Tjernlund PS1505 Fan Proving Safety Interlock Switch for Draft Inducer Fans

work as intended

Fixed a huge AC problem that could've cost me a fortune for just a couple of bucks and a YouTube video!

The review rating is for the lack of a sensitivity adjustment. The fan prover control, accessories and instructions are fine otherwise. If one of these is being installed in conjunction with an inline draft inducer, it interfaces with another control (Tjernlund UC1) that verifies whether or not the contacts on this switch are open. If the contacts on this control are closed, the main control thinks that the draft inducer is "on", and the control goes into lockout. The UC1 does not have a retry provision, only a setting to bypass the check to see if the draft prover contacts are open. The heat (or other connected appliance) is out until you notice a problem in this case and reset the UC1 control. If the inline draft inducer is connected to a furnace with a draft inducer, the furnace draft inducer may create enough pressure on its own to close the contacts on this fan prover switch. The problem here is that if the inline draft inducer stops functioning, the furnace draft inducer may be enough to keep the furnace operational, when it should shut down with the loss of the inline draft inducer. External (outside) winds may create enough air movement to close the contacts on this fan prover. This is the main problem I am having with my installation. I also experienced no furnace shutdown when I killed the power to the inline draft inducer (this is an important safety operational check). Both of these problems are mentioned in brief in the instructions for the UC1 control. The UC1 control has a provision to "turn off" the check for closed fan prover contacts. If activated, the connected appliances may continue to operate even if the inline draft inducer does not. In my opinion, this is a serious design flaw with the UC1 control used in combination with the recommended fan proving switch. The only appliance type that will shut down in any of the above scenarios would be natural draft and have a thermal safety switch on a barometric damper or draft hood, like a water heater or standard boiler. Tjernlund supplies what they cal a "linear limit switch" for these installations. I will have to source an adjustable pressure switch from another manufacturer to prevent these issues.