Should my boiler's automatic bypass be open or closed?

Belenus

My Potterton Powermax HE 150 boiler has an automatic bypass fitted. The picture shows it running horizontally from the flow pipe on the left to the return pipe fourth left. The silver part where it connects to the flow pipe is the actual valve. I presume that it opens when the flow pressure rises above a pre set level.

The valve has a screwdriver slot on the top which presumably allows it to be manually opened or closed by turning through 90 degrees. It is currently positioned to an almost but not quite closed position as seen in the picture.

The boiler manual states the following:

An automatic bypass valve is built into the boiler to allow thermostatic radiator valves to be fitted. This can be switched into or out of circuit as required.

I presume this means that, if there is at least one radiator on the circuit without a TRV, then the valve is not required and can be left closed.

But the manual also states:

The central heating pump is factory fitted and an automatic system by-pass is provided. The boiler can be used in systems controlled by thermostatic radiator valves (TRVs) but it is essential that the by-pass isolating valve is fully open.

Our hall radiator does not have a TRV. Every other radiator does have a TRV.

Which is correct? Should I leave the valve as it is, close it fully or open it fully?

Bendy_House

Ok, I don't KNOW, but looking at the manual ( https://www.manualslib.com/manual/946054/Potterton-Powermax-He-85-Litre.html?page=17#manual ) especially on page 17, it looks to me that your by-pass pipe (which it almost certainly is) is not 'auto'.

The valve with the screw slot on it looks like a normal manual non-return valve to me, in which case how much that screw is opened (and at the moment it's 'barely') directly determines how much 'by-pass' is going on.

The diagram in the manual seems to suggest that the actual 'auto by-pass valve' should be located on that pipe towards the RH end.

But, I don't know.

I'd ask Potterton.

NSG666

Belenus said:

My Potterton Powermax HE 150 boiler has an automatic bypass fitted. The picture shows it running horizontally from the flow pipe on the left to the return pipe fourth left. The silver part where it connects to the flow pipe is the actual valve. I presume that it opens when the flow pressure rises above a pre set level.

The valve has a screwdriver slot on the top which presumably allows it to be manually opened or closed by turning through 90 degrees. It is currently positioned to an almost but not quite closed position as seen in the picture.

The boiler manual states the following:

An automatic bypass valve is built into the boiler to allow thermostatic radiator valves to be fitted. This can be switched into or out of circuit as required.

I presume this means that, if there is at least one radiator on the circuit without a TRV, then the valve is not required and can be left closed.

But the manual also states:

The central heating pump is factory fitted and an automatic system by-pass is provided. The boiler can be used in systems controlled by thermostatic radiator valves (TRVs) but 

"it is essential that the by-pass isolating valve is fully open."

Our hall radiator does not have a TRV. Every other radiator does have a TRV.

Which is correct? Should I leave the valve as it is, close it fully or open it fully?

Fully open

Bendy_House

NSG, are you sure that valve is an auto by-pass and not just a non-return valve?

I don't know, and that's why I'm asking. It doesn't look like an ABP, but then I'm not sure what they look like! It's just different to what's shown in the instructions. Also, it's a flexi pipe - I wonder if that was installed by the fitter in lieu of the real thing?

NSG666

Bendy_House said:

NSG, are you sure that valve is an auto by-pass and not just a non-return valve?

I don't know, and that's why I'm asking. It doesn't look like an ABP, but then I'm not sure what they look like! It's just different to what's shown in the instructions. Also, it's a flexi pipe - I wonder if that was installed by the fitter in lieu of the real thing?

The auto bypass is built into the fitting at the RH end of the flexi. I found it in the doc your useful link took me to but I can't remember which page. 

Bendy_House

Cool - thanks, NSG :-)

Ectophile

That bit of braided flexible pipe looks like a filling loop to me.  The valves on a filling loop should be off.

[Deleted User]

NSG666 said:

Bendy_House said:

NSG, are you sure that valve is an auto by-pass and not just a non-return valve?

I don't know, and that's why I'm asking. It doesn't look like an ABP, but then I'm not sure what they look like! It's just different to what's shown in the instructions. Also, it's a flexi pipe - I wonder if that was installed by the fitter in lieu of the real thing?

The auto bypass is built into the fitting at the RH end of the flexi. I found it in the doc your useful link took me to but I can't remember which page. 

Is it? It may be a boiler pressure relief valve which dumps water into the unvented cylinder tundish but most ABVs dump nothing: all they do is provide a pathway from the flow to the return side of the boiler if all the radiator TRVs are closed. In the ‘olden days’, installers offered two options: one, a radiator without a TRV that was left on or, two, a gated valve that was left partially closed

A typical ABV looks like this:

NSG666

Ectophile said:

That bit of braided flexible pipe looks like a filling loop to me.  The valves on a filling loop should be off.

It looks like one but isn't

Dolor said:

NSG666 said:

Bendy_House said:

NSG, are you sure that valve is an auto by-pass and not just a non-return valve?

I don't know, and that's why I'm asking. It doesn't look like an ABP, but then I'm not sure what they look like! It's just different to what's shown in the instructions. Also, it's a flexi pipe - I wonder if that was installed by the fitter in lieu of the real thing?

The auto bypass is built into the fitting at the RH end of the flexi. I found it in the doc your useful link took me to but I can't remember which page. 

Is it? It may be a boiler pressure relief valve which dumps water into the unvented cylinder tundish but most ABVs dump nothing: all they do is provide a pathway from the flow to the return side of the boiler if all the radiator TRVs are closed. In the ‘olden days’, installers offered two options: one, a radiator without a TRV that was left on or, two, a gated valve that was left partially closed

A typical ABV looks like this:

See p30
Pressure Sensor/Pressure Relief Valve - Potterton Powermax HE 85 Litre Installation And Servicing Instructions [Page 30] | ManualsLib

Bendy_House

NSG666 said:

See p30
Pressure Sensor/Pressure Relief Valve - Potterton Powermax HE 85 Litre Installation And Servicing Instructions [Page 30] | ManualsLib

Cool - that's pretty conclusive! I didn't realise it was tucked inside such a small connector.

plumb1_2

 how can you have a auto bypass that is closed. Unless it’s manual and should be opened to suite the system.
Most manual bypass valves are lock shield valve or a gate valve with the head removed to prevent tampering once set