Operating Principles of a Honeywell Smart Valve – Part 1

by Ron Walker

Operating Sequence.

The operating sequence of Smart Valve gas ignition and control systems is very similar to  the older style, intermittent ignition systems which incorporate remotely mounted spark  ignition modules. Those older systems use a 12,000 to 18, 000 DC voltage source for ignition of the pilot gas. The Smart Valve system uses a 24 VAC powered hot surface pilot  gas igniter to ignite the pilot gas. The main gas is ignited in both instances by a pilot gas burner.

The Smart Valve system is self-contained, thus eliminating the need for a separately mounted ignition source, usually in the furnace or boiler vestibule.

Starting with a call for heat from the thermostat, the circuit usually (not always) will be completed to the R and W terminals located on an Electronic Fan Timer (EFT). A separately mounted 120/24 VAC transformer supplies power to the entire control circuit. Immediately the EFT energizes the induced draft fan. As quickly as the Air Proving Switch (APS) senses that there is a proper amount of draft air moving through the combustion chamber, the APS
closes its switching contacts in order to complete the circuit to the Smart Valve. As the circuit is completed, the igniter is powered and the pilot gas valve is opened with the igniter lighting the pilot gas. In those systems using an EFT the Smart Valve sends a 16 VAC signal to the EFT to start the time delay which dictates the amount of time that must pass prior to energizing the air handling blower.

The flame rectification / proving system proves the pilot flame is lit (in less than 2 seconds) and allows the main fuel valve to open and to simultaneously shut down the igniter. The flame proving system continues, monitoring only the pilot gas flame. The main fuel valve remains open until the call for heat ends. In those instances where some problem exists, the operation includes a 90 second Timed Trial for Ignition. If the system has not proven the pilot flame in 90 seconds of trying to prove the pilot, the system will shut down (not lockout) for 5 minutes and then will go through the starting sequence again. The system will go through this sequence until either the call for heat is ended or the burner lights off successfully…. to be continued.


About The Author

Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.