Unit Heater size

[dasto]

OK, I did a search and read a lot of posts. Still confused on size. From what I have read, you can oversize. I am thinking about a 75 BTU or 100 BTU. New post construction with steel interior walls and ceiling. R19 in walls and R30 in ceiling. Building is 28x48 with 12' ceiling. Location is central Indiana. Will keep the building around 45 degrees and turn up when I occassional use.

What size should I buy?

 

[jvitez]

7 watt/sq.ft will keep a well insulated building heated to an even temp, 10 watts/sq ft is average, 15 watts/sq ft if you need quick recovery. 1 watt=3.412 btu.

NG heaters are more efficient if they run longer, so oversizing is much more important for combustion heaters than electric. NG/propane unit heaters are generally 80% efficient. So.....

28x48=1344 sq ft. 1344 x 10 watt/sq ft= 13,440 watts x 3.412= 45857 btu needed. At 80% efficiency, 45,857/.80=57,321 btu.

You'll need a bit more if you want fast recovery after opening doors, and for bumping up the temp. 75K btu should be fine. 100 btu would be too big.

But, check out on line heat calculators for a more definitive answer. I'd rather undersize a NG heater and supplement for heat recovery with a second heater of whatever type (propane bullet, electric, kerosene, etc) than have an oversized permanent heater.

 

[teleman37]

If my math is correct, for my size of building a 75K heater should work. I used the 10 watt average in my calculations. I have a 36'x48' with 10ft. ceilings. R19 in the walls, R48 in the ceiling. I have two 10'x9' insulated garage doors, one walk in door, and two 45"x64" double pane windows. Is my math correct??

Sorry, I didn't mean to hijack your thread dasto.....I'm just in the same boat as you with trying to figure out what size of heater to buy.

Kevin

 

[dasto]

Jvitez: Thank you for the very useful information. I need the help as I am not experienced in sizing the unit. After reading your post I started researching 75 Btu units. The technical details state "up to 950 sq ft with 7-8 ft ceiling height." My ceiling height is 12'3" and as you computed I have 1344 sq ft of space. Now what do I do?

 

[Milton Shaw]

You also need some ceiling fans to get the air down from the 12 foot high heat storage loft. Set a couple of fans blowing down and keep the air circulated will make the garage a lot more comfortable and cut the heat bill also. The higher the ceiling the more you need fans.

 

[BadgerBoilerMN]

A proper heat load calculation is needed, this information is not even close.

Fans do not add to comfort while in heating mode. They "may" add to overall system efficiency but always at the detriment of comfort.

There is no useful rule-of-thumb for sizing heating and cooling systems.

 

[dasto]

A proper heat load calculation is needed, this information is not even close.

Fans do not add to comfort while in heating mode. They "may" add to overall system efficiency but always at the detriment of comfort.

There is no useful rule-of-thumb for sizing heating and cooling systems.

Thank you. I am not experienced in these matters and had no clue on what size heater to buy. Is there any other information that I could provide that would be helpful to assist with a recommendation from someone more knowledgeable than myself?

 

[jvitez]

A proper heat load calculation is needed, this information is not even close.....There is no useful rule-of-thumb for sizing heating and cooling systems.

A clear and harsh reply, spoken like a purist HVAC tech or engineer.....

Yes, a proper Manual J heat loss calc is always the correct answer. How many HVAC contractors will do one unless they get the whole job? How many will charge just for the calc? I assumed the OP was planning on doing his own installation by his question.

To make everyone happy, here, download this and pay $49 to get a precise answer:

http://www.hvaccomputer.com/

Precise, based on the parameters you input. How much fudge factor is reasonable? Usually 10%? What if the OP wants fast recovery after opening doors, etc? As anything, it can get very detailed if you want it to.

Yet, the accumulated wisdom of a ton of guys on here should be worth something. At least in my non HVAC professional opinion.

BTW, this wouldn't be your website, would it?

http://www.badgerboilerservice.com/

If so, you're an HVAC contractor. We always appreciate having professional trades people on GJ. They frequently give much helpful technical advice. So would you be willing to a Manual J calc for the OP and fellow GJ member?

 

[dasto]

A clear and harsh reply, spoken like a purist HVAC tech or engineer.....

Yes, a proper Manual J heat loss calc is always the correct answer. How many HVAC contractors will do one unless they get the whole job? How many will charge just for the calc? I assumed the OP was planning on doing his own installation by his question.

To make everyone happy, here, download this and pay $49 to get a precise answer:

http://www.hvaccomputer.com/

Precise, based on the parameters you input. How much fudge factor is reasonable? Usually 10%? What if the OP wants fast recovery after opening doors, etc? As anything, it can get very detailed if you want it to.

Yet, the accumulated wisdom of a ton of guys on here should be worth something. At least in my non HVAC professional opinion.

BTW, this wouldn't be your website, would it?

http://www.badgerboilerservice.com/

If so, you're an HVAC contractor. We always appreciate having professional trades people on GJ. They frequently give much helpful technical advice. So would you be willing to a Manual J calc for the OP and fellow GJ member?

You are correct in your assumptions. I'm just a guy that wanted some help. The gas line and shut off valve have already been run to the new building by a professional. I just thought that I was capable of hanging a unit from the ceiling with threaded rod.

 

[thooks]

If gas is available, I would go radiant tube heaters.

 

[dasto]

If gas is available, I would go radiant tube heaters.

Natural gas is now to the new building. I will be in the building during winter only occasionally. I read some posts about radiant tube heaters on this site. Based upon needing to quickly bring temps up from 40 degrees to 70 degrees, I understood that unit gas heaters were recommended over radiant tube. Is this incorrect?

 

[thooks]

I would mix some tubes and spot radiant heaters.

Superior Radiant Products has a great reflector on their tubes, where 99%+ of the rays are directed towards the space. Not all tube heaters are created equal.

It doesn't take as long to heat the space up as you think with the radiant tubes and spot heaters.