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Discuss Has hot water from expansion pipe damaged cold water storage tank? in the Central Heating Forum area at

  1. Venger

    Venger New Member


    Trying to get my head around what may have caused our, admittedly very old, plastic cold water storage tank to split and leak through the bathroom ceiling.

    We have what I believe is referred to as an open vented system - conventional boiler, cold water storage tank and small header tank in loft, hot water cylinder in airing cupboard.

    There is a small Drayton HTS 3 thermostat attached to the cylinder which is set to just under 60 degrees.

    To be honest, despite doing a lot of reading up online, I have several questions about how the various aspects of the system work and why, for example, we have an immersion heater in the top of the cylinder that has no power cable attached to it?!

    Anyway, this week we had water dripping through the bathroom ceiling so I go up into the loft and find an inch long split at the base of the tank, close to where one of the pipes enters the tank. Surrounding timbers and the base the tank sits on where all soaked.

    I can't for the life of me understand what's caused the split, if the pipe had been wrenched or taken a blow stressing the point it joins the tank, I could maybe understand but nothings fallen on the pipe and I certainly haven't touched it. However, something a friend of mine said to me has got me thinking - he said it's possible that hot water travelling up the expansion pipe from the hot water cylinder discharges into the cold water tank and if there was enough of it, it could raise the temp in cold water tank and potentially cause a split in 30-40 year old plastic.

    Is that feasible?

    Under what circumstances would water do this, is it when there is too much or too hot, water in the cylinder and it needs to go somewhere?

    This has got me thinking that perhaps we produce too much hot water, currently the hot water is on 4hrs in the morning and 4hrs in the early evening and there's only 2 of us in the house.

    Or am I confusing things, surely the boiler won't run continuously heating water, it'll run until the water gets to the temp set on the cylinder thermostat and stop?

    We went on holiday for the first time in years for a week recently and one thought was that 8hrs of hot water was still being produced everyday (and not being used) so would that mean it would be discharged into the expansion pipe and subsequently the cold water tank?

    Sorry for the long post.

    Thanks for any clarification you can give.

  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Normally just age or lack of a good tank stand base

    And no water isn't on for the whole 4 hours as the cylinder stat controls the boiler when to heat up (in the allowed time)

    And if it's lagged the water would stay quiet hot for a few days so the boiler might come on every other just to top the heat in the cylinder up

    Was your cold tank round ?
    • Agree Agree x 2
  3. Venger

    Venger New Member

    Hi Shaun,

    Thanks for the quick reply.

    Yes, the tank was round.

    So technically, we're not producing 8hrs of hot water every day, rather there are 8hrs in the day when, IF the hot water in the cylinder falls below the thermostat temp, it will activate the boiler to heat more water?

    Under what circumstances does water travel up the expansion pipe?
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Common either on the side or the base (due to minor expansion and contraction)

    Yes correct you could get away with prob 2 hours in the morning and an hour at night

    And if the thermostat fails it's a safety pipe so the boiling water has some where to go and not turn your cylinder into a pressure cooker with no safety valve
    • Agree Agree x 1
  5. oz-plumber

    oz-plumber Plumber

    As water heats it expands.
    That's why on sealed systems there is an expansion tank.
    In you situation - open vented - the hot water expands into the tank - minimal in your situation & not enough to heat the tank to a temperature where the plastic would be compromised.

    Your problem is what you stated 30 - 40 year old tank.
    Plastic doesn't last forever and goes brittle over time.
    See whether insurance will cover the costs.
    • Agree Agree x 1
  6. Venger

    Venger New Member

    Many thanks for the clarification Shaun, much appreciated.
  7. Venger

    Venger New Member

    Many thanks Oz...and good shout about the insurance, will check.

  8. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    More than welcome

    Good luck
  9. Best

    Best Trusted Plumber Top Contributor!!

    Bit late on my reply, but just want to say that it depends on a lot of things.
    The round tanks are the strongest design IMO and should last the longest. But that depends on the quality of the plastic, if it has a fully supported and level base, if sunlight can't damage the tank and if tank doesn't get much hot water into it. It also is a strong possibility that the tank was damaged many years ago when it was folded to squeeze into the loft, or the plumbers van, etc, and left a weakness that only failed now.
    Just thought of another possibility, - installer had heated a copper pipe to push a hole in the tank instead of using a drill with hole cutter. Heat will weaken the plastic around the hole.
    Plastic will stress and eventually crack on rectangular tanks, that's for sure. Round tanks don't have much stress points.
    The wood base can be twisted, but look okay and obviously twist the actual tank base.
    This all shows why it is important to turn your mains water off if house vacant, or when you go for a holiday, no matter how short a time
    Last edited: Aug 10, 2017
  10. Venger

    Venger New Member

    Hi Best, many thanks for the reply.

    We had a 50 gallon coffin tank fitted yesterday which, having now read your post and also just having a shower with much reduced pressure (will post a question about that in the shower forum) I'm regretting already!!
  11. Best

    Best Trusted Plumber Top Contributor!!

    Coffin tanks can be fairly strong, due partly to the fact that they are not tall and the weight of water is spread over a large area.
    Certainly the all inclosed coffin tanks are strong.
    Your head of water shouldn't have changed much, simply if you just had the old tank replaced with a coffin tank, but on same base level.
    If tank is on same base position, then perhaps a valve is not fully open, or partial airlock, or small bore valve(s) have been installed and need removed.
    Another possibility is some crud had got into your shower, blocking filters
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