Heater Sizing

[Spudland_Dave]

So, what I was thinking & calculating via various posts here and over the interwebs, plus googling, I was thinking for my 30x50x10 shop that I would need a Hot Dawg or Reznor 75k unit....My HVAC guy and I just spoke and he said after doing a heat loss calculation and such, he comes up with a 60k unit as being the right size.
You guys think he's downsizing the heater to be competitive for cost?

 

[CNGsaves]

All depends on how much insulation you have, and heat loss points (ie windows, garage doors, man door, etc).

IMHO the 60K Btu sounds too small for 30'x50' even with good insulation.

I'd keep shoping in the 75K Btu to 80K Btu range if I was in Maine with a shop your size. Good luck.

 

[Mike007]

There typically isn't a huge cost difference in small differences in capacity, so I doubt he's trying to save money that way.

How does Hot Dawg and Reznor list there models, by input or output? If it's input, 75K is 60K Btu net.

 

[Highbeam]

I think this depends on use. Are you trying to walk into a cold shop and get the temps from 40-70 in short time or are you trying to maintain 70 all the time?

Only use the 60k if you plan to maintain temps. Otherwise, the HP to quickly heat the shop will be money well spent.

 

[Spudland_Dave]

All depends on how much insulation you have, and heat loss points (ie windows, garage doors, man door, etc).

IMHO the 60K Btu sounds too small for 30'x50' even with good insulation.

I'd keep shoping in the 75K Btu to 80K Btu range if I was in Maine with a shop your size. Good luck.

Sounded too small to me too..hence the question here

He took the following info when he came over for a quote...30x50x10, R38 in Ceiling, R19 in walls, 3 insulated 8x10 overheard doors, 2 32"x48" windows and a single 36" 9-Lite Entry door.

There typically isn't a huge cost difference in small differences in capacity, so I doubt he's trying to save money that way.

How does Hot Dawg and Reznor list there models, by input or output? If it's input, 75K is 60K Btu net.

Couldn't tell you the input/output thing.

I think this depends on use. Are you trying to walk into a cold shop and get the temps from 40-70 in short time or are you trying to maintain 70 all the time?

Only use the 60k if you plan to maintain temps. Otherwise, the HP to quickly heat the shop will be money well spent.

Intended use...I'd like to keep the shop above freezing at all times so I don't need to haul all my chemicals and other freezables in the house. My dad says all I should need to keep it 35 degrees minimum would be a candle and then yes on a night or weekend I want to go tinker in there, crank up the T-Stat and take my jacket off.

 

[Highbeam]

The system properly sized to maintain 35 degrees will take forever to raise temps from 35 to 70. The system sized to quickly heat from 35-70 will short cycle when you're just trying to hold 35.

That's it, you need two heaters!

 

[Spudland_Dave]

That's it, you need two heaters!

ROFL! If we were talking ratchets I'd agree with you 100%

But seriously do you think that had anything to do with the professional sizing?
I'm of the notion that the heater shouldn't run much if at all ... HVAC Guy said he has a similar garage, smaller footprint but 12' ceilings. Close to same air volume with same insulation values and his never got below 40 last year with NO heat installed at the time.

But hearing what you said about the short cycling, do they make wide range T-Stats? In other words, kicks off heat at 35 degrees, brings the garage to 55 and calls it quits?

 

[walrus]

in most case you want the smallest furnace that can handle the load to avoid short cycling.

Another factor would be how much are you opening doors, bringing cold vehicles in and out, that might bump you up. The county is hardly a warm place in the winter

 

[Shop Specialties]

You will be just fine with 60k. If you were out there working 10 hours a day and the doors were going up and down 100x then I would say bump it up to 75k.

 

[CNGsaves]

Yes, they have low temp thermostat that will handle the settings you want to maintain garage.

What kind of quote was HVAC guy giving for the Hot Dawg 60K Btu??

Since you still are "off season" right now, I'd think that vendors would be willing to work you a deal.

When I was researching hanging garage heaters last year, best price I found was QC Supply in Schuyler, NE. They have FREE Shipping.

Their website says $668 for a 60K Btu NG. . and same price for 75K Btu.
http://www.qcsupply.com/modine-hot-...m_medium=cpc&gclid=COf5h42_nLkCFU1k7Aod4UsAMQ

If you find a better price, they will MATCH it . . .and still give free shipping.

 

[Spudland_Dave]

in most case you want the smallest furnace that can handle the load to avoid short cycling.

Another factor would be how much are you opening doors, bringing cold vehicles in and out, that might bump you up. The county is hardly a warm place in the winter

Not in the county anymore...Born & Raised up there, but now "Down South". Actually, there have been years where we have more snow and cold then up there.

You will be just fine with 60k. If you were out there working 10 hours a day and the doors were going up and down 100x then I would say bump it up to 75k.

So even at the minimal (less then 50 bucks it seems) cost of 60k vs 75k you would go with a 60k?

Yes, they have low temp thermostat that will handle the settings you want to maintain garage.

What kind of quote was HVAC guy giving for the Hot Dawg 60K Btu??

Since you still are "off season" right now, I'd think that vendors would be willing to work you a deal.

When I was researching hanging garage heaters last year, best price I found was QC Supply in Schuyler, NE. They have FREE Shipping.

Their website says $668 for a 60K Btu NG. . and same price for 75K Btu.
http://www.qcsupply.com/modine-hot-...m_medium=cpc&gclid=COf5h42_nLkCFU1k7Aod4UsAMQ

If you find a better price, they will MATCH it . . .and still give free shipping.

