S plan CH system with no bypass valve

[PT1984]

So we have a system boiler. Flow and return pipes above the boiler. Flow pipe goes to the upstairs tank cupboard. Then splits into two zone valves, CH and HW. CH pipe disappears. Then a return pipe from the tank coil.





The old boiler didn’t have pump overrun. The new Vitodens 100 system does. We found this out when there was a loud clattering noise from some pipes. And the manual pressure gauge on the supply pipe was slowing reducing to 0. Digital pressure gauze was 1.5 Barr. This must be on the return.

The install company advise they will be coming back to fit a bypass valve. But I’m losing trust in their ability.

Where should the relief valve be installed? I presume not above the boiler. So in the tank cupboard. Inlet to the valve on the main feed pipe before the pair of zone valves? But should the outlet of the relief valve be tapped into the CH circuit after the zone valve, or on the HW circuit on the return from the tank? Also what pressure should be set?

Cheers in advance.

Edited by PT1984 on Sunday 11th December 20:30

[LocoBlade]

The bypass should be between the flow and return before any zone valves etc, so if everything is shut off and the boiler fires/pump starts, the water can at least circulate around via the pressure relief bypass back to the return.

[PT1984]

In this instance:

-The hive shuts off the heating as temp is reached. Hive closes zone valve. Zone valve micro switch sends signal to boiler.
-Boiler turns off flame but continues to run the pump.
-Water has nowhere to go.

It appears some water is forced past the valves. As the pump stops the feed pipe loses pressure. Shows 0 on the gauge. When you turn on the boiler next time the gauge bounces back to 1.5 Barr and there’s some pipe clanging.

Currently have the HW zone valve set to open / manual so there is always a open loop.

Edited by PT1984 on Sunday 11th December 22:31

Edited by PT1984 on Sunday 11th December 22:32

[PT1984]

Sorry for the bump. I just want to make sure the bypass valve goes in the correct location. The tank cupboard rather than above the boiler? I have had several issues with the online installer. The engineer likes to take the easy option.


The only return pipe in the tank room back to the boiler is the return from the tank itself. I have read somewhere about reverse flow back through the tank being a bad thing. Or is this not an issue here as the flow pipe has the Honeywell valve which is normally closed?

Edited by PT1984 on Monday 19th December 10:34

[CharlesElliott]

As someone said above, the bypass needs to be between the flow and the return before any valves. So this could be either above the boiler, or in the tank cupboard as long as it is prior to the zone valves.

[PT1984]

Stupid question time. Inlet to the valve should before the zone valve. Exit from the valve to the return pipe work can be after the zone valve ( think it has to be?).

[LocoBlade]

PT1984 said:

Stupid question time. Inlet to the valve should before the zone valve. Exit from the valve to the return pipe work can be after the zone valve ( think it has to be?).

The bypass needs to create a direct unhindered loop to and from the boiler so water can if necessary go from the boiler flow, through the bypass and then straight back to the boiler via the return, so yep technically before the zone valve on the flow, and after everything on the return.

[PT1984]

Excellent. I was just getting confused as the only return pipe we have access to is the HW tank.

[B'stard Child]

LocoBlade said:

PT1984 said:

Stupid question time. Inlet to the valve should before the zone valve. Exit from the valve to the return pipe work can be after the zone valve ( think it has to be?).

The bypass needs to create a direct unhindered loop to and from the boiler so water can if necessary go from the boiler flow, through the bypass and then straight back to the boiler via the return, so yep technically before the zone valve on the flow, and after everything on the return.

I thought with a three port valve (CH only, HW only or CH and HW together) as long as one rad is in circuit you can never dead head the boiler??

Edited to add - Appreciate that this is not what the OP has

Edited by B'stard Child on Monday 19th December 20:55

[LocoBlade]

Not sure but you may be right, it was just the clearest diagram with a bypass that Google threw up

[miroku1]

Most boilers have a built in by pass so some what superfluous

[tux850]

miroku1 said:

Most boilers have a built in by pass so some what superfluous

It doesn't matter what most have; the question is: Does the Vitodens 100W system variant?

(I don't think it does, but I haven't dug deep)

[forest07]

B'stard Child said:

LocoBlade said:

PT1984 said:

Stupid question time. Inlet to the valve should before the zone valve. Exit from the valve to the return pipe work can be after the zone valve ( think it has to be?).

The bypass needs to create a direct unhindered loop to and from the boiler so water can if necessary go from the boiler flow, through the bypass and then straight back to the boiler via the return, so yep technically before the zone valve on the flow, and after everything on the return.

I thought with a three port valve (CH only, HW only or CH and HW together) as long as one rad is in circuit you can never dead head the boiler??

Edited to add - Appreciate that this is not what the OP has

Edited by B'stard Child on Monday 19th December 20:55

That’s correct, but the boiler manufacturers used to specify a bypass must be fitted. I’ve not been involved with domestic heating systems for some years now but it certainly was the case.

[PT1984]

B'stard Child said:

I thought with a three port valve (CH only, HW only or CH and HW together) as long as one rad is in circuit you can never dead head the boiler??

Edited to add - Appreciate that this is not what the OP has

Edited by B'stard Child on Monday 19th December 20:55

Believe sometimes towel rads are on an open circuit. But not here.

[PT1984]

tux850 said:

miroku1 said:

Most boilers have a built in by pass so some what superfluous

It doesn't matter what most have; the question is: Does the Vitodens 100W system variant?

(I don't think it does, but I haven't dug deep)

Correct sir. It does not!

[B'stard Child]

forest07 said:

B'stard Child said:

LocoBlade said:

PT1984 said:

Stupid question time. Inlet to the valve should before the zone valve. Exit from the valve to the return pipe work can be after the zone valve ( think it has to be?).

The bypass needs to create a direct unhindered loop to and from the boiler so water can if necessary go from the boiler flow, through the bypass and then straight back to the boiler via the return, so yep technically before the zone valve on the flow, and after everything on the return.

I thought with a three port valve (CH only, HW only or CH and HW together) as long as one rad is in circuit you can never dead head the boiler??

Edited to add - Appreciate that this is not what the OP has

Edited by B'stard Child on Monday 19th December 20:55

That’s correct, but the boiler manufacturers used to specify a bypass must be fitted. I’ve not been involved with domestic heating systems for some years now but it certainly was the case.

System in my house was installed in 1982 - boiler has been changed in 2009 from non condescending to condescending type

Installer did not mention a need for a bypass (despite the fact it is required in the installation manual) Quell Surprise!!!!

I have one ready to fit along with a Magnaclean unit and a restrictor for the HW heating coil (because the system pump needs to be run at more than 17L/min and the old boiler was happy with 12 L/Min so I had to up the pump speed) but I ran out of time before the heating was needed

I'll do all that in the spring now unless I have to intervene before then............