radiator btu calculation

micks_address

hi folks,

I'm looking for a btu requirement for a upstairs bedroom. Double glazed window, wooden floor, 300mm attic insulation above. Size approx 10 feet by 12 feet. The window is about 5 feet wide, 4 feet high

Would 3.2k btu cover it? I've tried some online calculators and they say it should but im not sure if they all equal

Thanks,

Mick

10-10-20

Mick, the count of external walls is a huge factor (plus you're using imperial (feet and BTU)). Anyway, 3.2k BTU is about 1000W.

What size rad is in there now, often that's the best place to start off from?

I sized mine using the stelrad site and it was there or thereabouts. Yours looks ok for a 3m x 3.2m, but all factors need to be taken into account.

micks_address

https://www.boards.ie/discussion/comment/120751924#Comment_120751924

1 external wall hollow block

micks_address

Existing rad is single 23 inches high, 54 long approx 58cm by 137cm

10-10-20

Match it on here to get an idea of the wattage.

https://www.stelrad.com/

micks_address

Just coming back to this radiator. It's 3268 if I run the boiler at 65c is that what it means?

10-10-20

https://www.boards.ie/discussion/comment/120770242#Comment_120770242

Yes, correct. Except using the delta T65 value is incorrect as your existing rads were probably sized for delta-T50 (so the temperature difference between the average ambient room temperature and the average boiler temperature). Delta-T50 would be the difference between 20 degrees C ambient and an average of 70 degrees off the boiler. Ideally moving forward you need to look at something between the delta-T30 value and T50 as the efficiency of modern boilers is better around the delta T30 to T40 mark. So try size the rad off something between T30 and T50.

I'd read that rad as being 2313 BTU (677w).

(I know, it's a bit of a mess)

micks_address

https://www.boards.ie/discussion/comment/120771621#Comment_120771621

cheers, so if the calcs say 3000 btu required it probably wont cut it.. ill ask my plumber to recommend a size.

10-10-20

https://www.boards.ie/discussion/comment/120771750#Comment_120771750

He'll have a table or a conversion factor for sizing. I can show you all of the links to how to do it, but it's a mine-field. This is a good link through - even if you are not going down the heat-pump route.

https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-for-a-heat-pump/

John.G

https://www.boards.ie/discussion/comment/120770242#Comment_120770242

NO, it doesn,t mean that.

Rads are normally specified at "T50", its the mean radiator temperature minus the required room temperature (usually assumed as 20C), the mean rad temperature is the (rad flowtemp+rad return temp)/2. If you have a rad flowtemp (boiler temp) of 75C and a return temp of 65C then the mean rad temp is,(75+65)/2, 70C, the required room temp is 20C so the "Deg Rad" is 70-20, 50C, a T50 rad. If you had a heatpump supplying water at 45C with a rad returntemp of 40C, the mean rad temp, (45+40)2, 42.5C, room temp required is 20C "Deg rad" is 42.5-20, 22.5C. a "T22.5" rad and its output is (22.5/50)^1.3 x 100, 35% of a T50 rad so would have to be oversized by a factor of 100/35, 2.8. You can read off the oversizing factor from the spreadsheet I built, or see it in the txt file.

Radiator Sizing Factor.docx

Radiator Sizing Factor.xlsx

10-10-20

https://www.boards.ie/discussion/comment/120772115#Comment_120772115

Good correction there from John. I should have said that T65 was the not the boiler output temperature, but I did calculate (average boiler temperature minus ambient room) it later on.

micks_address

ill get my plumber to recommend a btu requirement.. i was looking at a radiator on adverts someone had left over from a job but probably best just let him spec up a new one.. i suppose no harm specing for a heat pump if we ever went down that road.. our boiler flow temp is set to 60.. what would that rad emit btu wise at that?

John.G

It will emmit ~ 1410 BTU with a flowte of 60C and return of 48C.

micks_address

https://www.boards.ie/discussion/comment/120772591#Comment_120772591

thanks for clarifying - wouldnt be sufficent so - would probably need a double of the same size footprint