Water pressure


My son and his wife live in a Victorian terrace house and have suffered constant problems with their combi boiler tripping out due to low water pressure. The water pressure was low because, like many old terraced properties, there were half a dozen houses fed from a single 'communal' feed from the water main.
My wife and I have just paid out (almost 1k!) to have them connected directly to the water main. The water pressure has increased considerably but, if you have both hot and cold taps running on the bath, the hot water flow slows to a virtual trickle and I can't understand why.
Their boiler is a Worcester Bosch 240 combi - about 9 - 10 years old I think. Personally I think it is a bit underpowered. I have a W Bosch 30CD which is great - but if I turn on a cold tap when a hot tap is running, although there is some reduction in flow from the hot tap, it certainly doesn't slow to a trickle.
Assuming that the water pressure in my son's house is now up to standard - why should the hot water flow slow down so dramatically when a cold tap is turned on?
Ret.
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Ret. wrote:

Personally, I'd have demanded that the water board give you the minimum requirement of pressure and flow which from your description you never had (I think it is something like 1 bar and 7 litres/min) If I had needed to spend 1K, I'd have gone for the water storage option rather than continuing with a combi but easy to say in hindsight.. This aside, water pressure and water flow are different. A new supply should have sorted this but perhaps the flow gives one tap a decent output but two - less so. This may be hampered but a water filter within the boiler being blocked with crud since they installed a new pipe? Also, the the flow restrictor inside the boiler is probably choked given age and possibly crud too. You may wish to invest in a water softener (if your boiler is happy with them - ask manufacturer) as this will not only prevent scale build-up - it will slowly remove old scale too and this is not to mention the other advantages of these things. Good luck
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The Water Board (United Utilities) provided the feed from the water main to the pavement outside my son's house free of charge. The 900 odd pounds that we had to pay was for providing the feed from the new pavement stop-cock into the house and also for disconnecting the old pipe from the communal feed. The Water Board will not allow the old feed to be simply capped inside the house because (they say) this will leave a length of pipe from the old communal feed containing standing water which would turn stagnant. The charge was for digging up the pavement outside the house, taking the feed through the foundations of the house and also digging a hole in the back yard to disconnect the old feed. It was a two day job.

The previous occupiers had the combi installed when they converted the loft into a third bedroom. There really wasn't the room for a conventional system when they did that. My argument was, however, that whoever installed the combi shouldn't have done with the variability in the water pressure due to the communal feed from the water main.

Yes - something that we can look into it.

The water in our area is very soft and there is no scaling up of kettles etc at all. I don't even bother putting salt in our dishwasher - it just doesn't need it.
Ret.
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I agree. But I also have a combi, (albeit a more powerful, and condensing, boiler than my son's) and I don't have the same problem. If I have a hot tap running and then turn on the cold tap fully - the flow from the hot tap does decrease somewhat - but certainly not to a trickle like it does with my son's. Possibly, there is less flow restriction through my boiler because it can handle a greater flow and still mainting decent hot temperatures.

That's what he is doing - but I was trying to determine if the reduction in hot water flow when the cold tap is turned on is normal or not.
Ret.
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wrote:

With mine and three other I have access to, it is normal. So much so that we have the same embargo on tap use during showers as we had with the old mixer tap.
The amount of modulation that the boiler does on the flow is also a major factor.
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Thanks for that Eric - at least we know now that this is not a continuing problem with water pressure! I have been doing a complete bathroom make-over for my son and his wife to coincide with the direct link to the water main. For the first time today, since the job (the direct connection to the water main) was completed, my son was able to try out the new thermostatic mixer shower that I have installed for them. Frustratingly, although the boiler ran for much longer than had been the case previously - before the shower had been completed, the boiler over-heated and tripped out again. Clearly there is a problem with the boiler!
Ret.
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Ret. wrote:

If the flow is acceptable when only hot water is being run off - then the problem isn't flow restriction through the boiler.
If the inlet pressure was constant - it wouldn't matter whether the cold tap was on or not - the only effect that the cold tap /can/ have is to lower the inlet pressure to the boiler.
The water pressure at the inlet to the boiler is falling, a lot, as a result of the cold tap being turned on. Easy enough to check.
I've seen, overseas, a pressure tank next to the boiler with a non-return valve in the cold water feed. The tank sustains the pressure, so the hot water flow doesn't (instantly) reduce when a cold tap is turned on. I've never seen this sort of system in the UK though.
-- Sue
-- Sue
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We had a similar problem with our previous house, I am not a plumber so bear with me... we had a new kitchen sink and hot/cold taps replaced with a mixer..., aparently the fitter didn't put in a one-way-valve to the new mixer tap on the hot water side. This meant that when we "mixed" the water, all we got was cold water, apparently the pressure of the cold was somehow forcing its way into the hot system and pushing it back. !?!?
But... I don't understand much of this, so can only tell you what the (second) plumber told me. He fitted a one-way-valve to the hot pipe running into the kitchen mixer, and we had no problems afterwards.
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Thanks for that - we'll look into it!
Ret.
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Ret. wrote:

That will be where mains pressure cold actually goes back up the hot pipe where the hot water is significantly lower pressure. Won't be the case here as both are the same pressure as both fed off the mains supply.
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