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Gas Combi Boiler Fed Shower Acting Weird?

Postby George_A_ » Wed Dec 05, 2018 5:57 pm

Hi everyone,

I have recently entered a new property.

The property is served hot water by a gas combi boiler at a fixed temperature of 55oC.

The bathroom of the property has a shower mixer with a tap, as per the attached image.

From the beginning, this setup has been acting strangely.

If I run water through the shower tap, or any other tap in the house, I can mix the water by turning on the hot and the cold tap in order to adjust the water temperature to a comfortable level.

If I had water running through the shower tap and I switched the water direction to get water out of the shower hose on its own, this is where things started getting strange.

The same mixed water that used to come, say, at 37 oC out of the tap, came out of the shower hose much colder when the hose was kept at a low height and as I increased the height where I held the hose, the water temperature increased. When I moved the hose to head level (I am 6ft tall), only unbearably hot water came out of it. If I lowered the hose again, the temperature went down again.

But this was not all…

If while holding the shower hose low I connected a shower head to the shower head, only hot water came out.

I have a yearly service contract with British Gas that covers general plumbing issues, through their subsidiary, DYNO.

I called in a DYNO plumber, who came and inspected the shower and told me that the mixer had to be changed, as it was not working properly.

I agreed to that and the guy changed the shower mixer and told me everything should be fine, but he did not test his new installation before he left.

So, I am left with a brand new shower mixer with the exact same problem and some additional ones…

The shower hose that came with the new shower mixer has a much smaller bore than the old one and the shower head is also much smaller in diameter.
When I run the water through the tap, I can still mix it.

However, if I run the water through the new shower hose and shower head, turning on the hot tap makes water come out at full pressure. But when I turn on the cold water tap, suddenly the water flow coming from the shower head is massively reduced, but with only hot water still coming out of it. If I then turn the hot tap off, hot water keeps coming out of the new shower head for like one minute or so before it turns to cold.

I substituted this smaller-bore hose with my old one and the old shower head and I am facing the original problem again –water coming out of the hose increases in temperature with the height the hose is held at and connecting the shower head or partially blocking the flow of water from the shower hose makes only hot water coming through.

Does anyone have any idea what may be going wrong?

I thought I was good at physics, but I cannot think of any reasonable explanation about how the mixed water coming out of the hose is split into cold and hot water and only hot water comes out after I have connected the shower head…

Thank you in advance for your time,

Kind Regards,
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