Flow Temperature and Power Output relationship

Flow Temperature is the temperature of the water leaving the boiler circulating through the radiators, upto the underfloor heating manifold or through the hot water cylinder coil.

Power Output is the amount of energy your boiler outputs in the form of heat. Equally a 24kW of boiler heat output running for 1 hour would equal 24 kWh of energy used.

Comparing to a vehicle, the flow temperature can be thought of the speed of the vehicle, and the power output can be thought of the engine size. Just like a vehicle the power output can vary depending on road conditions, gear the vehicle is in and rpm etc.

In most modern boilers where a boiler is set to say 24 kW for example, it would switch on and usually ramp up immediately to 24 kW output and aim to reach a flow temperature of that may be set to 65 degrees celcius with a differential of usually 20 degrees (flow is 65 degrees, return is 45 degrees), although not always the case due to conditions of operation.

If the temperature of the water returning back to the boiler starts becoming less than 20 degrees, the boiler starts to modulate ie reduce or increase it’s flame and output until it can no longer reduce any further, at this point it turn’s off and may continue to circulate the water until it’s able to fire up again ie 20 degrees differential or the heating controls are turned off.

Modulation of the flame can also increase if the differential temperature starts increasing.

For additional information the ratio between the lowest power output and highest power output is known as the modulation ratio. Higher ratio usually means better efficiency this will be explained in another article.

Disclaimer: Information on this website is provided for informational purpose therefore we cannot accept liability for subsequent problems. All repair tasks should be carried out by qualified trades people. We may also receive commission payments when you are taken to external websites and complete purchases from third party sellers, once you leave our site, we cannot be held liable for your purchases.