roomstat not firing boiler?

[IanJ]

Hi

Had British Gas in last week to powerflush system and fit TRVs to most rads. Also improved airing cupboard pipework, replaced pump and HW tank thermostat, so generally a long-overdue overhaul (pipework full of 18 years of sludge and lots of problems with HW).

All seemed well for first couple of days, but now we're noticing intermittent problems - e.g. heating not quite right, and/or HW amount or temperature not quite right.

There's a existing digital room stat in the lounge with an indicator to show when it's calling for heat from the boiler. When the heating first comes on and this indicator shows, the boiler fires no problem. But later if the temperature drops slightly and the stat clicks back on, the boiler doesn't fire. Same if the hot water has been on and then the central heating tries to take over.

Whatever the individual TRVs are set at, my understanding is that the room stat is just a glorified switch, and if it's calling for heat from the boiler, it should always fire. Is that right or have I missed something obvious?

BG are coming back next week to check it out, but just wondering if there are any basic checks I can do in the meantime. Mid-position valve in airing cupboard looks to be working okay (at least it's switching when CH on, HW on or both on).

Cheers

 

[IanJ]

This morning, HW came on at 5.30 and partially heated up before CH came on at 6. Boiler then failed to fire until CH or HW switched off and on again. All working fine once that was done.

So it does seem to be a boiler switching problem. Any ideas?

Cheers

 

[Ronald0]

Turn off HW at programmer or turn cylinder stat right down so system is driven by room stat only. Turn up room stat, is demand reaching boiler?

If not problem is with motorised valve. Need to know if orange wire becomes live.

 

[IanJ]

OK - so at 8am this morning: central heating on (since 6am) and rads hot.
HW should have been on 5.30-6.30 but water only just lukewarm.
Roomstat set at 20 - had dropped to 19 so calling for heat, but boiler not firing.

Made sure HW off at programmer, and cylinder stat turned down.
Orange wire live at connection box near motorised valve. (edit - pump running too)
Can't get cover off motorised valve to check connection there - could a loose connection at the valve be the problem? It's a BGMVSP-23 (a couple of years old) - photo below.

Cheers

View media item 17866

 

[twgas]

What is the boiler stat set to, (and what boiler is it)?
Never had a loose connection in one of those valves, although I've had a few mechanical breakages.

 

[IanJ]

Boiler is 18-year-old Glowworm Fuelsaver. Boiler stat is set to max (no numbers - just 0, then min-max).

Turned HW on manually earlier - now piping hot. Just turned CH on manually, and seems fine. So it does seem that switching between them is a problem. But motorised valve looks to be working correctly - the positions of the little plastic lever aren't labelled, but it's at the front when just CH on, moves to the back when just HW on, and to the middle when both on.

Cheers

 

[twgas]

Balancing issue, is there a valve on the return out of the cylinder?

 

[IanJ]

There's a valve on the pipe coming out of the bottom of the cylinder - is that the return?

 

[Hugh Jaleak]

Flow and return from/to boiler will be same side of the cylinder one above the other. Bottom of these two is return, this is the pipe and valve you want. (The opposite side is the inlet from the cold tank in the loft. Valve on this shuts of Hot water to taps.)

 

[IanJ]

OK - I have the right pipe/valve now. What do I need to do to sort out the balancing?

Out of interest, how does the balancing affect the boiler firing? I'd assumed the boiler was simply switched on/off on demand, but obviously it's a whole lot more complicated than that!

Cheers

 

[Ronald0]

Did you sort problem out?

"OK - I have the right pipe/valve now. What do I need to do to sort out the balancing?"

This is the HW circuits version of the radiators lockshield valve and will reduce the HW flow and give bias toward the radiator circuit when MV is in mid position.

If you set your MV to manual and call the boiler do the HW and CH pipes either side of the MV both heat up?

 

[IanJ]

Hi - nope, problem not yet sorted but BG guy due tomorrow.
Both the HW and CH pipes get hot (very) when motorised valve in mid-position.

This morning at 6.30 - CH on since 6 but taking a while to warm up (but then it was minus 4 outside!), but no HW (but should have been on since 5.30). HW started to heat up after switched off then on again via boiler programmer.

So all in all it's hard to pinpoint the pattern and it's driving me nuts.
I'll report back here once the BG guy has been tomorrow.

Cheers

 

[Ronald0]

Override the clock on your programmer so that HW and CH are on.

Set the MV to manual via the lever (this will lock it in the mid position).

Turn the room stat down to zero and the cyclinder stat up high (to 65).

Boiler will fire. If both outlet pipes from the MV (one to the HW coil in the cylinder and the other to the rad circuit) are getting equally hot some way from the valve, you must be getting HW and CH.

 

[IanJ]

BG guy came yesterday, screatched his head a bit and eventually decided that the CH/HW switching problem could only be due to either the motorised valve or the boiler programmer (even though both looked to be working normally). He replaced the valve (because it was much older than the 6-month-old programmer) and agreed to come back if that didn't sort the problem.

Doesn't seem to have sorted the problem, although it will take me a couple of days to monitor how the system switches before I'll know for sure.
I suspect I'll be getting BG back after Christmas to replace the programmer as well...

Cheers - and happy Christmas!