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I'm having a boiler replaced and have chosen a combi boiler. One of the benefits is mains water pressure hot and cold water where previously I had low pressure coming from a tank. The boiler is in place and the hot water is plumbed and the pressure is fine, but putting the mains to the cold taps in the bathroom is a problem.

I'm the top flat of three storeys (ground floor; first floor; my floor) and there is a large water tank in the loft which serves the block. Due to access restrictions and the way the place was built the pipes go up in to the loft and then down in to the bathroom. We are going to do the same with a new mains pipe to the bathroom. However, we can't just drop a new mains pipe down in to the bathroom unless we rip the bathroom out because the pipes go to the mixer behind the tiling and we can't see a way to drop a new pipe down. It's a bathroom that is less than 5 years old so I'm loathe to start ripping it apart.

Could we cut the tank pipe and put the mains through it? This would give me and any other flats mains pressure for our cold taps in the bathrooms. Is there something that could go wrong with this?

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  • I'm assuming you are removing the tank supply, not connecting to it in addition. You don't mention what sort of piping is used for the tank supply. The only issue I can imagine is if the piping cannot handle the presumably higher mains pressure. Usually tanks are installed when mains supply is inadequate (due to pressure and/or volume), can I assume this situation has resolved itself? If the piping is up to current standards you should have no problems if the supply is adequate.
    – bcworkz
    Commented May 29, 2014 at 22:23
  • Thank you for all comments. The advice given made us re-think the approach. We found an alternate way to route the mains to the bathroom without the tank that was less destructive than originally thought. The tank is still in use for those potentially expecting low pressure and for my bathroom everything has a direct supply of mains cold and pressured hot from the combi.
    – Johnny Maelstrom
    Commented Jun 5, 2014 at 10:43

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Some taps and other fittings and appliances are rated for either high-pressure supply (direct mains) or low pressure supply (from a header tank in the building).

If any occupants have any fittings or appliances that are only rated for low-pressure, converting the building to high pressure could cause problems.

As others have noted, converting completely from low-pressure to high pressure means isolating and emptying the old header tank.

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Is the tank sealed? If it's open, you still won't get mains water pressure through the tank.

The other option would be to just bypass the tank - ie cut the pipes in and out of the tank and put a piece of pipe in to connect the pipes together.

Here in the UK, it's not usual (and might even be against the water regulations) to have a tank directly connected to mains supply. You need to have an air gap to stop water being able to syphon back into the mains. You might be able to achieve this with a check valve to stop backflow.

Basically, you don't want water from the tank getting back into the mains, as the tank water could be (or become) contaminated.

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  • I've accepted RedGrittyBrick's answer, but will actually try to find time to write both up as one if I can. Thanks.
    – Johnny Maelstrom
    Commented Jun 5, 2014 at 10:42

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