combi boiler with cold tank in roof

Discussion in 'Plumbers' Talk' started by camo, Jun 27, 2006.

  1. camo

    camo New Member

    I'm a bathroom fitter and have a query about cold water tanks remaining in use when a combi boiler has been fitted.

    Often I find that bathroom cold supplies are still being fed by the cold tank in the roof rather than cold mains.

    Is this because regs say that you should have a cold tank or is it laziness on the part of the chap who installed the combi?

    I often have the option of chopping out the old cold feeds from the roof and running cold supplies to bathroom services in mains cold. Any reasons why I shouldn't do this?

    Cheers.
     
  2. Walterfilter

    Walterfilter New Member

    who knows why it may be left, such an easy change over by taking the mains supply in the loft and conecting it to the cold feed to the bathroom....

    Either laziness or the homeowner wanted a separate supply in case of a water shortage??

    No reason why you shouldn't change it over, no regs either way, plus your customer then has potable water in the bathroom and a quick fill loo..........
     
  3. bentbolt

    bentbolt New Member

    Quite often find this in older properties. The trouble is by connecting to the old tank fed pipework with the cold main puts the old pipe and fittings under a lot more pressure, usually most of this pipe is not visible and usually the toilet ball valve is not up to the extra pressure. "Are you feeling lucky today"
    Pete
     
  4. Water Systems

    Water Systems New Member

    Quite often find this in older properties. The
    trouble is by connecting to the old tank fed pipework
    with the cold main puts the old pipe and fittings
    under a lot more pressure, usually most of this pipe
    is not visible and usually the toilet ball valve is
    not up to the extra pressure. "Are you feeling lucky
    today"
    Pete

    Best leave the tank and only have the toilets off it. In case of a cut the toilets work. All else off the mains and change the toilet ballcock to a high pressure one and have an overflow into the bowl siphon.
     
  5. mario500

    mario500 New Member

    Sometimes the combi itself can be tank fed! I know strange but true - Britsh Gas have a habit of doing this with their own brand boilers. Check the bar pressure first and see if there is a filling loop. The first time I came accross this, it threw me until I realised what they had done. A worcester (BG brand name boiler) will fire up and work correctly on a very low bar pressure.
    It is worth you checking these things first otherwise you might find yourself doing an alteration on the heating system too.

    hope this helps you.

    Allan
     
  6. Water Systems

    Water Systems New Member

    Sometimes the combi itself can be tank fed! I know
    strange but true - Britsh Gas have a habit of doing
    this with their own brand boilers. Check the bar
    pressure first and see if there is a filling loop.
    The first time I came accross this, it threw me until
    I realised what they had done. A worcester (BG brand
    name boiler) will fire up and work correctly on a
    very low bar pressure.
    It is worth you checking these things first otherwise
    you might find yourself doing an alteration on the
    heating system too.

    hope this helps you.

    Allan

    What did they do for showers with the combi fed from the tank?
     
  7. Moss

    Moss New Member

    Many mixer taps require equal pressure. If the hot supply to the mixer is from the combi at mains pressure, the cold must also be at mains pressure.
    Exceptions to this rule apply to mixers with two separate water channels when the mixing occurs at the tap outlet.
     
  8. Water Systems

    Water Systems New Member

    Many mixer taps require equal pressure. If the hot
    supply to the mixer is from the combi at mains
    pressure, the cold must also be at mains pressure.
    Exceptions to this rule apply to mixers with two
    separate water channels when the mixing occurs at the
    tap outlet.

    I'm not quite sure if these BG combis had the hot and cold fed from the tanks. If so then a power shower pump would need to be used for a decent high pressure shower.

    If only the hot (combi) is off the tank and the cold is off the mains then a Grundfos in-line pump can be used on the hot (combi) and a pressure equalising valve on the hot and cold for the shower. The valve ensures equal pressure at both hot and cold ports. Some mixers have these integrated these days.

    The only advantage is that a cylinder is not needed. Also water backup in case of cut offs too. The reduced pressure on the combi innards may add to longevity.

    Either way a pump has to be used.
     
