So I’m calling “The Guy” to find out about these room heaters, and his first question is how many BTU’s does it take to heat your house. I replied (in a most educated tone…) “I have no Idea”.
How do I figure this out. I understand that it is a function of space and air exchange, so I’m guessing I take the volume of my house, and take a guess at how often the air changes in it (I have no idea what this would be), then convert my ambient temperature to BTU’s and … Ok you guys are the pros, How do I figure this out???
Dagwood
Replies
How much fuel do you burn now for heat and what kind is it? If you do burn fuel you can make a direct conversion of fuel consumed x efficiency x a known conversion factor equals BTU's. If you use electrictiy it becomes more complicated to break out the heat portion but it isn't impossible.
Ron
I agree with Ron that if you have a track record on this house (in the form of fuel bills) that would be the most accurate. Then you only have to convert from the actual, average weather that month to the worst-case scenerio in which you want to keep the house (or a room) at 70. Worst-case for me is -40F. For most it is not as extreme.
Then, if you want, fudge that up 25% for a north side room or 50% if there's a lot more glazing there than in other rooms or an outside corner room. Or down a bit if it has good solar input or fewer exterior walls.
Lacking the fuel data, for the overall house, I use 0.1 BTU/degree F/sq ft for a very well insulated, very tight. Not just a house where you think you paid for that, but where you were on-site making sure it happened.
0.3 BTU/degree F/sq ft for typical modern construction
1 BTU/degree F/sq ft for 1960's, 1970's construction before modern energy-conservation codes were adopted in your locale.
3 BTU/degree F/sq ft for a pre-war (very leaky) house without any insulation to speak of.
For instance: modern house, -10F design temp: 0.3 x 2000 sq ft x (70- -10=80) = 48,000 BTU/hour. For an average 200 sq ft room in that house = 4,800 BTU/hour = 1.4 kilowatts.
Hey David, I used your formula and came up with the following. It seems way high to me, but maybe I'm overlooking something...
Living space to Heat is 350 sq ftColdest (design) temp is -30 Celcius (equal to -22 F)Airflow = 3
(3 X 350) X (70 - (-22)) = 96,600 btu/hr
The biggest of these wall heaters has a BTU input of 38,400 - 10,500BTU with Natural Gas. Now either I'm not reading the numbers right, or these things are waay too small for my application... Am I wrong?View Image
Like I said, check the nameplate rating of your existing furnace. Figure 50-60% efficiency, given how old it is. Use that to sanity check your computations.
96,600 BTU for a 350 square foot house seems pretty high, but may not be unreasonable if the house is leaky. We heat a 1000 sq ft footprint house in southern MN with about 100,000 BTU of capacity (and could probably get by on 60K, given the weatherization we've done over the years).
You're using the numbers right (but 3.0 is just a fudge factor not directly interpretable as airflow). My 1910 Seattle house was about that, maybe 2.0 or 2.5 for it's boxy design (square foot print, 2 story).
I've visited a lot of houses in SF from 10's and 20's in which wall heaters couldn't keep up with 45F and a stiff breeze.
Take a pro-rated fraction of floor space times your current heater nameplate rating as a double check.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for your help everyone. I think I'll take a closer look at the nameplate on the heater before I commit any money to anything.
Dagwood.View Image
Following David's and Ron's replies, look at the zero heat/cool months and you have a good idea of the base line energy you use in daily living.
Then go to the winter months and crank out the unit cost for X fuel,(over and above the base line),efficiency of heat source etc., and you can figure the real heat load for your house.It will not be perfect, as things like more heat needed to heat colder water, fewer hours of daylight require lights on more etc., will have a small impact.
Paul
Another thing to do (if this is an existing home) is to go down to your furnace and read the nameplate. It will generally give some sort of BTU rating (though it may be input BTUs or output BTUs, depending on the era of the furnace). Assuming that you never experience any serious shortcoming of the furnace (though possibly cold spots in the house due to poor heat distribution), then your peak BTU usage will be somewhere below the rating of the furnace.
What if the existing furnace was overspecified?
"What if the existing furnace was overspecified?"
Which most are. So check the duty cycle versus the outside temperature. Extrapolate to your worst case design temp for the size you need. i.e. it can run 100% at your lowest temp.
This has the advantages that 1) you won't buy more heater than you need and 2) the unit may last longer because it is not cycling on and off so often.David Thomas Overlooking Cook Inlet in Kenai, Alaska
> What if the existing furnace was overspecified?
Certainly a possibility. But the furnace rating still gives an upper bound on the BTU rating, and then using some estimate of % operation time will allow an estimate of actual BTU consumption.
With something like this it's usually best to come at it from two or three angles and look for the computations agree to a reasonable degree. If something seems wildly inconsistent then you can look for your error.
Thanks for yor replies guys, I think I can figure something out now...
As for going from the old furnace, well, it was very inefficient in that most of its heat went through the wall and out doors. I do not exaggerate here. If you stand inside with a hand over the heater and then go outside and do the same thing, you'll burn your hand when you go outdoors. The heater is over 50 yrs old, and parts are no longer available. Thus my theory that taking the BTU's off the old heater wouldn't be very accurate.
Cold spots are a real problem in the house but I see no way to alleviate that. its poor design, and unless I dig out the foundation, bust a hole through it, then run ductwork under the house, I ain't gonna never fix it. The house isn't worth the work.
The house was built during the depression, using the best economies of the time to build cheap. Yup its uninsulated. Yup it needs work. Yup it leaks like all hell Yup the lot is worth more than the house.
Thanks again guys,
DagwoodView Image
Specifics & basics that others maybe assumed you knew:
3412 BTUs per kW-hr. 100,000 BTUs per a therm . 19,000 BTUs per # of oil (off the top of head type number)
kW-hrs are 100% efficient as resistive, 300% + efficient as HP, gas is 92-94% condensing, 80% normal, oil about 80%.
Thus, if you had a gas bill with a condensing furnace that was $120 and your rate is 90 cents/therm, your electric bill at 10 cent kW-hr with a resistive heat would be .......... if you can't follow it and carry on the calc from here, go back and review high school math, I'm too tired to even reach for the calculator (but R heat will be higher at 10 cents kW-hr, about break even with HP).