Boiler Overheat

[penguinventures]

I have a Myson Economist 60/80 S Plan system which has started to occasionally overheat shutting off the pilot light. Seems to occur only when pump stops when boiler is flaming at max output. Boiling/rumbling can be heard in the heat exchanger as there’s no pump-overrun and the overheat sensor trips shutting off the pilot light

Boiler temp has been turned down to 2.5 which reduces occurrence. Test meter on boiler output pipe indicates boiler stat works fine as temp cycles never going above 58 degC on setting 2.5.

System is clean having new pump, power flush, inhibitor/noise reducer additives last year.

Any suggestions on what could be causing the problem. It's worked ok without a pump-overrun for many years?

Can a pump overrun feature be fitted? Can it be as simple as pipe thermostat fitted adjacent to the pump keeping the pump on till water cools. I am concerned with both valves closed is there going to be enough flow through the bypass as its a gate valve that's barely open. The system has no TRV's and hence no pressure bypass valve.

Any advice appreciated?

 

[MrTherm]

There was an extra permanent live terminal on the Myson Ranco boiler stat especially for the pump overrun - check it out.

The rumbling will happen for a short time after full fire on an S plan when both zone valves a closed if there is no bypass.

Indeed, the boiler stat set at 2.5 or 3 will raise contained (trapped) hot water above the 93 degree fixed limit stat setting to almost boiling and trip the interrupter.

An effective pump overrun would need a pipework bypass loop. No bypass restriction is allowed via a gate valve as full bore is required to clear the finned copper tube heat /ex' - low content always needs fully pumped.

One solution, just a suggestion, would be to replace the c'h zone valve with a three port valve specially wired so as to open and close via the room stat but to keep open a bypass loop when both c/h and h/w are satisfied and closed. The boiler/burner would be off and the overrun active per the Ranco boiler stat.

The Ranco boiler stat cuts off at about 88 to 90 degrees max if i remember correctly, keeping just below the 93 degree limit.

 

[penguinventures]

Cheers for the advice. I'll have a look for the terminal on the stat. Maybe its even wired in and become disconnected in the airing cupboard? If not hopefully there's already a spare core in the cable running from the boiler to the airing cupboard?

From what I've seen of the Heat Exchanger I think its all cast iron.

I see what your saying about the change to a 3 way valve and will mention it to the engineer.

On another S plan system I'm familiar with there's a pump overun controlled by a more modern boiler and this was originally routing water through a gate valve bypass until it got blocked and was replaced with a Honeywell DU145 automatic bypass. Could the same thing be done to this system or is this not good practise?

 

[MrTherm]

Yes, that would work although an effective bypass loop would need several metres of pipe run to dissipate excess heat rather than too close to the boiler with trickle valve between flow and return. The best place to dump heat would be in the system itself, either via c'h or cylinder coil, overrun times would normally be for just two or three minutes and would have little impact on temp' output settings.

other methods:-

(i) three two port valves (one for bypass via loop to boiler return)

(ii) replace two zones valves with a three port valve, there will always be one port open for heat dump.

However, of the various Myson models, if your boiler has the cast iron heat exchanger the problems will be minimal compared to the finned tube models.

These boilers will last for years if well maintained.

 

[penguinventures]

Thanks again for the support.

If an auto bypass replaces the current gate vale there would be a bypass loop of over 12 metres. Hopefully this would be enough?

I'm waiting now for a heating engineer to meet me at the property then it'll be clearer what is and what can be configured.

Cheers

 

[MrTherm]

12 metres sounds good, when the work is complete you should be able to turn the boiler stat higher without any pilot interruption.

Good luck