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

If you have no interest in software you may as well use the average figures for heat loss per square metre based on the standard components used in the building at the time of construction.


If it's a leaky 1970's building shoot for 120w/sq m for example. Add up the square meterage, multiply that by the required power density and you have your radiator output. Generally 20% is added as a design margin.


If you are fitting a combi-boiler and want to use low flow and return temperatures like 60/40 you'll need to size your radiators up by about 50% for the same total heat output.


Always fit TRVs to every radiator except one so that the boiler can always bleed heat off through an open circuit when required. This generally goes in the coldest room in the house or the bathroom.


An oversized radiator allows the room to heat up quicker before the TRV throttles down the flow to maintain whatever temperature you set it at.


You are sizing the system for the coldest weather conditions, at anything less the system will be running at far less than maximum output so even if you get it wrong you'll only have to worry about it for a few days each year when it's -5 outside.


The total heat loss for a house is anything between 2.5kW for a modern, well-insulated small house and 20kW for a leaky old large house. Any boiler you choose will be able to output that level of heat with no issues.


You can make it horrendously complicated if you like but there is really no point, it's actually very simple. You would have to try really, really hard to do something bad because all the radiators will have TRVs.


If you have a look at Table 18 of BSRIA "BG 9" Rules of thumb 5th edition you'll see the sort of thing i mean.


Hope that helps.

If you have no interest in software you may as well use the average figures for heat loss per square metre based on the standard components used in the building at the time of construction.


If it's a leaky 1970's building shoot for 120w/sq m for example. Add up the square meterage, multiply that by the required power density and you have your radiator output. Generally 20% is added as a design margin.


If you are fitting a combi-boiler and want to use low flow and return temperatures like 60/40 you'll need to size your radiators up by about 50% for the same total heat output.


Always fit TRVs to every radiator except one so that the boiler can always bleed heat off through an open circuit when required. This generally goes in the coldest room in the house or the bathroom.


An oversized radiator allows the room to heat up quicker before the TRV throttles down the flow to maintain whatever temperature you set it at.


You are sizing the system for the coldest weather conditions, at anything less the system will be running at far less than maximum output so even if you get it wrong you'll only have to worry about it for a few days each year when it's -5 outside.


The total heat loss for a house is anything between 2.5kW for a modern, well-insulated small house and 20kW for a leaky old large house. Any boiler you choose will be able to output that level of heat with no issues.


You can make it horrendously complicated if you like but there is really no point, it's actually very simple. You would have to try really, really hard to do something bad because all the radiators will have TRVs.


If you have a look at Table 18 of BSRIA "BG 9" Rules of thumb 5th edition you'll see the sort of thing i mean.


Hope that helps.