No Central Heating only - fine on HW and CH or HW only

[Wildeman]

Hello,

I am after some assistance with the issue I have with my Open Vented, Fully pumped Central Heating system.

I have a Glow Worm Ultimate 60BF boiler, Sunvic MoMo 3 Port Mid-Position Valve, Sunvic Digi Stat, Sunvic Programmer, TRV's fitted to all but 2 rads, Honeywell Tank Stat and a Wilo Gold RS pump

The issue I have is that the HW only works without issue and so too does HW and CH until the HW is up to temperature. Once the hot water is up to temperature or CH only is requested on the programmer the boiler fires for a minute or so and then goes out and the radiators do not get warm and cool water is circulated.

The only way I have been able to get this to work is to turn the cylinder stat up to 85 degrees and turn the boiler temp stat down but I have had a shower pump and kitchen tap newly fitted that have a max water temp of 65 degrees!!

I have had an electrician rewire all the components and the box in the airing cupboard, the pump has been replaced recently as to has the cylinder stat and mid position valve. The programmer too has been replaced and the electrician confirmed all the DIP switches are set right and it is set to fully pumped.

The pipework under the floor is 28mm reduced to 10mm close to the radiators where the 10mm tails run through the walls to the rads.

Could the issue be flow through the boiler causing the issue when the valve is in the heating only position as the attached boiler manual states "the flow through the boiler must not be allowed to fall such that there is a temp difference greater than 20 degrees between the flow and return" The manual for the boiler is attached.

If so how can this be resolved - balancing the radiators?

Any assistance would be greatly appreciated as I have ruled out the electrics, components such as pump and valve and the pipework appears ok so the only thing left is the boiler.

 

[jackthom]

Assuming the DHW is fully up to temperature and nothing is running, when the room thermostat is then turned up does the boiler fire at all and does the pump run ?

If nothing happens the culprit is likely the 3 port valve.

 

[Wildeman]

Assuming the DHW is fully up to temperature and nothing is running, when the room thermostat is then turned up does the boiler fire at all and does the pump run ?

If nothing happens the culprit is likely the 3 port valve.

The valve was only replaced 8 days ago due to the Valve body leaking - but to answer your question when the DHW is fully up to temp and the room stat is turned up with the programmer set to CH on the valve moves to the CH only position and then the pump runs and the boiler fires - the feed to the pump gets hot briefly then the boiler goes out and the water runs cool and remains like that. If the DHW cylinder stat is turned up the valve moves to mid position the boiler instantly fires and red hot water to the pump and the rads get hot.

As I say the electrics have been rewired as per Y plan by a qualified electrician.

 

[jackthom]

Does the pump continue to run indefinitely and if it does does the boiler ever fire up again for another brief run?

TBH I would want to have a multimeter connected to the boiler demand wire to check that the 3 port valve microswitch is supplying and maintaining the correct voltage to fire up the boiler.

It could be a flow problem but then I would expect the boiler to cycle on/off and not remain cold after a single burn.

 

[Wildeman]

Does the pump continue to run indefinitely and if it does does the boiler ever fire up again for another brief run?

TBH I would want to have a multimeter connected to the boiler demand wire to check that the 3 port valve microswitch is supplying and maintaining the correct voltage to fire up the boiler.

It could be a flow problem but then I would expect the boiler to cycle on/off and not remain cold after a single burn.

Yes the pump does continue to run indefinitely and tbh I am not sure if the boiler ever fires again for a brief run as not stood in the garage long enough - maybe that is the next step.

The electrician when troubleshooting to rule out the wiring he had a meter on the boiler demand wire and confirmed the voltage supplied and maintained was correct.

What is weird and rules out the wiring is if the valve is in the CH only position and the head is removed - once the cam on the valve body is moved slightly towards the mid position the boiler fires instantly and the pipe to the pump gets red hot - move it back to the CH only position and the boiler goes out and the water runs cool to cold eventually!! This is what leads me to believe it is flow or temp differentials at or through the boiler.

 

[jackthom]

There's clearly no problem with flow around the CH circuit itself as it gets hot when DHW and CH are on together. Sounds like the valve body isn't allowing much flow at all when in CH only position. The boiler will no doubt remain very hot for a while then eventually fire up briefly again when cooler.

Did this problem only start when the valve body was replaced?

 

[Wildeman]

The valve is a new valve fitted 8 days ago - how is it getting flow to the radiators in that case when in the mid postion?

The problem has been around for years but I used to have the DHW cylinder stat set to 80 and the boiler stat turned down but I have a shower pump/valves and a kitchen tap that cannot take water hotter than 65 degrees so I have turned the DHW cylinder stat down and the issue has reared its ugly head again!

 

[jackthom]

It is baffling but what you are describing sounds like almost zero flow when in CH only mode. Something the valve shouldn't be capable of doing.

Can you post some pictures of the layout?

 

[Wildeman]

It is baffling but what you are describing sounds like almost zero flow when in CH only mode. Something the valve shouldn't be capable of doing.

Can you post some pictures of the layout?

Yeah I thought either flow or temp differential - but if a lack of flow how is it all working in the mid position heating the DHW cylinder as well as the rads?

I will only be able to post pics from the airing cupboard where the DHW cylinder, valve and pump are and the boiler - is that what you need?

 

[jackthom]

Somehow the lack of flow is only when the CH port should be open by itself. This strongly suggests the valve is at fault but pics of the valve and what is connected to each of it's 3 ports might help rule out any other possibilities.

 

[Wildeman]

Somehow the lack of flow is only when the CH port should be open by itself. This strongly suggests the valve is at fault but pics of the valve and what is connected to each of it's 3 ports might help rule out any other possibilities.

I will get some pics later when I get home but the valve has been removed and checked to see if the CH port is opening as it should do and was. It was exactly the same and the same thing happened with the previous valve which was only changed due to the body leaking where the cam comes out to attach to the head.

 

[Wildeman]

Here the requested pics of the Valve as well as the airing cupboard pipe layout and the Boiler

 

[Agile]

Has it ever worked as you expect it to with just the CH on?

Tony

 

[Wildeman]

Has it ever worked as you expect it to with just the CH on?

Tony

It is hard to say for sure but I believe it did when we first moved in but for the last 9 yrs it has been like this - it was only really noticed when I turned the DHW Cylinder stat down from 80!!

 

[Agile]

Are you quite sure the timeclock is not set for gravity instead of pumped?