Condensing/combi boiler vs. pipework question

[Dave Phillips]

FOlks

Quick boiler question...

I currently have a Saunier Duval combi boiler installed in my house. The install dates from 1987, i.e. before I lived there. It has been running fine, although possibly not terribly efficiently of late (my perception).

At a recent routine service, the engineer (the same company as I've used for some years) says that the boiler will need some fairly major parts replacing: heat exchanger, pump etc, which will cost a few hundred quid. I was expecting this. He then looks at the pipework leading in and out of the boiler, and tells me that it is actually the wrong size (15 mil instead of a larger size- maybe 22mil?) and points to the various step-down pipe adaptors fitted there to enable just that fact. I actually knew about this already, since another engineer from the same company had told me the same thing some years ago, so I am not too surprised to hear it again.

My question is this. Is the wrong size pipework a major problem; should I have the current boiler repaired or should I have it replaced, and in the latter case, would I have to have a condensing boiler fitted rather than another combi? I believe there are new regulations about boiler efficiency coming into force next Spring?

One more thing. Bear in mind I will probably move house next year.

tia Dave P

[Andy Hall]

At that age, it is nearing the end of its useful life. I wouldn't spend several hundred quid on it.

In most domestic installations, apart from ion large properties, 22mm at the boiler flow and return is normal and adequate. Below this, the flow rate through the pipes becomes inhibited and it's difficult to transfer the heat. To some extent the pump deals with this, but to mode the same volume of water, it has to flow faster and more noisily. Beyond a certain point it's not really possible to go further. In short, the pipes from the boiler and forming the main trunk to the radiators should be in 22mm up to a point where enough branches to radiators are done to bring the flow requirement to a sensible level.

This isn't the decision.

You can have boilers without combi function that heat radiators and a cylinder or you can have a combi. Separate to that, you can have condensing or not in both types (i.e. 4 main permutations) Actually there are more, but this is the essence.

It has been mooted that the minimum SEDBUK efficiency will be raised to 86%. This is not achievable with the typical 78-80% conventional boiler of current manufacture. Whether that happens before next Spring is not completely clear, but it is the aim.

It is beneficial to get a condensing boiler anyway and yes you can get a condensing combi.

You may prefer to get a cheap non-condensing combi in that case.

.andy

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[Christian McArdle]

IIRC, 15mm is good for around 6kW, which will just about run a flat or a well insulated small house. Should the official requirement be greater than

15mm, it won't just stop working, but may get a little noisy, as you'll have to turn the pump speed up to compensate for the losses. This causes the water to rush faster and may disturb the smooth flow of the water, causing greater amount of noise than may be desiried. Provided that all the radiators heat up well, even in the depths of winter, and the noise isn't too bad, there is no real need to go repiping.

Not yet, although they are usually recommended. Also, as condensing boilers are typically designed for a greater temperature difference between flow and return, they can shift more power into the same size pipe runs, so may alleviate the pipe sizing issues, particularly if the radiators are oversized, allowing a 70/50 system, instead of the conventional 82/70.

You may be getting confused about the term "combi". The "combiness" of a boiler is unrelated to its condensing nature. You would need to replace with a condensing combi boiler. However, as you are moving out soon, you may find it financially more beneficial to pick some cheap bottom of the range traditional combi and hope it lasts until you move.

Christian.