Differential Pressure Switch For TrueSTEAM

My HVAC guy never installed this critical component when he installed my Aprilaire humidifier years ago. This meant that the humidifier was running constantly (and wasting water) whenever the humidistat called for more moisture even when the fan/blower was off. So, I installed this on the return side of the ductwork first and it worked like a charm. I also installed the tubing on the supply side since it was easy to do. This is easy to install. I drilled two small holes to mount this on the return ductwork and used the supplied sheet metal screws. Then, I used a holesaw to drill the holes for the tubing. Finally, I used a short piece of thermostat wire to place this switch is series with the 24 V transformer and humidistat. Easy. Like I said, this works great.

I bought this differential air pressure switch to install it with the new Honeywell Steam Humidifier we purchased. We have a variable speed furnace so the installation guidance was to install a probe in the supply and one in the return since the furnace can run at low speed. At low speed very little pressure is generated. The switch didn't work at all at low speed. Anyway, based on everything I've learned this is not a good product for variable speed systems that can run at low speed. There is a better and cheaper solutions. Cons: Air Sensing Probes require drilling a big 3/4" hole in both the supply and return ducts. Air Sensing Probes only measure static pressure (which is very low at low speeds). Air Sensing Probes are very short so they insert only about 1" into duct. Two probes are required in variable speed systems. The set point for the switch is not advertised and appears to be higher than required for low speed systems. Pros: None Better & Cheaper Solution: --------------------------------- Qty 1 of Cleveland Controls DFS-221-112 Fixed Point Air Pressure Sensing Switch with Barbed Connector (It has a low set point of 0.05" W.C.). Qty 1 of Cleveland Controls Total Pressure Probe (they come in different lengths. I used the Part #21122-112 which is 6", Barbed). Qty 1 of Flexible Plastic Tubing, 1/4" ID, 10ft. Pros: Drill only one small 9/32" hole in the supply side. Use only 1 probe in the supply side. Cons: None