Help please - am stuck

[JohnBailey]

Yesterday I drained my CH to replace a leaking drain cock beneath a radiator, all went OK until I came to get it running, Cyl stat calls for heat and boiler/pump work, but as soon as I put the room stat up to call for heat the boiler cuts out leaving pump running. Checking volts at the wiring box there is live on both H/W ON & HW Off connections when it's in this state!!
My guess is that it's the V4073A gone wrong, but am too confused as to know why I think thats whats wrong, anyway its a coincidence if it failed at the same time as doing the plumbing downstairs.
Any ideas?

 

[crockett]

Dunno what your pipework layout looks like, but may be an airlock on the heating side of the central heating system, which will cut the boiler out on high temp, because the water isn't being circulated.

 

[JohnBailey]

That would explain the 'coincidence' alright.

I just, checked it again and found that the boiler cuts-out at precisely the moment the room stat clicks on, I would have thought an overheat cut-out may take a few seconds to cut-in, is that correct?

 

[JohnBailey]

I don't know if it'll help, but I've uploaded a pic here...
//www.diynot.com/network/JohnBailey/albums/
This pipework is in the (upstairs) airing cupboard, the boiler is wall-hung downstairs. The nearest two 22mm pipes are CH flow and CH Return, the CH flow comes out of the back of the 3-port valve and round behind other pipes. I bled the 15mm drain-cock (sticking up at the back) until water came out. The CH pipes must split at some point under the upstairs floorboards to do the upstairs/downstairs rads.

 

[stumpydick]

check with meter, the voltage at the boiler switched live when room stat is turned up. most likely faulty 3 port valve powerhead [ is boiler a suprima ?]

 

[keego]

Have a look before the three way valve & see if there's a tee off it with a valve on the outlet (bypass), shut that valve & if you have another on the cylinder return then shut that as well if not then leave it on CH only & shut all your rad's down bar the furthest one.
It's definitely air locked so put your meter away!.

 

[JohnBailey]

The boiler is a Glowworm Space Saver 45-60, most likely installed when the house was built in '81, I moved in in '94.

The boiler switched-live is the same connection as the pump switched-live and it measures 239v when the either stat demands heat.

I'm confused about the HW-Off connection though, especially as it is sometimes live at the same time as the HW-On.

Also, after the boiler cuts-out (due to room stat clicking on), the HW-Off connector goes to 80v with the HW-On staying at 239v.

 

[JohnBailey]

Thanks keego, was typing when you posted.

There is no bypass or other valves, but your confidence about it being an air-lock makes me hopeful, I guess I've just got to figure out where and how to shift it. Unfortunately familys gone to bed so will have to try tomorrow after work.

 

[keego]

One more thing to try is to put a hose on the drain off you were working on & open it up a bit with the CH on & let it trickle out for a bit (other rads closed again). If it is an airlock then it's just a case of getting everything moving again.[/u]

 

[JohnBailey]

Now we're getting somewhere, I did as you suggested keego and it's going again. The boiler isn't completely happy as it has to be set on max to fire-up and cuts out after a few minutes, maybe the flow isn't good for some reason (still air trouble?).

 

[JohnBailey]

Good news!!!…

This morning the system was still not working properly, boiler lasting just a few minutes before cutting out (over temp I guess). I did a bit more draining (as per keego suggestion), then left it pumping, after about 40 minutes and several bouts of seriously loud gurgling the system decided to sort itself out and is now back working.

Many thanks to all who helped, especially keego, you’re a star.

Although I accept the explanation of what the problem was, I don’t understand how it manifests itself, if anyone can explain I’d be really interested.

The airlock was (probably) in the CH side somewhere. With only HW on everything worked fine, with HW remaining on I clicked on the room stat, the instant it clicked-on the boiler shut off, there was no delay for heat build-up so I guess it is a pressure switch in the boiler(??). The thing I don’t understand is how the situation with CH+HW is different, from the boiler’s perspective, from the situation with just HW, either in terms of pressure or flow.

The system is Y plan with a 28 year old Glow Worm Space Saver boiler, no bypass valves. There was an extra loop added (downstairs extension) 5 years ago, this was taken from the upstairs circuit adjacent to an original bedroom rad. They took the loop from upstairs because the flow downstairs at the back of the house has always been poor and it probably wouldn’t have coped very effectively with an extra 3 rads + kick-space heater.