Rube Goldberg here. I have an ERV operated by both demand switches and a percentage timer. The demand switches, in the bathrooms, run power open dampers with end switches that start the ERV. The percentage timer also operates a power open damper (balanced for airflow, although the bath ducts are not.) Small but aggravating hitch: two dampers are open more often than you'd think, and it degrades exhaust in the distant bath,
Can you buy a percentage timer with lockout rather than interlock, or is possible to rig the latter to interrupt the timed damper? Complication: The dampers are 115V.