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.
The surface area of the radiator.
The specific heat capacity of the air in the room, including any water vapour in it.
How well the air is circulated around the room - do you want to do computational fluid dynamics on that?
The thermal conductivity of the walls, floor, ceiling, windows and doors.
The temperatures on the other sides of those walls, floor, etc. (not necessarily the same as the outdoor temperature).
How much cold air leaks in around doors and windows.
What temperature that air is at.
A formula that is 90% accurate is better than a formula that is 50% accurate! In a world of rising energy prices and proactive measures to reduce CO2 emissions then plumbers have to play their part as well.
Another factor I haven't mentioned are the effects of gas fires on central heating systems. If a room has both a gas fire and a radiator then should the radiator be deliberately underpowered or not? Installing the main thermostat in the same room as a gas fire will confuse the central heating system when the fire is on and make the rest of the building colder as a result.
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.
The surface area of the radiator.
The specific heat capacity of the air in the room, including any water vapour in it.
How well the air is circulated around the room - do you want to do computational fluid dynamics on that?
The thermal conductivity of the walls, floor, ceiling, windows and doors.
The temperatures on the other sides of those walls, floor, etc. (not necessarily the same as the outdoor temperature).
How much cold air leaks in around doors and windows.
What temperature that air is at.
A formula that is 90% accurate is better than a formula that is 50% accurate! In a world of rising energy prices and proactive measures to reduce CO2 emissions then plumbers have to play their part as well.
Another factor I haven't mentioned are the effects of gas fires on central heating systems. If a room has both a gas fire and a radiator then should the radiator be deliberately underpowered or not? Installing the main thermostat in the same room as a gas fire will confuse the central heating system when the fire is on and make the rest of the building colder as a result.
We're about to take you to the IET registration website. Don't worry though, you'll be sent straight back to the community after completing the registration.
Continue to the IET registration site