wierd HW/CH problem

[mk2phil]

Both HW and CH are set to come on in the morningwith the CH coming on first. However, this morning the central heating didn't need to fire because the room stat was set pretty low so it never got going. When the HW was scheduled to come on - nothing. The Programmer was calling for heat but the boiler wasn't running. Went to the stat on the cylinder which is set to 60 degrees.

I turned the stat down so I could see/hear when it clicked to find out the temp of the water in the cylinder which was about 25ish. Turned the stat back up and the zone valve opened (its right next to the cylinder - gravity fed) and the boiler started.

However, when the central heating has been running beforehand there is no problem with the HW. I also turned the CH off on the weeken and left just the HW to come on and the same thing happened.

Could this be a faulty cylinder stat..?

 

[Softus]

Either that or an intermittent fault with the MZV.

Do you have two two port MZVs or a single three port?

 

[mk2phil]

I only have 1 MZV. HW is gravity and central heating is pumped - the supply is tee'd as it exits the boiler.

 

[scaby]

if the mzv is opening then your cylinder stat is working,it sounds like the micro switch is not making contact, or simply the wire (normally orange but could be different)has come loose , be careful, every cable could be at 230ac

 

[mk2phil]

the micro switch on the MZV which connects to the boiler and turns it on?

Would the stat be wired in series with the MZV and the boiler - i.e. the stat controls the Zone valve, which in turn calls for heat/cuts the boiler?

 

[scaby]

thats right , it works like this ,hot water on signal from programmer to cylinder stat , when stat calls for heat it energises motor on valve, valve opens making micro switch, which in turn switches on the boiler,

 

[mk2phil]

Cheers Scaby, much appreciated fella.