Do I need an automatic bypass valve?

[GreyBana]

I have a very old oil fired boiler, a Boulter Camray 40/50 to be specific, from the mid 90s. I'm in the process of a large renovation and part of that has meant moving the hot water cylinder and re-plumbing most of the central heating system (excluding the runs off to the radiators). When I was pulling out the old system, I didn't take as many pictures as I should have, but I don't recall seeing a gate valve or an automatic bypass valve between the pump and 3 port valve to the return. I've just read that I should have one. Is this correct or does this only apply to newer systems? It was my understanding that you should always have one radiator in each zone without a TRV, that's always open, which - as I understand - would do the same thing as an automatic bypass, or have I got this wrong?

I also have a 22mm overflow between the boiler and before the pump that goes to the header tank, which I feel also plays into this to some extent.

 

[Agile]

An auto bypass valve is needed when a situation can arise that there is no flow from the pump.

That might happen when there are two or more two port valves. But a three port valve always has at least one port open . So an auto bypass valve is not required.

 

[GreyBana]

Ah, thanks Agile! That makes sense. It was a Y plan system, but I've converted it to a 3 zone S plan system, so that's why I'm seeing it in all of the stuff I'm reading.

Just to avoid any ambiguity, am I correct in thinking that I should install the auto bypass valve as per the blue circle below (rectangles are zone valves), after the pump, before the zone valves, directly to the return:

Thanks again for your help!