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Plumbing

Cold water out of hot tap/when turned down low

Anonymous user 23/02/2024 - 3.40 PM

Hello, We have an issue of cold water coming out of the hot when the tap or shower are turned down low. When it's fully open it's fine, albeit runs cold I initially but I think that's down to hot water going cold in the pipes. I've tried the other hot taps in the house and they're the same, hot when fully open but when slightly open it runs cold. We also have pressure drop in the shower when another tap is on, a toilet is flushed or the washing machine runs (just like a combi) but I didn't think that happened to system boiler? The tank is 240ltrs. Thanks **Edit. The megaflow is an unvented indirect cylinder. Also I should have mentioned that even after running hot water, once I turned it off and back on again I get cold water initially. And it goes cold when I turn the tap down. ***EDIT 2*** I've done a little investigation and had Thames Water check the mains pressure and flow from the outside entering my property. He said it was good, 4bar pressure at 20litrs a mins on the garden tap. However by the time it reaches my kitchen the flow is halved to 10litres and it's 15litres in the utility room. Regarding the pipes on the mixers this is what I have found out. Family Bathroom Sink mixer tap goes very hot when shower is on cold. Otherwise the temperature is decent and not scolding. When the tap is fully open the pipe is piping hot. When the flow is lowered the temperature goes lukewarm on both the tap and the pipe. The same pipe supplies the mixer shower and it's the same, lukewarm when restricting the flow on both the water and the pipe. The mixer temperature were both set to the hottest setting. Loft Ensuite When both the shower or the sink tap are on the hottest setting the pipe goes very hot. When restricting the flow the temperature of the of the water goes lukewarm but the hot pipe still felt hot. Doesn't make sense to me but I hope it helps you? Also I have found that the pipes going into the ensuite have been restricted from 22mm to 15mm. The family bathroom is 22mm. Thanks Shad

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4 Answers

Dukes of Preston

No reviews yet

Preston

This sounds like you will need a site visit from a Plumbing & Heating engineer, There are multiple reasons this can be happenings, many different system types and designs that can effect the answer to the question, If you have unequal pressure's ( example tank fed hot water gravity and mains cold water / or you might have an unvented hot water cylinder fitted that has a pressure reducing valve without a balanced cold supply) you can get back feed's from mixing valves like shower mixers etc, best thing to do as a home owner is to turn shower mixer to fully hot or fully cold and try the taps again see if this improves the fault you may save your self some labour time if already found reason for the fault before you call the plumber. often you can hear the water being back fed, if it is an exposed shower mixer type you can get your hand to the hot pipe you will feel the cold pushing back down, Thanks Joe *** Edit. you have cold back fed into the hot pipe, You require a plumbing & Heating engineer with knowledge of unvented hot water systems According to G3 section of UK Building Regulations, it is a legal requirement to hold qualification to install, maintain and service unvented cylinders. The issue may not be linked to the cylinder but checking this will be part of fault finding ( Thermostatic mixing valves can cause this fault)

Answered7 June 2021

MCB Plumbing

No reviews yet

Stockport

It's because your taps aren't open enough to fire up the boiler and also when you turn the tap on its got to release what water it has in the pipes (cold) to produce more hot water. It doesn't stay hot for the next time you use it thats why you need the boiler to fire up In order for it to produce more hot water.

Answered7 June 2021

LLYN CAMBRIAN ELECTRICAL PLUMBING

Rating: 4.8 out of 5

Wrexham

Firstly has the system ever been ok? Secondly you say you have a system boiler connected to a 240 litre storage Is this a pressurised system or an normal open vented system Perhaps an edit to the post would help giving these details

Answered7 June 2021

Expert Plumbing & Heating

Rating: 5 out of 5

Coventry

With an unvented system your hot and cold water is fed via a balancing valve, which sounds like this could be the most likely problem, these unvented cylinder systems should be serviced by a suitably qualified engineer ( not all plumbers have this qualification, so need to check their credentials) every year, I strongly suggest you have this checked urgently.

Answered26 June 2021