Another BTU calc question

[JasonF]

I've read more on this forum than I care to admit but like so many others I am looking for opinions on BTU size for a hanging furnace in my newly built barn. I already have talked to 3 different contractors and used just about every BTU calc I could find but the numbers are all over the place.

The contractors I have got estimates from each come back with different sizes at 75k, 100k and a 125k. The online calculators I've used have kicked out anywhere from 45k to 135k btu's so they weren't of much help.

My barn is 32x48x12 but with scissor trusses so the center height is 14.5'. Walls are insulated with r-19, ceiling r-30. I have two insulated 12x10 overhead doors and two double pane 3'x3' windows. Floor is uninsulated concrete.
I'm in central Michigan for a temp reference and would like to heat the garage to around 60f a few times a week.

From what I've read its better to be a bit undersized but I'm not really sure. Was leaning towards the 100k btu, what are some thoughts on this?
Jason

 

[DonPowers]

The calculation for heat loss is Q=U x A x ∆T
Where:
Q= BTU per Hour
U= 1/R
A= Area in sq ft
∆T= Temperature Difference between inside and outside

So if R=19 and the area of one wall = (32 x 12) and ∆T = 60° - 5°
Q=(1/19) x (32 x 12) x (60°-5°)
Q= 1,112 BTU per Hr

Add up all the area that is R-19 calculate that then do the same for R-30, windows and doors. Since the floor is un-insulated and you will not keep it heated all the time, consider the ground to be frozen (32°) and the R=1.

Finally add up all the Q’s

There a lot of other considerations to factor into a heat loss calculation but this will give you a ballpark idea of your heat loss. If you set it up on a spreadsheet you can model options.

I believe that the heat loss through your floor may be the reason for the variation in unit sizes.

All three should give you the 60° you are looking for, the smaller one will just run more, the bigger one will burn more fuel per hour of run time. Is there a significant cost difference between the three?

 

[Shop Specialties]

100k would be good.

 

[zmaxmotorsports]

Id go with the 100,000 also,what kind of system are you looking at?

 

[Radix2]

I think the sizing standards are geared toward a full time heating situation. So a Heater that can hold and maintain an area on really cold days will have quite a long warm up time if starting from scratch. So while a smaller heater is more efficient in maintaining temperature perhaps, if you are using it intermittently, you may do better to have one that can get up to temp faster so you don't need to pre heat as often...or get warm when you have that emergency need for the shop.

 

[JasonF]

Thanks for the responses.
For the estimates that I have received, two of the units were Reznor and the third was a Modine. Prices for the install was all pretty close at right around $2k. I ran NG out to the garage this summer so prices are for the heater, install of the heater/vent/ and connection to my gas supply.
I'm considering putting the unit in myself. I've seen a Modine and Beacon Morris 100k btu heaters online for $900-$1000 and install looks pretty straight forward. Just have to buy or make myself a manometer.

 

[JasonF]

The calculation for heat loss is Q=U x A x ∆T
Where:
Q= BTU per Hour
U= 1/R
A= Area in sq ft
∆T= Temperature Difference between inside and outside

So if R=19 and the area of one wall = (32 x 12) and ∆T = 60° - 5°
Q=(1/19) x (32 x 12) x (60°-5°)
Q= 1,112 BTU per Hr

Add up all the area that is R-19 calculate that then do the same for R-30, windows and doors. Since the floor is un-insulated and you will not keep it heated all the time, consider the ground to be frozen (32°) and the R=1.

Finally add up all the Q’s

There a lot of other considerations to factor into a heat loss calculation but this will give you a ballpark idea of your heat loss. If you set it up on a spreadsheet you can model options.

I believe that the heat loss through your floor may be the reason for the variation in unit sizes.

All three should give you the 60° you are looking for, the smaller one will just run more, the bigger one will burn more fuel per hour of run time. Is there a significant cost difference between the three?

Thanks for taking to time to post this, I will save it for future reference. I just plugged all my numbers in and I cant believe how much of a heat sink my uninsulated floor is. Looks like 100,000 btu would work just about right.

 

[brewchief]

I punched the details given into my software and it comes out to about 45k, that may not be perfect but it should be fairly close.

Since the heater size will be input btus and most garage heaters are going to be 80% efficient a 60k-75k unit would be ideal, ideal until you need to open large doors or heat it up from 20 degrees. In your situation I would go with the 100k unit.

 

[zmaxmotorsports]

For that big of an area Id probably go with a forced air system,versus a heater hanging in the corner from the ceiling.
It would be a lot easier to put heat where your losses are with ductwork and central return,maybe install everything in the attic to save space.

 

[DonPowers]

Your welcome Jason.

With all three units being in the same ballpark price wise, I would also go with the 100K unit. It would be nice if it had a High - Low output selector.

Everything in the building will be cold and will take a lot longer to warm up than the air. This will cause condensation to form on surfaces. I use butcher block wax on cast iron tool surfaces and cover with a sheet of plastic over the winter to keep rust from forming. The wax cleans up nicely with mineral spirits. Just something else to consider.

