Mystery pipe

[Jerry1]

I have just moved into a new house and am trying to get my head around the CH/HW system and i have come across a pipe which appears to serve no purpose at all, i was wondering if anyone could shed any light on what it might be....

The layout is as follows....

Boiler hot pipe---pump---i---Zone control valve---HW(cylinder inlet)---HW (cylinder outlet)----i---boiler return line






For those who may be baffled by my crap drawing the mystery pipe is the small " i " . it is a 15mm bore and is vertical and is joined between the pump and the ZCV and runs to a point between the ZCV and the cylinder coil return to the boiler line. there is a stop cock near the top.

Does anyone know what it is for?????
i have tried to reason it out but i just cant figure it out - should it be open or closed?

any help would be appreciated as it may help to solve my other problem - see thread on 10th feb labelled pump/boiler problem

thanks

 

[ChrisR]

It's a Bypass. Possibly required by the boiler to help dissipate heat when the pump stops, or, eg, trv's are shut.
You'd have to refer to the mfrs instrs to be sure whether the boiler needs it or not, sorry don't have that one.
If you close the valve you may find the rads heat better, but the boiler may boil when the pump stops.

 

[Jerry1]

Thanks for the suggestion - had absoloutly no idea what it may have been for, that does seem logical now you think about it - thanks for shedding some light on it

If it is closed and the pump stops then you suggested the boiler may boil, the boiler is kettling at the moment but the pump appears to be running(humming/vibration etc.) but it is getting very hot - is there anyway of checking whether the pump is doing its job properly - am just trying to rule out the possibilty of poor circulation/inefficient pump.

If the valve is open and only the HW is on, how would the bypass effect the flow thru the cylinder coil and boiler - its unlikely to reduce the speed of flow and cause kettling is it?

 

[Prof.plumb]

Lets just make this clear a bypass shouldn't be fully open it should only be open a couple of turns.
To test a pump, whilst running, if it has a screw cap on the front remove this with a screwdriver. Have a towel or dustsheet below because it will leak water. Now try to stop the propeller with you screwdriver. If it is easy to stop it the pump is weak.
However, 9 times out of 10 kettling is caused by scale in the heat exchanger.

 

[chrishutt]

Lets just make this clear a bypass shouldn't be fully open it should only be open a couple of turns.

Where on earth do you get this stuff from? A by-pass should be adjusted to achieve certain minimum flow rates through the boiler heat exchanger. Depending on the circumstances and by-pass size and length, that could be fully open or nearly shut.

 

[croydoncorgi]

...and Best Practice requires it to be an AUTOMATIC bypass which adjusts according to back-pressure in the system.

If you have a fixed bypass adjusted too low, the boiler will overheat because the heat cannot get out of the boiler fast enough when there are no alternate paths through the system.
If it's adjusted too high, too much water will flow through it ALL THE TIME, so Return temperature to the boiler will be too high and it won't condense.