Vaillant Ecotec Pro 28 always runs the central heating

[ggabriel]

Hi, all,
I wonder if anybody can help with a problem that I have on a boiler that's about a decade old but in good condition. Basically, when I switch on the central heating, it will run regardless of the demand signal from the simple Honeywell thermostat that I have attached to terminals/clamps 3-4.
I have done some diagnosis:
- Checked for continuity on the cable that runs through terminals/clamps 3-4 and found that when the thermostat is below room temperature, there is no continuity; when the thermostat is above room temperature, I observe continuity. This is what I expect based on the diagrams provided in the manual. I don't measure, however, any voltage.
- Checked the diagnostic code d.08, which is always 1, no matter whether the CH is on or off, or the thermostat is above or below room temperature.
- Checked the PCB (from the back, haven't took it apart completely) and didn't find any obvious failures like burnt fuses or burst capacitors.

Has anybody experienced this/any idea of what I could do to further diagnose?

Thanks!

Gabriel

 

[Johnmdc]

Has it always done this? If yes then check that there is no link in the terminals marked RT24. If there is then remove it and that should cure the issue.

 

[ggabriel]

No, it hasn't always done this - I'll check the RT24 terminal though, just to be sure.
A slight behaviour update though: yesterday I left it running (as I needed to heat up the house a bit) and eventually the CH behaved as it should be, with d.08=0. I thought that maybe I was being impatient and the PCB has some faulty logic but eventually it starts working fine. However, this morning I did turn the thermostat all the way down, but the CH continued to work (d.08=1 again).

 

[ggabriel]

Josh - I admit that I can't find the terminals marked RT24 in the installation manual, and don't recall seeing anything bridged.
Having tested it a few days more, it appears that the boiler is behaving as expected, so it was [hopefully] a slight misunderstanding from my side that d.08=1 meant that the thermostat was sending "demand" only - it looks like d.08=1 when something else is generating demand.
...hopefully, and it isn't like the PCB is dying.

 

[Johntheo5]

If you have a indirect HW cylinder then its cylinder stat may still be demanding heat when the CH roomstat isn't. In a normal S plan system you will have two or more motorized valves, one of which is driven open by the cylinder stat, the boiler isn't switched on directly from the stats but when any motorized valve opens a set of contacts (end switches) close and send a boiler run signal. occasionally the end switches can fail closed so the boiler will continue to run even with no demand from either the CH or DHW.