BTU Calc - HVAC - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum > HVAC BTU calc
 User Name Remember Me? Password
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

 Thread Tools Display Modes
02-14-2012, 08:27 AM   #1
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

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

02-14-2012, 08:52 AM   #2
Member

Join Date: Oct 2007
Posts: 400
Rewards Points: 380

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.

02-14-2012, 09:08 AM   #3
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

Quote:
 Originally Posted by HVACDave 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

02-14-2012, 09:12 AM   #4
Member

Join Date: May 2011
Location: NW Burbs of Detroit Mi.
Posts: 1,107
Rewards Points: 500

Quote:
 Originally Posted by anesthes 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..

02-14-2012, 09:21 AM   #5
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

Quote:
 Originally Posted by hvac5646 ~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

 02-14-2012, 09:23 AM #6 Member   Join Date: Oct 2007 Location: Manitoba, Canada Posts: 400 Rewards Points: 380 So what size of furnace did you buy?
02-14-2012, 09:41 AM   #7
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

Quote:
 Originally Posted by HVACDave 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

02-14-2012, 01:53 PM   #8
Member

Join Date: Jul 2008
Location: NW of D.C.
Posts: 5,990
Rewards Points: 2,000

Quote:
 Originally Posted by anesthes 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?

02-14-2012, 02:00 PM   #9
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

Quote:
 Originally Posted by Yoyizit 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

 02-14-2012, 04:21 PM #10 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,471 Rewards Points: 4,522 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.
02-14-2012, 04:52 PM   #11
011010100110111101100101

Join Date: Jan 2011
Posts: 249
Rewards Points: 160

Quote:
 Originally Posted by Daniel Holzman 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

02-14-2012, 05:31 PM   #12

Join Date: Oct 2008
Location: Somewhere
Posts: 33,588
Rewards Points: 6,140

If that 18°F 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.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post anesthes Electrical 49 09-24-2011 06:43 AM ChrisDIY HVAC 2 05-31-2011 10:06 PM WillK Electrical 1 01-03-2011 04:09 PM qbert Electrical 4 01-23-2010 11:24 PM

Top of Page | View New Posts