    Paul James Gomes likes this.
  9. PTFE

    PTFE Member

    Where I live it was standard practice to install cold water storage cisterns to combat the problems with poor mains pressure, which I'm told became even more apparent when the men used to come home from work and the demand on the mains peaked at around 5pm. The CWSC allowed baths to be run etc.
    About 5 miles from me the houses are fed from a different resevoir and the mains pressure has always been spot on, so there are no CWSC in the loft and the bathrooms have always been mains fed, even in the very old terraces.

    I've beed helping a mate put central heating (2 x combis at the owners choicer) into a house that's being converted into 2 flats.
    The mains pressure is ****.
    We've pointed this out to the owner but after being quoted over £1000 by the water co. to install a second main he's decided to go for a 25mm alcathene replacement and tee it off inside the house for the upper flat.
    We've warned him again that he's asking for trouble but he's adamant. Ah well, you can only try .....

    PTFE
     
    Paul12345 likes this.
  10. Water Systems

    Water Systems New Member

    I've beed helping a mate put central heating (2 x
    combis at the owners choicer) into a house that's
    being converted into 2 flats.
    The mains pressure is ****.
    We've pointed this out to the owner but after being
    quoted over £1000 by the water co. to install a
    second main he's decided to go for a 25mm alcathene
    replacement and tee it off inside the house for the
    upper flat.
    We've warned him again that he's asking for trouble
    but he's adamant. Ah well, you can only try .....

    That should be OK. Only when running two baths together will there be problems. Assuming good flow and pressure of course.

    In each flat have a 22mm feed to the combi. At the stoptap have a tee and have all the cold taps off this. Divide and rule.
     
  11. PTFE

    PTFE Member


    That should be OK. Only when running two baths
    together will there be problems. Assuming good flow
    and pressure of course.

    In each flat have a 22mm feed to the combi. At the
    stoptap have a tee and have all the cold taps off
    this. Divide and rule.

    The flow rate at the first floor - presently via the original lead main - is awful. I haven't measureed it but I'd guess at around 6 or 7 lpm. The 25mm alacthene will obviously help this and we'll run 22mm as far as we practically can, but I still have my doubts in terms of usable pressure.
    The owner, who's 'doing the place up' seems to be doing things in a rather strange order anyway.
    He's installing a new staircase in at the back of the property to access the first floor flat, but has had us installing pipework in the area where he 'thinks' the stairwell won't encroach - but can't be sure. He's already got his brand new fridge freezer and stainless steel oven/hob, unpacked, in the kitchen which is still to be half demolished, replastered etc etc.
    I have a feeling we'll be back before the summer's out to either move or repair some pipework - or both! :)

    PTFE
     
  12. A common reason for leaving cold supplies on the loft cistern is because the mains flow is low. It makes a mixer shower difficult to control though.

    I dont see why but a combi manufacturer told me that it is against "regulations" to feed a combi from a cistern although I dont see any reason against it. His concern was legionella because the cistern was not well chlorinated.

    Tony
     
  13. r2d2

    r2d2 New Member

    All else off the mains and change the toilet ballcock to a high pressure one

    Dont you mean low pressure one Water Systems if it is fed from a gravity fed storage tank.??
     
  14. Water Systems

    Water Systems New Member

    A common reason for leaving cold supplies on the loft
    cistern is because the mains flow is low. It makes a
    mixer shower difficult to control though.

    I dont see why but a combi manufacturer told me that
    it is against "regulations" to feed a combi from a
    cistern

    Tripe.

    although I dont see any reason against it.
    His concern was legionella because the cistern was
    not well chlorinated.

    It is possible to have the combi outlet below 60C, the killing temp.
     
  15. mario500

    mario500 New Member

    Sorry I mustn't have explained properly what I meant.
    The cold supply to the boiler would remain the same as usual - (from the rising main coming into the property), so hot water outlet from the combi, has the same pressure as cold outlets on showers, basins etc.
    It is the central heating circuit that is fed from the f and e. Thats why the bar pressure reading is much lower than usual, but enough to make the combi fire up when required.
    So taking this out would mean that you would have to seal and pressurize the system as normal.
    Usually they would be fed from the original feed and expansion tank, that would have been in use before the installation of the combi - but I have seen them fed from the old cold water storage tank too. Maybe because it was an excuse not to take out the old tank.
    Hope I have clarified for all the confused!

