Boiler too hot!

[KetchupFace]

Does anyone know how to stop a boiler overheating the water? Despite being on the lowest setting, the water that comes out of the shower is scalding and you can't add cold water to it as then the pressure drops to a dribble. Can the boiler be reset somehow so it doesn't heat as much? I have mentioned this to plumbers in the past and not really got an answer.

[Balletgirlmum]

I reckon it's probably a pressure problem rather than your boiler. The wAter has to be stored at 60 & distributed at at least 50 degrees because of legionella.

[dementedpixie]

My shower heats its own water. Is the shower definitely linked to the boiler?

[KetchupFace]

It's definitely from the boiler and not a self-heating shower. It may well be a pressure problem although the boiler is mains-fed and there's no problem with pressure in the other taps but the bathroom cold taps seem to be fed from a tank on the roof rather than the mains. Is there a way of fixing the pressure?

[dementedpixie]

What sort of boiler is it? Do you have a hot water cylinder and water tanks in the loft?

[KetchupFace]

It's a combi boiler, no cylinders or tanks anywhere just this roof tank thing (which I've never really understood the point of).

[PigletJohn]

It's not normal to have a water tank in the loft when you have a combi boiler. How big is this tank, what shape, and is there any water in it?

A mixer shower, where the hot comes from the combi and the cold comes from a tank, will never be successful and is not sensible. It suggests a bodge install. You need a plumber to put some new pipes in.

If you have individual (non-mixer) taps to fill the bath, it doesn't matter for them.

As for temperature, is the hot water out of the hot taps hotter than usual? A thermometer would be useful.

[KetchupFace]

The tank isn't in the loft, it's cube shaped and actually on the flat roof outside covered in the same roof felt (there's no loft). It is full I assume as the overflow pipe leaks sometimes. The bath does have a mixer tap and the hot water from the taps is also v hot but it's less annoying day to day than the shower. Is this a weird arrangement then? It's never been commented on by plumbers that have serviced / flushed the boiler before.

[PigletJohn]

if you have a mixer tap, and run a high-pressure supply and a low-pressure supply to it, the high-pressure will always try to overwhelm the low, so it is difficult or impossible to adjust the temperature to your wishes.

Unbalanced supplies to shower mixers are a very familiar problem to plumbers.

[KetchupFace]

Thanks for the insight, what can they do then? Reroute the mains and effectively cut the tank off? Is that a big job?

[KetchupFace]

I keep thinking about the weird tank now, it's not likely to be dangerous is it like full of bacteria or dead pigeons or lead or something is it??

[PigletJohn]

If you keep running cold water off it to fill your bath, it will constantly refill with fresh chlorinated water from the main, so the pigeons and spiders will be quite diluted.

You are probably not drinking it because your kitchen sink cold tap will (should) be supplied straight off the watermain.

To identify the supply to a tap, put your thumb over the spout. If you can block the flow it is from a tank. If you can't it is off the main.

BTW water that has been heated in a HW cylinder has been pasteurised by the heat, and will generally be safe to drink. In some countries it is safer to drink water from the hot tap in your hotel for this reason.

[KetchupFace]

Thanks for the info, I'll try the supply test tomorrow.