I have 4 industrial boilers that range from 40klbs/hr to 60 klbs/hr. Currently, we have 2 types of air proving switches: Cleveland controls AFS-951 and an old Honeywell that I cannot read the part number.

My operating combustion air pressures run from 8-20"H2O. The Cleveland controls switch specs down to .5"H2O but when I bench test it (or the Honeywells) they close with pressure applied but tend not to open when pressure is let off them. They also aren't very repeatable.

I can't seem to find much else out there. Anyone using anything else that may work in my application AND be more reliable, repeatable?

Generally quality stuff:

Dwyer Instruments Home Page 2008

Search for "pressure switch"

Boilers of this type will have (2) or possibly (3) air flow switches. All switches have deadband of some value.

Calibrating them depends on if the switch (regarding logic) makes or breaks on increase or decrease of pressure/flow.

The "minimum air flow switch" proves that adaquate minimum air is being supplied. It is "closed" with adaquate air supply and "opens" when air supply/flow falls below a certain point 10-15% depending how the combustion is set up. Adjust the switch to open on falling flow.

The "purge air flow switch" makes when air flow is >70% of maximum flow and starts the purge timer. Adjust the switch to make on rising flow.

Some boilers especially with VFD's on the fans have a 3rd air flow switch that proves the air flow has decreased from purge to a "lightoff" value of <25-30% again depending on the combustion setup. Adjust this switch to make on falling flow at the proper setpoint.

The switches each play a particular role in the combustion logic and the deadband if minimal should not hinder it's operation. Again adjust the switches based on the rise or fall of air flow.

Terry D.