need sizing help for Big Maxx

[nirvana996]

Great site....first time posting.

Just finishing building my garage/office.
The garage portion is 45X30X9 foot ceiling.
Fully insulated 2X6 walls and R-30 in ceiling.

Looking to install a Mr Heater Big Maxx running propane.
Great deal locally for a 80kBTU for $700 CDN.

It looks like I am about 1350 sq ft.
I could probably get away with a 50kBTU version...but thought I would go with 80K given that I am eastern Ontario and we can get some cold days.

Looking to keep garage in the 45-50F range and then turn up to 60 for working.

I am probably oversizing it a bit going with the 80k...is this the right way to go.
I have look at some past threads and it looks like it might be the right choice.

Also, what size propane tank (or how many) should I be looking at for a single fill for the season. I was thinking a couple of 100g/420lb tanks.

Any comments or suggestion would be appreciated.

Thanks
Andy

 

[Pruittx2]

Hi there,,, take a stroll thru this thread of mine,, lot's of info regarding your question.

I've found a ton of info, and will be installing the same unit. Burn rates,, vaporization chart info, and lot's of suggestions.

https://www.garagejournal.com/forum/showthread.php?t=405376

 

[JOE.G]

I have a 28x28x10 two story shop and I went with a 80K Mr.Big and kinda now know I should have gone with the 50 K.

 

[nirvana996]

thanks for the reply everyone.

 

[wesst]

jOE G, can you elaborate a bit on why you wish you went with a smaller unit? I went with an 80K unit as well, but have not installed it yet. I have an “L” shape garage with 900 square feet and two stories with 10 foot walls. Just curious to as if I went too big. I appreciate any insight you may offer.

 

[JOE.G]

I just feel it short cycles a bit, I don't think it is terrible but I would like it to run a bit longer so moisture has a chance to burn off and what not. It does heat the shop very quickly, I leave it on about 45 when not ok there and then turn it yup to 65 or 70 while I am out there and it has it up to temperature fairly quickly, But then after it is up to temp and drops enough to kick on thermostat it runs in my opinion to short of a time to get it back up two or three deg.

 

[wesst]

Thank you for the response. I too am concerned with that aspect, and am considering exchanging mine for the smaller unit.

 

[AJ44PCT]

Gee....I got the 125 BTU Mr. Heater and I have the same dimensions 30 x 45 but with 20 foot ceilings and same insulation. I am in upstate NY. I have yet to install the heater. Sounds like I may have went too big? AJ

 

[D45]

Doesn't take much heat to burn off moisture

Ever put a laser temp gun on the parts inside a heater? HOT!

 

[D45]

I sold my smaller Hot Dawg 45K and bought a larger heater............

I ended up getting another Hot Dawg, just in the 75,000 BTU version (HD 75AS-0111)

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

The added BTUs and the longer heat throw was perfect

I sold my 45K for $400 and bought a new 75K for $575 shipped......a good price I think, plus I got the initial 45K heater for free

66% increase in BTUs was perfect and the heater runs less and take less time to heat up the shop!!

 

[yeldogt]

My studio in PA is 1700 sf. Heat it with a 40k 80% sealed combustion fan forced cabinet heater -- it's very well insulated .... maintain low/mid 60's most of the time. That's 32k Btu's. The old setup was twin 100g tanks. The property has been rebuilt and I now have a buried 1k tank for this an other propane uses. You need to consult with your propane supplier after picking heater -- they can quickly give you the tank sizes. You will not get a single fill w/ twin 100's especially with an 80k running very much. Remember -- they can only be filled to 80% and the draw down drops with the temp.

A few hundred gallons for a cold winter is normal for me .. the key to comfort is insulation and sealing up the doors. Is the office open to the garage space? How are you heating the office?

My memory with D45's build is that it was not very well insulated/leaky. With poor insulation and leaks -- the heater needs to be providing many more BTU's. Lot's of heat is escaping as the space is warming. With a larger heater more heat is lost as the stack effect works against you as well. It's a double whammy -- but with NG being so cheap the cost is still low. The BTU's needed to heat the same SF can be much different depending on insulation and especially leaks -- wind.

I have also found that most people with a well insulated space tend to keep it warmer -- the cost savings being small with large setbacks. It can take a long time for a well stocked garage to heat when everything is 40 degrees -- the air is hot .. all the tools are cold.

Prior to installing the propane heater -- The 1700 sf space was heated with two electric utility heaters. Max was 4000watts -- so I was getting 28k BTU's if they were both on high .. this was also enough to bring the space to 70 and keep it there. Although with my electric at almost .20 ... that's why I got the propane heater.

Back a few years ago when I was not using the property very much I would occasionally drop down the temps ... flipping on one of the electric's along with the propane gave me a bit of a boost .. and the couple hours of run time not expensive.

They make units today that have two step gas valves -- obviously more expensive .. but better able to handle the various loads based on outside temp and setback. I would get a modulating cabinet heater today now that they are available vs the single available when I purchased.

 

[biggziff]

You can always de-rate a larger unit....you cannot up-rate a smaller unit.