Pump goes off for a few seconds

[Raymundo]

Hi all. Wonder if anyone can suss this one?
I have an old Thorn Apollo boiler, in a system with 2 heating-zones, with Honeywell 2-ways and roomstats and a hotwater zone with Honeywell 2-way and cylinder-stat. No programmer or clocks. Only using HW now, of course. Replaced the overrun stat a year ago when it got down to about 10 second overrun, allowing some minor noise at shutdown. It's been happy at 2-3 minutes ever since and all quiet. No problems till this month, when the boiler started to kettle and pipes bang loudly, always at the same temp - I have a clip-on thermometer on the boiler output-pipe - of 55C. The noise lasts for just a few seconds, but it's quite scary!
Today, I checked the system when it fired up and as the dreaded temp. approached, went to the airing-cupboard to check for bangs there. I was touching the pump, which was running quietly, when I felt it stop. There was rattling and banging from the pipework, the pump came back on and all was quiet again. Normal boiler-cycling, heat-up and overrun followed. The pump was off for about 5 seconds. So I think I've found the fault, but not the cause!
The basic circuit-diagram suggest that it could only be the cyl-stat, but why would it lose contact for a few seconds and always at the same temp?
Any suggestions will be very welcome, so thanks in advance.
Raymundo

 

[Biguns]

I went through that same scenario recently with my Apollo. Turned out to be the pump.
Take the large screw out of the back of the pump, insert a screwdriver and see if you can easily stop it. Some water will come out. so use a dish or a towel
Or repeat your test using a DVM on the pumps wiring connections, to see if the power is cut

 

[bengasman]

Get a pro out to check the problem, and he can do a service and safety check at the same time. These things really should be checked every year.

 

[BoltGas]

I don't think I would recommend stopping the pump with a screwdriver to be honest... But perhaps I've mis-interpreted that advice?

If you don't set certain heat-up time periods for the hot water (&other heating zones really), it's only working off the cyl stat...

...I'm pretty sure that would shorten the life of the stat - unless its ultra-modern the Honeywell cyl stat is the bi-metallic strip type so as with any metal, the constant warming & cooling will cause fatigue and I really don't think after however many years of this type of constant use you'd get an accurate switch on or switch off at set temperature, and there must be a much increased possibility of that stat mis-behaving.

Thing is, once the stat is satisfied, surely it will close the 2-port in the primary flow to cylinder coil. So when the valve closes, where does the pump-overrun pump the water to? Probably only the bypass, assuming there is a bypass fitted to the system. If the pump is getting on a bit, it won't have the amount of push it should have and if the heat-exchanger is a bit clogged as is quite likely being that the Apollo is older than I am, perhaps it just can't push it through - if the pressure on the 'out' side of the pump is greater than other then the impeller couldn't possibly spin against that, could it?

Add to the mix the fact that over the years, the boiler's heat-exchanger will become more scaled due to this and it will become so difficult to dump/get rid of the heat it has produced and the amount of water in the HE will be so reduced due to the scale that it won't take long for it to overheat that water and when it overheats & kettles, there's every chance of the water turning to steam and causing the banging in the pipe-work due to pockets of steam working their way to vent.

When you say "Normal boiler-cycling, heat-up and overrun followed." Do you mean over-run when controls are satisfied, or once boiler has reached set temp and is distributing its heat through the pipe-work? I'm pretty sure the definition of Pump Overrun is to dissipate heat throughout the system or via the bypass after the controls are satisfied in order to avoid kettling and the heated water just sitting in the system, scaling. Until then the boiler will fire in and out topping up the heat of the water that is eventually transferring its heat to the required parts of the system.

There are so many variable's (and often people use slightly different terms or vice-versa, it can be tricky to determine exactly what you mean) that you really should find someone who knows what they're doing to give the system a thorough inspection, and don't be afraid to ask for explanations as any good plumber should but of course - the more time you keep him, the more it's going to cost. We don't pay thousands for training & registrations just to impart all our knowledge to Tom, Dick and Harry for free in the time we have to try and make a living. I come on here because I want to improve my knowledge and skills and answering peoples questions is a bit of a challenge that helps to keep my mind fresh with it all.

I'd really like someone to highlight where if I'm talking nonsense, as that will help greatly with developing my skills. I have rambled here. Never again.

 

[Biguns]

I don't think I would recommend stopping the pump with a screwdriver to be honest... But perhaps I've mis-interpreted that advice?
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See FAQ 14 here. //www.diynot.com/forums/viewtopic.php?t=175736

Much the same. Ive checked lots of pumps this way. If it stops with very little pressure there is a problem

 

[BoltGas]

Ah yes OK, check if its spinning with that method. Perhaps don't TRY to stop it but if it does when you're checking to see if it's spinning while applying little pressure...that's got to be the problem.

 

[picasso]

the only thing controlling the pump is the overrun stat, if its switching off I would check this first, as it seems to be temp related as well.

 

[namsag]

As picasso says it is probably the pump over run stat again or a bad connection on it or wiring harness (they sometimes go brittle) .
If you can check for voltage at the pump when it stops 240v there means it is a pump fault no voltage then its overrun stat

 

[Raymundo]

Thanks for all the quick suggestions, folks. Much appreciated.
Perhaps I should describe the process step-by-step, to give a clearer picture.
Cyl-stat calls for heat, boiler fires up and pump runs immediately. Boiler output temp rises steadily for about 10 minutes with no noises and all pipework warming where it should. As output temp hits 55C, pump stops for 5 seconds or less, causing loud noises, which stop as soon as pump restarts. Temp then rises to 65-70C and boiler burner goes off. Pump continues OK. Boiler reignites at about 60C and cycles happily until cyl-stat is satisfied. Boiler goes off and circulation continues through the fixed bypass until the overrun-stat is satisfied. Pump goes off and all is well. We get 2 or 3 heating periods a day, depending on water usage.

As I understand the system, the cyl-stat switches the 2-port valve on and off, which in turn switches the gas-valve via the low/high stats and the pump via the overrun-stat, through terminal 3. As the boiler heats up, at some point the overrun-stat switches across from term3 to term2, so that the pump will be fed directly until the cyl-stat goes off and the overrun-stat goes back to term3, which is then "dead". This suggests that the overrun-stat must move very quickly from t3 to t2on warm-up, to avoid the pump stopping, so if that movement is sluggish it could perhaps cause my problem. I would need to meter those terminals and the pump-feed to see what happens.

Does any of that make sense?

Raymundo

 

[Tipper]

Isn't this just the time lag whilst the zone valves motorover/changeover and make the aux switches? Or maybe I've missed something in all those words!

 

[namsag]

No it isnt time lag and you have not missed anything.

OP
It certainly sounds like the overrun stst as it changes over

 

[Raymundo]

Well, I fired things up again, waited until the temp was just under 50C, then sat with the mains-tester glowing on the pump live terminal and a hand on the pump. After a few minutes, the tester went off and back on instantly and I felt just the lightest jolt from the pump. It continued running sweetly until the cyl-stat was satisfied, plus about 2 minutes, then stopped. No bangs at all. So I guess it is the o'run stat messing about - switching instantly sometimes but mostly not. It's only just 2 years old, so I'm gonna check the warranty on that, 'cos they're not really cheap!

Thanks for all the input everyone,

Raymundo

 

[namsag]

Not cheap ????? £25 ish