Maximising boiler condensation operation

There is now a requirement in the Domestic Building Services Compliance Guide that states that where condensing boilers are installed, the system should be designed to operate at lower temperatures to give a return water temperature less than 55 degrees C to maximise condensing operation.

Boiler Condensation

There is now a requirement in the Domestic Building Services Compliance Guide that states that where condensing boilers are installed, the system should be designed to operate at lower temperatures to give a return water temperature less than 55oC to maximise condensing operation.

It states:

“Systems with condensing boilers should be designed to have low primary return water temperatures, preferably less than 55oC, to maximise condensing operation. Low return water temperatures can be obtained through techniques such as weather compensation and the use of low temperature heat emitters (for example correctly-sized radiators and underfloor heating elements).


When natural gas burns, it produces considerable quantities of water and this is vaporised in the combustion process (converted to steam).

This process absorbs quite a lot of energy (heat), in fact, approximately 14% of the heat produced by burning the gas is required to boil off the water – this heat is commonly known as latent heat, as it causes a change of state – from liquid to gas (steam).


So why do we need to get the return water temperature less than 55oC in order to maximise condensing operation?


  • The design of a condensing boiler enables a lot of this latent heat to be recovered by condensing the steam in the flue gases back to water (condensate), but this can only be achieved by getting the return water temperature down to 54oC – the dew point of gases.
  • At this temperature, the water vapour in the flue gases will start to condense onto the heat exchanger and give up its latent heat. This heat, plus the sensible heat recovered through cooling the flue gases, can amount to around 12% of the heat that would have otherwise been lost.
  • Reducing the return water temperature still further will recover even more latent heat.

Whilst commissioning your boiler replacement, you may have heard the boiler condensing through the plastic pipe leading to the drain within only minutes of operation, as the return water temperature is well below 55oC on start-up from cold.

As the flow temperature gradually increases and approaches the boiler thermostat set point – approximately 82oC, the burner will commence modulating down to low flame as it begins to match the heat load upon it, which may also switch off if a light heat load is put upon it.

At this point, the return water temperature is likely to be approximately 70oC and possibly higher.

Limiting the return water temperature to around 55oC means we must limit the flow water temperature to approximately 66oC.

With underfloor heating, where lower flow water temperatures can be used at the manifolds, weather compensation controller settings can be changed so that the maximum water temperature is 66oC when the outside temperature is say – 2oC.

Changes in the external temperature alter the temperature of the flow, lowering it if the outside temperature rises and increases it if it drops.

The constant monitoring of the external climate means the boiler is able to operate efficiently at the minimum required temperature and condense for longer.

With outside weather compensation controls, there is always a provision for running at maximum boiler flow temperature for domestic hot water heating, and this should be on a priority basis.

The optimum efficiency level that many newly installed condensing boilers can achieve is not being fully recognised.

Domestic heating accounts for around 14% of the UK’s CO2 emissions, the installation of intelligent weather compensation controls could help to reduce it by almost a quarter.

When installing radiators within new or refurbished properties which are insulated to current high standards, you can do exactly the same thing, but you need to oversize the radiators so that they will give the required heat output.

Identify the difference between room temperature required and the mean radiator water temperature, which determines the rate at which heat from the water can be transferred to the room.

This determines the value of the Delta T factor which is used to adjust the rated output of the radiator from the manufacturer’s catalogue rating.

Once calculated, multiply the manufacturer’s stated radiator output by the corresponding Delta T factor to show the actual performance of the radiator in those specific conditions.

Delta T factors for radiator- to- room temperature differences (figures may vary slightly depending on manufacturer):

Temperature difference (oC)Delta T factor


Heat emission required: 2000 Watts (2 KW)
Room air temperature required: 20oC
Mean water temperature in the radiator: 60oC

  1. Temperature difference = 60 – 20 = 40oC
  2. From delta T factor table 40oC = 0.605
  3. Divide the required heat emission by factor =
    2000 watts (2KW) required divided by 0.605 = 3305 watts (3.3KW)
  4. From the manufacturer’s selection table, choose the radiator rated at 3305 watts (or slightly more).