I have a Y plan system boiler . The boiler fires up for not water but will not fire up for central heating.

[Nick]

I have a Y plan system boiler the boiler fires up for not water but will not fire up for central heating

[Animal]

commonly caused by a faulty 3 way valve, but can be due ot the room stat or timer, or even wiring.

[Roger Mills]

Most likely the 3-port mid-position valve. The wet part could be seized, the actuator could be dead - or just confused. If the latter, you can sometimes fix it by removing *all*[1] power from the valve, and starting again - but that doesn't seem likely for your symptoms, but there's little to lose by trying it.

Can you access the valve to be able to remove the actuator from the wet part (usually 2 screws)? If so, do that, but leave the actuator connected electrically. See whether the spindle of the wet part can be turned by finger and thumb. It only moves about a quarter of a turn. if it's very stiff, see whether you can free it up by turning back and forth with a pair of pliers and dripping a spot or two of oil unto the shaft gland.

Then see whether the (removed) actuator does what it should by (a) switching on just the HW (make sure that the cylinder stat is calling for heat), (b) switching on just the CH (make sure that the room stat is calling for heat) and (c) switching both on at the same time. For (a) and (b) the actuator should be at the two ends of its travel respectively, and for (c) somewhere in the middle of its travel.

[1] To remove all power from the valve you need to turn off the whole system at the FCU - or wherever it's powered from. Just turning off HW and CH at the programmer isn't sufficient because it still leaves a power connection to the valve.