Intermittent or Elusive Issues

It is a good practice to make sure all of your dip switches are set to the factory position on all of your flow selector boxes during installation. If the dip switches are not set in the factory position you will have L11 and E17 error codes to follow. You can verify the factory dip switch settings by looking at access door on the flow selector box. The L11 and E17 error codes can also be caused by wiring issues, good wiring practices can prevent having problems.

Changing dip switches on your flow selectors is not necessary because all of your flow selector addresses and port numbers are set using and codes 00FE and 0105 on your thermostat. Making sure your flow selector address and your flow selector port address is correct will make the start up process much easier. If care is taken and you have a good understanding of the wiring and installation of a VRF system the start up process can be close to seamless.

L11- Detected indoor unit address, Flow selector unit not connected.

E17- Communication trouble between indoor unit and flow selector unit.

Meter leads on the DFT position 1 and the C terminal.

Schematic: The circles represent where meter leads were making contact for testing.

The Pipe Sensor:

Copper plumbing pipe with wires and a blue clamp inside an appliance cabinet

The pipe sensor monitors the temperature of the water going to a 2-pipe FCU, also known as changeover sensor, changeover switch, Aquastat, among others:

If the water is hot (> 83 F), it signals the thermostat that the system is in heat mode;
If the water is cold (< 65 F), it signals the thermostat that the system is in cooling mode;
User configurable for Open or Closed activation
Compatible with both on-Cool / on-Heat sensors (determined on 1st run setup)
Compatible with dry-contact pipe sensors (on/off), not analog. (ex. 10K ohm thermistor)

Dehumidify with Fan (Yes or No): when Dehum is active this setting specifies if you want the furnace/air-handler fan to run as well. This will activate (G) output on the t-stat at the same time. This can work independent of a cooling cycle.
Min Runtime Delta (2-10%): to reduce short cycling, the dehumidifier will run beyond the desired humidity by the selected amount (%). Default is 5%.
Dehumidify in Heat Mode (Yes or No): If humidity is a concern during the heating season and have a standalone Dehumidifier, this setting can be used to activate the Dehumidifier and will work independent of a heating cycle.
Dehumidifier Active (Open or Closed): this setting is dependent whether the HVAC system or standalone Dehumidifier requires it to be open or closed while active. This setting is part of the 1st run setup but can change afterwards in the Dehumidifier menu.

HVAC technicians often face challenges in pinpointing intermittent or elusive issues, as they can be harder to diagnose compared to consistent problems. Identifying and resolving issues related to intermittent malfunctions or sporadic performance can be more time consuming for technicians. I want to go over some of the most common intermittent malfunctions for gas furnaces that I have come across this winter, and I want to share and elusive issue I came across this past summer.

High Limit Fault

A common cause for a furnace to experience a high limit fault is restricted airflow. This can result from issues such as a dirty or clogged air filter, blocked vents, or obstructed ductwork. When air flow is restricted, heat builds up inside the furnace, triggering the high limit switch to shut off the burners to prevent overheating. Regular maintenance, including replacing air filters and ensuring proper ventilation. If the system has zoning, check to make sure all of the dampers are working properly. Take voltage readings at the zone board. I’ve see a zone board short cycle the blower motor in first and second stages. Test voltage and amp readings at the blower motor. If the system has a VFD drive, go through the parameters and make sure everything is set to the specifications to that particular motor.

Blocked vent or exhaust pipe: Obstructions, such as debris, bird’s nest, or other materials, can restrict the proper venting of combustion gases.
Faulty inducer motor: The inducer motor creates negative pressure, assisting in venting combustion gasses. If the motor is malfunctioning, it can lead to pressure switch issues.
Blocked condensate drain: in high efficiency furnaces, a blocked condensate drain can trigger pressure switch problems as it affects the proper function of the condensate removal system.
Cracked heat exchanger: A cracked heat exchanger can disrupt airflow and cause pressure irregularities, leading to a pressure switch fault.

To check the inducer fan motor in a furnace, You can follow these steps:

Visual inspection: inspect the inducer fan motor visually for any obvious signs of damage, wear or loose connections.
Electrical Testing: Use a multimeter to check the continuity of the motor windings. This helps determine if there are any electrical issues inside the motor.
Voltage Testing: Measure the voltage across the motor terminals while the furnace is running. This ensures that the motor is receiving the correct voltage.
Inspect the Fan Blades: Check fan blades for any obstructions or damage that might affect the motor’s performance.
Inducer Motor Amp Draw: Measure the amp draw of the inducer motor using a clamp meter. Compare the reading to the manufactures specifications to ensure it is within the acceptable range.
Check for binding: Ensure that the inducer spins freely without any resistance. Binding or stiffness may indicate a mechanical problem.
7.Inspect Venting System: Examine the venting system connected to the inducer motor for any obstructions that might affect airflow.
Verify Proper Sequence of Operations: Confirm the inducer motor starts and runs as part of the furnace’s normal sequence of operations.

Gas Valve

I’ve seen the gas valve cause intermittent issues. Use a manometer to check the inlet gas pressure and outlet pressure. Take pressure readings during first stage and during second stage. You can also take an Ohm reading across the terminals. Most gas valves have an Ohm rage between 20 – 40 Ohms.

I want share one elusive issue I came across this past summer. A homeowner had installed an air conditioning system to his fully finished garage a few years ago. The garage was professional finished with insulation and dry wall. The system worked great with no problems for years. This past summer was the first time they had any issues with it. It was now running constantly and there was sweat forming on the supply vents. I thought it could be and airflow issue or a refrigerant issue. We first checked to make sure all the ductwork was clear and free flowing, the filter was brand new, and the coil was clean. We then checked pressures, superheat, and subcooled temperatures. Everything thing was in range. We then asked the homeowner if there were any changes made to the house or the garage. It ended up that the homeowner had a large oak tree cut down and that was enough to change the load on the house.

Sometimes us HVAC technicians can’t be completely focused on just the system. When we get stumped on an issue, we need to take everything into account.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.

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