Hot water cylinder loses its heat way too quickly

[Frenchoid]

Hi,

In the last few days we've had a problem with our hot water system. We have a gas-fired boiler, and a bog-standard indirect system with heat exchanger - gravity fed. The boiler is firing up the radiators nicely, and heats the water in the airing-cupboard cylinder too...kinda!

The problem is that within a couple of hours of the boiler going off, the water in the cylinder goes from hot to barely luke-warm. Also we can't get a good hot bath out of it even when the cylinder appears to be full of hot water.

The bottom section of the cylinder is always cold and I can hear water running through a pipe somewhere - could the cold water tank be re-filling the cylinder when it shouldn't be? When the cylinder's hot the expansion pipe up into the loft seems really hot too, although that might be normal.

Now this might be completely unrelated but a few days ago the immersion heater got accidentally switched on and appears to have burned out. We've never used the immersion heater so I wasn't that bothered... unless somehow that's caused the problem here!

Any pointers gratefully received!

[burnhal]

My understanding is that the tank will only fill when there is space to fill it....and space to fill it occurs when hot water is drawn off- so logic says that something must be emptying the hot water from the tank.
Have a look at http://www.diydoctor.org.uk/projects/domestic_ hot_water_systems.htm#Stored%20hot%20water:
dont know if your system is similar but if the temperature relief valve was stuck open this would cause the symptoms. I guess if the immersion heater was left on for ages it could cause that to lift.
Alternatively we wait for a plumber to throw his two pennysworth in!!

[Frenchoid]

Thanks Burnhal! I'll take another look when I get home but I'm pretty sure the cylinder is fed from the tank in the loft rather than the mains, as in a vented system - which from the diagrams on that excellent web page seems to be the only type with a temperature relief valve.

[gasman]

There could be a number of things going on all at the same time here. Here is just a coulple of possible senario's.

The washer in the ballc0ck in the header-tank could be worn or the arm needs adjusting as it may be filling the header tank up too much & even to all-most overflow point. This will then alter the designed head of water calculated for the rising thermal of the expansion / vent pipe and it becomes comprimised and circulates round the feed and expansion pipes, too and from the hot water cylinder, thus constasntly replenishing the hot water cylinder with cold water.

or...

you may have a leaking tap washer under you're bath or a leak anywhere on the hot water pipe work you are not aware off, and again the hot water is constantly been replaced with cold.

There are other possible things but these are 90% of the times the main problems we encounter.


As you say the first poster did not recognise the system you have and their answer was for sealed systems only. You will find you have no relief valves installed.

[gasman]

oh yeah !!! and the immersion heater is unrelated as you suspected.

[Frenchoid]

Gasman, you are an absolute star - just checked the header tank and the valve is indeed letting water in constantly, and not to overflow point so it must be going down to the cylinder. I'll adjust the ballcock accordingly!

Many thanks again - !

[Panic Button]

Thank God for Gasman again!