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Energy bills have been increasing. There are still ways to reduce this.

Reducing the Flow Temperature for a Condensing Combi Boiler for upto 8 percent saving on your gas heating bills.

In this page we show you how to improve the energy efficiency for your condensing combi boiler if it has not already been set for efficiency. Please note this should not be used for a system, regular boiler with a hot water cylinder. This also includes a combi boiler that has a stored hot water cylinder as part of the system, this is due to legionella prevention requiring a slightly different settings or specialised heating controls.

You must be able tick the following questions before proceeding further.

    • I have a combi boiler with 5-6 copper pipes coming from below.
    • It’s a condensing combi boiler with a white/black/other plastic pipe coming from below
    • My plastic condense pipe goes into a waste pipe internally or it goes outside and is insulated / installed correctly with the correct slope, therefore not likely to freeze in extreme winter.
    • I confirm that I do not have a hot water cylinder as part of this system

Image of a Condensing Combi boiler showing pipes from below for identification purporse
Image of a example Condensing Combi boiler showing pipes from below for identification purporse; Vaillant Combi pictured.


Very simply, a condensing boiler working correctly, recovers additional heat from water vapour in the exhaust gases, from a process called “latent heat of vaporisation recovery”. See wikipeadia article for more info. . In order to recover this latent heat, your boiler heating return water temperature must be below the dew point temperature (54 degrees celcius).

As you can see from the graph shown below, as soon as return water temperature is below 54°C, you enter High Efficiency condensing mode as shown by the green area. This high efficiency mode will save you 6-8 percent on your heating gas bill (source: HHIC). Most condensing boilers from the factory are set at 75°C heating flow temparature. So the return temperature is usually within 20 degree below this flow temperature ie 55 degrees and above, it does not condense and therefore it will not run in high efficiency mode. Please note while we talk about return water temperature, it is usually the heating flow temperature and the system setup that decides the actual return temperature into the boiler. 

Condensing mode savings calculator

Combi Boiler Condensing savings calculator

Technique and howto.

So now you understand the theory, the next step is to get your return water temperature to below 54°C for as much of the time as possible. For this we will use a 1-step or 2-step technique. We will always start with 1-step and if you find in winter your house is not getting warm then we switch to a 2-step technique


You will set your flow temperature to 55°C [1] so that the return water temperature will always be below 54°C. Your home will take longer to warm up, so you will need to make sure your programmer timings and room thermostat setting allow for this. For this to work, your radiator sizing and/or home insulation levels must good enough to support this low flow temperature. Ie your radiators must be able to output enough heat to match your heat loss.

You will set your room thermostat to the level at which you are comfortable 21 degrees plus minus 2 is the usual. Lowering the room thermostat will obviously reduce your bills as stated by the energy saving trust, however this article is more about running your boiler more efficiently rather reducing the temperature of your home. If you want to know how to lower your flow temperature adjustments, see section below for your particular boiler in the table, or apply a search. For more advanced controls and techniques see Further Improvements section below.

[1] UPDATE: A 60°C flow temperature setting is an acceptable all round flow temperature that will work for most people and still provide a balance of efficiency and fast warm up times should you want a balance. This is based on Nesta research and Vaillant research.


In this technique, if you find in winter, your home is not able to stay warm with the 1-step technique,  you will find a flow temperature that is higher ie 65°C or what ever is needed for your home to stay warm and set your boiler to this. This way you have a summer and winter flow temperature. At these higher flow temperatures in winter your boiler may begin to condense less ie return temp may go above due point and waste a little more gas, however using this technique at least most of the year you will be able to take advantages of condensing boilers savings.

Flow temperature adjustments.

Search for or select your boiler model / type from the below table to be shown how to reduce your flow temperature. If your boiler is not shown below, consult your boiler manual (if you can’t find it, go to ) or get in touch with us to see if we can guide you or create a video.

BrandModels / FamilyBoiler TypeVideo
MainHE, Eco HE, Eco, Eco EliteCombiVideo
IntergasRapid, HRE and ECO RFCombiVideo
VaillantEcotec Plus/ Exclusive Pre 2012CombiVideo
VaillantEcotec Plus/ Exclusive Post 2012CombiVideo
Worcester BoschGreenstar i ErpCombiVideo


Further Improvements

If you want to improve and move towards a 1step for the entire year, you will need to increase the size of the radiators in order to work at lower flow temperatures or/and decrease heat loss from the building. This could be by sealing holes, replacing leaky windows, increasing insulation. Usually it costs less overall to increase the size of your radiators than to increase your insulation levels. Insulation over the very long term is more effective. This will be addressed in another article.

You can also improve efficiency automatically by using modulating controls combined with correct settings of these controls. See our article on modulating controls. Modulating A Rated Boilers with Smart Modulating Thermostats Some modern boilers that have remote monitoring, you can see how efficient they are running as they show you the return water temperature from an app or a dashboard accessible to you / or the heating professional.

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.