Instantanious gas water heater heat exchanger.

I have a frost damaged (obsolete) 32kW gas heat exchanger.
Can the copper pipe be soft soldered to fill the split?
If the heater is beyond repair is a combi boiler a practical option for direct hot water when there are no radiators to supply?
Any comments would be welcome.
HN
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Which one?

I would imagine so. It would be pretty impossible in the finned area because you can't really get to it, but if it's exposed pipe, it should be possible. I would cut a patch from a piece of copper pipe, form it to be a close fit over the split, clean up both mating surfaces with steel wool to be bright copper, flux, tie in place with copper wire (may need to be quite thick so blow lamp doesn't melt it), and solder with lead-free solder.
Some of these (certainly my 11 year old MAIN one) have internally teflon coated pipework to prevent scaling up (which seems to work briliantly as I've never had to descale it, whereas the previous one had to be descaled every 1-2 years). Not sure what the soldering might do to that.
Can you not get replacement heat exchangers?

Sometimes no, as some require the heating circuit to dump excess heat when you switch off the hot water tap. A long loop of pipework may be enough though.
It looks like MAIN still do two models - balanced flue one which uses only gas, and a fanned flue one which also requires an electricity supply for the fan and controls.
I replaced a much older and long obsolete MAIN water heater 11 years ago. The new one was designed to hang on any of the balanced flues of the long obsolete models, which made installation very easy. I only needed to rearrange the water pipework which wasn't in the same position. The brochure implies this still holds for their current balanced flue model. It only took about 2 hours to swap them over (and having done it once, I could probably do it again in an hour).
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Andrew Gabriel
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On Sat, 29 Jan 2011 10:48:10 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

An old Mersey Main.

Thanks, I may give it a try. It is the exposed pipe. There are no fins BTW, just a grey squarish metal box structure with the copper pipe brazed? to the outside.
As it is hollow in the center, I always did wonder how it could have any degree of efficiency at all.

I tried a dealer in Birmingham, they said it was unobtainable. Never tried anywhere else.

I have seen the Main unit. It seems tempting. The only thing is it is rated at 25.5kW with 23kW into the water.
Perhaps you could comment on the relative poers of your replacement?
The original was 32kW with 27kW to the water, and I would never have considered it a very effective supply from new.
I'm wondering if the lack of fins had anything to do with it's poor performance.
I assume gas input/ water heating output is what 32kW/27kW on the ratings plate means ?
Many thanks for the information, it was extremely helpful.
HN
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It sounds like you have not noticed the most significant part of the heat exchanger.
Across the inside top of the grey squarish metal box structure, the pipe snakes back and fourth inside a set of close spaced copper fins. That's the main part of the heat exchanger where it absorbs the heat from the gas burners. The pipework also thickens in this area to slow the flow rate and allow more time for the heat transfer.
You should periodically check the heat exchanger to make sure the fins are clear of debris, as part of servicing the unit. When I first moved in, I found lots of the fins had curled over along the edge partially blocking the flue gas path. I've no idea how that happened, but I bent them back straight and they didn't do it again during the following 14 years (at which point I replaced the unit with a newer model).

I noticed the new one was less powerful, but I found the original one (which had no proportional gas control) was far too powerful for a shower. In order not to burn yourself, you had to turn the flow rate up so high that if you accidently let go of the shower head, you could never catch it again, and the high pressure made water spurt out of all the water seals in the shower hose/head and you ended up with a dripping ceiling, etc.

If yours really has no fins, it's really badly broken.
You might also want to read... http://www.diyfaq.org.uk/humour.html#uninstalling
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Andrew Gabriel
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On Sat, 29 Jan 2011 12:49:01 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

Many thanks. I never really looked at the top bit. I suppose I assumed it was a bend to direct the waste gasses to the flue.
Aah well One lives & learns.
Again many thanks for the enlightenment, I thought the square box didn't look too fuel efficient.
HN
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On Sat, 29 Jan 2011 12:49:01 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:
SNIP

Just looked at this. Amazing, it happened to me some years back. It blew apart a soldered joint in the feed to the heater.
The cause was a pin from the top of the diaphragm to the gas valve having stuck in the "up" position.
I recollect cleaning the pin and resoldering the joint. I suppose I should have looked into the rest of the unit in more depth as there would probably have been further damage.
HN
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Same here. In my case it stuck full open. More commonly, these heaters fail such that it didn't fully close and there were a row of tiny flames left on the burners, resulting in a slow boil a little while after you stop using it.
Newer models have protection against this. An overheat detector on the top of the heat exchanger interrupts the pilot thermocouple circuit and causes the gas valve to instantly spring shut.
The pilot sensor on my old model was a bi-metalic strip, and that had a number of ways of not working properly. The most common one was that if the pilot light jet got partially blocked, the flame would shrink below the main burners, and the bi-metalic strip would half-close the main gas. When you turned on a tap, gas came out of the main burners at half the rate it should, couldn't find the pilot light because it was below the burners, until it filled the whole thing with a perfect gas/air mixture, and then it lit explosively, variously blowing itself right out, and/or blowing the end off the flue. This seemed to happen often enough that I eventually drilled a 1mm hole in the side of the burner pointing at the pilot flame, and that completely solved the explosive ignition problem. The new one (11 years old now) has never done this, and I suspect the problem with the old one was down to debris dropping from the heat exchanger due to whatever event happened to it before I took it over which had partially melted/burned the bottoms of the copper fins.
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