Correction to Oil Filtration Circuit Schematic

Document created by ectofix on May 23, 2015Last modified by ectofix on May 25, 2015
Version 12Show Document
  • View in full screen mode

While working on an in-house project, I found an error in a small schematic that is illustrated in several fryer manuals.  The error is in how the schematic depicts operation of the filter pump control circuit in relation to the two solenoid valves in the filtration circuit.


I’ve informed Frymaster of the error in the diagram and their response back is that their tech writing department has been contacted to request a change to it.


The schematic is located in the H50 Series Gas Fryer S&P Manual (P/N 819-0001, Jun 2005) on page 1-27. The other that I I've found is in the Pro H50/55 Series Gas Fryer S&P Manual (P/N 819-6083, Jun 2009) on page 1-29. The diagram is also included in the several technical reference (factory training) books that I possess.  The electrical configuration of the circuit appears universal in all current-production Frymaster fryer models which have built-in filtration systems, but that little schematic isn't in all manuals.  For instance, the RE Series E4 Electric Fryers S&P Manual (P/N 819-6150, Sep 2012) doesn't provide the schematic, but the configuration of the circuit is the same in those fryers, with the only difference being that the primary voltage to the transformer is different..


The error is seemingly minor, but it may lead to some confusion to newer techs when looking at the schematic. Here’s a brief overview of the filter pump circuit operation:


Frymaster’s current-production oil return system design employs two normally-open solenoid valves to allow any oil trapped in the oil return manifold to drain back into the filter pan.  The solenoid valve coils are connected in parallel to the filter pump motor relay coil.  Any given cam-operated microswitch (whichever fryvat is being filtered) will supply 24 VAC to those component's coils when turning the filter pump lever to the ON position.  As such -


  • When turning the filter pump lever ON, the pump motor relay and solenoids valves energize.  The solenoid valves then CLOSE while the filter pump motor is running, thereby directing oil through the oil return manifold to the opened oil return valve of the fryvat being filtered.


  • When the filter pump lever is turned OFF, the pump motor relay and the solenoid valves de-energize. The two solenoid valves OPEN.  One of the solenoid valve allows oil to drain back to the filter pan while the other solenoid valve serves as a vacuum-break to allow air to be drawn into the oil return manifold as the oil drains out.


As seen in the picture below, the left-hand diagram show the solenoids connected to 24 VAC AT ALL TIMES, while the microswitches appear to supply power to JUST the pump motor relay. This diagram is essentially the same one you'll find in the aforementioned manuals and is INCORRECT.


The right-hand diagram is my "unofficial" correction to that schematic, which depicts the solenoid valve coils connected in parallel with the pump motor relay coil AFTER the microswitches.  As such, the solenoid valves receive power at the same time as the pump motor relay.

This is how the units are actually wired.  I've traced a wiring diagram out just to be sure.  Additionally, I've observed a unit's operation and that IS how they work.


I've added a few other details of my own into the right-hand diagram for accuracy and clarity, such as actually depicting TWO solenoid valves, colorizing the source voltage and 24 volt circuits and how the relay is illustrated.




The graphic shown below illustrates how the filtration system operates to control oil flow.


  • The solenoid valve to the left is the air-vent valve.


In the illustration I'd chosen to use here, the solenoid valve connects to a T-fitting by way of a 2" long nipple extending upward so the valve remains the highest point in the plumbing.  On some units, this valve may otherwise be connected directly to the top of the rear oil return manifold with that 2" long nipple.

The air-vent valve's other port connects to one end of a Teflon tube that is routed to the front of the fryer for connection to the top of the drain manifold.  That Teflon tube is connected there for air because, if the valve so happens to become stuck open when the pump is running, the hot oil that is leaking by will be harmlessly diverted back into the the drain manifold.


  • The solenoid valve to the right is the drain-off valve.


That valve connects between the oil return manifold and the filter pan.  When it's de-energized, that valve allows residual oil to drain back to the pan once the filtration system pump has been turned off.




Despite the flawed schematics within the service manuals, I hope I've provided some clarity in this system's electrical operation and oil flow,  With that, you will find that maintaining the oil return system and inspecting its operation is simple.  Additionally, you may further understand the importance of certain components in the system to not only determine whether it's operating properly, but functioning safely as well.