BTU 'Rule of Thumb' (Stolen From Electric Baseboard 'ROT')

lsayre · Post by **lsayre** » Sun. Dec. 18, 2011 12:11 pm

There are loads of suggestions online for how to determine the requirements by room for electrical baseboard heating units. I've condensed them as follows:

1) For an Old/Uninsulated home: Use 12.5 Watts per square foot of floor space
2) For a typical pre 1980 home with some insulation: Use 10 Watts per square foot of floor space
3) For a post 1980 home with decent insulation: Use 8 Watts per square foot of floor space
4) For a modern highly insulated post 2000 home: Use 6 Watts per square foot of floor space

Since 1 Watt = 3.412 BTU's, it seems that this rule of thumb guide should be capable of being extended to calculate an entire homes heat demand in "output" BTU's.

Example one: A 1,500 square foot non-insulated older home
Use category 1 above. 1,500 x 12.5 x 3.412 = ~64,000 output BTU's required (an old 65% efficient 100,000 BTU furnace should work well here)

Example two: A 3,500 square foot post 1980 home
Use category 3 above. 3,500 x 8 = 3.412 = ~96,000 output BTU's required (a 96% efficient condensing NG or Propane furnace rated at 100,000 BTU's should work well here)

Example three: A 2,500 square foot 1970 home with some insulation
Use category 2 above. 2,500 x 10 x 3.412 = ~85,000 output BTU's required (a modern non-condensing 85% efficient 100,000 BTU furnace (NG, propane, or oil) should work well here)

Example four: A 4,650 square foot modern and extremely well insulated 2011 home
Use category 4 above. 4,650 x 6 x 3.412 = ~95,000 output BTU's required. (a 95% efficient condensing furnace rated at 100,000 BTU's should work well here)

In all 4 examples, a 100,000 BTU rated furnace (of one form or another) should suffice. It shows the overall variability of heat demand to be huge. It's also one thing to get it wrong for a single room, and another thing to get it wrong for an entire house.

nortcan · Post by **nortcan** » Sun. Dec. 18, 2011 5:58 pm

Thank for the infos and the good work.

ValterBorges · Post by **ValterBorges** » Sun. Dec. 18, 2011 6:22 pm

Is this right?

5700*8 = 45600w
45.6kw *9.5c = $4.33 /hr
24*4.33= $103.99 /day
103.99 *26w*7d= $18,913/season

And this is just generation.

Post by **Rob R.** » Sun. Dec. 18, 2011 6:40 pm

Use these "rules of thumb" if you want to, but I think it is well worth the effort to calculate the radiation based on the HEAT LOSS of the room. Just using the dimensions of the room is just asking for trouble. You could have two rooms the same size, one with a single exterior wall, the other with two...and one of those walls made of glass. Guess what happens if both get the same amount of radation? The guy in the first room will have the window open, the guy in the other will be in his Union suit trying to stay warm.

There are plenty of "rules" to account for the glass and # of doors in a room...or you could try the very easy to use software from Slant FIn.

lsayre · Post by **lsayre** » Sun. Dec. 18, 2011 8:06 pm

ValterBorges wrote:Is this right?

5700*8 = 45600w
45.6kw *9.5c = $4.33 /hr
24*4.33= $103.99 /day
103.99 *26w*7d= $18,913/season

And this is just generation.

Only if your heat requirement was for your furnace to run at its maximum possible rated output non stop for an entire heating season would it be right. About 25% of that would likely get you to a closer ballpark guess of the annual dollars required. Or about $4,700.

ValterBorges · Post by **ValterBorges** » Sun. Dec. 18, 2011 8:28 pm

Ah ok. That is better than propane at 6k, but not as good as coal at 2k.

Same as if I had calculated on running the S260 at 20lbs of coal an hour 24x7 which would be a half ton a day

When in reality its more like 1/6 th of that.

lsayre · Post by **lsayre** » Sun. Dec. 18, 2011 8:39 pm

ValterBorges wrote:Ah ok. That is better than propane at 6k, but not as good as coal at 2k.

Same as if I had calculated on running the S260 at 20lbs of coal an hour 24x7 which would be a half ton a day
When in reality its more like 1/6 th of that.

Propane is generally the most expensive way to heat a home. It's always a regional thing to some greater or lesser degree, but in general it seems to me that from highest to lowest cost they shake out as:

propane
#2 heating oil
electricity
wood (or wood pellets)
anthracite coal
natural gas
bituminous coal

If you are near the region in Eastern PA where anthracite is mined it is probably still more economical than NG. But as you get farther away from the source of anthracite it moves up the list, eventually perhaps exceeding even propane. I'm basing this list purely upon my perspective, which is from a location that is about 320 miles west of anthracite mining central.

SMITTY · Post by **SMITTY** » Sun. Dec. 18, 2011 8:53 pm

Yeah out here electricity is #1 and propane is a close #2 in terms of expense. People up here heating with electricity in an average sized home (for MA that's 2,500 - 3,000 sq.ft.) see $600/mo. + electric bills.

Berlin · Post by **Berlin** » Sun. Dec. 18, 2011 10:37 pm

I would say that in most areas propane and electricity compete for the top spot, heating oil would generally be below both of them especially in the northeast.

CapeCoaler · Post by **CapeCoaler** » Mon. Dec. 19, 2011 12:40 am

Heat loss calculaction is the best way to figure out what your house needs...
Depends what you have as the design temp also...
Measure your walls, windows and insulation and do it properly...
My rental cottage needs 24,000 btu/hr at 0* outdoor, 75* indoor and 15 mph wind...
Just shy of 600 sq ft...
Main house I don't want to know...
But the DS at 130,000 btu/hr has yet to not keep the house above 68* on the nastiest days so far...