need math help please

[83trekker]

For my garge in floor heat im going to go with a gas water heater, it has an input of 40,000btu and i think its 79% effiecent, can somebody tell me what the btu output would be? I just cant figure it or work it out. Math is not my stong suit i tell you. I dont know if it matters but just in case our gas is mesaured in gigajoules and we pay $7.20 a gigajoule. Thanks

 

[CraigFL]

Her's what I use when I can't remember....

http://www.onlineconversion.com/energy.htm

=40,000 * 79/100 * 0.000001055 * $7.20 = about $0.24

 

[83trekker]

Acutally im trying to figure the BTU output of a 40,000 BTU input water heater. I like the link you have though thats helpfull

 

[CraigFL]

Sorry I didn't make that clear.... 40,000 BTU input at 79% efficiency = 40000 * .79 = 31,600 BTU output

 

[83trekker]

wow i thought it was a big conversion to do that, thats easy, thanks man.

with this equation =40,000 * 79/100 * 0.000001055 * $7.20 = about $0.24 where does the 0.000001055 value come from? and what is the end result work out to be .24 cents per hour? per btu? Sorry i should have payed alot more attention in school, im really kicking my self now.

 

[fefarms]

It's 40000 BTU per hour coming in. 31600 BTU per hour is useful heat imparted on the water, 8400 BTU is wasted. You pay for the gas input, not the useful heat output.

One gigajoule = 0.000001055 BTU

40000 BTU per hour * 0.000001055 gigajoule per BTU * $7.20 per gigajoule = about $0.30 per hour operating cost running full blast.

But what you really want to know is how much its going to cost to heat your garage over the course of a winter. This depends on the heat loss, which depends on the climate, the insulation, and the indoor set point. (The water heater will not be running full blast).

Calgary has about 9600 heating degree days per winter. Rough guessing, I'd say your garage loses 400 BTU per hour per degree difference between inside and outside (might be as little as half that if really well sealed and insulated, could be double or triple that if really leaky).

Let's assume you keep your garage at 65 degrees 24 x 7 through the whole heating season.

9600 degree days * 24 hours per day * 400 BTU per hour per degree =
92160000 BTU per year heat output

92160000 BTU out / (0.79) = 116600000 BTU gas needed per year
1166000000 BTU * .000001055 gigajoule per btu = 123 gigajoules
123 gigajoules * $7.20 per gigajoule = $886 per year

It will cost you $886 per year to keep the garage at 65 (f) (18 (c)) on a 24x7 basis

 

[83trekker]

wow, thanks thats great info to know and have to work with, i appreciated it!

Well i found out that the water heater is actually only 59%effiecent. Also i did a heat loss on my garage with some thing i found on the net it worked out to be 26,300 btu heat loss @ 18 degree C. Does that seem high? Its a 23 x 26 garage , 12 foot ceiling in the center (scissor trusses) r12 walls (2x4 walls), r38-r40 for the roof, 16x9 R8 insulated garage door, 2 good windows, 1 insulated man door. But no insulation under the pad??