A mostly old Y plan system.... Symptoms of dwindling circulation occurred, lower rads not heating, upstairs output tailing off then the system quit entirely. Boiler said dry fire error, it was filled but presumably no circulation. Filter has been emptied of muck, pump has been opened & is clear. What else is likely to have failed the circlation? The 3 way valve isn't blocking rad flow in HW only mode, but getting the bottom part of it off the pipework to fit a new ball looks like a right mare of a job. Access is terrible & the pipework around it allows it no movement afaics. I don't see that causing the problem. Clues welcome, right now I lack one!
So no circulation at all, including HW cylinder primary. Earlier you said, "The 3 way valve isn't blocking rad flow in HW only mode" implying there was flow here?
Are you sure the system is correctly filled with water and there are no airlocks?
I've seen the symptoms you describe once (without the electronic warnings), on a neighbours system which was all but empty- the gate valve from the header tank was closed. When I opened it and bled the radiators, all was well.
Blockage or airlock sounds the most likely. You could start with a drain down and refill. If that does not help, then a mains water flush (temporarily block the vent and F&E pipes).
If you are (for example) injecting mains water into a disconnected rad tail, and draining from the other, you want all the flow to go through the pipework and rads, not to be forced into the F&E tank where it might overwhelm the overflow.
Update: Today I set the room stat to minimum so the boiler didn't fire & switched the system on. Previously I found the pump wasn't getting power from the boiler, so temporarily rewired its live to permanent live so it runs all the time, which would at least get the thing running. So the pump's been on. It's definitely spinning, I can hear that clearly & can hear bubble or cavitation type noises.
However... the primary circuit & downstairs rads are all cold, despite running through the HW cylinder that's hot from the immersion. So I can only conclude that primary circuit water is not flowing thru the HW cyl heat exchanger. Why I don't know.
The system was drained & refilled the other day. The plastic header tank connected by copper pipes contains a LOT of rust pieces & dust. The system is nearly 40 years old and I don't know whether there has been an increase in recent times in rust deposition. A magnetic filter was fitted 2 years ago.
Oh, when it first went wrong I discovered the header tank overtaken with biofouling. That has gone without a trace now, the chemicals wiped it out.
So something somewhere has to be blocked. I guess the first moves to make are to blast what I can with mains water & to add cleaning chemical. However this system had all the rads flushed out 2 years ago & has been cleaner chemicalled a few times... it plainly hasn't solved it. I'm beginning to think the only solution is to take all the plumbing apart & blast every bit out. I hate the thought of doing that though.
PS one downstairs drain points drains freely, the other barely manages to dribble.
Just to be on the safe side, you might check that the pump is actually pumping. That would be one thing checked off. If you could swap in a known-working pump from somewhere else, or plumb this one into a test circuit?
I'm not sure how. But I'm also not sure how it would fail to pump given that it's making all the right noises, fizzy ones as well as spinny ones. I checked its ports were clear.
My central heating boiler water pump failed recently. I did find that when I removed the screw in the middle alot of water came out because the rotor was loose. The pump was vibrating slightly but not doing nything useful. I was a bit slow to realize that the pump was faulty. When I tried to buy a replacement I was told that they just had the heads in stock, which actually proved very easy to change.
Those were my thoughts, but NT was initially giving mixed signals of whether there was flow into the HW cylinder primary, or not.
I think its time to take off the pump and see if it run. If it has a screw on the end of the rotor on its removal it should be possible to see it turn under power.
Of course all the vanes could be missing, which is likely given the tales of gunk in a very old system. I've seen an old BMC water pump otherwise working ok, missing its vanes, causing the engine to overheat.
no there's certainly no flow there. If there were it'd extract DHW cylinder heat & send it to the rads at low level.
I know it turns. The noises indicate it does, it doesn't just hum.
This pump is 2 years old, has a plastic rotor with narrow water passages & the rotor is pristine & there's no blockage or debris there. The rotor turns without problem. I really don't believe it's the pump.
I plan to mains water blast the drain point that only dribbles. It might be worth taking that rad outside to flush it, it's not been there very long but if there's no flow I guess that rad is the prime place debris would collect.
I must fit a valve to the header tank, mucking with it in the loft is a pita.
Another update. There is certainly primary flow. It can be heard loud & clear from the filter. That means there's air in there, which is odd since from the filter it flows through the boiler then through a pot with F&E pipes out the top, and it's been running much of the day. But it's running.
The boiler is still displaying F72 fault code, NTC sensors out of range, though they've been replaced. It won't fire up. So it's looking like a PCB problem and a blocked HW cyl heat exchanger. Sounds expensive :/
My VW Passat started to overheat whenever the car went up a hill. When I eventually took the car to pieces I found that the plastic impellor had completely disintegrated. I was pleased with the replacement as that had a metal impellor.
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