    Allan.
     
  16. Water Systems

    Water Systems New Member

    Sorry I mustn't have explained properly what I
    meant.
    The cold supply to the boiler would remain the same
    as usual - (from the rising main coming into the
    property), so hot water outlet from the combi, has
    the same pressure as cold outlets on showers, basins
    etc.
    It is the central heating circuit that is fed from
    m the f and e. Thats why the bar pressure reading is
    much lower than usual, but enough to make the combi
    fire up when required.
    So taking this out would mean that you would have to
    seal and pressurize the system as normal.
    Usually they would be fed from the original feed and
    expansion tank, that would have been in use before
    the installation of the combi - but I have seen them
    fed from the old cold water storage tank too. Maybe
    because it was an excuse not to take out the old
    tank.
    Hope I have clarified for all the confused!

    Allan.

    Few combis can be open vented and fed from an F&E tank. The BG was made by W-B for them. I can't think of a combi that is open vented. It is a case of getting touch with the makers to identify a model that can.

    Open vented are better in many ways, especially if there is no way of getting a pressure relief pipe to the outside.
     
  17. Water Systems

    Water Systems New Member

    Sorry I mustn't have explained properly what I
    meant.
    The cold supply to the boiler would remain the same
    as usual - (from the rising main coming into the
    property), so hot water outlet from the combi, has
    the same pressure as cold outlets on showers, basins
    etc.
    It is the central heating circuit that is fed from
    m the f and e. Thats why the bar pressure reading is
    much lower than usual, but enough to make the combi
    fire up when required.
    So taking this out would mean that you would have to
    seal and pressurize the system as normal.
    Usually they would be fed from the original feed and
    expansion tank, that would have been in use before
    the installation of the combi - but I have seen them
    fed from the old cold water storage tank too. Maybe
    because it was an excuse not to take out the old
    tank.
    Hope I have clarified for all the confused!

    Allan.

    Few combis can be open vented and fed from an F&E tank. The BG was made by W-B for them. I can't think of a combi that is open vented. It is a case of getting touch with the makers to identify a model that can.

    Open vented are better in many ways, especially if there is no way of getting a pressure relief pipe to the outside.

    Using an existing 50-100 litre cold water storage tank as the F&E tanks is not good practice at all. It means that extra inhibitor has to be added to compensate for the cold water tanks volume. That could be 3 or 4 one litre cans.
     
  18. Water Systems

    Water Systems New Member

    A common reason for leaving cold supplies on the loft
    cistern is because the mains flow is low. It makes a
    mixer shower difficult to control though.

    Many people want to get rid of cylinders as they take up valuable space and look unsightly, but the roof tank may takes no usable space at all. If the mains flow is poor but enough for the hot water then a cold tank may be left in to cope with the cold supplies - with the cold mains to the tank throttled down so it doesn’t rob the DHW.

    Then the DHW is not affected when toilets are flushed or the kitchen tap run.

    To get the cold supply up in pressure to the cold side to a shower, a Grundfoss in-line pump can be added (about £80) and a pressure equalising valve before the shower mixer, if one is not inside the mixer valve itself.
     
  19. Paul James Gomes

    Paul James Gomes New Member

    Hi, I'm facing a similar issue, and would like your advise. I have a potterton combi boiler that feeds from main, but in the bathroom the cold comes from tank in the roof. this creating a problem in the shower. I have a thermostatic mixer shower, and its gets totally cold after a few minutes, stays cold for 20-30 secs, then gets hot again. What are my options? I cant get rid of the cold water tank as we have low to almost no water problem during the morning. Can you please advise?
     
  20. KIAB

    KIAB Well-Known Member

    Do your neighbours have same problems with mains pressure, how old is house.
     
    Paul James Gomes likes this.

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