How can I calculate the BTU output of existing radiators

[FozzieBear]

I am sure this question has been asked a thousand times but when I search for radiator outputs all I get are calculators that work out what size radiators I need for a specific room volume.
We have 5 large (2000m x 600 approx) very old steel double panel radiators without convection fins in our community building. These are more than adequate to raise the temperature to a set value in a given time. However they are very heavy, dropped on the supporting brackets and causing air leaks in the pressurised system. In addition they project a long way into the room (140mm) is which causes an obstruction for activities like badminton etc.
We are currently getting quotes for boiler replacement/upgrades and at the same time replacing these rads with modern slimmer convector rads. All three companies quote different rads and it's difficult to guess if they are similar of oversized for the task.

One company has quoted for 6 Henrad Compact rads which for a single panel convector only projects 83mm which is perfect. I know the output from the spec sheet for the Henrads but I want to calculate the output from the existing rads to compare total overall output. Is there an easy way to do this? I have no idea of the make and they were probably installed in the 1950/60's so cant go to the manufacturer to get data.
Any help would be appreciated

 

[muggles]

They're about 2kW each (no idea in BTU, watts has been the standard measure of heat output since the 1970s).

Any new boiler replacement since June 2022 requires a full heat loss calculation, which will also give the required output of the radiators for each room. There should be no variation in sizes of radiators quoted by your potential installers, provided they are doing proper calculations and not guessing. Same with boiler heat output. Any who are guessing should not be employed. www.heat-engineer.com is an excellent professional grade heat loss calculator which will give required boiler and radiator heat outputs for your building

 

[FozzieBear]

Thanks Muggles. What they should do and what they actually do are two different things. I do not doubt that they have done a calculation of some sort and have specified the "correct" radiators. What I am trying to do is to decide on which quote to go with based on the initial requirements i.e. similar or greater output but slimmer profile.
In order to check the quotes against the specification I need to be able to ascertain what the existing output in kW is to compared against the radiators being specced. I have no doubt they will provide adequate output but may be oversized double panel as being readily available/cheaper whereas my preferred single panel convector radiators Henrad may be more than adequate.
Building Heat loss calculators are great for modern construction but almost useless in our building scenario which is a 17th Century listed building of multiple construction methods, materials and unknown thermal loss. We would have to employ a specialist in listed building to calculate the heat loss. What we know for definate is the heat output from the current radiators is more than sufficient. I just need to ascertain what that is. Surely there is a methodology for calculating the thermal output from a rad?

 

[DetlefSchmitz]

I found this: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjeg7q5_Pf_AhURUsAKHYK7Av8QFnoECB8QAQ&url=https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf&usg=AOvVaw09Qqu1QvSPloP8xuZ6_-sF&opi=89978449

Not the kind of radiators you have but...

I was just searching for 'estimate radiator output'. You could try variations on the theme.

Also 1 W is equal to 3.41 BTU/h.

 

[muggles]

Thanks Muggles. What they should do and what they actually do are two different things. I do not doubt that they have done a calculation of some sort and have specified the "correct" radiators. What I am trying to do is to decide on which quote to go with based on the initial requirements i.e. similar or greater output but slimmer profile.
In order to check the quotes against the specification I need to be able to ascertain what the existing output in kW is to compared against the radiators being specced. I have no doubt they will provide adequate output but may be oversized double panel as being readily available/cheaper whereas my preferred single panel convector radiators Henrad may be more than adequate.
Building Heat loss calculators are great for modern construction but almost useless in our building scenario which is a 17th Century listed building of multiple construction methods, materials and unknown thermal loss. We would have to employ a specialist in listed building to calculate the heat loss. What we know for definate is the heat output from the current radiators is more than sufficient. I just need to ascertain what that is. Surely there is a methodology for calculating the thermal output from a rad?

See the first line of my response. Your existing radiators are about 2kW each.

Heat Engineer heat loss software can cope with calculating heat loss for all types of buildings, not just modern ones.

 

[kidgreen61]

An easy way is to look in Screwfix catalogue and find siimilar size rad, the kw output will be in the info

 

[FiremanT]

They're about 2kW each (no idea in BTU, watts has been the standard measure of heat output since the 1970s).

Any new boiler replacement since June 2022 requires a full heat loss calculation, which will also give the required output of the radiators for each room. There should be no variation in sizes of radiators quoted by your potential installers, provided they are doing proper calculations and not guessing. Same with boiler heat output. Any who are guessing should not be employed. www.heat-engineer.com is an excellent professional grade heat loss calculator which will give required boiler and radiator heat outputs for your building

Are you sure? I started in '76 and learnt Btu/hrs. Boilers were all designated by Btu's, it was later that Kw kicked in. In fact, I still have a (broken) Mears calculator on BTU/hrs.

 

[FiremanT]

An easy way is to look in Screwfix catalogue and find siimilar size rad, the kw output will be in the info

Years ago, radiator radiator design improvements resulted in significantly increased output. So the current figures will be a lot less than than those for the actual old rads