Water Tank Problems

2 problems have recently started with my hot water tank.
1. Every time we have a bath and the tank fills up it starts to drip from the overflow. Running the hot tap for a minute or 2 causes it to stop. 2. At the same time this started I also noticed that I dont get as much hot water into the bath until it starts to run cold.
I seem to remember one time someone told me your tank can start to fill with dirt in the bottom of it. Is this the problem and is it a case of removing the element and scooping out the dirt on the base of the tank. Its awkward because its up high on the wall of the bathroom but I replaced the element about 10-15 years ago so I can do it ok!
Or is the water running cold too soon simply a case of a new element reqd? I always thought they just stopped dead and dont remember the water getting gradually colder in this manner.
Any help or ideas appreciated!
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Conventionally,
This might be caused by the insulation drooping and preventing the ball float from rising up to shut off the water, otherwise ....
It is caused by the disintegration of the rubber washer in the ball valve on your cold tank in the loft - typically caused by age and a build up of calcium on the moving part. Locate your stop cock and switch off - good time to find it now and make sure you can turn it since it will be -135 centigrade by Wednesday. If you are DIY proficient, remove the whole assembly from the tank, then pull out the split pin and barrel slider that contains the washer should pop out. Remove te old washer and clean off all the crud with wire wool and kettle descaler. re-Assemble in reverse order and refit. Houses built from mid 80's on will probably have cheapo plastic version. Alternatively get a new (mains pressure) ball float assembly for about 8 and chuck away the old one.
However I am confused by mention of an element. Do you have a combined hot and cold tank ?. This looks like one unit but is in fact two separate tanks. If so I still think the problem is the ball valve assembly on the cold tank which is on top.
--
Andrew

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snipped-for-privacy@sPydata.uklinux.net says...

Purleeease!
Part 2 float valve, http://www.screwfix.com/app/sfd/sea/searchresults.jsp?n 518 under 3 from Screwfix, probably double that locally. Don't faff around repairing old piston valves or whatever, unless you've seriously got time to spare on your hands :-)
Remember to fit a new fibre washer (tap connector washer) to the bit that connects the pipework to the inlet to the float valve (unless it's a connector with an olive like half a compression joint) and you could do worse than fit an isolation valve like http://www.screwfix.com/app/sfd/sea/searchresults.jsp?n 483 in the feed pipe.

Scale: you can end up with bucketsful in an old HW cylinder.

Nope, elements do sudden death not reduced output.
I suspect the float valve has been getting slow at letting in water as well as not stopping it when full, so the cold water storage [part of the] tank is emptying quicker than if there were a good flow replenishing what's being drawn off. What happens when you run out of hot water? Does the hot water stop flowing, or does it continue flowing, but running cold? If the former then my theory could be right, if the latter it's not. But replace the float valve anyway and see.

Sounds like a combined ('fortic' type) unit to me, too.
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