Thermostatic pump overrun temperature?

[brian_mk]

I managed to find a brand new replacement thermostat.
It was advertised as being for a Ravenheat boiler rather than a Glowworm Fuelsaver MK2 but it's actually the exact same Ranco C77p0102.

When it arrived, I checked the calibration using pan of water and an accurate thermometer.
The results are with the temperature control set to the highest setting (=5).
The boiler switches off @ 77 degC.
The pump overrun switches at 70 degC.

I was surprised that the pump overrun temp is so high.
Now I'm wondering if the calibration is within spec.

I still have been unable to find any documents that specify the correct factory settings.

 

[John.g]

It would (will) be interesting when you test the old one and see its switching point, based on your new one its probably around 70C so if you are running with a boiler SP of anything below ~ 65C C then probably a pump overrun start delay with this one as well?

 

[Ben-gee]

It’s possible the thermostat you have bought has a higher sp of 70 when fitted to a ravenheat; but a lower factory setting intended for your boiler?

 

[John.g]

Ideally I suppose one wants the pump to kick in immediately when the boiler shuts down so the stat should be set at, or slightly lower than the minimum boiler set point which on a condensing boiler may be ~ 40C but on non condensing boilers is around 60C so this stat, even though still apparently calibrated high at 70C may be specifically designed for a non condensing boiler.

 

[brian_mk]

A further development...

This evening I discovered that if I turn the room stat right down, the boiler keeps running and the radiators still get hot. I believe the room stat is working. I can hear it click when I turn it.

The only explanation I can come up with is that the 3 port valve is not operating correctly. The HW cylinder gets hot, so could the valve be stuck in the mid position?

While I was trying to diagnose the pump overrun problem I took the cover off the 3 port valve to check it. I thought it was working as I could hear the motor run and I could see the gear wheels turning. Is it possible that the rubber ball may have disintegrated over time?
If the valve is not working correctly that may explain the 'thump' noise in the pipes I sometimes get when the room stat turns off.

The valve is a Honeywell V4073A1039.

See:-

Honeywell motorised valve faults - Free Heating Advice - https://www.freeheatingadvice.com/articles/honeywell-motorised-valve-faults/#Honeywell_3-port_mid-position_valve_operation_V4073A1039_and_V4073A1088

For valves made before 1985, you have to drain the water before you can remove the power head. I think the one I have falls into that category. Just my luck!

You can't replace the entire valve without first draining the HW cylinder, disconnecting it and shifting it sideways to allow the pipes to pull out of the compression fittings. I'm hoping it may be possible to just replace the power head.

Can anyone confirm that it's possible to change just the head without having to remove the entire valve?

 

[ShaunCorbs]

does the head (the part that removes to get at the insides eg synchron motor) has a dimple ?

 

[brian_mk]

does the head (the part that removes to get at the insides eg synchron motor) has a dimple ?

There is no dimple so I'm pretty sure the head cannot be removed without draining the CH system.

I suspect the shaft may be jamming and/or the ball is not seating properly, so the valve body almost certainly needs replacing.

If you look at the earlier photo of the inside of the valve, it would appear that you can remove four screws and replace the top plate, valve ball and drive shaft from a new unit without having to disconnect and replace the main body. Obviously the large O-ring would need to be swapped over from the new unit. Has anyone tried to do this?

As already explained, in my situation there is little free movement available in the pipework. Hence replacing the entire valve would involve draining, disconnecting and shifting the HW cylinder to allow the pipes to be removed from the valve compression fittings. That would involve a lot more work (See the photo below).

 

[ShaunCorbs]

Yea no dimple so 4 screws so not removable sorry to say

you should be able to remove that

loosen all joints

undo the back pipe, push the pipe out of the 3 port spin the body so it’s out of the way

undo left and right nuts

push left pipe out of the valve and at the same time remove from cylinder

if you can don’t remove the cylinder connection

 

[brian_mk]

I've ordered a replacement valve.
In the mean time I re-checked the operation of the suspect valve.

The shaft definitely turns and it returns to the end stop under the force from the springs when no power is applied. In that position, hot water from the boiler should feed the HW cylinder only.
I've tried setting the programmer to HW only but the rads still get hot.
It suggests the problem may be the rubber ball inside the valve. Has anyone taken an old one apart? Does the rubber harden and no longer provide a seal? Does the rubber disintegrate?

 

[ShaunCorbs]

Sometimes they fall off or yes they wear and can let water pass