Why is our water going hot a and cold?

[KitKat1985]

Just that really. Moved into our new house last week. Everything fine for the first couple of days. FIL came and visited at the weekend to see the house. He likes 'tinkering' with things and said to DH (unbeknownst to me) he would do something to water system so we could have higher pressure when we have a shower (no idea why he thought this was necessary as we had no issues with water pressure). Since then we can't get any consistent hot water. So for example when we have a shower it'll be fine for a few seconds, and then be freezing cold for a few seconds etc etc. DH has been trying to work out what he's done all week so we can fix it but to no avail. I'm on the verge of calling a plumber out. But to be honest I could really do without the bill after an already expensive month. Any ideas wise Mumsnetters?

I've just bled all my radiators and it sorted this problem out for me as there was a lot of air in there.

Okay thank you Againfaster we can definitely try that! x

You've got a combi boiler, I presume? If not, it will be a pressure imbalance.

It has nothing to do with bleeding the radiators.

I think you need to ask your FIL what he did.

If your shower comes directly off the water system (I.e is not an electric shower) the balance is set according to pressure. So your FIL’ ill advised tinkering will have sent it out of kilter.

Maybe.

How is it if you use it when no other water is running? No taps, no toilet cistern filling, no washing machine on? Ours used to go hot and cold when other water was running.

Sorry I should have said - it's a conventional boiler. I had a combi boiler in my previous house and I don't really understand this new conventional boiler!

Piglet John - A pressure imbalance would seem most likely I guess given that FIL was tinkering with the water pressure. Is there an easy way to fix what he did, or do I need to get a professional out?

What colour is your hot-water cylinder?

I expect the shower has high-pressure cold and low-pressure hot. This causes the cold to overwhelm the hot supply for a mixer. If the mixer has a thermostat, e.g. many showers, the shower will then try to turn down the cold. The cycle will then repeat.

In this case, the correct action is to re-pipe the shower supply. As a temporary bodge you can try turning down the service valve on the cold supply. It will not be satisfactory. You can also get a pressure-balancing valve.

Have this issue right now too.

Combi boiler - have to switch off heating for an hour to get water for bath.

We do have a radiator getting fixed so air in system can do it according to Scottish Power guy and doesn't help we also have a cold water tank in attic.

Get your FIL to pay for it - he shouldn't mess with your house

Gryff

Your description does not match a combi.

Have you got a hot-water cylinder?

What does the loft tank supply?

Thanks PigletJohn. The hot water cylinder is a light green colour.

PigletJohn- can't the OP just undo whatever tinkering her FIL took it upon himself to undertake?

yes, by tightening the service valve on the cold water supply. It will then be as good as it was before, but if I am right, it was wrong before and it is an unsatisfactory bodge.

Light green is a low-pressure cylinder, fed from a tank, probably in the loft.

Okay, thanks Piglet John. Without wanting to sound really dense, how do I tighten the service valve on the cold water supply?

I know long-term it will need a 'proper' fix, but we've literally just spent all of our money on this move, so will have to just go with the 'bodge job' option for now.

there might be a service valve on the pipe to the shower. It's the cold one you'd have to throttle. It might look like one of these horrible cheap things which you turn with a screwdriver (it might start to leak if it sees you approaching)

there is a faint chance it will be a better one

If you can't find one, you can turn down the main stopcock, (observe the unique shape) which is horrible, as it constricts all the incoming water to the house

If you can't repipe it, you can get a pressure reducing valve. Caleffi is a popular brand. If your hot water is supplied from a loft tank the pressure will be half a bar or less (ten metres head of water gives 1 bar) and the cold water from the main might come in at 3 to 10bar, which is why it overwhelms the hot and may cause leaks and overflows.

BTW

To see if the cold tap pressure is higher than the hot tap pressure, turn the hot bath tap on. Press your thumb on the spout. Can you stop the flow?

Now do it with the cold.

What happens?

It is possible, but rare, for the shower cold to be taken from the tank to prevent pressure mismatch, even if the other cold taps are high pressure. It mostly varies with regional custom. Plumbers are very traditional. In London, bathroom cold taps are usually from the tank, if there is one, for historical reasons (Jonathan Swift, author of Gulliver's Travels, experienced this when he was Dean of St. Paul's, but London's plumbing practices are much older than 300 years).

That's brilliant advice. Thank you PigletJohn. I really appreciate your help.