Need for automatic bypass ?

Postby notfocused » Tue Jan 26, 2010 9:26 pm


Currently my Hot water cylinder water temperature is being controlled by the boiler temperature, i have the boiler set at 75 deg, i know this is to hot and would like to add a cylinder thermostat/motorized valve to regulate the water temperature.
I currently have one motorized valve on the heating circuit controlled by a room stat.
As the cylinder flow and return are part of the central heating pipework i believe i would have to add a second motorized valve onto the pipework going into the cylinder,and then add a cylinder stat and wire it into the valve and programmer, this would then convert my set up into a 's' plan set up.
My boiler is a ferroli optimax he 31s, in the manual that came with it it states that there is an automatic by pass within the boiler and on the 's' plan diagram there is no automatic by pass shown, so do i need to add an extra automatic by pass for this set up to work or is the boiler bypass sufficient ?
Here's the link to the manual.

Google ""

Page 15 shows the s plan set up, page 32 shows the boiler set up with the automatic by pass,

Any help greatly appreciated
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Joined: Tue Jan 26, 2010 9:06 pm


Simply Build It

Postby plumbbob » Wed Jan 27, 2010 4:34 pm

Page 11 of the installation instructions say, "A minimum flow of 6 litres/min is required through the heat exchanger, calibrated on site. An automatic
by-pass is fitted to the boiler which will provide this flow rate
in most circumstances".

That would suggest a further bypass is not required. However, if you did wish to include a slightly open 15mm gate valve between the feeds near the cylinder, it would certainly do no harm.
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Joined: Wed May 07, 2008 9:59 pm

Postby notfocused » Wed Jan 27, 2010 5:03 pm

Hi plumbob thanks forthe reply

Would you mind explaining in a little more detail about the gate valve.
Do you mean by adding a gate valve that i tee off the 22mm flow and returns into and out of the cylinder with 15mm pipe and put the valve between the two ?
Would this be put in in front of the motorized valve ?
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Joined: Tue Jan 26, 2010 9:06 pm

Postby plumbbob » Thu Jan 28, 2010 9:24 am

All "S" plans need a bypass because if the boiler/pump are active then a flow of water must be maintained. In most systems this is simply achieved by teeing between the flow and return just before the motorised valves. Linking the two will make a circuit.

To prevent this link robbing the main circuits, fit a 15mm gate valve, close it, then open it between one and two full turns - until you sense water can pass. This then acts just as a lockshield valve does on a radiator.

Once the valve is set, remove the hand wheel to prevent accidental adjustment.
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Joined: Wed May 07, 2008 9:59 pm

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