Online BTU calculator

PieCa

Hi there!

I am looking for a reliable online BTU calculator. I found many online and all of them yield a different result when I punch the numbers. Is there a to-go place online for this type of calculation?
I need those BTU to pick the right size of steel rads.

Thanks!

Ironman

SlanFin has a free app that's reliable - if you enter the correct data.

PieCa

Thanks! What is "Exposed wall" and "Cold partition"? I have ideas but I think I'm wrong. So I'd rather ask

Jamie Hall

An exposed wall is just that -- exposed to the rest of the world. A cold partition is a wall which has the space you are heating on one side, and an unheated space -- such as perhaps a garage, or a mud room -- on the other.

And yes, Slant/Fin's is a very good calculator.

Various ones will give slightly different answers, but the more information they require the better they will be -- provided, as the old saying goes, it's good information -- not "garbage in, garbage out"!

PieCa

What should I put in outdoor temp though? Avg temp of the winter? The coldest it gets?
Is there a ideal indoor temp too?

EBEBRATT-Ed

Most people use 70deg for indoor design. Outdoor design depends on where you live. The outdoor temp in Main is going to be different than Florida.

Tell us you location and we can help. Where I live in Ma. we use 0 deg around here

PieCa

South of Ontario. Toronto area.

PieCa

So, I want to replace the cast iron rads from my master bedroom (and another room, but let's focus on this one).

Currently the rad is a 4 tubes / 19 sections / 26in h x 7in w. The boiler water temp is currently set to 145F. I never lived in the house, I just bought the house, so I don't know if that setting was sufficient or not. According to some cast iron calculation sheets I found, it looks like the output of this rad is between 4500 and 5000 BTU (depending on which sheet I use). So I was thinking to replace it with a modern slick looking StelRad S2 series and wanted to pick one which yield the same BTU.

But then I wanted to confirm and calculate what was required for the room instead of calculating what was already there.

The room is 12ft x 11ft. Ceiling is at 8in3. The room has two wall exposed to the exterior (west and north). Under the room is a heated living room. Above is the attic (currently not really insulated, the original layer from the 40ies, but it will be soon insulated with at least an 8in layer). The outside wall are 10in bricks. there is no insulation in the walls. There is just a 1/2 dry wall panel with a layer of plastic film (I don't know the technical term) between the plaster and the brick). There is a 27sqft window. Double glazed (not too old either, installed 8 years ago). Because I am in the Toronto area (south of Ontario Canada), the temp can get quite cold out there. The avg temp throughout the winter is around 20F. When I punch the number, I am getting like a 9000 BTU requirement... So I am guessing there is something wrong either with my fingers, or with the original rads choice (and the attic was not even insulated yet). What do you say?

pecmsg

Do the improvements first otherwise you will have to change the rad again after the improvements!

PieCa

No worry, I will not move in until all the reno are over, and that includes the new insulation in the attic. That's pretty much the only variable that will change anyways. So I use an insulated attic in my calculation in the app, and that is the results I shared of higher than 9000BTU.

Jamie Hall

Toronto area? I'd use -5 F (-20 C) for your design.

There is no guarantee that the original radiation was adequate... on the other hand, if the original was run rather hot (say 180 F), which is quite possible, and your figures are based on 140 F (you don't specify either the water temperature used or the EDR of the radiator), it's also quite possible that the actual output was indeed quite adequate.

Now all that said -- when you are dealing with hot water heat, remember that the output BTUh of a radiator is widely variable, and depends entirely on the average water temperature in the radiator.

Alan (California Radiant) Forbes

The design temperature for the Totonto Buttonville area is 1°F.

https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

PieCa

Well, I am a layman here... I am not sure how to account the variability of the temp in the rads. I know that 180 is the max temp of the boiler.
Based on the data I mentioned, what would you say is a the BTU requirement for the master bedroom?

Jamie Hall

Very broad ballpark, back of the napkin... 5 K. But you should run it through Slant/Fin's calculator.

And there is a very rough way to figure the BTUh output of a radiator for hot water. Take the average water temperature (input and return averaged) in degrees F. Subtract 70. Divide by 140. Multiply the result by 240. Multiply that by the EDR of the radiator. That will be an approximation of the BTUh output of the radiator.

That's also back of the napkin stuff, but isn't that far off.