Dwyer Mark II system component

First time I have seen this device.Is this monitoring the amount of moisture to the humidifier? Can someone explain how & what this does?

I also Have a crawl space question. The exterior vents are blocked and they are heating with the supply ducts of the furnace. Am I right in noting that the crawl requires proper ventilation. What is the requirement? !sq. ft. of vent for how mant feet of crawl? I think they are wasting a lot of heat at the crawl. Walls & subnot insulated.



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That is an inclined manometer and measures pressure in inches of water column. It is probably being used is a monitor for the air filter.

What was the pressure manometer hooked up to? It most likely is hooked up to a damper in the system that opens to relieve pressure when it gets above 0.5" w.c., which is the practical limit for pressures in residential HVAC systems. That could be related to several purposes, including monitoring filters (bypasses if pressures built due to clogging) or possibly a supply-return pressure reduction bypass for a zoned system.