Grant Vortex Oil Fired Boiler micro firing?

[John Carroll]

 

Came across this problem on another site, someone may have words of wisdom.

 

The system is Y plan with a MPV, if either CH or HW is selected then the boiler cycles normally, if both CH and HW are selected, then the burner switches on/off every few seconds until CH or HW is again selected, it seems to have all the symptoms of micro firing. I thought it could possibly be due to the MPV hunting in its mid position interrupting the power to the boiler but a few tests on my own Firebird (S plan) seems to rule this out, when I interrupt the power to the burner for a second or two, the burner cuts out then does its 12 sec (pre) purge and refires, if I interrupt the power when its say 6 sec into its purge then it again just does a full 12 sec purge before refiring, if I interrupt the power immediately after refiring but before the extra 5 sec for flame detection has elapsed, it does exactly the same, a full 12 sec purge and refire, all as expected really?. 

I have suggested selecting HW only on, then manually latching the valve in mid position to see what happens.

[dpmiller]

what make of burner is on the Grant? Is it blueflame or modulating in any way?

 

/ what happens on yours if power is interrupted for a shorter time even, half a sec or less?

[John Carroll]

Don't know what type of burner but no modulation, my Firebird will do the full purge as fast as I can switch it on/off, i have experienced this micro firing only a few times in 18 years while the boiler was running normally but cannot see how running with CH&HW should affect this Grant except the violtage is being pulled down or something like that.

[ProDave]

Y plan = 3 port motorised valve.  the work of the devil.

 

I bet of you go and look when it is selected for DHW and HW, the 3 port valve has not settled in the mid position but is hunting backwards and forwards, and in the process the call for heat to the boiler is turning on and off.

 

Replace valve control head or bin the 3 port valve and use two 2 port valves instead, FAR more reliable.

[John Carroll]

The work of the Devil allright but a most ingenious device.

The MPV hunting was/is my chief culprit too but judging on the results I got when interrupting the supply to mine then doesn't match what the Grant is doing, apparently stopping/starting every couple of seconds, I'd be surprised though if it should behave much differently to my 18 year old vetern in that regard, more tests are being carried out tomorrow, will inform.

[ProDave]

This is a good description of how the 3 port valve works  http://wiki.diyfaq.org.uk/index.php/Three_port_mid_position_valve

 

I still suspect yours is overshooting and not reaching a stable mid position

 

"In the "water and heating" state, white is energised. If the valve has port B open, the motor will wind it until A+B, whereupon SW1 changes over, DC is applied to the motor via SW2, and it will stall. If it overshoots, or if it port A was open, SW2 will be operated as well, removing all power from the motor, and allowing the spring to pull the valve back to A+B. It is fun to watch this happening: as the spring pulls the valve back from A open to A+B, the motor acquires quite a momentum and overshoots. It then winds forward a little, and stops in the correct position."

 

Check the voltage coming out of the orange wire.  (boiler demand)  I bet you will find it is pulsing on and off.

[dpmiller]

I suspect the off time will have to be extremely short- arcing-  if the  burner continues to run without a flameout and purge. I'd be more inclined to say loose connection near the TPV, or a *failing* microswitch

[John Carroll]

Something like that but the boiler run signal is still the same with the valve in mid position? so the hunting valve still makes the most sense to me.

[dpmiller]

any chance the motorised head is slightly loose from the valve?

[John Carroll]

On 18/09/2024 at 21:40, ProDave said:

This is a good description of how the 3 port valve works  http://wiki.diyfaq.org.uk/index.php/Three_port_mid_position_valve

 

I still suspect yours is overshooting and not reaching a stable mid position

 

"In the "water and heating" state, white is energised. If the valve has port B open, the motor will wind it until A+B, whereupon SW1 changes over, DC is applied to the motor via SW2, and it will stall. If it overshoots, or if it port A was open, SW2 will be operated as well, removing all power from the motor, and allowing the spring to pull the valve back to A+B. It is fun to watch this happening: as the spring pulls the valve back from A open to A+B, the motor acquires quite a momentum and overshoots. It then winds forward a little, and stops in the correct position."

 

Check the voltage coming out of the orange wire.  (boiler demand)  I bet you will find it is pulsing on and off.

 

Tests so far, not mine or not my boiler

Can you or someone identify each terminal connection, especially the switched live and the permanent live on the boiler terminal Block drawing, attached.

Terminal 1 appears to be connected to the MPV? yet the reading is constant for all three positions, if the orange wire is pulsing (CH position only) then the M/meter isn'n picking it up.

 

HW+CH (boiler hunting)
2(N) - 1 240v constant
2(N) - 4 all over the place. Probably about 58. is most consistent reading
2(N) - 7 240 through cycles. Shows OL for split sec when switching back on

HW only (no problems)
2(N) - 1 240v constant
2(N) - 4 all over the place
2(N) - 7 random figures as low as 6v and up to 180v

CH only (no problems)
2(N) - 1 240v constant
2(N) - 4 all over the place
2(N) - 7 240v constant

No room stat just TRV on rads.
 

Edited September 20 by John Carroll

[ProDave]

I am not sure what you are measuring there, your 2(n) on everything confuses me?

 

You want to measure terminal 10 on the wiring centre or terminal 1 on the boiler, with respect to neutral which can be found on terminal 2 on the wiring centre.

 

This is the call for heat to the boiler that should be on all the time HW, CH or both is demanded.  This is the one I think you will find is pulsing on and off.  Verify that first.

 

And testing on someone elses system won't get you closer to finding what your system is doing.

[John Carroll]

2 hours ago, ProDave said:

I am not sure what you are measuring there, your 2(n) on everything confuses me?

 

Not sure why the 2(N) etc but I'm quite happy that what was tested (not by me, but suggested by me) was 1 to N, 4 to N, & 7 to N.

 

2 hours ago, ProDave said:

 

You want to measure terminal 10 on the wiring centre or terminal 1 on the boiler, with respect to neutral which can be found on terminal 2 on the wiring centre.

 

 

The measurement 1 to N was taken from Ter 1 on the boiler terminal Block to Ter 2 (N) on the boiler terminal block which I would think should show any pulsing?

2 hours ago, ProDave said:

 

And testing on someone elses system won't get you closer to finding what your system is doing.

 

I am only trying  to assist some one else, my 19 year old Firebird with its Riello G5X burner is faultless and has never tripped even once.

 

Thanks for your help,  I will suggest testing between 10 & 2 on the wiring centre.

 

 

Edited September 20 by John Carroll

[John Carroll]

It was found that the MPV would go to and  stay in mid position when CH only was selected, a replacement new Honeywell ST9400C programmer appears to have solved the problems.