Hot water heat: zone doesn't run unless another zone is also calling for heat
I have a hydronic baseboard system in which the zone relay built into the aquastat stopped working. Instead of replacing the aquastat, I decided to add another Honeywell R845A and move the thermostat and circulator to that. The new relay is at the end of the "chain" of relays. I am using the provided diagram for a multizone system, which matches the way the existing relays were wired.
Each zone has its own circulator. The problem zone's circulator is hooked up to terminals 2 and 6. Terminal 2 is also hooked up to the common of the line power. The line hot is connected to terminal 3, which is jumpered over to 5. Terminals 1 and 4 are hooked up to 1 and 4, respectively, from the last relay in the line (which ultimately ends up at ZC and ZP on the aquastat, of course). The thermostat is hooked up to the thermostat terminals.
When the thermostat in the problem zone calls for heat alone, nothing happens. When another zone calls for heat while the problem zone is also calling for heat, the problem zone starts working as expected. When another zone calls for heat while the problem zone is not, only the other zone runs just as expected.
I use a Honeywell digital setback thermostat, but I tried changing its batteries and also testing with the old Honeywell round one that it replaced with no change.
I measured 120V between terminals 2 and 5 (and 2 and 3), but measured 24V across the thermostat terminals only when another zone is calling for heat. When no other zone is calling for heat, there is no power on this relay. So this appears to be the cause of the problem, but I don't know how to fix it.