Baxi Boiler overheating!?

[bon98uk]

Hi All,
I have Baxi Bermuda Inset 2 Boiler 50/4E and Graivty fed HW/Pumped CH system. I have roomstat, BoilerStat and 2port Honeywell valve.
System has been wired using Honeywell C-plan schematic.

Untitled

• bon98uk
• 23 Mar 2008
• 1

(Note the red spots indicate which positions are live under various conditions, above shows HW-Off, HT-ON).

I have tested the configuration under the HW-On, HT-ON through to HW-Off,HT-Off and all the right locations activate.

The system performs as expected, in that if RoomStat is satisfied Boiler and Pump switch off and if CylStat is satisfied the Valve closes and the auxilary switch changes poles.

So what's the problem; The problem is that if I set the roomstat high so it takes a long time to satisfy (or set it so it will neve be satisfied i.e. short relevant switch.) Then I expect the BoilerStat to cut in and stop the water in the flow overheating. This isn't happening! So conclusion the boilerstat is duff.

However the boiler does cycle and when near Roomstat satisfy the boiler 'short cycles'. On for a few seconds/ switches off the a minute or two switches off again.

If the roomstat is well off satisfy then the boiler does come on for a decent period of time.

So is the 'short cycling' near heat satisfy normal. I.e. Homing in.

Is the boilerstat really broken, (if I leave RoomStat high I eventually get some pump over into F&E. By eventually I mean after about 5 hours.)

Any ideas??

PS: I have seen on another thread comments about the C-plan wiring about the Grey wire could be positioned at 6 and by the same token the Boiler live feed could be moved to 9 I guess. But from a switch matrix logic these are equivalent, in that the end result would be the same.

 

[seco services]

whats the boiler stat set to ?
turn it down low and see if it cuts out
what speed is the pump on?

 

[gas4you]

First impressions are that the boiler is working ok.

Can't see how any external wiring can affect the operation of the boiler stat.

Pumping over could be a seperate fault, sludge build up or something.

 

[bon98uk]

Thanks,

Dave, I just wanted to specify the wiring because the original wiring was awful and did not work as an interlock. (Boiler would appear to stay on even when Room and Cylinder were okay.)

I doubt it's sludge because the inhibitor is religiously updated every two years and I do a steel nail/copper pipe in jar check each time. Never had any issues. (Though I will probably do a full flush etc. this summer, maybe even powerflush.)

I am just trying to eliminate possibles. I currently have the Boilerstat set just off it's minimum value. (Total of 9 rads, 2doubles, 4 singles, and 3 single short ones.)

Even here the Flow appears to be getting hotter than 82C, I can't see how sludge is doing this. (That may be my own lack of understanding if so help me out here.)

 

[seco services]

you just said your boiler stat is down low so you should see the boiler cutting in/out.

 

[gas4you]

I was just pointing out that external controls could not affect system water temp if boiler stat was working.

Now that you say you are getting 82C with it set at minimum i would suspect the boiler stat.

Has the boiler been serviced very recently? If so the boiler stat phial may not have been pushed back in fully, or replaced at all.

Unfortunately as checking this requires removal of the fire front and therefore disturbing gas connections, you will need an RGI to check things further for you.

 

[bon98uk]

whats the boiler stat set to ?
turn it down low and see if it cuts out
what speed is the pump on?

Thanks Seco I missed your post so a quick follow up.

Boilerstat is set to minimum, at present. I will do some more 'scientific' twiddling with it shortly.

Pump Setting; I have found following when system is at operating temp and well before the 'overheating' occurs.

Speed 1: Delta T (FR) = 11-12 degrees
Speed 2: Delta T (FR) = 7-8 degrees
Speed 3: Delta T (FR) = 5-6 degrees

Based on these I have the pump set to 1; (Boiler specs are Delta T 11degrees).

Additionally the Pump is in the return pipe not the flow. (i.e. system is not under +ve pressure.)

 

[seco services]

if you boiler stat is on 1 and not cutting out. could be faulty or not pushed in fully as mentioned.

 

[bon98uk]

Hi Dave and Seco,

Thanks for your replies very helpful. Yes the boiler is due a service and of course I will use a RGI. I'm happy and confident doing both plumbing and electrics but the rules are use and RGI if gas is involved. So that's fine.

I'm really in 'trying to understand' mode. With the boilers own thermostat set to minimum and the Room calling for heat (Orange neon switched live is on) the boiler swtches on for a few minutes (green pilot neon lights and burners fire up) then they switch off but the Orange light is still on. This cycle repeats itself many times (Dwell time is about 1minute or so - i.e.on for 1 off for 1).

Eventually the flow pipe reaches temperatures in excess of 82C. (I switch off programmer when it gets to 87C) If I leave it I will get pump over.

 

[gas4you]

Have you properly checked that the pump is actually spinning/circulating?

(See FAQs)

 

[bon98uk]

Hi Dave,

A good thought! But I use the old screw driver to ear trick and I can definitely hear the pump turn. (I also switch the pump speed during this check to ensure the pump speed changes.)

So yes I am confident the pump is spinning when the system tells it too.

 

[gas4you]

Thats OK, but as said hundreds of times before, a pump can spin but not circulate

 

[bon98uk]

Fair comment! If I undo the bleed screw I get water. So I don't believe the pump is spinning in air but that's definitely worth checking further.

Any other checks you can suggest to verify circulation other than checking the rads do actually heat up and that water escapes from the bleed screw.

 

[bon98uk]

Okay,

So I tried the boiler stat Potentiometer. Didn't seem to change anything. If I set it to max and wait till boiler fires and then turn it to min. Nothing happens.

So I set it to middle and put pump on 2 and turned up RoomStat to 25C.
Two hours in the flow pipe reads 95C (I'm using an infrared thermometer pointed at a black piece of insulation tape to ensure the emissivity is caibrated. I also have a pair of spring clip on pipe thermometers. These are not very accurate!)

But still the pipe is flippin' hot! and I have turned down the Roomstat to deactivate the boiler.