An old Y plan system failing to fire up when CH & HW both called for.
Checked programmer output: both lines are indeed live
Checked stat output: its live, and switches correctly
Checked boiler: no power getting to boiler, or 4v ac to be precise.
Checked wiring cent other than the E&N terminals, ALL terminals except the pump/boiler one are live.

My conclusion, and I'm no expert on Y plans: I very much suspect a problem with the 3 way valve. However an examination reveals no such valve anywhere! The funny voltages on terminal 7 (not on terminal 3 which powers the boiler & pump) make it fairly clear that there is a Y plan 3 port somewhere, but it has to be either under or behind the HW cylinder AFAICS, or less likely under floorboards.

But... after failing to fire, while I was searching for the 3 port something went click and the system fired up. Looks like the system is suffering from sludging, so perhaps that's causing the 3 way to misbehave. I hope so, as getting to the 3 way looks likely to be impractical. So I think the first move is to drain, refill with cleaner, redrain refill & reinhibit.

Suggestions/feedback welcome!


NT