When you look at heat loss calculators, normally they don't account for that scenario.

Maybe some do? It would be the heat loss per hour PLUS the amount of BTU's required to heat all of the air in the garage from 50 to 70 times two (to account for getting it done in 1/2 hour).

Phil

 

[HoosierBuddy]

I like being a bit oversized on these as in a garage, we often use large setbacks when we aren't using them (say 20 degrees) and want enough extra capacity to heat the space up quickly (say within 30 minutes or so) even on a cold day. When you look at heat loss calculators, normally they don't account for that scenario.

Maybe some do? It would be the heat loss per hour PLUS the amount of BTU's required to heat all of the air in the garage from 50 to 70 times two (to account for getting it done in 1/2 hour).

Phil

OK...since I got curious I'll answer my own question.

OP's garage holds 6048 cubic feet of air.
To heat one cubic foot of air 1 degree it takes .018 BTU's.

So 20 degrees X 6048 cf X .018 = 2177 BTUs.

To raise 20 degrees in 1/2 hour you'd need an extra 4355 BTU/Hour capacity above the heat loss amount. You'd actually have to gross up that number for efficiency losses to be exact, but I can't find that number for the Big Maxx heaters. Assume 90% takes you to a bit under 5000 BTU/Hour extra needed.

Phil

 

[red94chev]

All I know is that i just put the 80k Big Maxx in my garage and it seems just about perfect.

My garage is 32x42 with 9 foot ceilings and insulated but not quite as well as yours.

Also, I got my vent pipe from build.com, they have great pricing on it. I went with Duravent brand and have no regrets there.

 

[ripperd]

50k will provide more efficient heat, with less heat blast and longer cycle times. But, with slower temp rise when you bump it up.

Those are the pluses and minuses. I bet either would be able to maintain your desired temperature, even on the coldest days. Although I would insulate that one uninsulated garage door!

Also, is this attached or detached? Attached means at least one entire wall is actually already warming the area instead of cooling it.

 

[cadunkle]

Heating well-insulated 720 sqft, 10 ft ceilings, with a 45K Sterling on propane in mid Ohio. The 30K unit would have been more than enough.

Same 720 sqft and 10' ceiling, 45k Sterling but on natural gas. I don't know that I'd call mine well insulated. R-13 walls and R-22 ceiling, insulated man and overhead door (7'x16'). Good bottom seal, drafty around the rest of the door.

The 45k is adequate. I keep it at 50*, it takes over an hour to get to 70*. Not a big deal with a wifi thermostat I can turn up ahead of time, even a 10-15 minute head start makes it tolerable. Temps inside drop fast when the rollup door is open. I'm happy with the 45k, though I think the 60k unit would have been nice for a quicker recovery time. Which you choose probably depends more on how you use the space. Always at a moderate temperate and bump it up when it use vs heat off when unused and on only when occupied, and how often you open doors.

Either way, the 45k is appreciably more comfortable than the old 115k furnace I had in there. Sure it would heat up real quick, but then a bunch of short cycles before warm air filled the space and even then the relatively short cycles had a noticeable cold/hot swing. Too big is a lot better than no heat, but not nearly as comfortable as something in the ballpark for the building and typical use.

 

[gmcgeo]

you need 41,808 btu output to heat at a normal insulation on your garage. that is if you only want it to 65 deg and would be down to -5 deg out side. your input would be 50,000 btu on the heater but what is it actually producing on the output? modine heaters for instants, say you buy an hd45 input is 45,000 but output is 36,900 so when i size the garage i want my output to handle what the load is calling for.

by what you have wrote i would say a 50k would be ok since you have a decent insulated garage. big max heaters i am not familiar with so i do not know the output on a 50k or what the max heat throw is on it

 

[PWC Repair]

another vote for the 50k. It will be enough.

 

[Bowman85merc]

You all did some pretty good convincing, I picked up the 50k Big Maxx over the Black Friday weekend, it was $150 off and they threw in a $50 gift card for ordering online, couldn't pass that up! From what I've gathered it is about 81%-85% efficient, I believe I saw those two numbers somewhere regarding this heater.

Now the only thing holding me back is my non-existent garage!

It is a detached garage, and the slab is being poured on Friday and there's a big cold front moving in on Monday. Looking like winter is now fully upon us. Hoping the weather gets "nice" around the time Menards has their next 11% off sale, since that's where i'm buying the garage "kit" from.


Thanks for the input everyone, now lets see if I can get it installed by the END of winter.

 

[jvitez]

Good choice!

But you need more insulation. R30 ceiling is pretty light for Iowa winters. I'd go R40 at least, R50 would be better. And since it's not built yet, you should look into adding 2" foam board insulation on the outside of the wall sheathing, then whatever wall finish you're going to use. Styrofoam is expensive (extruded polystryrene or XPS). Explanded polystryrene (EPS) is much cheaper. BASF Halo Exterra is graphite impregnated EPS and R5 per inch, same as XPS and cheaper.

https://buildwithhalo.com/product-p...ard-insulation-that-does-it-all-halo-exterra/