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"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.

DW Heating 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.

Is your condensing boiler actually condensing?, Is your condensing boiler actually condensing?, Is your condensing boiler actually condensing?