Radiator BTU/Wattage calculation questions

Lulu58

I've been looking online to calculate the radiator/heated towel rail BTU.  I've looked at three websites and the values are quite different:
1. 4474 BTU, 131143316 watts
2. 5014 BTU 146955900 watts
3. 6016 BTU 176347080 watts. 
I was wondering whether someone could explain why the values aren't more closely aligned when the measurements I input were the same?
The existing radiator in this bathroom is 1190 BTU, 349 watts which seems to be very underpowered when compared to the figures above.
Can anyone advise me on how to calculate the measurements for a bedroom with a gable roof so I can check the BTU?

bris

You need to know the cubic feet of the room. Then you can get the correct size. You mention bathroom and bedroom, is it one or both? For bathrooms towel radiators are pretty poor for heating the room as they are for towels so I just recommend the biggest one your wall will take.

Lulu58

bris said:

You need to know the cubic feet of the room. Then you can get the correct size. You mention bathroom and bedroom, is it one or both? For bathrooms towel radiators are pretty poor for heating the room as they are for towels so I just recommend the biggest one your wall will take.

I've used online calculators where you enter the length, width and height plus information re windows, outside walls etc.  I thought the BTU would be fixed regardless of what calculator I used so I'm a bit baffled.
I am looking at two rooms - a bedroom with a gabled roof, and a bathroom.
Thank you for the advice regarding the towel radiators.

Grenage

Some websites are a bit disingenuous use different Delta T ratings (the water temperature going through).  Looking at the detailed info should identify their baseline, so check that when comparing radiators of the same size.

You'd need to measure your room and throw the info into a calculator, but err on the large size.

ic

You must live at Sellafield if you can consider radiators that deliver hundreds of megawatts of heat!

Doozergirl

The calculators use different levels of detail.  I use the one at Best Heating because it asks more questions. 

In order to calculate the gable, presuming it is equal both sides, just add half the height of the gable triangle to the height of quadrangular area of the wall and use that as the overall height of the room. 

eg.  Eaves height is 2 metres, overall height is 3 metres.  Split the 1 metre difference and add it to the eaves - overall room height = 2.5 metres

TELLIT01

ic said:

You must live at Sellafield if you can consider radiators that deliver hundreds of megawatts of heat!

Definitely a decimal point missing somewhere in the wattage figures.

Lulu58

ic said:

You must live at Sellafield if you can consider radiators that deliver hundreds of megawatts of heat!

I was a bit thrown by the figures but wasn't sure whether it was just a question of round them down but that didn't make sense in terms of the existing radiator being 349 watts. 

southcoastrgi

Your watts for your btu's are
1308 w
1466 w 
1759 w 

Lulu58

Doozergirl said:

In order to calculate the gable, presuming it is equal both sides, just add half the height of the gable triangle to the height of quadrangular area of the wall and use that as the overall height of the room. 

eg.  Eaves height is 2 metres, overall height is 3 metres.  Split the 1 metre difference and add it to the eaves - overall room height = 2.5 metres

Thanks, Doozergirl.
To make sure I understand you, the eaves (two side walls?) in the gable room are low at 106cm.  The highest point of the gable is 300 so the difference is 194, divided by 2 = 97 + 106 (for the eaves wall) = 203cm.  Is that right?

Lulu58

southcoastrgi said:

Your watts for your btu's are
1308 w
1466 w 
1759 w 

Hi southcoastrgi and thanks for posting.  Out of interest, how do you get those figures as it's clearly not just rounding them down as I thought it might be?