hot water too hot - thermostat or valve or something else?

[AlF2008]

Yet another question regarding our old system. Its an open vented system with a Baxi back boiler. At the moment the hot water is verging on scalding. I've turned the thermostat on the front of the cylinder from 70 degrees downto 50 degrees but it doesn't appear to have made much difference.

I need to get under the floor boards and check the plumbing from the boiler to the cylinder as I cant see exactly what goes where. In our system there are 2 pipes with 2-port valves both with the flow heading away from the boiler. I suspect one must turn back to the cylinder but cant see enough to be sure.

Apart from the obvious health hazard from the hot water, our heating bills seem a bit excessive so I guess this could be related.

I'm guessing the problem is either with the thermostat (old Honeywell) itself or the motorised valve which feeds the cylinder. Does that sound right? Could it be anything else?

Am I right in saying that when the timer is on for HW the thermostat controls whether the valve is on or off and hence whether you get hot water from the boiler to the heating element of the cylinder?

Whats the best way to diagnose which is faulty?

The system is going to be drained down this weekend to move a radiator and ideally I'd like to fix this problem at the same time to save having to drain the system down again in the near future.

Thanks once again!

 

[gas4you]

Sounds like the HW zone valve is stuck open and firing the boiler.

Check the manual lever on it. If it is free to move back and forth then it is open.

Also a chance that cyl stat has failed 'on'.

 

[AlF2008]

Sounds like the HW zone valve is stuck open and firing the boiler.

Check the manual lever on it. If it is free to move back and forth then it is open.

Presumably if the HW is turned off at the control timer, the valve should be closed and hence prevent hot water going through the heating element of the cylinder?

Also a chance that cyl stat has failed 'on'.

Would it normally fail "off"?

thanks

 

[gas4you]

1) Yes

2) could fail either way.

 

[AlF2008]

Great. Thanks for the advice.

 

[AlF2008]

Got a bit further with this. I didn't manage identify all the pipes due to limited time and cramped spaces. Anyway what I did manage to find out was by running some hot water out and hence ensuring there was a demand for it to heat up the water in the cylinder. Then turning off the HW at the timer and I can hear the valve closing. So it appears that the valve is working fine and hence its got to be the thermostat thats causing the problem.

One other question though. If the thermostat doesn't turn off the valve wouldn't the water eventually get too hot and go up the overflow? Or is the heat from the boiler generally insufficient to cause that kind of problem?

 

[DP]

At the programmer, if you select CH, does HW also switch on? Do you have a thermostat fitted to the cylinder? It looks like a small box with cable fitted to it and located about a third height from the base of the tank and usually has a metal band or cable holding it in position.

If yes to first question and no to second, turn down the boiler thermostat setting midpoint.

 

[AlF2008]

At the programmer, if you select CH, does HW also switch on? Do you have a thermostat fitted to the cylinder? It looks like a small box with cable fitted to it and located about a third height from the base of the tank and usually has a metal band or cable holding it in position.

If yes to first question and no to second, turn down the boiler thermostat setting midpoint.

I've turned the thermostat on the front of the cylinder from 70 degrees downto 50 degrees

The CH can be independently turned on and off via the programmer. Thats how I know that the 2port valve is working as I can hear it shutting off when the HW is turned off.

 

[ChrisR]

You should be able to hear clicks from the cyl stat when you turn it up and dow, if the water's up to temp. The hysteresis is pretty wide though.

Thw iring is simple if drawn like this: the valve should be closed when there's no volts on it, though sometimes they "let by" because of sticking or a "soggy ball". If you take the head off the valve and turn the shaft with a baby spanner you should gfeel it move freely and stop definitely.

Also check the thermostat is tight to the cylinder wall, otherwise it won't work at all.

the boiler stat should stop the water in the boiler and hence the cylinder getting above 80°, max. Until you fix it you could turn your boiler stat to LOW which is still hot hot water temp.


goes to seek wiring diagram.....

 

[AlF2008]

thanks for the diagram. I presumed it must be wired up like that but its always nice to have it confirmed.

I cant hear anything from the cylinder thermostat despite the water being very hot when turning it up and down.

How do you get the top off the valve? I didn't see any screws so I presume there are clips of some description?

 

[AlF2008]

And another quick question. The current thermostat is a Honeywell L641A1005 (from memory). I take it there is nothing special abuot it and that something like this would be fine as a replacement:

http://www.screwfix.com/prods/40051...-Controls/Honeywell-L641A-Cylinder-Thermostat

Thanks
Al

 

[DP]

You should have two motorised valves. What make are they.

Let the system cool down. Then fire up system by selecting CH only (according to your reply, HW zone will stay inactive). Feel the pipe connected to cylinder coil- it should remain cold.

As mentioned earlier, reduce setting on boiler to moderate HW temperature until fault has been fixed.

 

[AlF2008]

I've turned the boiler down earlier and will check out the port operation (and make) tomorrow.

 

[AlF2008]

The valves are Honeywell V4043H1056.

I've just checked the operation temp of the flow pipe at the cylinder and it was warm (heating had been on) to start with. I put on the CH and there was no change. Turned on the HW and it got hot (and you could feel the water going through the pipe).

Is there anything else to try?

I've ordered a new thermostat anyway which should be here in the next day or so. For the small cost it seemed worth ruling it out.

 

[twgas]

you have to leave hot water turned off for quite a while as if the motorised valve is letting by it may take some time before the 'seep' gets through to cylinder.

and have you checked immersion heater turned off?