boiler too hot and cutting out

[JackRussell99]

Boiler runs ok for 15 mins if heating hot water but longer if just rads on, approx 1hr, then cuts out. Tried reading through some of the prev threads but not sure if the same faults.

Boiler is Apollo, not sure of type.

System looks like S plan - open tank for feed and vent, 2 x 2 port valves, room and tank thermostats, separate timer. Rads have TRVs except one. New pump fitted 6 months ago (wilo gold) and system drained down and refilled with Fernox MB1. Water seems reasonably clear when I bleed the rads. Rads heat up quickly and evenly.

Boiler seems to gradually overheat causing steam in the pipes to increase (tinkling sound) until it eventually sends a rush of very hot water/steam up the vent, at the same time the boiler clicks OFF extinguishing the pilot light as well. Pump continues to run for a while.

I've tried Hi and Lo settings on boiler, similar effect just quicker on high.

The boiler is old but serviced regularly. Can't afford to change at the moment. Any suggestions on what the problem is how to keep it going for now would be much appreciated.

 

[gas_man_gaz]

do the rads get really hot before it overheats? could be ya thermistors/stats, common cause for overheating on these

 

[JackRussell99]

Yes they are hot.

 

[D_Hailsham]

Boiler runs ok for 15 mins if heating hot water but longer if just rads on, approx 1hr, then cuts out.

When did this problem start?
Have you made any changes to the system, e.g. fitting TRVs
Do you have a bypass valve? (after the pump feeding into the return)
What is the speed setting on the pump?

 

[JackRussell99]

Problem started after having TRVs fitted as part of efficiency grant. Fitters not that interested in problem. Bypass is 15mm pipe with manual valve, this is wide open. Pump on speed setting 2.

 

[D_Hailsham]

Problem started after having TRVs fitted as part of efficiency grant.

I guessed it might be that.

Fitters not that interested in problem.

Have you complained to the company who gave the grant? The fitters should have changed the manual valve for an Auto bypass and set it.

Bypass is 15mm pipe with manual valve, this is wide open.

So the bypass is acting as a short circuit.

Pump on speed setting 2.

Not the real solution, but have you tried setting the pump to 3?

May I ask who you got the grant from?

 

[JackRussell99]

I should have said, the bypass opened by me in an attempt to get more hot water. It seemed to be overheating without rads on, pump straining when water very hot.

Just tried pump on 3. It lasted about 1 hr 20 mins before I switched off to prevent it venting over again. Every thing really hot. I had the room stat set at about 23-24C so was still trying to acheive that. Pump stopped about 15mins later.

 

[gas_man_gaz]

still think it'll be ya thermistors, if it is you will need a pro because they are situated inside the combustion chamber

 

[JackRussell99]

if its the thermostats is it expensive?

 

[gas_man_gaz]

not really, theres 2 and they are £2.95 each.. one for the hi setting and one for the low settin, jus labour then - tht is if it is tht at fault

 

[namsag]

AS gaz says its the stats that have gone and the overheat stat is doing what it should and cutting the boiler off. If you need the part numbers you will find them here www.partsarena.com/baxi

EDIT removed

 

[JackRussell99]

Many thanks for your wisdom. I will see if I can combine a service wth fitting the stats.

One last thing should the bypass be closed some and how much?

 

[gas_man_gaz]

i norm shut it fully then open it up around 1/2 a turn

 

[JackRussell99]

Thank all for your help, will get engineer to fit stats as soon as poss.
Will let you know what happens.

 

[gas_man_gaz]

cheers, its nice to hear how it goes