Is it the 3 port valve?

24 Oct 2014
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United Kingdom
I have oil fired boiler (Trianco about 15 years old as is I suspect the heating system), honeywell frost stat (wall mounted) honeywell 3 port valve, Grasslon Chron QE2 programmer. The bathroom rad and HW are "on the same system" ie when heating HW then bath rad also heats up. Ambeint temp is about 6deg at the moment. Previously when ambeint fell below 2deg HW/bath rad kicked in.

With the programmer off and frost stat at 2deg no CH/HW. With programmer off and stat above ambient have CH but no HW/bath rad. With programmer on calling for CH/HW CH on no HW/bath rad. Programmer on for HW only boiler does no fire even with frost stat above ambeint.

Is it the 3 port, programmer, both. Any suggestions?
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