How to Improve Your Mitsubishi Troubleshooting

Hello Everyone! Mitsubishi makes some of the most efficient and comfortable systems available. If you have ever installed a Mitsubishi unit, then you know it is a pretty easy task. However, it seems that when it comes to diagnosing a problem with the units, many techs shy away from them. So, how do you go about diagnosing a system that you are not familiar with?

Have you heard of Mitsubishi’s website, With this site, you can find specs on the equipment, advertising info, product matchups, and service manuals. There is also an incredibly helpful tab called ‘M&P Troubleshooter’, which provides you with a step-by-step diagnosis for the exact system you are working on. All you have to do is follow the instructions step-by-step, and it will give you the necessary information to properly diagnose the system.

As always, if you need any assistance, please feel free to contact the Famous Supply HVAC Tech Department at 330-434-5194.

Proper Start Up

The summer is in full swing and I’m sure you are staying busy. With that being said, please take time to do a proper start up on that brand new AC unit you just installed. As I have said many times, we don’t live in the age of just hooking up an AC and letting it run. Things have changed! We must pay special attention to the process we use to install and start that new equipment.

When doing the initial install, you need to flow nitrogen through the line set as you braze to prevent oxidation on the inside of the lines. Then when pulling a vacuum, use a micron gauge to make sure you have reached the correct level of dehydration. Now that the system is installed, the job is not done yet. Make a point to check and adjust the cooling speed for the correct CFM per ton. Typically use 400CFM/ Ton. The static pressure should also be verified to ensure that the ductwork is delivering enough air to the conditioned space. Then and only then connect the gauges to verify the refrigerant charge.

By following these steps, you can feel comfortable that the system will perform as expected and deliver the comfort your customers expect. By installing a system that makes your customers not just happy, but ecstatic, you will have a much better chance of that customer giving you rave reviews to prospective buyers.

If you need any assistance in the start up process, please call the Famous Supply HVAC Tech Support Team at 330-434-5194

The Right Tools For The Job

If you have been following along in the previous Tech Tips, we have talked about testing and diagnosing, but what do you need in your tool belt to perform these tests? With today’s high efficiency furnaces and air conditioners, you need more than a big screwdriver and hammer to get the job done. Newer equipment requires some finesse.

When installing an air conditioner or heat pump, you must pull a vacuum to remove any moisture. Simply running a vacuum pump for 45 minutes will not work; you must use a micron gauge. This will verify that you have reached an optimum vacuum level so that the system is properly dehydrated. A digital gauge set makes the charging process a snap. When adding refrigerant, a digital scale is needed to make sure you add the right amount.

How about once the system is running? In order to make sure it is cooling correctly, you need to know the wet bulb temperature in and out of the evaporator coil. This will require a digital psychomotor. So now that the system is cooling, it is now necessary to make sure all that nice new ductwork delivers the cool air the system is producing. The right tool to grab would be a dual input digital manometer kit, as it comes with everything you will need. Using this tool will tell you your static pressure, and it can also be used to test pressure switches and gas pressure. A great tool to pair with this would be a mini vane air flow meter to verify your CFM.

Having these tools in your arsenal will make your installs perform correctly and will make any problems easier to diagnose. If you need any help in choosing the right tool for the job, or how to make the tool work for you, feel free to call the Technical Services Department at 330-434-5194.

High Head Pressure on Heat Pumps

Diagnosing a heat pump that is tripping on high head pressure in heating mode can be a challenge. With three service ports to choose from and many different “valves” and metering devices, it can be difficult to narrow down what exactly is causing the system to trip.

  1. Verify that the system is listed through AHRI as a viable match. This is especially important on newly installed equipment. An undersized indoor coil can cause the system to run high head pressure.
  2. Check airflow at the indoor unit. This can be verified by measuring the static pressure on the duct. Restricted airflow/a dirty coil will cause high head pressure, the same way that a dirty outdoor coil will cause high head pressure in cooling mode. Be sure to also check filters and make sure that register grilles have not been closed off in certain rooms.
  3. Take pressure readings at all three ports on the outdoor unit. If this is a split system, the outdoor unit will have a third port that sits above the two service ports. This is called the true suction port. When measuring pressures in heating mode, the big pipe will show your discharge pressure, the little pipe will show your liquid pressure, and the true suction port will show your suction pressure back to the compressor. If there is a restriction at the indoor coil, or in the line sets, you may see a significant drop in pressure between the discharge and liquid lines.
  4. The last item on the list is charging. Charging a heat pump correctly is critical, especially when in heating mode. If you are forced to charge a system in heating mode, weigh the charge in per the manufacturers’ specifications. An over-charged system will cause efficiency problems and can cause component damage, but the most obvious sign of an over-charge in heating mode, is that it will trip on high head pressure.

