Measuring radiator BTUs: Are these calculations correct?

paperclap

Hi all,

I've been toying with the idea to replace all of our central heating pipework and radiators for a while now. But now I'm seeing I can only fit 69mm skirting board beneath the existing pipework, I think the decision has been made for me! Replace and relocate higher.

We've currently got type 10 radiators (single panel, no convector) throughout four of our rooms. Looking to replace these four with type 21 (double panel, single convector) radiators, for better heat output.

I'm leaning toward using Stelrad radiators, as I've heard good things about them. Probably the Homeline range, as Wickes stocks them in a plethora of sizes.

Which, has led me to use Stelrad's BTU calculator:
http://starsapp.co.uk/basic-heat-loss-calculator/

I'll use one of our bedrooms as an example:

• 3.7m x 3.3m room, with 2.4m ceiling
• One external wall, with PVC double-glazing window (3.9m squared window)
• Cavity brick walls, with insulation
• Solid concrete floor
• Insulation in loft space (we're in a bungalow)

It is giving me a reading of 4746 BTU.

I've tried a few other calculators, all within the same / similar criteria, and I'm getting readings of around 3500 BTU.

What are your thoughts? Are Stelrad way off the mark, or are they "oversizing" the radiators as a lot of people suggest to do (by around 30%)?

Thank you in advance!

Doozergirl

I've used the one at Best Heating for a long time and it's not let me down. 

I tend to choose radiators based on the ideal space available rather than deciding that everything is a 2 1 or whatever.  

NSG666

Better too big than too small, the TRV will control the heat. Screwfix rads often come with decent free TRVs and free delivery. As a rough guide to check the output of your current rads find the output of a similar sized single panel with convector and divide by 1.75.

paperclap

Doozergirl said:

I've used the one at Best Heating for a long time and it's not let me down. 

I tend to choose radiators based on the ideal space available rather than deciding that everything is a 2 1 or whatever.  

I have just tried this calculator with all the same variables. Got a result of 2774 BTU.

How can two calculations give such different results?

Its_not_paranoia

I also found that the stelrad calculator came up with big values compared with some others on the internet. 

Also, the heat loss calculators are (I understand from my plumber) based on a 30 degree difference between the outside and inside (so you can get to 25 degrees inside when it's minus 5 outside). 

I get that the calculator is designed to produce a result that will definitely keep you toasty. You might take a little more risk in order to get something that looks nice and fits the space - e.g. If you're happy with only being at 20 degrees when it's minus 5 outside, you can knock 16% off straight away. 

paperclap

Its_not_paranoia said:

I also found that the stelrad calculator came up with big values compared with some others on the internet. 

Also, the heat loss calculators are (I understand from my plumber) based on a 30 degree difference between the outside and inside (so you can get to 25 degrees inside when it's minus 5 outside). 

I get that the calculator is designed to produce a result that will definitely keep you toasty. You might take a little more risk in order to get something that looks nice and fits the space - e.g. If you're happy with only being at 20 degrees when it's minus 5 outside, you can knock 16% off straight away. 

I think there may actually be a bug with the Stelrad calculator.

I put in all the same variables, got a reading of X (example). Changed one variable, changed back to the original variable, and got a reading of Y.

Its_not_paranoia

laurencewhymark said:

Its_not_paranoia said:

I also found that the stelrad calculator came up with big values compared with some others on the internet. 

Also, the heat loss calculators are (I understand from my plumber) based on a 30 degree difference between the outside and inside (so you can get to 25 degrees inside when it's minus 5 outside). 

I get that the calculator is designed to produce a result that will definitely keep you toasty. You might take a little more risk in order to get something that looks nice and fits the space - e.g. If you're happy with only being at 20 degrees when it's minus 5 outside, you can knock 16% off straight away. 

I think there may actually be a bug with the Stelrad calculator.

I put in all the same variables, got a reading of X (example). Changed one variable, changed back to the original variable, and got a reading of Y.

Agreed - it doesn't give consistent results which shows there is something wrong with it.