programmer restarts when DHW and CH on together ?

[growfins]

Hi all,
I've got an odd issue with the central heating in our home. When the programmer is set to have CH and HW on at the same time, the programmer keeps switching off then on again (in the order of HW off first, then CH, then both back on). The cut out only lasts for about a second, but occurs frequently. We've got the following installation:

Ideal Icos HE18 condensing boiler ( jibes will be accepted on this btw)
Old Drayton Tempus 7 programmer
Honeywell v4073a 3 port valve
Grundflos pump
B gas RS1 room stat
Unknown cylinder stat


As a DIYer, I've checked some of the obvious mechanical stuff I know about, noting the following:

- Independent CH or HW work normally when set on the programmer. CH and HW operate at a good temp, Room and cylinder stats seem to operate normally to call and turn off the boiler when satisfied.

- Had the cover off the 3 port valve and checked the motor moves across the 3 positions a,b and midpoint depending on what's being called. All seems OK.

- Pump turns on and off when needed and sounds smooth.

- Fault occurs when the boiler is turned on or off.

- Checked for loose connections in the programmer backplate and wiring centre. The programmers definitely the oldest part of the system and I've got a replacement on the way.

So now I'm out of knowledge. I'm thinking it's probably time to call in a pro with a multimeter, but before I do so, I'd like to hear any opinions on where the fault might likely be located, and anything useful I can check that I've missed ?

Thanks in advance

 

[Whn1]

Sounds like you already know the answer.... New programmer

 

[growfins]

Sounds like you already know the answer.... New programmer

Thanks for the super fast reply. That was my first suspicion as the fault's evident there, but not sure whether some other connection would cause it to trip, such as when the valve's in its mid position on in the stats.

Is it easy to take the programmer out of the loop to test the CH and HW running together ? On the backplate I've got the following connections:

L
N
1 HW off
2 empty
3 HW on
4 CH on

In the Live terminal, there's a yellow wire connected as well as red. Would like to know what that's doing there ...Today's job has been tracing the wiring to see how it fit's together in the junction box

Thanks