H/W stat not firing up boiler

[pmx_super]

This feels like a job for a professional, but since it's not critical I thought I'd take time to do a little research as to what the issue might be. I thought I had a decent understanding of the CH/HW system in my house but I'm flummoxed.

I have an open vented system with a h/w cylinder. Something in the H/W system isn't triggering the heating system (the circulation pump or the boiler) to come on. The CH works fine, so it looks like the boiler, pump and timer are all OK. What's more, the h/w stat seems to operate the Honeywell valve ok (so if the CH is on the water gets heated hence not a critical issue). I just don't get what could have failed.

Is that enough to go on?

 

[Hugh Jaleak]

Do you have a 3 port valve or 2x 2 port valves? Problem may still lie with the valve(s).

 

[pmx_super]

Do you have a 3 port valve or 2x 2 port valves? Problem may still lie with the valve(s).

2 x 2 port valves.

 

[Armo74]

Are you sure you don't have a 3 port valve? as you say you get hot water when the heating is on.

 

[FiremanT]

Are you sure you don't have a 3 port valve? as you say you get hot water when the heating is on.

Jammed 2 port HW valve, in a mid on position (if you know what I mean)

 

[ianmcd]

sounds like the synchron motor on the HW valve has seized mate a very easy fix for a competent Diyr, easy to test turn off all power and go to both valves there will be a manual bypass lever on both , try moving this and see which one wont move over

 

[Hugh Jaleak]

It is possible the motor is opening the valve partially, but not fully. The motor needs to wind the valve over fully to operated a microswitch within the valve that then energises the boiler and pump.

HW alone wont activate the microswitch hence no hot water, but if the CH is on, power is supplied to the boiler/pump via the CH valve, HW valve is open partially so hot water from the boiler goes into HW circuit as well as the CH.

It may be possible to replace just the valve head or motor (depending on what has failed), subject to valve type. (Older Honeywell versions will need entire valve changing IIRC).

 

[pmx_super]

It seems my "decent understanding" was a "woefully incomplete understanding". Interesting that there's a microswitch in the valve.

I think a jammed valve is the answer - though it appears to move freely between closed and just-short-of-mid-position, rather than being stuck permanently in mid position. I'm not in front of it now, but I seem to remember an unusual amount of resistance when I tried to move it beyond that mid-point to the manual latched position, and giving up. I didn't think to try the CH valve to see if that was normal.

I'll report back with the honeywell valve details so hopefully somebody can advise whether it's a new valve or new valve head. I could probably just about handle the latter, but I'm not going to try to get that thing out of the pipework myself.

 

[pmx_super]

I pushed a bit harder and the manual level went right across to the manual position, and lo and behold the pump and boiler fired up. That appears to have freed something up because it's working ok again now on its own. It's clearly on the way out though, so I'm thinking of replacing anyway so I'd still like to know if it's a recent enough model to be able to just do the head. The serial number on it is:

4043H1056

On close inspection I noticed that the CH 2-port valve is different, probably newer. It has a white label saying that the head can be replaced with part # 40003916. Unfortunately my HW valve doesn't have this label, instead the serial number is embossed into the metal. I also noticed that the CH one has a "nipple" on the top of the metal case, but the HW one doesn't. Are there any other tell-tale signs that'll say whether one of these: http://www.gwsupplies.co.uk/p/6266/honeywell<i>motorised+valve+head.htm can be fitted without draining down?

 

[Hugh Jaleak]

If memory serves me correctly the older version didnt have the dimple (nipple) on the head and therefore will need entire valve changing. I would stick with Honeywell though, cheaper valves dont always last, and with newer version head can be replaced should it ever be required.

Just for interest there are some good animated diagrams here explaining the path the power takes on a 'S' plan system. http://octaveblue.co.uk/c_heating/index.htm May give you a better understanding for future reference.