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 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.
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)
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.
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.
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).