Bypass valve? What does it do?

[Olivias Grumpy]

hi
I’ve been battling a problem with my sons system. See earlier post re circulation

System has a Potterton Profile boiler with a header tank
2 zone valves and Grundfos pump.
Total of 11 rads with TRVs 1 rad with lockshields in hall where room stat is located. Plus 1 towel rail in bathroom with lockshields.

There is what I assume to be a manual bypass valve fitted, just after the pump connected to the return.

QUESTION. What is it supposed to do?
With 2 locations on lockshields, do I actually need a bypass valve as well, or can I close it?

Thanks

 

[Terrywookfit]

It allows flow during overrun when Z V's close.....So no.

 

[stem]

With an S Plan system with two port valves, if your boiler has a pump overrun then yes it should stay, as there is nowhere else for the water to flow. If it doesn't have a pump overrun, then maybe not, as long as the two radiators without TRV's allow a sufficient flowrate according to the boilers specification.

A boiler needs a certain amount of water flowing through it. So, when most of the radiators have TRV's fitted, as the property warms up and the TRV's start to close the amount of water flowing can be reduced below the boilers designed minimum, so the by-pass is there to maintain the required flowrate.

However, when several of the TRV's are open, the by-pass is unnecessary and is actually preventing full flow through the radiators, so an automatic version is better as it will only open when actually required.

 

[Olivias Grumpy]

Thanks Stem
Appreciate you detailed explanation.
I’m getting poor circulation to the extremes of the house.
I’ve tried cleaning , balancing etc with little improvement

I haven’t touched the bypass yet.
What issue would I see if
Valve is fully open?
Valve is shut or blocked?

Cheers

 

[stem]

If the valve is fully open, most of the water from the boiler will flow through the by-pass rather than take the route through the radiators with a much higher resistance to flow. There are other causes of the symptoms you describe though, so it may not only be the fault of the by-pass.

If the valve is not open sufficiently, insufficient flow through the boiler could cause it to overheat and even trip the overheat lockout.

Swap it for an automatic by-pass if there's a problem, then it will only allow flow through it when necessary. Manual by-passes can no longer be fitted to new installations because of their inefficiency.

 

[Terrywookfit]

If the valve is fully open, most of the water from the boiler will flow through the by-pass rather than take the route through the radiators with a much higher resistance to flow. There are other causes of the symptoms you describe though, so it may not only be the fault of the by-pass.

If the valve is not open sufficiently, insufficient flow through the boiler could cause it to overheat and even trip the overheat lockout.

Swap it for an automatic by-pass if there's a problem, then it will only allow flow through it when necessary. Manual by-passes can no longer be fitted to new installations because of their inefficiency.

Fitting an A B V is not going to cure his circulation problem.
A few system details would help.
Pipe sizes, layout, Rads on drops, pipe length runs. etc.

 

[Olivias Grumpy]

Thanks again
I’ll fit one anyway for piece of mind.
Would you know if they are same length as a 22mm gate valve?
If so it’s a straightforward swap out
Cheers

 

[Olivias Grumpy]

Hello Terry
Not sure about pipe sizes under floors
To and from boiler 22mm
Flow and return to rads 22mm into upstairs floor space
22mm to and from ground floor then they disappear into floor and all rads on 15mm rising tails

I’m pretty sure that the 22mm should have been extended further. But it has worked ok up to last summer

 

[Terrywookfit]

Is it open vented.
Have you checked pipes around feed and vent with a magnet.

 

[Olivias Grumpy]

Hi Terry
By open vented do you mean header tank in attic with cold water feed to system, and and a 22mm pipe I refer to as an expansion pipe over the top of the tank , then yes it is.

Haven’t tried a magnet anywhere, what would I be checking for?

Thanks

 

[Terrywookfit]

To see if there was magnetite in the pipework around pump and feed/ expansion pipes causing blockage.

 

[Olivias Grumpy]

Ok

I have had the pump off the system and it is all clear
Very little evidence of black crap etc
I am fairly sure all pipes are clear
Some rads get warm when I shut some down.
When I shut the warms one down and open the cold ones I do get heat but not enough
When all are open upstairs they seem ok but no matter what I do I cannot get heat to everywhere downstairs
I’m flummoxed

 

[Terrywookfit]

Are you sure its not a 1 pipe.
A strong magnet test is quick and free.
Is it better with h/w off ??
Photo of pump m/v's etc ?

 

[EAST61]

Worth noting that some boilers with pump overrun have integral ABV fitted and do not require an external one. I’ve no idea if Potterton’s do or don’t.

 

[Wildcat1961]

Worth noting that some boilers with pump overrun have integral ABV fitted and do not require an external one. I’ve no idea if Potterton’s do or don’t.

Potty profile utilises pump overrun stat, no integral ABV on the profile/Prima.
Profile/netaheat/Prima were a very common boiler back in the day, many are still going strong.