hot water in bath goes cold but hot water available

24 Jan 2004
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United Kingdom

Can someone explain why when I try to run a bath with hot water, the hot water coming from the tap gradually goes cold? It cannot be that I don't have enough hot water in my boiler because when I turn the hot water sink tap hot water comes out.

My bath taps are not the mixer type. I also have a separately installed shower above the bath and produces constant hot water.

When I turn off the bath hot water tap and turn on the sink hot water tap to check for hot water and re-turn on the bath hot water tap the hot water reappears but then disappears shortly afterwards.

Any help greatly appreciated.
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Hello Makkie
I seem to be having the same problem, i've checked the boiler and pump, the automatic flow valve, and the flow and return pipes to the cylinder and they are all ok (pipes are good and hot). There is no air in the rads, the only other thing i have noticed is, when i first turn on the hot water tap there's a short gurgle of air, which didn't happend prior to a roof conversion (which means the contractors had to turn the water supply to the water tanks off to replace them, these by-the-way are a block
of flats). I'm thinking may-be it's some kind of airlock within the cylinder, the only thing i carn't get my head round is why does water still come out
of the hot water taps,if surly if it was an airlock problem, i would't get any
water, would i?

Did you manage to find out what the problem was?

have you got a combi boiler or system with boiler + hw cylinder
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Makkie, the problem as i suspected, an airlock, but not the variety which
shows itself with big gaps in the flow of water (air burst's, then water
releasing or no water at all) from the hot water tap, but because a new
water tank was fitted above a newly built level of flats.

The problem was caused with the ball valve's pressure flow nipple being
the wrong size apparently(not enough flow into the water tank) and thus,
the demand of water leaving the water tank was greater than the supply
to it, this means that the water line was regularly just under the outlet
pipe and dragging small amounts of air as it flowed away into the cylinder.

The air builds within the tank, but is not enough to cause a noticeable
airlock, and only a fraction of the water is being heated by the heat
exchanger (coiled pipe in the cylinder tank), as soon as the hot water tap
is opened, the air's released you get a small amount of hot water and the
rest is cold.

The other possibility that was pointed out to me, is that sometimes the
heat exchanger in the cylinder ,is caked in so much scale that there is
hardly any exchange of heat, and so again, the water flows but there
is very little hot water. If you run your boiler (for 'Hot water' only) and run
it for a great length of time (more than you would normally have to, to
heat the water), making sure you don't run any of the hot water taps
while the boiler's running (say three or six hours), if you then get a full
tank of constant hot water, this probably means you don't have an
'airlock' type problem, but more likely, a cylinder that's full of scale, and
needs changing.

I hope this of some help to you


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