Central heating 3port valve?

[Kev2632]

I was at a job today where there was a central heating comprising of 3port valve, what i couldn't understand was why is there HW off connection to be used in the programmer and also a third connection to be used on the tank stat, also this system also wanted a 2port valve incorporated into the HW pipe feading the coil to the tank ?? Why is that all? Could anybody help me understand this?

 

[TaffyDuck]

It sounds like a Y plan which needs HW off and both 1 and 2 connections on the cyl stat to operate the system.

 

[TaffyDuck]

Google Image Result for http://www.electriciansforums.co.uk/wp-content/uploads/2012/02/Y-Plan-Biflow-Wiring-Diagram.png

 

[Kev2632]

Yea that's correct but why do they need those connections though what does off HW off do? Why is it needed?

 

[electric will]

Could be a pressurised cylinder, I'm sure you can't just rely on a 3 port valve to control these, so a 2 port is also fitted for safety.

 

[markythesparky]

The 3 port valve doesn't have the spring that a two port valve has. It has an extra connection to 'bring back' back the valve. A two port valve springs back when there is no power supplied.

The cylinder stat has 'call' and 'satisfied' terminals. I.e. normally open and normally closed. This is to operate the 'bring back' the valve when satisfied for heat.

The extra two port is unusual but if its on the pipe going to the coil, then all it will do is shut the water feed off when satisfied. Are you sure that the three port does HW and CH and not two CH zones?

 

[Kev2632]

The 3 port valve doesn't have the spring that a two port valve has. It has an extra connection to 'bring back' back the valve. A two port valve springs back when there is no power supplied.

The cylinder stat has 'call' and 'satisfied' terminals. I.e. normally open and normally closed. This is to operate the 'bring back' the valve when satisfied for heat.

The extra two port is unusual but if its on the pipe going to the coil, then all it will do is shut the water feed off when satisfied. Are you sure that the three port does HW and CH and not two CH zones?

Thanks for your reply, yea it does the HW and CH and the two port valve was fitted for the feed going into the coil as well? Pretty weird to be honest

 

[TaffyDuck]

Or perhaps the two port valve is controlled by a pipe stat to allow HW from a solid fuel back boiler to heat the tank perhaps?

 

[snowhead]

If it's an unvented cylinder, mains water fed no storage tank (with expansion vessel and Temperature and pressure relief valve) it must have a 2 port valve connected via the Over temperature stat on the cylinder.
The valve comes with the kit supplied by the cylinder manufacturer.

 

[Kev2632]

If it's an unvented cylinder, mains water fed no storage tank (with expansion vessel and Temperature and pressure relief valve) it must have a 2 port valve connected via the Over temperature stat on the cylinder.
The valve comes with the kit supplied by the cylinder manufacturer.

Yea that's the exact it!! And why is thus 2port valve need exactly???

 

[markythesparky]

If it's an unvented cylinder, mains water fed no storage tank (with expansion vessel and Temperature and pressure relief valve) it must have a 2 port valve connected via the Over temperature stat on the cylinder.
The valve comes with the kit supplied by the cylinder manufacturer.

What's the idea behind this then. What does it do?

 

[snowhead]

What's the idea behind this then. What does it do?

Cuts the heat source off to the cylinder from the boiler under control of the over temperature stat on the cylinder.

If the normal Hot water zone valve failed and the unvented cylinder continued to heat up, it could potentially become a bomb ready to discharge steam when it explodes.

The 2 port valve is part of a package of safety features, associated with unvented cylinders, designed to prevent this.

 

[markythesparky]

Oh ive learnt some thing there. Don't Mega flows have pressure release valves then?

Ive not seen these two port cable set up before on unvented cylinders. Is it a new addition?

 

[TAZ]

The main reason the additional valve is there is to prevent the hot water from ever going above 100'c. The cylinder stat is the first line of defence followed by the over heat stat which would operate at 85c & would cut of the supply of hot water to the cylinder, hence the extra valve. If the temperature were to go above 90c the temperature & pressure relief valve would start to open & let in more cold water to keep it from going above 100c As the pressure rises in the cylinder so does the temperature as it is above atmospheric pressure. A pressure of 1bar inside the cylinder would result in a temperature of 122c. As the pressure goes up so does the temperature Unless one of these unvented cylinders has not been installed correctly it would vertually be impossible for one of these to explode as there are so many safety features it would have to overcome for that to ever happen. Most modern boilers have overheat devices as well

 

[PEG]

Found a good few of these set ad-hoc or addition of solid fuel input and the big risk is at overheat temperature ANY burst or even leak vents pressure and entire contents above 100c flashes to steam at 1600 times volume,can level like a gas explosion. Vented systems much easier to remain tame...

 

[Stroppy]

I've seen the result of a cylinder overflow being run in plastic pipe ("well its designed to carry hot water!") when the cylinder overheated and the safety valve opened to discharge the pressurised well-over-boiling-point steam the plastic pipe melted and jetted across the loft to hit the opposite wall over 30 feet away. When we got there 20 minutes after it happened the wall was still steaming...

Exploding Unvented Hot Water Cylinder.wmv - YouTube

Impressive slow-motion at 1:25 Exploding Unvented Hot Water Cylinder.wmv - YouTube

 

[snowhead]

Oh ive learnt some thing there. Don't Mega flows have pressure release valves then?

Yes, but that's the last safety feature.
Relying only on the Temp and Pressure Relief would be like having bare live cables and saying it's O.k it's RCD protected.

Ive not seen these two port cable set up before on unvented cylinders. Is it a new addition?

Can't name the date, but they've been around quite a while.