Glowworm Fuelsaver MK2 Pump overrun

[brian_mk]

I have an old Glowworm Fuelsaver MK2 in a Y plan installation.
It's still working ok apart from an issue with the pump overrun...

As of 2/3 weeks ago, when the room stat switches off the boiler, there is sometimes a thump in the vent/expansion pipe as hot water / steam is pushed up into the F&E tank. It doesn't happen every time. It suggests an intermittent problem with the pump overrun.

I had a similar problem some years ago and replaced the boiler thermostat which cured the fault.

This time, to help diagnose the fault, I connected two neon lamps into the terminal box.
One shows when the pump is on, the other shows when the switched live to the boiler is on.

This is what the lamps indicate:-
When the boiler is running normally, both neons are on as you would expect.
When the room stat reaches temperature, both neons turn off together. This happens immediately.
After a short delay of maybe 3/4 seconds, the pump neon switches back on.
(This is when the changeover microswitch in boiler thermostat connects the pump to permanent live rather than switched live).
The pump then runs for about 25 seconds and switches off when the water temperature in the heat exchanger drops and the changeover microswitch connects the pump back to the switched live.

Twenty five seconds seems like quite a short delay.

Question 1: Is 25 seconds pump overrun delay normal?
I did have the boiler temperature control set low when I ran the test (position '1' on the dial that goes from '1' to '5').

The boiler thermostat is a Ranco C77P0112 (glowworm part no 416189).
These are now obsolete and it's impossible to find a replacement.

There is an adjustment screw on the thermostat that controls the 'set point' temperature for the pump overrun switch.
Question 2: Should I try backing the adjustment screw by 1/2 turn to increase the delay?

Please no advice about replacing the boiler with a modern, more efficient one. I don't want to do that until next year when the weather is warmer!

 

[brian_mk]

Question 3: Is the 3/4 second delay before the pump turns back on due to the movement of the 3 port valve?

It appears that the pump overrun adjustment screw has some kind of thread lock to prevent tampering. It cannot be moved. If the 25 second overun delay is too short then it looks like the only option is to replace the thermostat. Unfortunately they are no longer obtainable.

Question 4: Does anyone know if there is an alternative replacement for the C77P0112 thermostat that will fit?
I've been told that all the Ranco C77 range are similar. The main difference seems to be the size and shape of the bulb.

 

[Last Plumber]

I wouldn't alter the boilers component parts from the original design without obtaining the manufacturers written permission or finding out whether they have upgraded/altered the design to something they have provided or deemed acceptable.

I can't think of a reason why you couldn't add an external one. Pump over runs are better if they go by temperature and time in my experience so you could use a timer relay (capable of carrying the pump motor) and a pipe stat on the boiler flow (close as possible). The design/wiring needs some thought! It needs to run long enough to dissipate heat obviously and I would want it to bring the boiler temperature down below 60 deg Celsius.

If you are not competent and experienced in electrical work or boilers, I think you would be wiser asking a heating engineer or electrician who works on central heating to do it for you.

Boilers that boil/create steam can be extremely dangerous as you may know so it needs to be dealt with immediately.

 

[brian_mk]

Hmmm. Interesting idea.

Maybe this eBay timer module could be adapted?

AC 110V 220V 230V 240V Delay Turn OFF Module Relay Timer Control Switch UK | eBay

Power Consumption: <1.0W standby zero power consumption. 1 x Relay Module. If need start button, the button knows capacity. Screw Hole: center distance 83 31mm, aperture 3.2mm. Easy to use. Long service life.

www.ebay.co.uk

'L' would connect to permanent live.
'L2' would connect to the switched live that turns the boiler on and off. That would start the timer.
'FAN' would drive the pump.

The 'FAN' relay would probably need to be swapped for one with heavier duty contacts capable of switching the pump. The contacts would need to be single-pole double-throw so that the pump either connects to a permanent live or to the existing pump overrun connection from the boiler stat.

 

[Last Plumber]

I meant more like this. You can buy them from most electrical merchants or online.

 

[brian_mk]

I think the cheap eBay module should do the job.
Like most stuff made in China it may not use the best quality components.
It uses a simple capacitive power supply rather than a transformer to save space and money.
That should be ok as long as the capacitor has a suitable voltage & current rating.

The relay is a JQV 3FF which is rated at 10A 277VAC so it should easily handle the pump.
It has SPDT contacts as required.

The timing may not be that accurate but for this application it doesn't need to be.

It just needs a box with fuse and maybe a LED or neon to show when the delay is active.
For only £6, I might get one to try.

 

[brian_mk]

This is what the lamps indicate:-
When the boiler is running normally, both neons are on as you would expect.
When the room stat reaches temperature, both neons turn off together. This happens immediately.
After a short delay of maybe 3/4 seconds, the pump neon switches back on.
(This is when the changeover microswitch in boiler thermostat connects the pump to permanent live rather than switched live).
The pump then runs for about 25 seconds and switches off when the water temperature in the heat exchanger drops and the changeover microswitch connects the pump back to the switched live.

I just repeated this test. This time I got a different result:-

When the room stat reached temperature, both neons went off together.
This time both the boiler and the pump remained off. There was no thump from the vent pipe.

I'm puzzled. Could this be because the boiler stat had already switched off the burner at the point when the room stat switched off? In this case the water in the heat exchanger could have cooled such that no pump overrun was required i.e. the overrun microswitch remained in the 'cool' state with the pump connected to the switched live rather than the permanent live?