Formulas to calculate the required power output of a radiator for a room of a specific size. Do they exist?

Simon Barker:

I'm sure that, given a bit of time, a physicist could give you an near-exact formula to calculate the size of radiator you would need to maintain the temperature of a given room.


The trouble is, it would have many variables that you don't have the exact values for.  Plugging in wild guesses would give you a result no more accurate than the plumber's rule of thumb.


Off the top of my head, you would need to know:

The temperature of the water in the radiator (and do you want to account for the temperature drop from the inlet to the outlet?).

The thermal conductivity of the steel and paint of the radiator.

. . .

What temperature that air is at.

 

I could add a few more things to this list, which could be endless.

• What about the insulation on the pipes feeding the radiators, or lack thereof? (I was appalled to find in my house many uninsulated water pipes below floor level.)

• Is there any reflective material on the wall behind the radiator. (Does this really make much difference? I suspect not.)

• What about factoring in the pipes gradually becoming furred up over the years?

• What about people who constantly alter the thermostat? (Not for bad reason. People want the room air warmer when they feel cold and vice versa.)

A formula needs to be straightforward, in view of all the unknowns, even though it may be based on rigorous thermodynamic analysis.


Let me give an analogy. An electrical engineer working for a electricity supply network is to design a substation to supply a new, large office block. How should he assess the MVA rating of the transformer? I don't think he would seek a detailed inventory of all the lights and appliances expected to be used, tot these up and apply a diversity factor. Over the years reliable statistical information has been produced from which one can assess the volt-ampéres required per square metre of floor area. It would depend, of course, on the type and purpose of the building.


In a similar manner, a plumbing-cum-heating engineer can assess the rating of a boiler for a house of given size, and judge the size of radiator needed according to the size of the room. If house insulation level is judged to be poor then I suggest this is where attention is required rather than just boost radiator sizes to compensate.

Arran Cameron:

. . .

It's notable that the central heating trade stubbornly sticks with BTU/h (more often than not erroneously quoted as just BTU) but not all central heating installers know the definition of a BTU or that it is convertible into watts, and vice versa.

 

I am not sure the trade is as stubborn as Arran suggests. I have seen in boiler specifications ratings in the dated BThU/h, usually as a bracketed alternative to kilowatts. When we had our central heating boiler replaced last year, the conversation about the rating required took place in kilowatts. Some installers are moving with the times. In the end it was a case of installing a boiler just slightly larger than the previous one, which served well while it worked.


In answer to Arran's original question, I think the current range of calcuating aids plus others suggested by contributors to this conversation are about as good as he will get.

Simon Barker:

I'm sure that, given a bit of time, a physicist could give you an near-exact formula to calculate the size of radiator you would need to maintain the temperature of a given room.


The trouble is, it would have many variables that you don't have the exact values for.  Plugging in wild guesses would give you a result no more accurate than the plumber's rule of thumb.


Off the top of my head, you would need to know:

The temperature of the water in the radiator (and do you want to account for the temperature drop from the inlet to the outlet?).

The thermal conductivity of the steel and paint of the radiator.

. . .

What temperature that air is at.

 

I could add a few more things to this list, which could be endless.

• What about the insulation on the pipes feeding the radiators, or lack thereof? (I was appalled to find in my house many uninsulated water pipes below floor level.)

• Is there any reflective material on the wall behind the radiator. (Does this really make much difference? I suspect not.)

• What about factoring in the pipes gradually becoming furred up over the years?

• What about people who constantly alter the thermostat? (Not for bad reason. People want the room air warmer when they feel cold and vice versa.)

A formula needs to be straightforward, in view of all the unknowns, even though it may be based on rigorous thermodynamic analysis.


Let me give an analogy. An electrical engineer working for a electricity supply network is to design a substation to supply a new, large office block. How should he assess the MVA rating of the transformer? I don't think he would seek a detailed inventory of all the lights and appliances expected to be used, tot these up and apply a diversity factor. Over the years reliable statistical information has been produced from which one can assess the volt-ampéres required per square metre of floor area. It would depend, of course, on the type and purpose of the building.


In a similar manner, a plumbing-cum-heating engineer can assess the rating of a boiler for a house of given size, and judge the size of radiator needed according to the size of the room. If house insulation level is judged to be poor then I suggest this is where attention is required rather than just boost radiator sizes to compensate.

Arran Cameron:

. . .

It's notable that the central heating trade stubbornly sticks with BTU/h (more often than not erroneously quoted as just BTU) but not all central heating installers know the definition of a BTU or that it is convertible into watts, and vice versa.

 

I am not sure the trade is as stubborn as Arran suggests. I have seen in boiler specifications ratings in the dated BThU/h, usually as a bracketed alternative to kilowatts. When we had our central heating boiler replaced last year, the conversation about the rating required took place in kilowatts. Some installers are moving with the times. In the end it was a case of installing a boiler just slightly larger than the previous one, which served well while it worked.


In answer to Arran's original question, I think the current range of calcuating aids plus others suggested by contributors to this conversation are about as good as he will get.