 

[bzinsky]

I think the sizing standards are geared toward a full time heating situation. So a Heater that can hold and maintain an area on really cold days will have quite a long warm up time if starting from scratch. So while a smaller heater is more efficient in maintaining temperature perhaps, if you are using it intermittently, you may do better to have one that can get up to temp faster so you don't need to pre heat as often...or get warm when you have that emergency need for the shop.

please listen to this post.

Size it as big as you can. Your main priority should be get the place as warm as you can as fast as you can. Having the ability to make the place comfortable in 10-15 minutes is a great thing, it completely negates the need to maintain any sort of heat while you're not in it.

I have one duct in my garage, just a bunch of vents in the duct, the return duct is about 4 feet long and goes straight up and open (just to keep it from sucking debri off the floor).

I have a timer switch and no-tstat. I turn it on 20 minutes before I get in, let it run for 40 minutes, just turn it back on for 20 minutes whenever it starts getting cold. Love it.


Also this is the setup I have in my garage, I have no condensation issues whatsoever.

 

[2slow]

Size it as big as you can. Your main priority should be get the place as warm as you can as fast as you can. Having the ability to make the place comfortable in 10-15 minutes is a great thing, it completely negates the need to maintain any sort of heat while you're not in it.

I have one duct in my garage, just a bunch of vents in the duct, the return duct is about 4 feet long and goes straight up and open (just to keep it from sucking debri off the floor).

I have a timer switch and no-tstat. I turn it on 20 minutes before I get in, let it run for 40 minutes, just turn it back on for 20 minutes whenever it starts getting cold. Love it.


Also this is the setup I have in my garage, I have no condensation issues whatsoever.

If you are going to keep it warm all the time, err on the small side of 60 to 75k. If it is going to be heat you only turn on when you are out there, err on the large side, 100k should be fine, but bigger will be OK.

I used a 125k in my 30x40 MI barn with insulated walls, and minimal (foam board) insulation in roof. My brother went with a 200K unit he got a good deal on via craigslist and his barn is smaller and better insulated, though it has 14' ceilings... Larger units will cycle more often once they reach temp, but they will reach temp more quickly.

If you are maintaining constant temp, large units lead to nuisance cycling. If you only want heat when you are out there, oversizing the unit will help you get up to reasonable temp in a short time period.

 

[JasonF]

Like others have said I only plan to work in the garage occasionally so I think I'll go with the 100k btu and have it heat up more rapidly. My last garage was the same size but with only a 10ft ceiling and I used an 80k btu home furnace which seemed to work just about right.

I have a question about venting now though. In looking through the Reznor install instructions it states that when horizontally venting that the vent must be a min of 3' below a soffit. Mine would come out approx 1' below the soffit. I couldn't find anywhere this was addressed so was wondering what the proper venting method is for this without going straight up through the ceiling? Can you take the B vent out horizontally to the edge of the eve and turn it up veritical until its above the max snow depth?

 

[Mr onetwo]

I would look at a 2 stage unit of around 100,000 gross output on 2nd stage.That way you can idle along on 1st stage most of the year and save fuel.Sunstar makes very nice units.

 

[2slow]

Two stage units are the best of both worlds, but cost more. For your purposes it may not be worth the expense.

Are your soffits vented? The 3' requirement is to prevent exhaust from being sucked into the building. I don't think it has to be 3' below to achieve this goal, you could extend the pipe so that it sticks out 4', but that might be kinda ugly.

I don't like the idea of turning with an elbow outside and going up as you will likely trap condensate at the elbow. Horizontal vents are often sloped down to allow condensate to drip out and adding an elbow would prevent that.

Can you make a turn in the building and go out a different wall that does not have a soffit?

 

[brewchief]

Like others have said I only plan to work in the garage occasionally so I think I'll go with the 100k btu and have it heat up more rapidly. My last garage was the same size but with only a 10ft ceiling and I used an 80k btu home furnace which seemed to work just about right.

I have a question about venting now though. In looking through the Reznor install instructions it states that when horizontally venting that the vent must be a min of 3' below a soffit. Mine would come out approx 1' below the soffit. I couldn't find anywhere this was addressed so was wondering what the proper venting method is for this without going straight up through the ceiling? Can you take the B vent out horizontally to the edge of the eve and turn it up veritical until its above the max snow depth?

In most cases you cannot vent horizontally with B-vent, you will have to use stainless steel to vent horizontal.

Why not go straight up?

 

[JasonF]

In most cases you cannot vent horizontally with B-vent, you will have to use stainless steel to vent horizontal.

Why not go straight up?

Not much of a reason other than I don't want to cut a hole in my metal roof, then deal with keeping it sealed up. Don't know why I didn't think of it earlier but I can run the vent just a little bit further and hit my gable wall so I should be ok.

 

[Shop Specialties]

Straight up and a Dektite rubber boot to keep it sealed up nice.

 

[This2ShallPass]

Sorry to ask another question, but what is a two stage unit please? is it heat only or also AC ? I sure am getting an education here!!

Thanks!

T2

 

[DonPowers]

High and low settings for heat.