Room heating

General calculation of heating requierment

There are numerouse factors that affect the peak heat input needed to heat a room. The following calculation give a relatvely good guide to the heat input needed to maintain 68°F 20°C during the typical winter in the UK. For high and exposed locations which go below 0°C, for long periods figures can be 50 to 100% higher.

Heat input estimation
Size of room in ft3  
Multiply by 4.5  
If windows face North add 10%  
If more than 1 external wall add 10% per additional external wall  
If ceiling hight exceeds 8 ft add 10%  
If ceiling hight exceeds 9 ft add 10%  
If ceiling hight exceeds 10 ft add 10%  
If loft/unheated room above add 10%  
If loft above not lagged add 20%  
Total for heating requierments in BTU  
Deviding total by 3,412 gives kW  

The result from the above can be further adjusted for variouse lifestyle or usage factors.

Usage adjustments
Factor BTU kW
Values obtained from basic estimation    
For 75°F 24°C add 25%    
For 71°F 22°C add 10%    
For 65°F 18°C subtract 10%    
If heating not constant {time clock} add 25%    
Adjusted radiator rating for room.    

Repeat for each room.


Radiators transfer heat from the boiler to the indevidual rooms. Manufacturers will give a heat output figure for there radiators in either BTU or kW. This figure is normaly quite prominant on the literature supplied.

You then need to check through the small print to find out how they obtained the rating. This is usualy given as a mean water temperature above room temperature. with the radiator connected in a TBOE {Top Bottom Opposite End} configuration, and no valves.

So if the manufacturer says mean temperature 60°C above room, above room TBOE and you want a room temperature of 20°C. The rated Output would be obtained with the boiler supplying water at 90°C to one of the top corners of the radiator and the return being from the oposite bottom corner at 70°C.

So if anyone touches it ouch!!!!!

The manufacturers data sheets usualy give a set of derating formula to calculate the radiators output under other conditions.

Radiator derating adjustments
Factor BTU kW
Nominal rating of radiator in test configuration    
Less 10% if connections are BOE {Bottom Oposite Ends}    
Less 20% if connections are TBSE {Top Bottom Same End}    
Rating in installed configuration    
Derating for a mean water temp of 40°C.above room.
(Test Mean - Actual mean) / TestMean = (60-40)/60 = 33%
Derating for a mean water temp of 30°C.above room.
(Test Mean - Actual mean) / TestMean = (60-30)/60 = 50%
Derating for a mean water temp of 20°C.above room.
(Test Mean - Actual mean) / TestMean = (60-20)/60 = 66%
Derating for a mean water temp of __°C.above room.
(Test Mean - Actual mean) / TestMean = (60-__)/60 = __%
Net output    

Condensing boilers generaly will not work at their rated efficency unless the return temperature is below 50°C. And for all boilers efficeny increases as the return water temperature drops.


When pumping hotwater from a boiler around a building tere is a tendancy for virtualy all of the hotwater to flow through the radiator closest to the pump and virtualy none through the one furthest from the pump.

To prevent this you can balance the flow through the radiators. to do this pick a nice cold day. You need a pipe thermomiter and a pair of pliers or a spaner. Use the thermomiter to check the return temperature of the water from the radiator. If the temperature differs by more than say 10°C from the temperature of the return pipe to the boiler adjust the fixed valve on the radiator. That is the one that does not have a knureld handel for adjusting by hand. Wait 30 minutes for the temperature to stabalize and check again.

Thermostatic radiator valves

Thease by continuously adjusting the water flow against the room temperature will regulate the room temperature and balance the water flow against the heating requierment of the room.

They are not without their snags however. When the system is turned on with the rooms cold all of the thermostic valves will be fully open. Unless the system has been atleased partialy balenced using the fixed valves virtualy all of the water from the boiler will pass through the first radiator until its room has warmed up. In the worst case heat loss through internal walls to naboring "unheated" rooms may prevent the room from warming sufficently for the thermostat to close and transfer heating to the other radiators.

These problems can be reduced by oversizing the radiators by about 20% and atleased roughly balencing the water flow without the thermostats in operation.