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Condensing boilers with single pipe systems


 
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Malcolmkeith

from United Kingdom

Joined: 06 Jul 2004
Posts: 37
Location: United Kingdom

PostPosted: Tue Jul 06, 2004 11:12 am Reply with quote

I live in a 4 bed detached house (1935) which has an ancient Potterton oil boiler (it looks about 40 years old at least). This wonderful old boiler has given me good service for the last 15 years but last winter's oil cost was around 1000 pounds and this seems much too high to me. Also on any night close to zero degrees we have to light a coal fire to stay warm.
I have improved the insulation as best I can and now I am turning my attention to the boiler.
I would appreciate advice on the following points.
a.Should I stick with oil? (There is no gas in the house and BG quoted 500 pounds to install it.
b.A condensing boiler seems to be sensible and will be compulsory soon but I have a single pipe central heating sysytem and I have been told that condensing boilers wont run in condense mode on a single pipe system.Is this correct? As I understand it the returning water temperature has to be low enough to trigger condensing but surely there is a way to do this even on a single pipe system
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croydoncorgi

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PostPosted: Tue Jul 06, 2004 12:34 pm Reply with quote

I think there will be other issues.

Most (all?) condensing boilers expect (if not, require) a 'sealed system', ie. pipework with no Feed & Expansion tank. I suppose in theory there's no reason why a one-pipe layout could not be adapted but I've never come across one and there may be problems....

Main operational mismatch will probably be high return temperature. I don't see how, with a one pipe system, you will be able to limit it and still get half-decent performance out of the radiators. (From your comments, performance is already questionable.)

Even if you solve that, there's still the filth issue!! This is a very old system, presumably with at least some steel pipes and cast iron radiators. Chances are, that parts of the system are mostly FULL of sludge. If you put a low-water-content, high efficiency boiler on the end of that, the sludge will fill up and ruin the heat exchanger in hours if not days! Trying to powerflush a one-pipe systems effectively would be difficult, if not impossible: even if you get most of the muck out of the pipes, there's no practical way of putting pressure across each rad to shift the sludge in these.
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kevplumb

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PostPosted: Tue Jul 06, 2004 9:50 pm Reply with quote

totally agree with croydencorgi plus i wouldnt even contemplate pressurising a system that old
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Malcolmkeith

from United Kingdom

Joined: 06 Jul 2004
Posts: 37
Location: United Kingdom

PostPosted: Wed Jul 07, 2004 9:24 am Reply with quote

>>Even if you solve that, there's still the filth issue!! This is a very old system, presumably with at least some steel pipes and cast iron radiators. Chances are, that parts of the system are mostly FULL of sludge. If you put a low-water-content, high efficiency boiler on the end of that, the sludge will fill up and ruin the heat exchanger in hours if not days! Trying to powerflush a one-pipe systems effectively would be difficult, if not impossible: even if you get most of the muck out of the pipes, there's no practical way of putting pressure across each rad to shift the sludge in these.<<

I can understand your concerns here but maybe the situation is not quite as bad as it sounds. The heating system was extended 15 years ago when the house was extended (just before I bought it), so half the radiators were new 15 years ago and I assume it was flushed then. The remaining radiators are steel but a lot older. I have already replaced 2 that were rusting so if need be I can replace the rest of the old ones. There are no cast iron rads or steel pipes, it is all copper. When I replaced the 2 old rads I did not notice any sludge in the water.
So what would you recommend. I think from what you say a condensing boiler is out but I notice that even some of the non condensing boilers are 92% efficient which must be a big improvement on my old boiler.
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kevplumb

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PostPosted: Wed Jul 07, 2004 7:29 pm Reply with quote

got to be better than what yuove got ,i would think , still have the system cleaned tho
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croydoncorgi

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PostPosted: Wed Jul 07, 2004 11:10 pm Reply with quote

The 92 percent efficiency can ONLY be achieved under perfect conditions! In fact on MOST systems, even well-designed ones, the boiler will only spend part of the time in full condensing mode and therefore achieve 90% + efficiency.
This is why return temperature is important: if the whole heat exchanger is hotter than the 'dew point' (56 degrees) no condensing will occur at all - it can't break the Laws of Physics! Ideally, the whole system should be designed to work at a lower temperature than a 'conventional' one.
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Malcolmkeith

from United Kingdom

Joined: 06 Jul 2004
Posts: 37
Location: United Kingdom

PostPosted: Thu Jul 08, 2004 11:00 am Reply with quote

croydoncorgi wrote:
This is why return temperature is important: if the whole heat exchanger is hotter than the 'dew point' (56 degrees) no condensing will occur at all - it can't break the Laws of Physics! .


This part I understand. The bit I don't understand is why a 2 pipe system achieves it and a one pipe can't. Surely if there are enough radiators you can arrange the return temperature on a one pipe system to be 56 degrees can't you?
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croydoncorgi

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PostPosted: Fri Jul 09, 2004 9:15 am Reply with quote

Problem is that the RADS on a one-pipe system need a big temperature difference between water in rad and water in pipe to create convective flow into the rad.

The single pipe is probably in fact a complete (set of) loops, from Flow on boiler, round house, through pump and back to boiler Return connection, without the water necessarily going through a rad. With this layout, it may not be possible to square the circle between decent performance from the rads and low enough temperature at the Return.
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Malcolmkeith

from United Kingdom

Joined: 06 Jul 2004
Posts: 37
Location: United Kingdom

PostPosted: Fri Jul 09, 2004 9:20 am Reply with quote

I think you have just convinced me to update the system to two pipes. Thanks for your help, it's not easy to find this kind of information anywhere.
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