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!

 

[Agile]

Yes thats the usual correct position to fit it.

But if it was for me then I might fit it across the two port for the HW cylinder so that if it did operate the heat could be stored in the cylinder.

But pedantics might claim that in a fault condition the hot water might exceed its temperature as set on the cylinder stat. I would say that would be very unusual.

 

[Mottie]

But if it was for me then I might fit it across the two port for the HW cylinder so that if it did operate the heat could be stored in the cylinder.

If the hot water was at its set temperature and the heating was continuing to switch on and off all day (forcing the bypass valve to open), would that not have the potential to bring the hot water in the cylinder up to a dangerously high temperature?

 

[Agile]

Well in theory it might as I mentioned in my third paragraph.

However the flow temperatures for modern condensing boilers is usually set at 70 C and hot water for 55C to 60C so I do not see much serious scope for a very excessive over temperature.

For the ABV to open under normal CH operation then most of the TRVs would need to be virtually off.

 

[ericmark]

I have a C Plan with two pumps and two zone valves, mainly to stop reverse flow. In theory the valve powers the pump, so the pump can't run if valve not open, in practice have had the micro switch stick, so the pump ran with valve closed. Pumps and valves on the return, so I assume pump likely cavitated?

But also have TRVs on all radiators, and three wall thermostats. I can see with TRV heads connecting to the Wiser hub, the boiler/pumps should activate unless the TRV is open, but the wall thermostats could activate boiler/pump when TRVs are closed, I would hope this would never happen, but that relies on TRV being set so wall thermostat will not switch on before they open.

Boiler no problem, as it will heat DHW then turn off, but it may damage the pumps. How I could detect cavitation damage, I don't know. To correct, the whole system would need to be re-plumbed, since pumps on the return, so think this

is all I can do? Maybe some pressure sensor can be used to turn off pumps? Interested to see replies. It has been running this way for 5 years, so I would assume I am getting away with it?

 

[GreyBana]

Thanks Agile. Definitely an interesting idea, I like the thinking; always open to hearing different ideas!