Glowworm micron 60FF reset

[Paul Carlton]

Hi, I have a Glowworm Micron 60FF boiler, which keeps resetting. Reset light on solid. It happens once or twice a day. In between the resets, the boiler works fine. It makes little difference where the boiler thermostat is set. The reset light indicates overtemp, and I've monitored the temp on the feed pipe, and it normally heats up to about 65, before turning off and cooling, and back up to 65. Every so often it goes up to 85, followed by the reset. The board, thermistor, pump and overtemp stat have all been replaced, but the fault remains. As far as I can see it has to be the thermistor or board. The old and new thermistors test ok. Old board was 2000227135, new board is 2000801991. I did consider scale inside the boiler preventing the thermistor from getting a good reading, but I've measured the voltage across the thermistor during use and the voltage continues to drop as the temperature rises, until it resets. I suspect I may have a faulty new board, but it's unusual to get the same fault on a new component. The voltage across the thermistor is quite erratic, which seems wrong, but I don't have a good one to compare. Any ideas? FWIW I'm an electrical engineer, so have a fair understanding of electrics, but can't find much info on these boards.

[John Rumm]

Have you checked that the pump is not under performing intermittently? Might be worth trying it on a different speed to see if that makes a difference.

A chat the CET might be a good idea, they recondition boiler PCBs and know most of the common failure modes. They can also do recon exchange boards if you want at a significant cost saving over new ones:

[Paul Carlton]

Hi, I tried a different speed, and then fitted a spare pump, so I don't think it's the pump. A refurb board is an option, but the board I fitted was new and sealed, so I just want to check I'm not missing something, before ordering another. I know the boards can be temperamental. Regards, Paul

[John Rumm]

ok fair enough - was just looking for things that could cause an unexpected temperature rise.

CET are usually open to discussing faults and diagnostic options (it's run by a former frequent poster to this group)

[Roger Mills]

You haven't told us what sort of control system you've got - how many and what sort of motorised valves, by-pass circuit, pump over-run arrangements etc. [See

It sound to me as if the flow through the boiler is stopping dead when the demand is removed - but that the boiler needs the flow to continue for a bit to take away the residual heat and stop it from overheating.

If all else fails, use a bathroom fan timer to keep the pump running for a couple of minutes after the boiler stops firing. [See the bit about adding pump over-run in

[Paul Carlton]

Hi Roger. I have Tado controls, and a 3 way valve for hot water, Y plan. We've lived here for 14 years without many boiler issues, and although the boiler has an overrun facility, it wasn't wired in. After investigating the fault, I thought it might be useful, so I wired the pump back to the boiler, but the fault remains. I've been monitoring the flow temp and thermistor voltage and I can see that the boiler still has the burner on, when the water is much hotter than normal, and only stops when the overtemp stat shuts it down, followed by the reset lamp.

[Roger Mills]

In that case, it sounds as if whatever the thermistor is connected to - PCB? - isn't responding to the set temperature being reached and turning off the burner. I suppose there's an outside chance that the gas valve is sticking open, but if that were the case, even the overtemp stat probably wouldn't stop it!

[Paul Carlton]

Thanks Roger and John for your replies. I'll have a chat with CET and then get another circuit board. Regards, Paul

[John Rumm]

Measuring the mains supply to the gas valve ought to verify that.

Another things to check is that the wiring loom is ok, since correct operation required decent connections between the PCB and the various sensors and activators.

[Paul Carlton]

Thanks for your replies. The new board should arrive tomorrow, so I'll have another look at it when I get home at a sensible time. The difficulty with measuring things is that the fault only occurs once or twice a day, but sometimes goes 3 days without tripping. I've checked all the connections to the board are ok, and connections to devices that I think could contribute to an overheat, but I'll check them all. There is only the gas valve and air-flow sensor left to check. Cheers, Paul

[John Rumm]

Yup, unless you have some kind of data logging multimeter it can be a pain trying to catch intermittent faults.

I have found a couple in the past, but to be fair both were not too tricky due to their nature. One was a modern combi locking out infrequently with an overtemp fail code. The odd thing was that it would do it at power up when cold. That turned out to be a poor connection to a thermistor at a multiway plug. The other was an old floor standing cast iron lump that would just stop firing about 15 mins after it got started. That turned out to be a faulty gas valve, and it was easy to see it still had mains in to it when it decided to close.

[Fredxx]

<snip>

I've tried and failed to implement a pump overrun timer using a staircase style timer relay[1]. This one looks as if it might work.

Why the minimum 3 minutes? For a lightweight heat exchanger I would have thought a minute would be sufficient to dissipate any residual heat. Anything would be better than no overrun!

[1] These rely on a floating control input to turn the timer off. Connecting it to either live or neutral will simply keep the timer-relay on.

[John Rumm]

Probably because boilers that lack pump overrun as standard tend to be older designs with heavier high water content heat exchangers.

[Fredxx]

A modern Baxi heat only boiler has no internal pump or facility for overrun.

The EcoBlue wiring diagram wires the pump in parallel to the boiler.

[charles]

My boiler is 30 years old and has a pump overrun. For our village Hall, I had to design an overrun system in the late 70s.

[John Rumm]

Even 30 year old boilers are quite often still relatively modern designs, many include electronic controls and pump overrun.

Our first house in the early 90's came with a glowworm fuelsaver F that had pump overrun controlled by an internal thermostat. Having said that, whoever had installed it, had ignored it and wired it like a traditional W plan system cutting off all power to the boiler when the call for heat from the stat was satisfied. Needless to say it all worked much better when re-wired to give it a permanent live, and the pump powered via the boiler. (changing the diversion valve to a mid position one helped as well since the cylinder was not fast recovery, and could not handle the full output of the boiler).

[Paul Carlton]

A bit of an update in case anyone is interested. I tried a board from CET, but got the same fault. There was no noise from the boiler, but in desperation I tried F3, and have had no resets for 2 weeks. I did a mains water flush yesterday. My theory is that there was a partial blockage in the boiler heat exchanger, so the thermistor wasn't always getting the true temperature, but the overtemp stat was reading the correct temp, hence the tripping. Thanks for your suggestions.

[John Rumm]

Sounds like a good result then. Yup, it is easy to overlook that there might be something else broken that is not really part of the boiler's logic or control system.