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Thermostats and bosch zwb combi boiler control

Installed a ZWB boiler, two zone, one external circulating pump, configured as a primary-secondary mixed system. Nest Tstat.
Trying to figure out the controls.
When the thermostat is installed, but set to heat off, the boiler still sees a call for heat (service code C.d =01). When i remove the stat from its base plate, call for heat goes to 0.
Service function 6.b Room Temp control, current voltage, terminal 2 reads 24 volts regardless of stat condition.
My concern - the external circulating pump runs continuously regardless of call for heat from the stat. It is connected to pin 2 and 4 as per manual. Service function 5.E is set to 02 - external heating zone pump in circuit w/o three way valve.

What i anticipated was T stat controls call for heat, turns pump on and boiler modulates to maintain supply water temp. If stat is satisfied, pump and boiler turn off. ( No OSA reset control installed - I manually set max supply temp).
What seems to happen is that as long as the boiler is powered on and the heat switch is on, the external pump runs and the boiler controls to the set supply temp. Meaning pump and boiler cycle even when OSA is 80F! And from what I’ve seen so far, the pump runs continuously. Unless i take the stat off the wall.
Low mass system - baseboards and panels, wood frame house, wood floors, not particularly well insulated or air tight. Reasonably sized boiler, estimated peak load about 80% of rated capacity. I’m in the mountains and OSA mins hit negative numbers frequently in the winter.


  • markman77
    markman77 Member Posts: 7
    One more, probably not related, problem:
    The low water cutoff is in fault mode i.e. normally closed contacts are open and i had to reinstall install the jumper (pins 8-9). Brand new, boiler is full, 15 psi, well bled. Don't feel comfortable running without LWCO but …
    It is wired correctly, powered up, etc.