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Hi, I have a Glowworm 30 SXi combi boiler and hot water tank run by a Honeywell ST6400 and Honeywell CM61 thermostat. I wante to upgrade the system to a multizone system such as Honeywell Evohome. I have read that the relay BDR91 is used - does this replace the St6400 or the Cm61? Could...