Rayburn overheating (again)

[Burnerman]

Good morning all
This rayburn thats haunting me is still overheating, causing a reset stat to trip on its control panel. Unfortunately it has to be manually reset, which is a bore.
The scenario....switch the boiler on, pump starts, all jolly and cosy.
When the boiler time switches off, water in the boiler naturally gets hot, reset deploys.
The boiler is sending a pump overrun signal to the wiring centre in the loft, so all ok there.
However, the pump stops running because the signal is blocked by something in the circuit - hence the overheating. It is an S plan system.
Would it be in order for me to take a live from the boiler and link it to the pump live - to keep it running for as long as the boiler overrun stat tells it to, or am I giving myself other problems?
As you can tell, the theory is fine but I don't really know what to do for the best.
Good day all! John and thanks for reading.

 

[slapper]

Could it be that the cylinder stat is switching the pump off irrespective of the overun facility,also is there a bypass on the system to dissapate X amount of btu,s.

Is there a flow temp switch fitted?,just food for thought.

 

[picasso]

disconnect the pump overrun and fit a pipe stat, this will keep the pump running dependant on the temperature of the water,

connect pump live to common on stat, permant live to no and switched live to nc, what have you got as a bypass?

 

[Burnerman]

Thanks for the replies, both of you. This is what I have done.....
The boiler, deciding that its too hot, sends a signal via the pump overrun stat to the wiring centre (spaghetti) in the loft. The pump is here, together with 2 2 port valves and a cylinder stat.
However, when the boiler switches itself off, the pump stops too so the boiler overheats and needs to be reset manually.
So, I found a live, sent from the boiler overrun, and I've connected the pump to this. So, the pump now runs for as long as the overrun stat tells it to. So far, so good. I've got heating, and the cylinder stat still controls its own MV so there should be HW too.
My concern though is that the room stat is now bypassed (not sure yet)
but even this shouldn't be too much of a problem because the rads have TRV's and the boiler stat works.
I really don't know what bypass there is....it could be 1 of 2 bathroom rads or a bypass valve hidden under the floor. I cant access this because of the quarry tiles on top of the flooring.
I'm going to run the thing right up to temperature this afternoon, so I'll post back.
Cheers again John

 

[Tipper]

Are you sure it's the water overheat stat that's tripping.

Many cookers have a cooker 'dry' overheat too that trips when the cooker body gets too hot. On the likes of 'Alpha Cookers' it kills power to the whole thing iirc.

BTW do they still make Alphas? Aga bought them out a few years ago....I assumed to get them off the market.

 

[Norcon]

With an S plan system , when the pump over run occurs motorised valves may be closed so no where to dissipate heat. A normal by-pass linking f&r (if one fitted)piping will be insufficient.
An electrical wiring solution is needed or a mechanical one.
The mechanical one is easier and I have used this method in the past on a Stanley cooker (which was overheating and tripping out) on the advice of the commissioning engineer and it solved the problem.

 

[Burnerman]

Here's an update.....
Now I've got the pump running as I want to, I turned the boiler stat 3/4 full to see if the rads got hotter and set the system going. When i returned to it, the boiler overheat trip had tripped out - but I don't know how long it took for this to happen. The pump is running on position 3.
I've unearthed a gate valve, which will be the bypass, I assume. It was fully open, and hadn't been touched for years.
I've closed the valve, and then opened it a couple of turns.
My theory is that the very hot water, far from being dissipated by the rads, was returning to the boiler and overheating it.
The system is running now, and I'll pop back over in an hour or so. Luckily its just next door!
Please keep the comments coming.
Cheers John

 

[Burnerman]

Norcon - would you be good enough to tell me about the mechanical solution you used on the Stanley cooker?
John

 

[Norcon]

Yes no problem.
A spring loaded by-pass valve across the HW cylinder motorized valve which dissipated enough heat to avoid the cut out.

 

[kevplumb]

Yes no problem.
A spring loaded by-pass valve across the HW cylinder motorized valve which dissipated enough heat to avoid the cut out.

how fiendishly simple good idea

 

[Norcon]

Thats what I thought too Kev.

 

[Burnerman]

Yes no problem.
A spring loaded by-pass valve across the HW cylinder motorized valve which dissipated enough heat to avoid the cut out.

Now that....is pretty brilliant! Wouldn't it be better across the CH MV instead of the HW one?

Further update.....
With the gate valve bypass turned down but not shut, the rads are much hotter than before. So, I've turned the boiler stat down (rads almost too hot to touch) so I'll see in another hour or so if the reset trips again.
Obviously this system has never worked properly, but I want to get it as right as I can as there will be a family renting the place in a week or so, and one of the children is disabled, so I hear. Therefore the place must be safe and reliable.
Cheers John (Make believe heating engineer )

 

[oilhead]

Is the Rayburn wired up as it should be? The supply to the pump should come from the Rayburn wiring centre and not the motorised valves.
From your description, it seems as though it may never have been wired properly.

 

[Burnerman]

Is the Rayburn wired up as it should be? The supply to the pump should come from the Rayburn wiring centre and not the motorised valves.
From your description, it seems as though it may never have been wired properly.

Thats the million dollar question, mate....the house is a bungalow, the flow / return pipes from the Rayburn head for the loft, where the pump and HW cylinder plus the 2 MV's are, and the pipes then descend back down to the rads. The wiring centre is there too...the live from the overrun stat wasn't connected to anything, but its driving the pump now.
Initially the pump would stop once the boiler timed itself off, then the reset stat for overheating would come into play. I've now got the pump running, controlled by the boiler overrun stat and I've played with the bypass - discovered hidden in the loft. My fear is that if the CH MV shuts when the boiler times off, the boiler will get too hot again and I'm back to square one...Testing it now!
John

 

[Norcon]

Wouldn't it be better across the CH MV instead of the HW one?

The Stanley in question was a twin burner. The problem arose with the cooking burner. I think the output to the water heat exchanger is only a few kilowatts so the HW by-pass worked OK and also he would have been using it for cooking in the summer so the HW is more beneficial than losing the heat to the rads.
He also held off the HW programming schedule when he knew he would be using the cooking burner.