Ng 45 or 75,000 heater?

[Tinsnip]

Hello I have run a natural gas line to my garage and I am having trouble deciding the size of the unit needed to operate efficiently. The garage is a cinder block 32x34 with 10 ft ceiling. Ceiling is insulated and drywalled. Inside walls are bare. I have two windows newer energy star and insulated entry door. The overhead doors are insulated as well. I live in norther Virginia.
I have done the btu calculations and 48,160 is what I came up with. The cinder block part is throwing me off. I would be in the garage 5 days a week for 8 hours a day. I would like to heat it when not in use to prevent things from freezing etc.
Looking at bigg maxx 50 and 80,000 heaters as I see they no longer offer the other models. Hot dawg has a 60,000 heater to split the difference as well. I'm not concerned with price of the unit. Just don't wanna go overkill and alternatively be under gunned. I'm torn like a old sweater. Any help or opinion you may have in this matter would be greatly appreciated

 

[regguy1]

I think the 60K would be about right. I have a 45K in my 26' x 30' wood frame, R13 in walls a ceiling.

See everything here: http://www.garagejournal.com/forum/showthread.php?t=140674

If you're not price sensitive consider a Reznor: http://www.djsonline.com/reznor_UDAP_Unit_Heaters.htm

 

[Steevo]

If those cinder block walls are un-insulated, you may want to go with the larger (75/80k) unit just to be sure.
In a fully insulated space that size, a 45/50k unit would be plenty, even in cold climates.
I have a Mr Heater 75k in my 24x40x12 shop, and it is more than enough. I am sure now that I could have used the 45k, but the building has R21 walls and R60 ceiling insulation, insulated doors, etc.

 

[Angelfire]

If you're after 48k btu to heat the space, you'll need a unit with output close to that. A 45k btu unit (they are all named per their Input value, not Output value) will only get you ~36K btu output based on efficiencies of 80% (which most unit heaters are, or close to). Altitude above 2k will impact that as well although in VA you may not need to consider this. A 60K unit sounds like it's just what you need.

 

[Vet65te]

Got a couple questions to ask. I sorta understand that there is a 'loss' of efficiency due to altitude, can someone tell me what the percentage is? I got a reply from another Garage Journal member who said that a 75K unit would really only kick out around 60K BTU's. He also mentioned something about cutting back on the NG or propane flow since it can be hard on the unit. I intend on building a house/garage in Prescott AZ, which is around 5400 foot elevation. Secondly, could I get a clarification on that last comment about ratings based on input value and not output?
Thanks
Mike T.

 

[Angelfire]

I'll try. Heaters are typically rated at Input value. ie. If the unit were 100% efficient, it would consume say 75K BTU in input potential and output 75K BTU's. Most of these unit heaters are about 80% eff. So in the case of a 75K BTU heater, you would effectively get 75K x 0.8 = 60K BTU's actually heating the space.

Altitude...I'm in ABQ which is similar to your altitude. I've heard varying factors but in general what I've seen is for every 1000 feet above 2000', factor in an additional loss of 4%. So for 5400' you'd be at something like this: 5400 - 2000 = 3400. 3400/1000 = 3.4 3.4x4% = 13.6% Subtract that from the units efficiency and that will give you your overall efficiency. So for an 80% heater, you'd want to calculate your numbers using an efficiency of 66.4%. Using the 75K example above (input!), you could expect to see 75k x 66.4% = 49.8K BTU output. Make sense or did I just cause more confusion!?

 

[Vet65te]

That does make sense, thanks for the input. One other thing to ask though, along with the unit's 'rated BTU's', do their specs also include the actual efficiency percentage or is it a given that the majority of these heaters operate around 80% efficiency?
Mike T.

 

[Angelfire]

They should. If not, call the manuf. and they should be able to tell you. I found most of mine online using the tech sheets they have available.

 

[nwav8tor]

You already installed the gas line BEFORE you decided on the size of the unit heater you'll install???

Hopefully you used a large enough line to supply a 75K heater, but if not, your heater size might be limited by the size of the supply line.

Paul

 

[rponfick]

Was your question on altitude relating to operating efficiency, or the fact that most units call for adjustment for altitude due to less oxygen in the air?

My Big Max 80K unit specs. calls for a lowering of operating gas pressure above 2000 ft. I think this may be due to the unit operating hotter at altitude than designed at sea level and burning out the heat chambers quicker than normal. Don't know if there is any danger involved with not adjusting the operating gas pressure downward.

Ralph.

 

[Steevo]

Was your question on altitude relating to operating efficiency, or the fact that most units call for adjustment for altitude due to less oxygen in the air?

My Big Max 80K unit specs. calls for a lowering of operating gas pressure above 2000 ft. I think this may be due to the unit operating hotter at altitude than designed at sea level and burning out the heat chambers quicker than normal. Don't know if there is any danger involved with not adjusting the operating gas pressure downward.

Ralph.

I have been wondering about this myself.
My unit is installed at over 5000' elevation.
I never adjusted the pressure down to compensate, because I didn't read that part of the manual until after it had already been running for a year with no problems. I am now wondering if I should get a gauge that can measure the pressure, and set it down as per the manual.

 

[rponfick]

Steevo, I am concerned also. I just ran my new Big Maxx 80K NG unit, since it is pretty cold out. The first bend in the heat exchanger tubes were glowing a dull red. Is that normal on your's?

I plan to call our utility co. Monday and see what they say. I heard somewhere that the utility my dilute the BTU's they supply at altitude to compensate for these issues.

Ralph

 

[Steevo]

Steevo, I am concerned also. I just ran my new Big Maxx 80K NG unit, since it is pretty cold out. The first bend in the heat exchanger tubes were glowing a dull red. Is that normal on your's?

I plan to call our utility co. Monday and see what they say. I heard somewhere that the utility my dilute the BTU's they supply at altitude to compensate for these issues.

Ralph

I've never noticed the tubes when running, but will check when I get back home in a couple of weeks. I don't want to send my wife out there to stand on a ladder in the dark watching to see if they glow

 

[nwav8tor]

My 75K unit does the same thing (only been in operation for 2 days).

Paul