Arghhh no hot water problem: need help please!! Maybe it's the boiler??

[Fierceflora]

Ok we have a vaillant ecotech boiler which we had installed in about 2009. Over last few months weird stuff has been happening with our taps. I should mention now that the boiler is in cupboard in bathroom which is next door to kitchen. We have no bathrooms or sinks etc upstairs.
Firstly sometimes when we were running the taps in the kitchen doing the dishes, we would turn off the tap and suddenly the hot water tap in the bath would suddenly come on! Where it had been fully turned off before (someone may have had a bath say an hour previously), it would now be turned on.
Other odd things- hot water tap in bath would run lukewarm or perhaps even old, but turning the over head shower on for a few seconds then turning bath hot tap on again would result in brilliantly hot water again. Odd. But ok as at least it would give us hot water.
Now today, usual prob with bath running lukewarm, did the showers trick and had hot water. Then started also running kitchen tap. Bath tap went cold. Then kitchen tap went cold. Bathroom sink tap also cold. Usual shower trick didn't work. In the end I turned boiler off for 5 mins and then back on. Bath tap still cold UNTIL I ran sink hot tap too (but ran cold) but simultaneously the bath tap then ran HOT but only when sink tap was running!
Oh should also mention heating is totally FIne through all this weirdness
If you got through all that and understood It you deserve a freaking medal! Please please can any plumbing or boiler experts tell me wtf is going on??? Thank you Muchly cxx

It sounds like this is a combi boiler you have, and perhaps you live in a hard water area.

The sticking taps are I think either very worn, or possibly scaled up with lime (or both). Perhaps they are older than the boiler and are not coping well with the extra pressure a combi gives.

They are probably the type with a capstan head that screws up and down by several turns between off and on. It is tiresome to take taps right off and replace them, as it involves lying on your back and squeezing under baths, while trying to undo nuts you can't see, so consider learning to take them apart, clean, rewasher and reassemble with silicone grease on the moving parts and PTFE tape on the gland and screw threads. This can all be done from above, and once you learn how, you will enjoy it so much that you'll want to do all the taps in the house. Or you can fit tap revivers at higher cost, which swap the working parts of the tap and the capstan head, for new ones.

But if you employ a plumber, fit new taps throughout. Bristan taps are especially good. Unnamed imports, especially if they are ceramic taps with a quarter-turn from off to on, may need replacement parts that are unobtainable.

The other problem you describe, about the temperature of the water from the hot taps varying when other hot or cold taps are turned on an off, is due to the flow switch in the boiler reacting to the hot water flow, which varies when taps are turned on. Perhaps it is also worn or scaly. A boiler engineer can replace it. He may also comment on other aspects of the boiler, ask if the heat exchanger seems scaled or dirty, and ask him to test the dynamic flow of your incoming water supply. It should be in the region of 12 to 20 litres per minute. More flow is better than less.

If the pressure is excessively high, your taps may wear out faster.

Vaillant is a good brand of boiler. Ask whoever normally services it to take a look. Boiler engineers have a much higher hourly rate than plumbers, so if you can find a well-recommended and experienced plumber, ask her to do the taps.

Opinions differ, but I am not a combi enthusiast, and would argue that these problems would not occur with a cylinder-based system.

test the dynamic pressure and the flow of your incoming water supply.

Thank you very much pigletjohn that's very helpful, am going to try and call boiler man out tomorrow I think. It has been serviced before the start of this winter so it's disappointing it's gone wrong now. Yes you're right it's a combi boiler. Is the other type you mention the type that would have a water tank?

I will bear in mind your suggestion maryshelley. It's a very old house so could be ,right?!

Ooh is it also worth mentioning we had a brand new mains water pipe installed from road to house in September?? Can't remember if problems may have been since then....

older hot-water cylinders are fed from a cold tank in the loft, but more recently, unvented cylinders, for example the Megaflo brand, are getting more popular. They are usually large, and can provide unsurpassed hot water. You do need to have good incoming flow for that, and some older houses with a half-inch lead waterpipe benefit from running a new supply pipe out to the pavement for a better flow.

it's possible that changing the pipe dislodged grit, limescale or sediment. The pipes ought to have been flushed through.