Gas boiler / condensate query

Discussion in 'Plumbers' Talk' started by Mosaix, Nov 29, 2016.

  1. Mosaix

    Mosaix Member


    I am at my daughter's. She's got a Worcestershire Bosch gas condensing boiler. Her washing machine is immediately underneath. The condensate drain is tapped into the same plastic pipe that the washing machine drain is connected to.

    She's just had the boiler maintenance engineer visit and he's left her with a warning notice:

    "Siphon inside boiler is draining and filling whilst washing machine is on. There is a potential for P.O.C to come(? can't really read this word) back into the boiler."

    He had the cover off the boiler when he was here and the washing machine was on / pumping out. He showed my daughter the level in the condensate tank(?) rising and dropping whilst the washing machine was on.

    Is the water from the washing machine backing up into the condensate pipe? Can't see how it can as the condensate tank(?) is four inches above the open end of the pipe where the washing machine drain pipe hooks inside. If it was back flowing that high wouldn't it also back up over the top of the open drain pipe?

    Maybe the level in the tank is rising and dropping due to air pressure as the washing machine pumps out?

    Thanks in anticipation.
  2. KIAB

    KIAB Well-Known Member

    P.O.C. Products of combustion.

    You can get carbon monoxide poisoning .
    Last edited: Nov 29, 2016
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  3. KIAB

    KIAB Well-Known Member

    Some boilers can pump CO back through the condense if the trap is empty of water, so carbon monoxide poisoning .
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  4. Mosaix

    Mosaix Member

    Thanks KIAB, appreciated.

    Is he saying that it needs a separate drain?
  5. KIAB

    KIAB Well-Known Member

    Yes, definitely.
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  6. Mosaix

    Mosaix Member

    Hmm, difficult. It's an ex-council first floor flat. There's a single boxed in cast iron drain pipe that runs the height of the block with a single tap off point in each kitchen for a drain. Hence all the drains - sink / dish washer / washing machine / boiler - sharing a single drain pipe.

    Any ideas?
  7. KIAB

    KIAB Well-Known Member

    Don't dabble with gas myself, but others on here that do, & will have a solution.
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  8. Mosaix

    Mosaix Member

    Thanks, KIAB,
  9. Hi Mos.

    I think it's exactly as you say - the pumped water flow from the W/M is causing changing air pressure which is in turn making the condensate trap level bob up and down (like when you look in to a toilet bowl on a windy day... or is that chust me?)

    I would say there is no risk of the W/M flow actually going up that pipe, for the reason you said yourself. However, it clearly ain't good and needs sorting - and I guess it is possible for the W/M flow to empty that trap through suction, although that would take some doing.

    'All' it needs is an air break, a bit like the way the W/M hose drops in to the open ended pipe - the gap around the hose allows air to follow in and prevent 'glugging'...

    So, a similar arrangement could surely easily be done for the condensate drain - a bit like a 'tundish' arrangement. But, it would obviously need to conform with the boiler's installation guidelines.

    I'm sure GasSafes will come on and suggest summat.

    Any chance of a photo to show the W/M outlet and also how the condensate pipe is connected - and where? (Keep it under 2MP).
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  10. Glad its Friday

    Glad its Friday Active Member

    I've seen this before but not an issue to fix fortunately.
    As DA says, you need an air break. The condensate should be teed in to the w/m waste but run the 22mm down inside the 40mm pipe without sealing it. Then as the w/m empties it will just draw air down rather than trying to empty the condensate trap on your boiler.

    Would have been quicker to just fix it than fill in the warning notice. British Gas by any chance?
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  11. Mosaix

    Mosaix Member

    DA, thanks for your reply. Unfortunately back home now so photo not possible. Tundish sounds like a great idea but see my reply to GIF below.

    GIF, also, thanks for reply which seems absolutely the answer. One query, that may also apply to the tundish: the W/M outlet pipe flows straight into a ubend 'trap' with the condensate pipe connection 'downwind' of the trap. Wouldn't any air break be the wrong side of the trap allowing the possibility of foul air back into the flat? Could I introduce a second ubend trap and allow the condensate pipe to drain directly into it with an air gap of course? Come to think of it why couldn't the condensate pipe share the same trap as the W/M? Spot on with British Gas BTW although, to be fair, what I didn't say is that between the boiler and the W/M is a work surface with some very tight clearances.

