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Old 02-14-2012, 08:27 AM   #1
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BTU calc


I'm curious if I can get a calculation based on the following information.

Outside temperature is 18* F
Inside temp is 56* F.

Building is being heated with a 1200w (4000btu) electric heater.

From this information, can I determine the required BTU to maintain 72* ?

Is the heat loss linear?

-- Joe

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Old 02-14-2012, 08:52 AM   #2
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BTU calc


You can't predict that your heat loss will be linear. As the temp difference rises between indoor and outdoor your heat loss will also increase. This doesn't say anything about infiltration air due to wind,doors and building openings, exposure variables etc... You need to have a heat loss for the building completed to give you an accurate number.

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Old 02-14-2012, 09:08 AM   #3
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Originally Posted by HVACDave View Post
You can't predict that your heat loss will be linear. As the temp difference rises between indoor and outdoor your heat loss will also increase. This doesn't say anything about infiltration air due to wind,doors and building openings, exposure variables etc... You need to have a heat loss for the building completed to give you an accurate number.
I did. I bought the super special amazing annex D software, plugged everything in, and I have a room to room cfm/btu estimate for my duct and register sizing.

However, I was curious about what I'm actually seeing. Because the the software calculated more BTU per room than what appears to be lost. The area in question, roughly 700 sq ft, had a calculated loss of 21k at a design temp of 72* and 20* outside.


-- Joe
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Old 02-14-2012, 09:12 AM   #4
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BTU calc


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Originally Posted by anesthes View Post
I did. I bought the super special amazing annex D software, plugged everything in, and I have a room to room cfm/btu estimate for my duct and register sizing.

However, I was curious about what I'm actually seeing. Because the the software calculated more BTU per room than what appears to be lost. The area in question, roughly 700 sq ft, had a calculated loss of 21k at a design temp of 72* and 20* outside.


-- Joe
~SSppllluurrrtttt~(coffee all over the place) ...you got bite by the "Fudge Margin" built in to a lot of software..
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Old 02-14-2012, 09:21 AM   #5
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Originally Posted by hvac5646 View Post
~SSppllluurrrtttt~(coffee all over the place) ...you got bite by the "Fudge Margin" built in to a lot of software..
Hahah!

Well the furnace is purchased, so that is that. What I bought the software for was mainly to size the ducts accordingly. The science behind forced hot air, and AC duct/register sizing is absolutely amazing.


-- Joe
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Old 02-14-2012, 09:23 AM   #6
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So what size of furnace did you buy?
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Old 02-14-2012, 09:41 AM   #7
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So what size of furnace did you buy?
70k (output) propane.

3 ton AC

96% mod/con.

Oh and it's 3/4hp, 1500 cfm.

-- Joe
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Old 02-14-2012, 01:53 PM   #8
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BTU calc


Quote:
Originally Posted by anesthes View Post
I'm curious if I can get a calculation based on the following information.

Outside temperature is 18* F
Inside temp is 56* F.

Building is being heated with a 1200w (4000btu) electric heater.

From this information, can I determine the required BTU to maintain 72* ?

Is the heat loss linear?

-- Joe
Knowing sq. ft, you can, with this
http://www.kouba-cavallo.com/bnchmrk.htm
Your one day temps give you HDD, you know the 4000 BTU and so you can figure your avg. heat loss.
Then rework the BTU to get the higher temp. diff.

It's more accurate if your temp. diff. is averaged over a month or so.

What's your sq. ft?
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Old 02-14-2012, 02:00 PM   #9
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Originally Posted by Yoyizit View Post
Knowing sq. ft, you can, with this
http://www.kouba-cavallo.com/bnchmrk.htm
Your one day temps give you HDD, you know the 4000 BTU and so you can figure your avg. heat loss.
Then rework the BTU to get the higher temp. diff.

It's more accurate if your temp. diff. is averaged over a month or so.

What's your sq. ft?
That section of the house is 640 exactly inside. Thats the new, R38 ceiling, R30 floor, R21 walls, and energy eff. windows.

The old section of the house, is around 900-950 sq feet. An 80k wood stove will produce 86+ degree temps on a 15 degree day if I don't choke it down. I've done a heat loss calc on the old part based on windows and insulation, and it will require a bit more BTU than the new section of house.

-- Joe
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Old 02-14-2012, 04:21 PM   #10
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BTU calc


You can get a reasonable approximation to your original question as follows. The steady state heat loss through a wall, based strictly on conduction, is directly proportional to the temperature difference between the inside and the outside. By increasing the inside temperature from 56 degrees to 72 degrees, you are increasing delta T from 38 degrees F to 54 degrees F, which means you need approximately 40 percent more BTU/HR to maintain temperature.

This ignores several complexities, including the fact that the walls, floor and ceiling all have different R values. It ignores the fact that conduction and convection do not follow the same heat loss equations as radiation. And it ignores transient temperature issues, such as heating up and cooling down the space. But the answer, namely 40 percent more BTU/HR required, may be close enough for your use.

By the way, unless your electric furnace runs at full capacity continuously, you cannot assume that you use the full 4000 BTU/HR, you may use significantly less. But if it runs continuously, my computation suggests you need about a 5600 BTU/HR heater to heat to 72 degrees F., assuming the geometry and insulation in your house do not change.
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Old 02-14-2012, 04:52 PM   #11
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Originally Posted by Daniel Holzman View Post
You can get a reasonable approximation to your original question as follows. The steady state heat loss through a wall, based strictly on conduction, is directly proportional to the temperature difference between the inside and the outside. By increasing the inside temperature from 56 degrees to 72 degrees, you are increasing delta T from 38 degrees F to 54 degrees F, which means you need approximately 40 percent more BTU/HR to maintain temperature.

This ignores several complexities, including the fact that the walls, floor and ceiling all have different R values. It ignores the fact that conduction and convection do not follow the same heat loss equations as radiation. And it ignores transient temperature issues, such as heating up and cooling down the space. But the answer, namely 40 percent more BTU/HR required, may be close enough for your use.

By the way, unless your electric furnace runs at full capacity continuously, you cannot assume that you use the full 4000 BTU/HR, you may use significantly less. But if it runs continuously, my computation suggests you need about a 5600 BTU/HR heater to heat to 72 degrees F., assuming the geometry and insulation in your house do not change.
Thanks for the information.

I'm assuming the electric space heater is running full blast 24/7 because the temp only rises as it gets warmer outside, and falls as it gets colder outside. But I don't know the intended duty cycle of the heater, as I don't have a manual for it.

Thanks again!

-- Joe
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Old 02-14-2012, 05:31 PM   #12
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If that 18F outdoor temp is at night time when there is no solar gain. You can do a linear plot to determine heat loss for design conditions, and be some what reasonably accurate. Infiltration will increase as the indoor temp increases due to stack effect. So it will be slightly low.

Since 1200 watts is maintaining a 38 degree difference from outdoor temp. 2400 watts (8,191 BTUs) should be able to get you to 72 when its 18 degrees outside, and not have to run 24/7 to do it.

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