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Is your condensing boiler actually condensing?

Posted by Cathy Debenham on 11 July 2012 at 9:23 am

"The majority of condensing combi boilers don't condense in hot water mode" said a speaker at a conference I attended recently. I pricked up my ears. Is this a problem? Are people being conned? How much does that affect their performance? I wanted to know. And I also wanted to know how people should make sure they are getting the most energy efficient option.

My first point of call was the Energy Saving Trust, to find out what the criteria for their Energy Saving Trust Recommended boilers are. I found out that whether or not a boiler condenses in hot water mode isn't part of the criteria.

EST asks for SEDBUK 90% or higher and standby under 10W. "We also ask that they declare the hot water heating efficiency for a combi boiler.  If a boiler has a keep hot facility we specify it must be able to be controlled by the user," said a spokeswoman.

"For our next revision, we’ve talked about specifying a level for the hot water heating efficiency – if the boiler is not condensing in hot water mode, this may effect this efficiency value – but we don’t have any data on this at the moment."

So my next step was to talk to the engineers who run SEDBUK to find out more.

Paul Balmer of Kiwa GASTEC at CRE told me that a large number of condensing boilers never condense, because they are set up with the wrong conditions. The dew point is at 55 degrees, and for the boiler to condense the temperature of the water that returns to the boiler must be 55 degrees or below. Many are set up with a flow and return temperature of 70 degrees.

"On the positive side," he said, "modern boilers will always be more efficient than the older boilers they replace, but not as efficient as if they were condensing."

Some boilers also have flue gas economisers to take the heat out of the flue gas and use it to heat the hot water.

And make sure that your room thermostat is sited sensibly. If you put it in a cold, draughty hall, then the system will still be calling for heat when your living room and kitchen are already too hot.

More information about efficiency and boilers from YouGen

Heating and hot water information page

Can I have a solar thermal system if I have a combi- boiler?

Combined heat and power information page

Biomass boiler information page

Photo by Chris Valentine

This blog was edited on 16 November 2012. The assertion that it didn't matter if boilers don't condense in hot water mode, as it's only a small part of the overall output was challenged, and we have removed it.


If you have a question about anything in the above blog, please ask it in the comments section below.

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1 comments - read them below or add one

anne miller

anne millerComment left on: 26 July 2012 at 3:34 pm

A simple way to maximise the efficiency of your condensing boiler is to set the flow temperature as low as is comfortable for you.

 For example, most of the time we set the flow temperature for our central heating at 55C or less.  This means the radiators are pleasantly warm but not "red hot".  When its freezing outside, or if we want to heat the house up quickly because we've been away for a weekend,  we increase the flow temperature to say 70C. 

The higher flow temperature lets the radiators push out more heat which is useful if otherwise they were struggling to reach the temperature set by the room thermostat or TRV.  It also means the house heats up faster.   t.

Similarly don't have the hot water coming out of the taps at a higher temperature than you need.... 

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