Thermostat - Hot water too hot

[Dixie]

I have a problem in that the hot water is too hot. The water suddenly got very hot. I have changed the cylinder thermostat (Honeywell L641A1039) but has made no difference. I have checked continuity between the terminals in the stat & the cable connections in the control box & these are ok. The cable is connected neutral to C terminal & live to no. 1 terminal as per the original stat. The earth is not connected. The new stat clicks as if working ok. With the cyl water hot & the stat cold the stat heats up when placed in position & seems to go up to 60 degrees before clicking off. The thermostat on the boiler (Ideal Classic) is set on 4 but i haven't changed it as I don't know what it controls. Any help would be appreciated before the wife scolds her hands doing the washing up.
Thanks

 

[Gas2Air]

Depending on what system you have it's likely to be your hot water motorised valve passing, the thermostat for hot water will have no effect on temperature without demand from your clock.

 

[CH4]

When your system is in hot water only mode, does the cylinder stat switch off the boiler before the water overheats? If so, then the cylinder stat is probably ok, and the fault lies within the motorised valve - allowing hot water flow to the cylinder when not required, whenever the central heating is on.

 

[CH4]

Sorry Gas2Air, I seem to have duplicated your answer. I didn't notice it while I was typing mine.

 

[brumylad]

check the immersion heater is not turned on by mistake.
Maybe the red light in the IH switch can fail ?

 

[Dixie]

Thanks for the replies. I have checked in hot water mode & the stat is ok. It starts/stops the boiler on raising/lowering the temp on the stat. The water is at 60 degrees so I assume this must be the heating temp. Is there a way that I can check the motorised valves. There are two on the system.

 

[CH4]

When your system is cold, switch it on in central heating only mode. The motorised valve that feeds the hot water cylinder coil should NOT pass water. If it does (you will be able to feel the pipe that connects it to the coil heating up) then the valve may have seized in the open position. The position of a lever on the side of the valve's head should also indicate whether it is open or not.

In normal operation, a micro-switch within the valve's head only turns the boiler on when the valve has moved to the open position. It takes a few seconds for the valve's motor to open the valve. So, if the boiler starts immediately that hot water only is switched on, then this further indicates that the valve is stuck open.

 

[Dixie]

It seems that you are right CH4. With the system on heating only, the pipe to the cylinder got to same temp as the main pipes. Can you tell me what sort of job it is. Is it a partial or full drain down of the system. Is it a job better left to a man who knows rather than one who doesn't.

 

[CH4]

It might be worth trying to free the motorised valve before you replace it, by giving the moving parts a squirt of lubricant (WD40) and sliding the lever back and forth a few times. If this works then you may be able to postpone replacing it until after the heating season is over.

If the valve is readily accessible then replacing it is not a particularly big job. You may be able to achieve this without draining down the system, by fitting rubber bungs in the cold feed and expansion pipes - at the header tank. Bungs for this specific purpose are usually available from plumber's merchants (and possibly B&Q). Provided that both open vent pipes have an airtight seal, you may only need to catch/mop-up a couple of cupfuls of water when you remove the motorised valve. Otherwise, you will need to drain the system down to a level below the motorised valve.

How difficult it is to remove the old valve depends on how much 'give' there is in the pipework. The pipes go into the valve's body about a centimetre or so. Hence, you with have to move the pipe by at least that amount to get it out. Some people reuse the old nuts and olives when fitting a new valve, but it's best to replace them (you can get the old olives off by part cutting through them with a hacksaw and then splitting them with the twist of a screwdriver - inserted into the cut). After fitting the new valve, remove the bungs, check for leaks and bleed the air from the vent on top of the pipe that connects to the upper end of the cylinder coil. If the pump is on the same level as (or above) the motorised valve, then you will have to bleed it as well.

All that remains then is to wire in the new motorised valve in place of the old one.

 

[Dixie]

I'll give it a try with WD & see how it goes. If it doesn't work I think I will wait until its warmer to get it sorted. Thanks very much for your help, its much appreciated.