    After I posted yesterday and as I left for home I looked back at the flats. All the ground floor flats have the condensate pipe exiting through the outside wall into plastic gutter-to-grid drain pipes. None of the first and second floor flats have this - they must all be internal. There is a plastic drain pipe just a couple of feet away from my daughter's boiler. I suppose if I fancied an hour or two twelve feet up a ladder in this weather I could do it. On the other hand a heating engineer might be the best bet.

    I would appreciate both your thoughts on my idea of sharing the same trap as the W/M. If it's a no no, then fair enough.

    Again, thanks both. I am always pleasantly surprised that professionals are prepared to give up some of their time to help out the likes of me with a bit of advice.
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  12. KIAB

    KIAB Well-Known Member

    The plastic drain pipe is a better option, you can use a long masonry bit & drill a 8mm pilot hole from the inside out, then drill it out to the right size with a 22mm or what ever condensate pipe is, most from the inside, but you will to drill outside a bit tostop break out of brickwork.

    Discharge condensate pipe into hopper or maybe a use a strap on boss with 22mm bung.

    Mcalping do a boss with 22mm outlet to fit rainwater pipe.
    Last edited: Nov 30, 2016
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  13. Jitender

    Jitender Well-Known Member

    Can the condensate discharge into the drain pipe, if it is a rainwater pipe?

    Have seen it done around the area, and most if not all carried out by council contractors.
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  14. KIAB

    KIAB Well-Known Member

    Yes, have it done same way here, drains into rainwater hopper.
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  15. Jitender

    Jitender Well-Known Member

    Could have used a condensate soak-away as flat is near to the ground.
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  16. Jitender

    Jitender Well-Known Member

    Found this.

    Says that an air break is requited externally as well.
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  17. Jitender

    Jitender Well-Known Member

    Some of the boilers fitted around my are they have just drilled a hole in the down pipe and made good with silicone.

    One plumber a few years back I watched burned a hole using the mapp torch and then stuck 22mm pipe into it. May have used 22mm copper first?

    one installation recently has used a 112 downpipe branch insted of hopper.
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  18. Jitender

    Jitender Well-Known Member

    Some people mentioned using an acid neutrailser if the system is purely for rain water, not foul.

    Something like this.

    But have seen inline ones which look neater, don't know if these need topping up with refil?
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  19. GiF is the pro here, Mosaix, so I'd be inclined to go with his idea. Also saves you having to drill through your exterior wall - best avoided if you can...

    At the moment you have a W/M trap which consists of a vertical length of 1 1/2" pipe which goes in to a U-bend? Ok, what I believe GiF is suggesting is a very simple addition of a 'tee' piece in to that W/M vertical pipe, nearish the bottom where it fits in to the U bend. Basically, you cut the W/M's vertical pipe a few inches above that U bend and insert a tee, with the middle connection of the tee coming out sideways. On to this middle tee connection you fit an elbow (short length of pipe needed to join them unless it's a 'socket' elbow) and then a further short length of pipe in that elbow so's it comes up vertically alongside the W/M pipe to a similar height.

    So, you'll have two vertical pipes effectively going in to that same U bend. One takes the W/M hose and t'other the condensate pipe - both will have natural air gaps around them. Jobbie jobbed.

    Ok, important points - the W/M pipe's top will now be a few inches higher than before 'cos it has that extra tee in it, so you need to trim it until it's at the previous height (assuming that was the correct height - check your W/M installation instructions!) Next point is, you need to ensure the vertical pipe for the condensate is tall enough to take the condensate pipe safely with it going in, say, a good inch or so, but with the end of the condensate pipe not going down too far where it might touch the backflow from the W/M pumping out. In theory, the W/M 'flush' shouldn't come much higher up it's vertical pipe than the height of the drain pipe on the other side of the U bend - does that make sense? In practice, with a powerful pumping-oot, the W/M water might temporarily 'bob' up a few inches above the U bend's top height, so you need to ensure that the end of the condensate pipe doesn't go in anywhere as far as this.

    Pretty sure that's what I would do. Provided it's 'legal'...

    If you type the make and model of your boiler into Google and add 'installation instructions', you will find diagrams showing the various ways the condensate pipe can be connected to your drain.
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