Thermostatic pump overrun temperature?

[brian_mk]

A simple question, but so far I've not found a definitive answer:-

Some older boilers use a thermostatically controlled pump overrun switch built into the boiler thermostat.
They are generally factory set and not adjustable.
At what temperature should it switch?

I'm specifically referring to the Glowworm Fuelsaver MK2 boiler with a Ranco C77p0102 thermostat, but I imagine they are all set to operate at a similar temperature.
I suspect it should be 65degC.
Can anyone confirm this?

 

[oz-plumber]

Not familiar with the boiler in question, but are familiar with what you are asking.

I have seen boiler stat pump run on settings as low as 40 C, but most I encounter are 50C.

I doubt yours would be 65 C. I would assume 50 C or lower

 

[brian_mk]

I think the one in question is currently switching at around 65degC.
The boiler thermostat is several years old so there is a good chance that the pump overrun set point has drifted over time and is now higher than it should be.

The result is that if the boiler thermostat is set fairly low (e.g. '2' on the '1' to '5' scale), then the water temp in the heat exchanger can remain below the pump overrun threshold. When the room stat turns off the boiler, the pump turns off at the same time (i.e. no initial pump overrun). The water temp in the heat exchanger continues to rise for a while and after a few seconds the pump switches back on. It only remains on for about 25 seconds and then is switched off by the pump overrun thermostat. When this happens there is sometimes a disturbing thump noise in the vent pipe as a pocket of water vapour is expelled up into the F&E tank.

If the boiler temperature is increased slightly (e.g. setting '3'), then the problem goes away because the water temperature in the heat exchanger is above the pump overrun threshold and so the pump remains on for longer when the room stat cuts off the boiler.

The pump overrun threshold is factory set and is not intended to be modified: The adjustment screw has thread lock to stop it moving. The problem is that the thermostat is obsolete and I cannot buy a replacement.

Using an old thermostat, I have found that a soldering iron applied to the adjustment screw softens the thread lock and allows it to turn. I think I need to remove the thermostat from the boiler, place the phial in a heated pan of water and use a thermometer to adjust the screw so that it switches at a lower temperature.

Hence my original question about the correct temperature setting for the pump overrun.
You would have thought this information would be published by Ranco but I've been unable to find any documentation online.

 

[John.g]

If its set and operating normally as you suggest at 65C and the boiler SP is on 60C then you might expect a delay between the room stat switching off the boiler and the pump starting on overrun until the Hx rises to 65C, it should then run on until the stat hysteresis switches it off?

 

[brian_mk]

If its set and operating normally as you suggest at 65C and the boiler SP is on 60C then you might expect a delay between the room stat switching off the boiler and the pump starting on overrun until the Hx rises to 65C, it should then run on until the stat hysteresis switches it off?

Yes I agree.
I can't really explain the 'thump' noise in the vent pipe unless the pump is sticking when it's switched back on by the overrun stat and the pump is hot. It's hard to pin it down because it doesn't happen very often. I'm sure the pump doesn't stick when the system first starts up and it's cool.

The only difference that I can see is that when running with a low boiler temperature, the pump switches off for a few seconds before being switched back on by the overrun stat. Even when that happens, it doesn't always make a bang in the vent.

 

[John.g]

Also you say the thump occurs when the pump stops which means the the temperature is or should be "normal" but strange that no thump when boiler SP increased to ensure pump overrun without delay until presumably the temp again falls to the same "normal".
I have seen thumping being caused by excessive pump pressure combined with a mid position valve but one would think that this should occur during normal heating operations as well and the diverter valve should be closed anyway during pump overrun.
You might look at the ABV setting (if fitted)

 

[brian_mk]

Also you say the thump occurs when the pump stops which means the the temperature is or should be "normal" but strange that no thump when boiler SP increased to ensure pump overrun without delay until presumably the temp again falls to the same "normal".
I have seen thumping being caused by excessive pump pressure combined with a mid position valve but one would think that this should occur during normal heating operations as well and the diverter valve should be closed anyway during pump overrun.
You might look at the ABV setting (if fitted)

I can't be sure if the thump happens when the pump starts or stops. It happens infrequently and I've been unable to force it to happen while I'm monitoring the pump. All I know is that it happens shortly after the room stat turns the boiler off.

I did wonder if the 3 port valve could be affecting things. I think the HW cylinder would already have reached its required temperature so the valve would be in the 'heating only' position (port B open, port A closed) at the point when the room stat switches off.
My understanding is that the valve would then move back to its default position with port B closed, port A open i.e. 'HW only'. I'm pretty sure the diverter valve can never close off both ports at the same time. The valve movement would take a few seconds. I'm not sure what effect it would have when the pump starts up while the valve is moving between the two positions? I would not have expected it to give a problem.
I don't think the valve is sticking - I've removed the cover and watched the gears and spindle moving.

 

[John.g]

More than likely thump is when the pump starts alright, if the circ pump is external can you see what mode/setting its on.

 

[brian_mk]

It's an old Grundfos pump with three (manual) speed settings.
I usually have it set to the lowest speed where it runs the quietest.
This is the setting where I've heard the thump.
I've also tried it on the middle speed. I haven't heard the thump on this setting, but because the problem is intermittent I can't really be sure if pump speed is a factor.

 

[John.g]

One would think that lowest speed should be best, also if you have both CH & HW on, the valve is in mid position (A+B) then with no demand for HW the valve will motor towards the heating only (A) position and if then no demand for heating (or HW) will remain with the heating port A open but boiler off so I don't think the valve will be changing over during pump overrun under these conditions.