I have a combined CH & indirect hot water system, 20 years old but Glowworm boiler only 10 years old. At last service mid-summer hot water wasn't getting hot so the motorised valve (Honeywell V4073A) was changed & appeared to fix the problem. Now even worse. I have followed the checkout procedure for the valve & I'm sure it is working ok & I can hear the valve moving as I alter the jacket thermostat. Boiler runs ok, CH plenty hot enough, pipes in the airing cupboard hot to the touch BUT hot taps in bathroom only run lukewarm. Taps downstairs run a bit hotter. Have tried various combinations of boiler output temperature & jacket thermostat settings to no effect. Has anyone out there got any ideas please? In particular, is it possible for a fault to develop in the hot water cylinder that lets the cold water inlet mix with the hot water outlet? Any help would be most welcome, Tony.
Thanks so much for your help, really useful. Have run hot water only all day, CH off. Feed into heat exchanger coil same temp as return and both too hot to touch. Altering cylinder thermostat switches boiler & pump on & off. Have set stat at 70, which I feel is higher than it should be, & as boiler isn't running all the time I assume cylinder is as hot as it's going to get. Turning on hot tap still produces only warm water, about 40 degrees, & nowhere near as hot as the feed into the cylinder. Looks as though your suggestion of a furred-up exchanger is correct - presume that means a replacement cylinder? Last possibility, could the cylinder thermostat be faulty & cutting out about 40 even when set at 70?
Have a look in the cold water storage tank that feeds the hot water cylinder. It could be that the end of the vent pipe into it is sitting beneath the liquid surface allowing cold water to be syphoned out along with the hot water to the taps. This could happen if, for instance, the washer in the ball tap is worn causing the water level in the cold water tank to be higher than usual.
I wouldn't advise spending that amount of money based simply on advice from this forum. It really needs a second opinion which can only be given on site.
Limescale is possible, but pretty unusual. You need to positively discount other possibilities such as a faulty pump, sticking valve, restrictor valve shut too far, airlock and of course a faulty tank stat, (but that should be pretty much discounted if you can hear it click off, and when on and the pump is running, the water still does not get warm).
Check Dave's suggestion by putting your hand on the centre top connection of the cylinder where the hot water leaves the cylinder, it is pretty much a good indicator of the water temp inside the cylinder.
PROBLEM SOLVED. Many thanks for your advice, particularly to Dave from Leeds who must have a brain the size of a planet. Fault was the vent pipe submerged in the header tank, syphoning cold water into the hot feed whenever hot tap turned on. Thanks too to plumbbob for advising caution over expensive repairs. Have lowered float valve in header tank at zero cost, best possible result, so thanks all round. Tony.