Optimum condensing temperature?

13 Sep 2004
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United Kingdom
How do you persuade a condensing boiler to condense.

I read a 'ravenheat' document suggesting an output temperature of 60-65 degrees is most efficient.
Other articles I have read say they only condense when warming up and become less efficient when ticking over.

Is there an optimum temperature for condensing boiler efficiency.
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The flow temperature is not relevant. Its the return temp that needs to be below approx 55c. A decent condensing boiler will always try to adjust its output to achieve this.

The installer should have designed the system to suit. If its a retrofit then there are some compromises. A condensing boiler should be no less efficient when ticking over than when starting up.
Yes it will. Connect the condensing boiler to underfloor heating where the flow is at 50 degrees and temp drop 20. Boiler will be in condensing mode at all times.

Connect the boiler to an existing installation requiring a flow of 85 and drop of 11 degrees stretched to 20 degrees, fat chance of condensate runoff.
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citizen said:
How do you persuade a condensing boiler to condense.

I read a 'ravenheat' document suggesting an output temperature of 60-65 degrees is most efficient.

Just installed a Ravenheat primary condensing boiler myself, and I'm suprised that you were even able to acertain the suggested temperature from their installation manual, as in my view, the manual is "pants".

Holding the return temperature down to 55 degrees or less is a much more difficult problem than most engineers seem to realise. OK - if you're starting out with a new design including nice low-temperature applications like underfloor heating - great, no problem at all. But where you've got conventional radiators, even new ones, there will be problems straight away, starting with the spec of the rads themselves. In many cases, the output of a rad is quoted as 'x watts at delta T of 50 degrees' - in other words, room temp 20 degrees, average rad temp 70 degrees. To design for a lower water temperature you need to get adjusted figures for rad outputs with water at average (say) 60 degrees. And even in the case of underfloor systems, there's often a parallel requirement to heat the water to 55 with a reasonable recovery time, which tends to push the required flow temp the other way, to at least 75 degrees. So the underfloor system (which of course has a blender valve and its own circulator anyway) gets fed with water at 75 or 80 degrees and the boiler spends most of its time not condensing.

In the case of retrofits, where a condensing boiler is added to a system with existing radiators sized based on 70 degrees (at least), there have to be even bigger compromises between effectiveness and efficiency.

And just because there's a pretty, ecologically-sound steam plume coming out of the flue, this does not mean that the boiler is actually condensing efficiently. Sod's Law dictates that the coldest part of the boiler will be at 57 degrees and all the condensing will be happening in the flue, not in the heat exchanger.

Don't get me wrong - I've had a condensing boiler for the last 5 years and I'm very happy with its performance. But I always run it at a low setting and I'm happy to tolerate long recovery time for hot water and slower response to heating demand.

Government 'initiatives' to force condensing boilers into every conceivable situation (new-build, upgrade, retrofit, ..) are bound to create lots of unhappy customers who will not be able to understand why their systems seem so unresponsive!

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