Do combi's condense HW and CH water?

18 Aug 2008
Reaction score
West Midlands
United Kingdom
I was reading through the Bosch literature and for the CDI boilers it states it condenses HW and CH water inferring that their other boilers condense CH water only?

Is that true, do most boilers condense CH water only? If so I guess HW efficiency would be 70-80% and Ch water the 90% quoted in SEBBUK.

But does the sedbuk test not include any elements of using HW?
Sponsored Links
Someone more knowledgeable will be along shortly but the condensing [from my understanding] is a by product of combustion not that the actual heating water is condensed.

I may, of course, have misunderstood the question.
That WB statement is 90% marketing BS, an effort to differentiate the CDi from the rest of the range.

Condensing occurs when the heat exchanger, or a portion of it, is below the dewpoint of the water vapour in the gas combustion products. When this happens, the water vapour condenses and gives up a considerable amount of heat to the exchanger. Clearly the heat exchanger is more or less at the temperature of the water flowing through it. Typically this water will be at a particular temperature entering the heat exchanger and will be hotter when it leaves, frequently starting out below the dewpoint and leaving above it. Because of this and because not all the water vapour will be able to condense out without a very long pass over a cold heat exchanger, all boilers only partially condense if they condense at all. That is, they only extract a portion of the available energy from the water vapour.

So the statements by WB (as well as by many other manufacturers) that their CDi boilers "fully condense" is meaningless and inaccurate. Similarly the inference that the other boilers in the range do not condense in DHW water mode is also meaningless and inaccurate. Both partially condense. Although incoming water much colder than the dewpoint, it doesn't come into contact with the heat exchanger. In all WB combis (and most other domestic boilers), the cold water passes through a secondary heat exchanger where it is heated by water flowing round the inside of the boiler through the main heat exchanger. Differences in the flow rate of this water and the size of the main heat exchanger (much bigger in CDis) mean that the CDi boilers are able to condense considerably more water vapour than the other models. The controls are also set to limit the maximum temperature of supplied hot water to 60C so that water flowing into the main heat exchanger is always below the dewpoint.
Sponsored Links
Thanks Ian for a very comprehensive reply - I have manufacturers that lie/bend the truth to promote their products.

Does SEDBUK not include an element of heating DHW because the differences in efficiency for the i, si or CDI boilers are all within 1% IIRC

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local