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HVAC question

wbrian63

Well-known member
Joined
Mar 31, 2010
Messages
843
Location
Houston, TX
I need to add HVAC to my shop.

I'm not looking to make the space as comfortable as my home. 60-65f in the winter is plenty warm, and 85-90f in the summer would be acceptable.

The conditioned space is 24x50 with a 16x25 loft at the back that's enclosed. I don't need to condition the loft area all the time - it's mainly for storage at this point. When I do need to be up there, a space heater will handle the heating requirement, and I've got a roll-around A/C unit that will work for cooling it.

The ceiling is insulated with about 9-1/2" of blown fiberglass and sheetrock.
The walls are OSB built inside and away from the exterior of the building about 6", but there is no insulation.

The space is on the northeast side of the building, but most of it is "middle space" (tenant to the south, west, and a "short" stall to the east).

I have enough electrical capacity to run electric heat, but I'd rather use gas, as the unit costs are generally lower, and I'd like to use this unit when I move out in 2014.

The problem with gas is that there isn't a city gas meter on the property, so I'd be forced to use propane.

I've been told that 4 tons of A/C should be sufficient, but I'm not certain about the BTU's for the furnace.

Regarding the BTU's - once I have that #, I need to know gas consumption to figure out how big a propane tank I'd need to feed the unit, as well as how much gas it will consume, which of course drives the cost of operation.

Truth be told, we don't have much of a winter down here, and the shop is only used briefly during the week, and frequently on weekends.

I would probably like to keep the space around 50f when not occupied, just to make it easier to warm it up when I want to use it.

So - the questions:

How to calculate BTU requirements for the heater?
Is there a difference between electric resistance heat and gas in terms of the size of furnace required?
How do I figure out how much gas is required to fuel a given furnace size? I assume it's going to be a constant value - a furnace of X btu will consume Y cu/ft of gas per hour based on the BTU capacity of the gas (natural or propane).

Assistance and suggestions are appreciated.

Regards
 
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Falcon67

Well-known member
Joined
Jun 11, 2009
Messages
18,371
Location
Merkel, TX
I can't help with all of that and if it's 50F in Houston, it much be damn cold elsewhere LOL. Especially out here 400m NW of you.

Here's a conversion to help figure costs
http://www.energystar.gov/ia/busine...finder/help/Energy_Units_Conversion_Table.htm

Which works a full 250 gallon propane tank out as 22,911,000 BTUs.

Our 1660 sq/ft house here has a 51K BTU electric heat system with a 3.5 ton AC unit. Our 1500 sq/ft house that we are leaving has a 75K BTU gas system and a 2.5 ton AC unit. Both seem to work well and the old house is about 3/4 of the walls insulated R-13 with R19 plus in the attic. Those readings are BTU/HR, so total use depends on cycle times. If a 75K ran 15 minutes per hour every day, you'd get maybe 50 days out of a tank - more or less. Having lived in Houston, I'd bet a 250 gallon tank (most common size) would run you all "winter", maybe two depending.

Real live heater/AC sizing is damn complicated - there is a whole book on it that the pro's refer to. Some googling (yes, I know it's not really a verb) would come up with at least some crude heat loss calculators that would give you some idea, otherwise you'd want a pro to come give a real estimate.
 
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