Originally Posted by
DBMandrake
If you have a hot water zone valve and a heating zone valve (eg an S-Plan system) there are two basic wiring and configuration options available.
Option 1 - two BDR91's - one connected to each zone valve, these are bound in the hot water configuration as heating and hot water relays. The orange wires from both zone valves are used to fire the boiler when the switches inside the zone valves close. No direct connection from Evohome to the boiler.
So when either zone valve opens, (hot water demand or heating demand) the switch inside the zone valve closes and fires the boiler. (Both switches are wired in parallel)
Option 2 - three BDR91's - the same as above except the orange wires from the zone valves are not used. Instead the third BDR91 is connected directly to the boiler and is bound as "boiler control" in the Evohome system, in addition to the other two relays being bound in the hot water config as before.
This is the system I have. There are a couple of advantages to using 3x BDR91 - one is that you can make use of the hot water overrun feature which doesn't work if you only have two relays. It also gives more wiring flexibility for some non standard configurations, for example if you have a custom pump overrun timer. But the two relay system can work fine as well.