Baxi Back Boiler 50/4E Noisy after being on for a while

Hi All,

I have a Baxi Back boiler 50/4E Gravity fed hot water paumped CH system with a two port (Honeywell) valve on the hot water return.

Right that's the system, heres the problem.

Timer 1 7am-9am Boiler fires up heating and hot water works fine swithces off at 9am no problems.

Timer 2: 5pm-9pm, Boiler fires up, cylinder reaches temp valve shuts of the gravity return. Heating works fine, noise free and nice an snug. Roomstat appears to work okay. Activtes/deactivates boiler.

Issue is that on timer 2 run after about 2.5-3hrs the radiators get very hot and noise begins to appear, clunking and banging. If I check the pipes they appear to be getting too hot.

Tests and checks done;
Balance seems okay; F-R 10C diff. same as flow return to boiler.
Pump is set on Low, (setting on 2 drops this diff to 7C.)
Flow Temp is 80C Return is 70C (when 'working' okay)
All TRV's have TRV bodies removed so all are fully open.
Pump activates/deactivates on Roomstat reaching temp.
Motorised valave operates on Cylinder reaching temp.

What happens is the boiler appears to start short cycling and overheating even though the roomstat and Cylinderstat are no longer calling for heat.

My logic concludes there is a boiler interlock fault and maybe the connections (for Valve perhaps) aren't switching off the boiler. This would be okay (though not ideal) as the boilerstat would kick in and switch off.

But if the Boilerstat is faulty AND there is an interlock issue then I guess these two together could give the symptoms I'm seeing.

Anyone with greater experience in CH and issues pertaining like to confirm my analysis or suggest other possible reasosn for the fault?

firstly test the boiler stat by running on the low setting
does the the boiler respond by shutting down at a lower temp?

Hi Baxpoti,

Many thanks!

I will be trying this test today. Though in above sequence I omitted to add that during the various heat up cycles I did increase the Boiler stat from from low to high. This seemed to increase the temperature of the Flow/Return as the boiler 'burnt' for a while. (As measured with my ETI Infrared Thermometer onto the pipes with black insulating tapes to increase the emmisivity to a sensible value.)

I have another view at the moment and feel my original analysis may be in error. The 'truth table' I have is:
CylStat RoomStat Boiler
1 --------- 1 ------------1
1 ----------0 ------------1
0 ----------1 ------------1
0 --------- 0 ------------0

i.e. 1 means On (call for heat/on) 0 means Off

It might be I have condition 0+0 but for some reason the boiler is still electrically alive and so if the Boilerstat detects water temperature drop then it fires up. This would mean the controls have not been connected correctly and the system was always wrong.

If this is true I have been wasting money on firing up a boiler to do nothing except get the water hotter and hotter. (by small increments).

The Motorised Valve is off and the pump is off so the water expands it's way slowly up the vent pipe and then pumps over causing noise etc.

This is a hypothesis at the mo' I will do further checks tonight. Not being a heating engineer I am seeing if anyone corroborates this theory. Or at least tells me why it's wrong!!

regards,

bon

HI

i have come across this before it was faulty boiler stat

Thanks for that, I did have that thought at one stage.

However it is my understanding that the boiler should be controlled in such a way as to only fire up on demand. The 'on demand' signal comes from either the Cylstat, roomstat or both.

The purpose of the boilerstat isn't so much to add control but to ensure the heat output of the boiler is in the right range so that when a demand for heat is called for it can supply it.

I'm taking this understanding from two places: 1) The Baxi 50/4e Install manual and 2) 'Central Heating: Fault finding and repair' by John Reginald.

So my understanding is that when the 0+0 condition occurs the power supply to the boiler is off and so it will not fire anyway even if the water temp drops *until* the call for heat is made by either or both of the control stats.

In checks tonight; the pump was off because the roomstat was 'satisfied' and the motorised valve was closed because the cylstat was 'satisfied'. However the boiler still fired on occasion (sometimes short cycling) and the water temperature exceeded aceptable levels. (I checked with my IR thermo and it was at one point over 100C!)

If the Boilerstat was faulty I could understand but the other two work and should have prevented the boiler firing. This is why I think I have a possible control wiring fault.

if boiler comes on when you think timer is not calling for heating or hot water then something is giving boiler a live feed,could be your 2 port valve as sometimes they are wired using five wires being permanent live,neutral,earth,hw on and hw off,could be picking its live up from the permanent,could also be faulty clock,try replacing both one at a time ,doesnt sound like boiler stat,definitely electrical.

i would suspect the mv is only connected to cyl stat and not boiler. ie has a seperate fused spur, probably in the airing cupboard

Okay final test tonight.
Got the water up to temeprature and the heating. Then set the Roomstat to minimum and likewise the Cylstat.

So technicallythere should be no call for heat. The MV was closed and the pump was off as expected.

I set the controller for hot water only. Within 10-15minutes the boiler kicked in. (The MV was still closed at this time and the Heating off).

The boiler went through a few cycles like this before the timer shutdown occured.

I guess this proves it that there is a wiring fault and the boiler is still getting a live feed when it should be off.

check back from mv/cyl stat to see where the wire is connected

Thanks twgas,

That's my next step. The MV is being operated from the CylStat but I am now convinced that the auxilary switch isn't connected correctly (i.e. I think it's bypassed by the wiring) an so although the valve closes when the tank reaches 60C the live feed to the boiler is not disconnected.

I will check this out shortly and fix!!

(Trouble is I still have a day job! )