Boiler Blues

[aurora83]

The problem I am having is that the hot water seems to run out very quickly. Despite the boiler being on for hours, and on max temperature, the water takes a long time to heat up. Then when it finally heats up, it only last a very short time. Sometimes the water comes out the tap quite warm, but then after a few seconds goes cold again. This happens upstairs and downstairs. The central heating seems to be ok, sometimes takes a while to get going but fine once hot.

It is a conventional boiler, with the boiler in the kitchen, and the cylinder upstairs. The boiler is quite old (about 20 years), but still in good condition as told by a couple of plumbers about a year ago.

I just need some advice as to what the problem could / might be. I will have to get a plumber in, but it's best to know a little bit before calling someone in. There a better chance of not getting ripped off.

[buildersmate]

If it is an old system, the circulation of the hot water in the 'boiler' part of the system to the cylinder may be gravity feed. This means that the water circulates only through the heat exchanger in the cylinder by using of the physical property that hot water is less dense than cooler water, so rises. If the circulating pipes start to become scaled or sludged up, it doesn't take much to impede the flow of hot water to the heat exchanger. Try feeling the temperature of the 2 pipes into the side of the cylinder. The upper one should be hot - the lower one cooler. Usually the upper one goes into the side of the cylinder about half way up, the lower one is close to the bottom.

If the system doesn't work by gravity feed, there is most likely a three port diverter valve somewhere in the CH pipework - usually close to the cylinder. This switches the boiler water to feed either the CH system or the cylinder, or both, as demanded by the thermostats in the system. If you have this sort of layout you will most likely have a thermostat mounted on the side of the cylinder. Is this working? It will have a temperature dial on it to enable you to change the hot water temperature. Try twiddling it upwards. Does this have any impact on the water temperature after say one hour?

Based on what you say, the most likley scenario is sludging up of the system. You could desludge it yourself using chemicals from B+Q or a plumbers merchant. It could be you haven't got the thermostat high enough (as above). Both these you can probably deal with yourself before calling in a plumber. Other possible fault mechanisms are a failing pump, or a failed three port diverter valve, for which you probably need the help of a plumber. HTH.

[pilotlight]

Just to add to buildersmate's most helpful reply. As the boiler is approx. 20 years old, there is a possibillity of domestic hot water thermostat failure.( that is if your boiler has 2 thermostat controls,one for heating, one for hot water ). If so, as you turn the control clockwise / anticlockwise, listen for a small " click ". If no click --- thermostat replacement required.

[stanleyman]

Is there an air vent approximately halfway down the cylinder on the hot feed pipe going into the cylinder? If so, when the system is off and cold, loosen this to allow any air to escape.

Check that the hot water timer control is actually switched on and is not just running on the immersion heater.

[aurora83]

Thanks for all your answers.

Felt the two pipes, the upper one is warm and lower cold, but i am sure i have felt the upper one much hotter, even to the point where you cannot touch it.

According to wht you have said, it looks as though I have a three port diverter valve as there is a thermostat fixed to the lower part of the cylinder. I will try as you said turning the cylinder thermo up, it is at 50 and goes upto 80.

Will maybe try de-sludging.

The thermostat on the boiler seems to be working, makes clicking sound, and fires up when turned up.

I can't see a vent on the cylinder.

A few years ago we had a problem which involved replacing a a small silver coloured box upstairs, connected to the pipework near the cylinder. There are two off them, but i don't really konw what they are?

[stanleyman]

If you have two silver boxes then that is an S plan system and they are motor valves, most probably the syncron motor has failed in the motorvalve controlling the hot water and won't open. You can open the valve temporarily by locking the little lever on the side open, but the hotwater may only work when the heating is on.

You can get a syncron motor from B&Q or a plumbers merchant. They are fiddly to fit, remove cover, remove usually only one screw holding the motor assembly, don't drop the screw, turn and wiggle the motor to remove. Two wires need to be reconnected to the new motor, reverse procedure to refit. Switch off the power to the boiler before starting.

[aurora83]

Thanks Stanleyman