3 way valve problem?

[SullyTheGoldfish]

Hi

We're building an extension and have had a new boiler fitted but getting the heating system back to normal is posing a few problems. The system is a Y plan one with Honeywell contols bits and the new boiler is a Rehema Avanta condensing one. For some reason the hot water is always on (ie it takes no notice of the hot water part of the timer) so we turn the cylinder stat up and down to control the hot water. The three way valve was clicking and has been replaced.

Unfortunately, the heating will not work unless the hot water is requesting heat. So to get heat to the rads in the morning we go and turn cylinder stat up. So i was thinking that the wiring must be wrong and trying to get the plumber to look at that (he is scratching his head a bit).

But this evening i discovered that I could also get the heating to work by turn the cylinder stat down, well past the hot-water off point.

So, in summary, the heating works if i turn the water stat up (by virtue of the water firing up the boiler and pump and the heating being on with room stat requesting heat) and if i turn the water stat right down (by virtue of the three way valve making the right connections). But it doesn't work if i only turn the water stat down to just off. Does this mean the three way valve is not working correctly?

Thanks v. much for reading and for any advice.

SullyTheGoldfish

 

[gavinda]

get whoever extended and rewired the system to go back to college.

 

[SullyTheGoldfish]

heehee. Thanks for that.

Regardless of the plumber, is it obvious why the three way valve would behave like this?

SullyTheGoldfish

 

[gavinda]

yeah..it sounds like its wired up wrong.

 

[D_Hailsham]

The system is a Y plan one with Honeywell contols bits and the new boiler is a Rehema Avanta condensing one. For some reason the hot water is always on (ie it takes no notice of the hot water part of the timer) so we turn the cylinder stat up and down to control the hot water. The three way valve was clicking and has been replaced.

Presumably you have the Avanta 18V heat only boiler. Did he wire it in accordance with the manufacturer's instructions for a Y-plan? See Avanta Schematics page 24

the heating will not work unless the hot water is requesting heat.

That means the valve is in the mid position; and in this position the boiler gets its power through the HW thermostat.

I could also get the heating to work by turn the cylinder stat down, well past the hot-water off point.

This means that the system thinks the HW is satisfied (up to temp) so the valve moves across to CH port only open and the boiler now gets its power through the orange wire.

But it doesn't work if i only turn the water stat down to just off.

Water stats don't have an off position. Sounds as if there is no wire from the HW OFF terminal of your timer/programmer to the grey wire from the MPV. There should be three wires connected together: grey, HW OFF and HW stat terminal 2

 

[SullyTheGoldfish]

Hi D_Hailsham

Sorry I didn't reply to this... I didn't get a notification in my email inbox so didn't realise that anyone other than Gavinda had replied.

Well, if anyone should care, the timer was replaced (so the hot water wasn't on constantly anymore) and the cylinder stat also. I think what was happening was that the cylinder stat didn;t make the off connection unless you turned the stat right down. Since the HW was on constant the system spent all it's time near to the requested HW temperature and thus the cylinder stat was never sending an off signal to the 3 way valve.

With a new timer I now need to mount it on the wall as it isn;t the same make as the original... but the 5 core wire is too short to reach the right spot on the wall.

So I'm going to buy some 5 core flex to reach but one small detail eludes me.... what size cores do i need? the options are 0.5, 0.75 and 1.0mm, which seem to be for 3, 6 and 10 amp uses. I assume that 0.5mm is sufficient but could somebody please confirm? This is to connect a timer into a Y plan system.

Thanks very much for reading and answering,

SullyTheGoldfish

 

[D_Hailsham]

0.5mm is sufficient. The complete heating system should be protected by a 3 amp fused spur or an un-switched 13 amp 3-pin socket with a 3 amp fuse in the plug.

 

[cso30]

It sounds like there is definately a wiring fault and possibly a fault with the installation of the 3 port valve.

Check the markings on the 3 port valve the port marked 'a' should be the flow to the heating and the port marked 'b' should be the flow to the cylinder.

It also sounds like there is a permanent live wire going to the cylinder stat common (c) wire and not a switched live from the timer.

Also check the 'CH ON' and 'DHW' on wires from the timer are not crossed in the timer or in the junction box where they meet the 3 port valve.

 

[SullyTheGoldfish]

D_Hailsham and cso30,

Thanks for your replies.

D_Hailsham, thanks for confirming the size of wires. Yep, a 3 amp fused spur protects the lot.

Cso30, thanks for your thoughts. Just to clarify what I wrote yesterday, the original problems were sorted by replacing the timer and cylinder stat and everything works fine now. I was only posting yesterday to get advice about the right flex to extend the from wiring block to timer.

Here's to a fully working system with new boiler, timer (with lovely longer flex , cylinder stat and three way valve. Hopefully many years of reliable heating and not having to figure out heating problems start here.

Cheers

SullyTheGoldfish

 

[Homesafe]

poor gavinda

not a mention

 

[gavinda]

im not here for the glory

btw cable for room stat wiring