There are two reasons for installing a bypass (as Mike say's only auto allowed now).
1. To guarantee the minimum flow rate through the boiler as required by the manufactures, when the heating is on & coming up to temperature the TRV's will be closing down restricting the flow of water around the system. bypass is required at this point to open & allow water to pass from the flow to the return without going through the emitters but as the return pipe will be close to the return temp the boiler will quickly shut down on the thermostat.
2. An auto bypass is also required on a system where the boiler requires an open pathway to dissipate heat from the main heat exchanger when it shuts down. This is most likely when an S Plan (2x2port zone valve) has been installed & the boiler has to control the pump i.e. it has a Pump Live wiring terminal. I this case as previous postings it may need to be some distance away to allow the heat to diapers.
As every read the manufacturers instructions (MI's), something that a lot of people seem incline NOT to do !! at least until things go wrong.
Now you know what they are & how they are used, the next question is how do you set them up so that they work correctly i.e. remain closed in normal operation to keep boiler in condensing mode & only opening when required ? Have a think & let us know.
. How come if it's an auto bypass at the end of the run how will it waste heat?
