Looonnng post a-comin...
1) You should seriously investigate WHY the room is so negative before proceeding with this elaborate band-aid. Sounds to me like they need bigger intake dampers, but the control scheme was an easier sell than cutting walls open, and more profitable. Was there just no wall space to do this?
2) If you add a fan, are there going to be other intakes still open to vent the pressure, or will this become the only intake path? If so, you are blocking the airway even worse when the fan is down. Just something else to consider if the fan fails.
3) You are adding a lot of components that can fail to an otherwise simple situation. Yes, liability does factor in.
4) Are these natural or induced draft boilers? Either way, if successful in controlling the room, you will alter the burner stack mixtures by changing the room pressure. The boilers should be checked afterwards to avoid monoxide, flue, or exchanger condensate and damage. Any good boiler man investigating a rotted out exchanger will notice the changes in stack pressure that a controlled fan will cause and point it out to the owner. If you do pressurize, it best not hunt badly, or odd things may result.
5) Are you going to interlock the intake fan with the boilers so it doesn't run unless needed or will it run always? Pros and Cons either way. The drive will have an enable dry contact input point.
6) The pressure sensors will have to be in the 0-0.1 range or lower. If you put a tenth of an inch of static in the room (if at all possible) you will get a door in the face. My guess would be adequate control around 1 hundreth to 5 hundreths (0.050). You are just leaning on the positive side.
7) Best to use proportional control only in this application. This will allow you to tune the thing in to get fast reaction time with minimal hunting when the burners start and change room pressure, or when a door is opened/closed. Stay away from Integral or Derivative unless you are experienced in using them. Then again, this would be a fun application to learn with...
8) Is the motor on the fan rated for drive service? A "B" motor insulation class is a dead ringer for a burnout on drive service. The slower the motor runs for extended periods, the shorter the service life if it's not rated for it. Talk to the drive rep and the motor manufacturer for advice here.
9) Is the motor closeley rated to the application. If the fan is rated to do the "calculated" CFM at 1 or 2 inches of static then you have way too much fan HP for the fraction of an inch static that you desire. This will result in a crawl of motor speed, and may overheat the drive and the motor.
10) Is the drive HP closely rated to the motor HP? Do not oversize motors or drives in an effort to "derate". Derating just doesnt work this way on a drive, it makes things worse.
11) You should have the facility to lock the setpoint of the control from curious hands for the benefit of fan and boilers.
12) Will the owner have facility to run the drive manually or directly on line (bypass contactor) if either the drive or controls fail? This sounds like a pretty sizable fan, does a big building depend on it?
13) What will the pressure sensor reference on the low side connection? (Every sensor has two connections) An adjacent space or outside air? OA would be the most sensible pick but will give you some buffeting from wind effects at such low pressure sensing ranges.
14) If control is un-stable, consider adding a considerable length of poly tubing to buffer the room pressure signal and especially the outside reference. This is detailed in the instructions for the Kele OA pressure sensor probe.
15) The DC power supply? Get the whole works from Kele, They cost less than your time to make the regulated supply that is necessary to operate the transducer accurately.