Boiler cycling Worcester Si24

[Bravo33]

The fault:
The boiler cycles on and off about every 3-5 minutes; Pump running continously with both hot water & heating in OFF cycle.
System:
Open vented; Worcester Bosch 24Si; Sunvic 2000 Pro programmer; Drayton cylinder stat; Grasslyn Thermio touch room stat; TLC TL MP3 mid position 3 port valve; pump.
History:
Moved into the house 6 months ago; Upgraded system of the past few years. Boiler installed 2005;
3 port valve, room stat, programmer, cylinder stat all new fitted within the last 6 weeks.
System drained and refilled approx 3 times with 2 replacement rads fitted and additional rad to the front entrance; system balanced and heat to rads working correctly.
The 3 port valve was changed as it would appear that it was sticking.
Further investigation showed the programmer was at fault as would not allow water & heat at the same time.
Cylinder stat changed as precautionary and appeared to be some 20 years old.
The system was re-wired last week as a precautionary measure.
Symptoms:
During the day the system seems to be stable however after the evening cycle with the hot water OFF at 18:30 and the heating OFF at 22:00 with this confirmed by the programmer indicator lights the pump is still running with temperature rise to the house thorughout the night.
I am getting up at 2am to switch power off to the system. Switch the boiler back on and reset it.
This solves the problem and the water and heat cycles appear to continue normally for the rest of the day.

 

[ambrosia]

bit a long post, but are you saying boiler keeps firing when there is no demand

sounds like a wiring fault to me, call your engineer back

 

[glenno1]

I would agree with a wiring fault probably involving the programmer

 

[Bravo33]

Wiring re-checked. All ok apart from a residual 145v across the 3 port mid position valve.
it would appear that this is energising the boiler as when the ST2 LR is disconnected from the boiler the boiler stops cycling. Really scratching my head on this one. 3 port valve already been changed.

 

[ambrosia]

145v is a lot, many countries and in fact building sites run on 110
the input to pcb goes as low as 24v

something sounds very wrong with your wiring

 

[Crinkmeister]

Did you ever try a substitute programmer? Which terminals are you measuring 145V on? You are on AC volts setting I assume.

 

[Nostrum]

It's normal to have 145v on a 3 port valve when in the mid position, but not on the orange wire which should be firing the boiler?

Sounds like a wiring fault.

 

[Bravo33]

Changed the 3 port valve. When boiler power turned off and then reset voltage is zero. Call for heat then 240v which is correct. No voltage on the orange wire. Then call for heat. 240v to the boiler. Heat satisfied. Voltage now 60v across orange. It's the same on the hot water. Cannot figure it out.

 

[Bravo33]

Ok gents, here is the solution.
worcester Bosch and I understand Ariston boilers will cycle when there is a residual current on the boiler call even when the programmer is in an off cycle or HW & CH are satisfied.
note that this is not an issue with 2 port valves as they are either ON or OFF.

Following discussions with the 3 port valve manufacturer pretty well all 3 port valves are based on Honeywell technology with a residual voltage of between 40v to 80v. Worcester Bosch have changed there PCB ,I believe in the past year, to allow for up to 50v residual. Anything above that and the boiler will continue to cycle.

SOLUTION:
install an inline relay on the boiler call wire. Ensure the release value is within the voltage range required.
I've just completed this and it works a treat.

 

[gassafe]

Nice to have the answer posted it helps peeps in the future...

there used to be similar faults on old Ri boilers, the stray voltage would cause a clicking relay sound or be enough to constantly run the boiler even with no demand