Garage Unit Heaters and Temperature Rise

[ChipperG]

Need some help trying to figure out a shop unit heater - separated combustion. My sq footage and ceiling height calcs BTUs at 60ish per foot. I'm in Montana and with 3 bay doors I lean on the higher side although shop is well insulated. The issue I'm having is with rise. The rise on popular units (Modine, Reznor etc) is around 40-45. If I understand correctly, this is the degrees the unit can raise input air to combusted air. If I'm working with outside air that is 10 below zero and the unit has a rise of 40.....how do I ever heat the bldg above freezing?

-10 degrees plus 40 degrees of rise is 30 degrees. What the heck am I missing??

 

[The Cobbler]

rate of rise is how much the air is heated as it passes the heat exchanger . you're not bringing in outside air to be heated, you're bringing in outside air to be used for combustion

 

[PoorUB]

That is temp rise across the heat exchanger, nothing to do with raising the temp in the building. So if you have 50 degree shop air the discharge air from the heater would be 40 degrees warmer, or 90 degrees.

What are you building and what calculators did you use to come up with 60 BTU per square foot?? We never do more than 30 BTU per square foot here in North Dakota, unless there is little or no insulation!

We did a 10,000 square foo shop with 25 foot scissor trusses at 25 BTU per square foot. Another building, a truck shop, was right around 30 BTU per square foot.

Assuming normal building pratises and reason able insulation 30 BTU will be more than enough.

 

[cruzer75]

as the air circulates through the heater it will rase the temperature - start with a cold building 10degres - first pass will be ** higher, then when it goes over the heat exchanger again it will go up more.... i would take 30 or over -10

 

[ChipperG]

Thanks for the replies very much appreciated -- typo on BTUs should've been a 4 ....40ish per foot. Bldg is only 32x40 with 20 ft lid and 3 bay doors - I'll be working with epoxies and laquers which require consistent heat so I'm overestimating on BTUs.

Making more sense now -- as you can tell I'm not an HVAC guy and just trying to make sure I understand specs. If I understand you guys correctly, the combustion air is not part of the heated air created across the exchanger? This makes much more sense.

 

[PoorUB]

Thanks for the replies very much appreciated -- typo on BTUs should've been a 4 ....40ish per foot. Bldg is only 32x40 with 20 ft lid and 3 bay doors - I'll be working with epoxies and laquers which require consistent heat so I'm overestimating on BTUs.

Making more sense now -- as you can tell I'm not an HVAC guy and just trying to make sure I understand specs. If I understand you guys correctly, the combustion air is not part of the heated air created across the exchanger? This makes much more sense.

Well, I have dabbled in HVAC for the last 20 plus years!

Seriously, if the building is built decent 30 BTU per sqft will be plenty, more than plenty.

 

[ChipperG]

PoorUB
Building is constructed very well - 2 inches of core-bond with bat insulation on top. inside 2x6 walls and 2x10 truss top chord. I appreciate your input on BTU sizing. thanks!

 

[The Cobbler]

.........If I understand you guys correctly, the combustion air is not part of the heated air created across the exchanger? This makes much more sense.

correct. in a sealed combustion furnace the combustion air is drawn in from outside and exhausted back outside . the air inside your shop that you're heating is passing over the heat exchanger and being heated . basically each time it passes over the heat exchanger it gets hotter until the desired temperature is reached .