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Old 15-05-2008, 11:28 PM   #1
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D/ Hot Water too hot!

In need of some inspiration please. Domestic hot water getting way too hot for about a week now.

Have conventional vented fully pumped system. Replaced both hot water cylinder stat and motorised valve actuator as precaution but problem still exists. Cylinder stat fittted 1/4 - 1/3 up on cylinder and cylinder wall by stat is hot, cooling towards bottom.

Raise cylinder stat and pump and boiler fire up; Turn down and both switch off. Thought it may have been a sticky mechanical valve but when I've removed the actuator having switched the supply off, the mechanical valve spigot seems to be at 90 degrees to the pipe, indicating it's fully closed.

Boiler stat set to low setting and turned cylinder stat down to 50 which is perhaps a bit too low but got a hunch it won't help.

I keep coming back to a sticky mechanical valve but if that were the case, would the boiler not perhaps be on more continuosly or conversely be induced to switch on and off more ' rapidly '?

Any thoughts greatly welcome.

Thanks John
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Old 16-05-2008, 12:37 AM   #2
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You havn't got an electric immersion heater switched on in your hot water tank with a dodgy thermostat insert have you? Normally the immersion thermostat would switch off as the central heating raised the water temperature but if it was faulty? Otherwise you got to suspect the new thermostat or the switching valve sticking despite the new actuator.

Last edited by grober; 16-05-2008 at 12:39 AM.
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Old 16-05-2008, 08:53 AM   #3
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Quote:
Originally Posted by grober View Post
You havn't got an electric immersion heater switched on in your hot water tank with a dodgy thermostat insert have you? Normally the immersion thermostat would switch off as the central heating raised the water temperature but if it was faulty? Otherwise you got to suspect the new thermostat or the switching valve sticking despite the new actuator.
Thanks Graeme. Immersion element was replaced a couple of years ago but will remove the switch fuse to eliminate. As you've implied, I keep returning to a sticky mechanical valve also. If it is that it will be a nuisance to replace as I'm tight for space and it has female threads whereas I think they only make them these days with male threads.

Thanks once again. John
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Old 16-05-2008, 09:00 AM   #4
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Quote:
Originally Posted by Parrotman View Post
In need of some inspiration please. Domestic hot water getting way too hot for about a week now.

Have conventional vented fully pumped system. Replaced both hot water cylinder stat and motorised valve actuator as precaution but problem still exists. Cylinder stat fittted 1/4 - 1/3 up on cylinder and cylinder wall by stat is hot, cooling towards bottom.

Raise cylinder stat and pump and boiler fire up; Turn down and both switch off. Thought it may have been a sticky mechanical valve but when I've removed the actuator having switched the supply off, the mechanical valve spigot seems to be at 90 degrees to the pipe, indicating it's fully closed.

Boiler stat set to low setting and turned cylinder stat down to 50 which is perhaps a bit too low but got a hunch it won't help.

I keep coming back to a sticky mechanical valve but if that were the case, would the boiler not perhaps be on more continuosly or conversely be induced to switch on and off more ' rapidly '?

Any thoughts greatly welcome.

Thanks John
Does your system do the hot water and heating independently ? ie, you can switch the heating on without the hot water ?

There are two thermostats that cause the boiler to fire up - the room thermostat and the cylinder thermostat. Nothing else would cause the boiler to fire up.

Your system will either have a three way valve, or two two-way valves, one for the hot water and one for the heating. These are controlled by the timer settings - heating, hot water / hot water and heating.

Turn the cylinder thermostat from one end to the other very slowly and you will find the point where the boiler fires, depending on the water temperature in the tank.
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Old 16-05-2008, 10:33 AM   #5
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Does your system do the hot water and heating independently ? ie, you can switch the heating on without the hot water ?

There are two thermostats that cause the boiler to fire up - the room thermostat and the cylinder thermostat. Nothing else would cause the boiler to fire up.

Your system will either have a three way valve, or two two-way valves, one for the hot water and one for the heating. These are controlled by the timer settings - heating, hot water / hot water and heating.

Turn the cylinder thermostat from one end to the other very slowly and you will find the point where the boiler fires, depending on the water temperature in the tank.

Thanks Desilva. Yes, both are independent and boiler fires up and down as normal. I'm now looking at the immersion heater and the hw cylinder. Though the immersion heater may be off there is a chance the contacts may be faulty and welded together. Have removed the switch fuse so there is no power at all to see what happens.

The other possibility could be a scaled up hw cylinder which if the cylinder walls are scaled ( don't know how likely this might be) then the cylinder stat might be calling for more hot water which would result in excessively hot tap water.

Thanks John
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Old 16-05-2008, 11:16 AM   #6
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I get very twitchy about stuff like this after a failed boiler thermostat on our system caused the contents of the heating circuit to cyclically blow out through the expansion tank. By the time I heard the "boom .... boom .... boom" noise and killed the boiler it was too late, we had hot water pouring out round the loft hatch
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Old 16-05-2008, 11:42 AM   #7
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Think I've found the cause. No power to immersion heater so I know that's dead. Set cylinder stat to 60. Switched boiler on and took car out to check tyre pressures.

Returned to find tap water scalding hot; top of hw cylinder hot but cool where cylinder stat is located. Can only be (?) scaled up hw cylinder and because the cylinder wall is 'cool', the stat keeps calling for hot water.

HW cylinder is at least 30yrs old so it's about time it was replaced.

Thank you for your replies, it all helps to narrow it down.

John
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