Plumbing help. No hot water coming out of new thermostatic mixer tap

[8MinutesToSunrise]

Hi, I'd really appreciate any help or advice. Single mum, super tight budget so trying to fix things myself.

My electric shower broke, so as an alternative I fitted a new thermostatic mixer in the bath last weekend (gravity fed). Worked alright for a few days, there was a delay in getting the hot water to come through but I could hasten it by running the kitchen tap hot. But as of this morning I can't get any hot water out of it at all. Cold is fine and hot water is fine out of the sink taps.

Any ideas really appreciated. Had a cold shower this morning, but really don't want to make that a regular thing!

have you got a hot-water cylinder? What colour?

Or a combi boiler?

Do you mean, no water comes out? Or do you mean the water is not hot?

Put your thumb over the bathroom basin hot tap. Turn it on. Can you stop the flow?

Now try it with the basin cold tap. Can you?

Does your bath shower mixer have two knobs? Or one lever? Photo would help.

Thank you for replying.

Yep, I've got a hot water cylinder. Not sure which colour. Cant get to it at the moment.

When the thermostat is turned to 38 just a drip comes out, if i turn it cold water comes out fine. Hot water elsewhere is fine.

With my thumb I can stop the flow of the hot but not the cold in the sink.

It's a Drench Clara Thermostatic Bath Shower Mixer, it's a bar style, one side is on/off, and the other controls the temperature.

"With my thumb I can stop the flow of the hot but not the cold in the sink"

That tells me you have high-pressure cold water from the mains, and low-pressure hot water from a tank.

The mixer you have used is unsuitable and will not be satisfactory. It is also quite likely that internal leaks will allow the high-pressure cold to force its way up the hot pipes and into the cylinder, causing the tank to overflow.

This is not unusual.

Sorry.

Bugger. That was an expensive learning curve.

So, would a normal mixer tap work, or do I need to go back to single taps?

best results will be with single bathtaps.

Traditional UK designs are likely to have larger waterways, which give better flow from low-pressure supplies.

if you really want a mixer shower, you probably need a pressure-limiting valve added on the cold supply to the bathroom. If you go that way, you could try it with your current mixer.

Something like this Honeywell

The gauge allows you to adjust the pressure to match the pressure of the hot (which you can calculate from the height of the cold tank above your tap. Ten metres is about 1 bar). There is an Italian brand whose name escapes me but seem good. The Honeywell one looks about the same.

An alternative is to run a new cold pipe from the cold water tank in your loft, to the bath or shower outlet. Sometimes this is easier. But pressure from a tank-fed shower is poor unless you add a pump (more expense, and noisy)

There are (or used to be) some rare mixers that can accomodate high-pressure cold and low-pressure hot. But it is fundamentally unsuitable.

Or give up and get another electric shower fed from the high-pressure cold supply?

update

I see the Honeywell has a minimum pressure of 1.5 bar, which is probably too high for you.

I've remembered the Italian brand. Caleffi.

Thanks