Quote was $2,600 installed...which I about dropped my cell phone when I heard it. Not saying he was high, he does my furnace cleanings and does great work and always has been priced fair, just thinking that the unit itself is 800ish bucks and the only other thing materials wise was a bit of plumbing...For 2600 he'd come hang it, plumb it (fuel & exhaust) and then its up to me to deal with the oil company to come install the tank and fire it up. Pretty sure I will buy one online and DIY the install...I think I prefer the Reznor, just debating if I should go with the separated combustion model vs normal air combustion model.

 

[Highbeam]

But hearing what you said about the short cycling, do they make wide range T-Stats? In other words, kicks off heat at 35 degrees, brings the garage to 55 and calls it quits?

There's a name for that range of temperature between off and on, I think they call it the "swing". You do not want a stat trying to hold an exact temp to the degree because it will cycle too much. Some stats allow for an adjustable swing but I don't know that they allow for 20 degrees of swing.

5-10 degrees would be more logical to prevent short cycles. Another way to set the swing is to time how long the heater runs during the cycle. You want a good 30 minutes of run time to heat it up well enough to burn condensation out of the heater and the flue.

 

[bd8134]

What temp do you think you will be working in the garage at and what will you allow the temp to drop too?
This will help decide on the heater size.
If the heater is too large, it has the same effect as a heater or ac unit unit for a house, noticeable temp swings and sometimes larger energy consumption or maintenance. A unit that is too large can cause condensation in the tubes and frequent heater cycling is not good. Too small and it takes a long time to heat up.
I have a 34 x 44 with pitched ceiling, highest 16', maybe 14' average, with a 45k heater which is too small but not too bad. Minimum at 50F, when working I set it at 72F. In the fall and spring it will get to a nice even temp in about 30 mins, I just turn it on a couple of mins before I go out there. In the winter at 0F outside, the heater takes a long time to get the garage warm, sometimes 90 mins or longer, I just wear warmer overalls for a while. However one advantage of having a smaller heater is that it makes the garage a pretty constant temp throughout. The min 50F heater setting melts any snow that is on the cars and by morning they are dry.

 

[Spudland_Dave]

What temp do you think you will be working in the garage at and what will you allow the temp to drop too?
This will help decide on the heater size.

I'd like to work in there at 65ish to 70...T-shirt & Jeans fall type weather we'll say, I'm NEVER cold, always hot.
I'd probably set the T-Stat to maintain it at 40 degrees all the time.

 

[Shop Specialties]

If the price is that close go with the 75k.

I would set the thermostat higher than 40* if you are going to kick it up to 65*-70 when you are out there. I would say at least 50*, when it is all closed up it should maintain temp good and you will not have the long run time waiting for everything to heat up.

 

[Spudland_Dave]

Thanks for all the input...so I've got another question... HVAC Guy said 60k would do it, we all agree 75k unit for the less then 50 buck difference in price.

What would happen if I could get a 45k unit for free? I'm not planning on running out and buying a 45k unit, but I might have the opportunity to get a 45k unit as trade on some stuff I have.

By what would happen I mean I know its small, and it would take some time to heat the place quickly, but price would be right... I guess the question is I know the downfall to oversizing an LP/NG unit is condensation in the vent and rusting issues, what would be a mechanical downfall to undersizing?

 

[pseudorealityx]

75%* of the time, it would work fine.

25%* of the time, your building would just slowly cool down, as the heat loss through walls/roof/floor is greater than the heat gain from your heater. It will also run constantly vs. intermittently, but that's not important.


* - those percentages are just guesses. Depends on temperature, time of day, wind speed, sun orientation, etc etc etc. It's just to give you an idea.

 

[Highbeam]

the question is I know the downfall to oversizing an LP/NG unit is condensation in the vent and rusting issues, what would be a mechanical downfall to undersizing?

No downside other than you'll have to listen to it run more often and the slow temp rise or possible inability to raise the temperature. You can't work it too hard.

 

[CNGsaves]

Free is always good !!

When you are not in shop and only job that 45K Btu has to do is maintain 50 degrees it will be fine. I'd say go for it but put in exhaust flues that are sized to work for 75K in case you get tired of the 45K unit after going through a winter season.

Could always supplement the 45K Btu hanging LPG heater with a portable Mr Heater heater running on LPG propane 20 lb bottle (these are 3 speed and put out up to 18K Btu). Ten minutes before you want to work out there, just fire up the Mr Heater and you'd be good to go. This would also let you put the heat in portion of shop where you need warmth.

+1 to go with free 45K heater, especially if you can install yourself as sounds like your HVAC quote was trying to **** and maim you with $2600 bill !!

 

[James-W]

I have a 24 X 36 garage with an 8 foot ceiling. There is R30 insulation in the ceiling and R19 in the walls. The 16 X 7 overhead door is insulated with no windows in it. I have one energy efficient window and one insulated entry door. I had a Hot Dawg 60,000 BTU natural gas hanging heater installed shortly after I built the garage by the same company that installed our new furnace and central air.

During the Winter months I keep the temperature at 50 degrees when I am not working out there, and when I have a project to do I crank it up to 72 degrees. Depending on the outside temperature, it takes somewhere between 10 - 20 minutes to raise the temperature up to 72 degrees. Like I said, the outside temperature, as well as the wind-chill factor, makes a difference in how long it takes to bring the inside temperature up to 72 degrees.

I would not go lower than 50 degrees during the times you are not out there working. My thinking is that in a well insulated building the fuel cost difference to maintain the temperature at 50 degrees versus 35 degrees would be minor. But the time it would take to raise the temperature would be a lot less if you maintained it at 50 degrees rather than 35 degrees. Those are my personal thoughts for what it's worth.