Converted to fully pumped, and back-boiler not firing consistently ...

[eco_mark]

So custard had an old potterton fuego super backboiler on gravity HW pumped CH, and as part of a phased change over to other heat sources has put in a thermal store and converted to fully pumped.
BB used to have two 28mm pipes for HW and two 22mm for CH. The 28's have been capped and 22's now feed into the thermal store, with the old CH pump remaining on those 22's and now acting as the boiler pump. CH now fed with new pump and new pipework from store to where the 22's where disconnected from old pump.

Problem they've got is that the boiler won't fire up consistently. Turn the cylinder stat up and the pump comes on but boiler doesn't fire. At seemingly random times it may come on for a minute or so but doesn't stay on and doesn't seem to be any pattern.

I've gone in and bled everywhere I can on the store-boiler pipework, including uncapping the 28's and bleeding, but no change. I've checked the wiring and that seems fine - when HW is required, and cylinder stat says store isn't hot enough, then live signal arrives at a terminal that both pump and boiler are wired into. Pump comes on, but boiler doesn't. Tried removing wires for pump just to see if only having signal received by boiler made a difference (stab in the dark) and it didn't. The fact the boiler will occasionally fire makes me think there isn't anything fundamentally wrong with the boiler (like a broken gas valve), just that some fault condition exists ...

Found installation manual for the boiler ([DLMURL]http://www.partsarena.com/baxi/System/DATA/Dx/DS1/installation/2050/I01-2050/I01-2050.htm[/DLMURL]) but the troubleshooting doesn't really help. I'm now just drawing a blank as I've not dealt with many gravity systems. Anyone have any reasons why a boiler would not consistently fire when giving a live signal? If it used to be gravity fed then I wouldn't think it would be fussy about flow rates, but what else could it be? Nearly given up on this, and told custard as much, but sure would be nice to surprise them with a fix
[Princess Leia voice] Help me UK Plumbing Forum, you're my only hope.

 

[diamondgas]

It doesn't sound like the heat is being removed from the boiler? What's the F&R pipe temps? Does the Flow get hot at all? I'd be checking that the power supply is constantly at the boiler then checking to see if the stat is turning on and off through normal operation reaching temp or not? If the water's heating but not being circulated suspect some blockage/faulty pump... All depends on whether the boiler is lighting up and reaching temp or not in the first place yeh? If it isn't then it depends on the boiler component operation, stat or gas valve...!

 

[eco_mark]

It's funny you should mention that
I've just gone back, to double-check that power is really getting to the boiler thermostat when it should (and no surprise it is, because it does fire occasionally), but while doing that I had to gain access to a boxed in section of pipework: the rising 28 and 22 pipes that appear in the cylinder cupboard. As luck would have it the boiler fired while I was there testing and went out after around two minutes. The 28mm pipes in this boxing got very hot, so it seems that even though the 22mm is being pumped the heat is still lingering in the 28's and causing the boiler thermostat to stop the firing. As they're capped in the cupboard above the heat has nowhere to go.
As this is part of a phased change over (with the back boiler being replaced by a wood boiler stove in the coming months) I think I'll suggest they uncap the 28's and plumb them into the store too. It's a bit odd, having pumped and gravity supply from the boiler, but the pumped bit is going to be ripped out in a month or two anyway.

 

[eco_mark]

What's the F&R pipe temps? Does the Flow get hot at all?

And that's weird actually, because the 22 F&R don't seem to be moving any heat at all ...

 

[diamondgas]

You could link them I supose for now or use the 28mm Flow that fed the cylinder as your primary flow?

 

[diamondgas]

And that's weird actually, because the 22 F&R don't seem to be moving any heat at all ...

Would it take much to use the old gravity F&R's as the primaries? Are you sure the old pump is working?

 

[eco_mark]

Yeah I'm thinking that as they'll need 28mm pipes anyway for the wood stove, they might as well just uncap and connect them to the store now. When the time comes to rip out the BB and put in the stove then they'll already have pipework from the fireplace to the store which is suitable for gravity solid fuel stove (apart from small modification for heat-sink control), and the 22mm stuff with pump can be ripped out then too.

 

[diamondgas]

If the heat isn't getting away from the old BBU then the older graviy pipes would be best utilised I'd suspect eco_mark ... Defo sounds like theres an issue with the old heating flow/pump yeh?

 

[eco_mark]

If the heat isn't getting away from the old BBU then the older graviy pipes would be best utilised I'd suspect eco_mark ... Defo sounds like theres an issue with the old heating flow/pump yeh?

Yeah you'd think so - although as best I can tell it is actually moving water around. When on (and I've tried it on all 3 speed settings) you can definitely feel/hear water moving through the 22 f&r. It just doesn't seem to be moving heat away from the BB along with that water, which is puzzling. Thanks for your help, because it has helped just to bounce the problem back and forth

 

[diamondgas]

No probs mate ... Hope you get it sorted