I just had a new raypak 400k cupro-nickel natural gas heater installed to replace my old Laars unit...the old heater would not fire if there was not flow through the heater. This new heater seems to not care if there is flow or not; when the heater was installed, I had the installer put in a couple of valves so I could isolate the heater when I need to lower pH for AA treatments, and when I isolate the heater, it will still fire when I expected it to error out due to no flow. Does this heater behave differently by design, or is something wrong preventing the flow sensor from stopping the heater from igniting?