Megaflo Y plan problem

[Gareth Parker]

Hi there,

I have a Megaflow wired to spec here (bottom of p21), i.e. in a Y plan with additional 2-port valve. When CH is on and the stat switches to satisfied, heating sometimes stays on, sometimes switches off as commanded. I thought it might be the 3-port's head so replaced that; is it maybe the programmer? That's a very old Honeywell ST712. I've checked the wiring - no faults that I can see there.
Help very much appreciated!

 

[Taylortwocities]

You are going to need a test meter. Guessing and observing no faults visually will not find an electrical fault.
It’s not like plumbing where a puddle will indicate the location of a fault.

 

[Midwest]

Has the system previously worked fine, or has some alterations been made, and now you have this fault?

 

[Gareth Parker]

Has the system previously worked fine, or has some alterations been made, and now you have this fault?

It's never worked correctly - but it took some time to notice - a warm house not as spottable as a cold one.

 

[Wilko]

Hi - I had a look at their diagram, and maybe it's just me but the 3 port valve white should probably be connected to the room stat - the missing link to pin 10. Just a thought.

 

[buzzlightyear]

It's never worked correctly

not wired correct.

 

[Midwest]

What controls etc, does your Megaflow have?

 

[Devonchris]

Why do you have an additional 2 port valve on a Y plan and what is it doing?

 

[buzzlightyear]

HEATRAE SADIA MEGAFLO INSTALLATION MANUAL Pdf Download. - https://www.manualslib.com/manual/928142/Heatrae-Sadia-Megaflo.html this better .

 

[Grant webber]

Why do you have an additional 2 port valve on a Y plan and what is it doing?

To stop the unvented cylinder over heating if there's a fault on the 3 port valve I think.

 

[jackhammerJIM]

Why do you have an additional 2 port valve on a Y plan and what is it doing?

Over heat stat and 2 port valve are standard safety device on any unvented cylinder . Positive shut off can't be achieved with a 3 port valve hence the additional 2 port .

 

[rolyberkin]

Never assume that changing a head will sort out a dodgy three port valve, had this last week and changing the head didn't solve the problem as the valve body was knackered. Changed the whole valve, problem sorted!

 

[buzzlightyear]

Most valves pack up is the water
in the system no additive for sludge and corrosion.