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Wiring a fan in a can and pressuretrol height?

Hi folks, I appreciate all the great comments on the site, hoping to find a couple answers.

I have a 1970's Burnham boiler that is closest to their current IM6 with a PA404A1033 pressuretrol, float LWCO and Honeywell VS820A-1054 millivolt standing pilot gas value.

1) I've closed in the boiler area and will add a Field Controls CAS-4mV fresh air intake fan.  The instructions show adding the fan control circuit to interrupt between the thermostat and gas valve but do not specify whether it should be before or after the pressuretrol and LWCO in that circuit.  If I connect it upstream of both controls then they are fed by 750mV (same as now) coming out of the fan module, and if I put the fan after the controls (still before the gas valve) the controls are fed by the 24v thermostat side of the CAS-4mV.  Both pressuretrol and LWCO appear to support both 750mV or 24v, so is there any advantage/disadvantage to where the CAS fan interrupts the circuit?

2) Is there a maximum pipe/height length to be concerned about when mounting the pressuretrol above the water line for accuracy?  The original installation has it mounted on a horizontal pigtail off the top of my LWCO, but it's barely above the waterline with the boiler at rest.  Is there any accuracy loss if I add a 6" pipe extension into the top of the LWCO to bring the pigtail and pressuretrol higher away from the water line?

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