Isolating a Furnace, Bypassing the Thermostat

Every day we receive many calls from frustrated technicians who are dealing with error codes on furnaces that, seemingly, can only be explained by a faulty control board. These codes may vary, but the consistent description is that the board is giving a code that “cannot exist”. One such example would be a pressure switch code when the pressure switch is clearly closed to the board.

One of the first tests that we will have technicians run is simply disconnecting all of the thermostat wires from the control board and jumping “R” to “W”. This may seem overly simple or elementary, but it is amazing the number of furnaces that are made to run correctly simply by removing the thermostat and thermostat wires from the system.

Keep in mind, the thermostat may be “functioning” properly; it may be calling for heating, cooling, or fan as commanded. But there are cases where the thermostat is found to be directly interfering with the boards’ performance. In these cases, the thermostats will need to be replaced.

We recommend that, if jumping “R” and “W” seems to allow the furnace to run correctly, you cycle the furnace multiple times before replacing the thermostat. This will ensure that the furnace itself is actually consistently working correctly.

Jeff Rosenblum
Technical Support
16 Years Industry Experience
Cell (330) 962-2491

Static Pressure

Whether performing an installation in a new construction home or a system change-out on a 50 year old home, knowing the static pressure on your duct system may give early detection of problems that may not present themselves for several years.

Measuring static pressure gives a solid indication of the volume of air being moved throughout the system. Most residential systems are designed to operate within .5 inches of water column.

To measure the static pressure in your duct, you must have either a digital or magnahelic manometer.

  1. Drill a small hole in the return ductwork on the blower side of the filter (after the filter)
  2. Drill a small hole in the supply ductwork close to the unit
  3. Insert tubing from the manometer (+ on the supply side, – on the return side) and turn the thermostat to “fan on”
  4. If the return measures -.25 and the supply measures +. 25, the static on the system would be .5 inches of water column.

If the numbers show anything higher than the manufacturers recommended ratings, check the sizing of the ductwork (both return and supply), and the filter to verify that the system can handle the rated pressure drop.

Jeff Rosenblum
Technical Support
16 Years Industry Experience
Cell (330) 962-2491

Testing a Single or Two Stage Gas Valve

Gas valves are an integral part of any furnace. They supply fuel to the burners, which enables ignition to take place. Gas valves have many integrated safeties in place to prevent dangerous failures. This can sometimes make them difficult to diagnose. Keep in mind that there are two key factors that need to be checked when diagnosing a valve.

  • Gas Pressure – Many technicians make the assumption that because the furnace has been running for many years, the gas pressure to the appliance must be sufficient. Both low and high inlet gas pressures can wreak havoc on a furnace. If the gas pressure is too low, the heat exchanger may soot up. If the pressure is too high (over 14iwc), the gas valve will lock out and will not open. Be sure to check the inlet gas pressure and adjust as necessary.
  • Voltage – Voltage to a single and two-stage gas valve should only be present with a call for heat. On most gas furnaces, this is a 24v signal from the ignition control. Some gas valves have resistance ratings through the coils, meaning that one can measure resistance through the two low voltage terminals (the coil of the valve) and find a potential failure. A good way to check for proper voltage/a failed valve coil is to connect the two low voltage wires from the gas valve to a 24v coil (such as a contactor coil) and attempt to fire the furnace. If the contactor pulls in, the ignition control is energizing correctly and has not failed.

The final thing to consider when dealing with gas valves is the “on” and “off” switch which is located on most valves. Be sure that this switch is turned “on” when attempting to light the furnace. Many technicians have wasted many hours and good gas valves due to overlooking this simple switch.

Jeff Rosenblum
Technical Support
16 Years Industry Experience
Cell (330) 962-2491

Measuring CFM’s Using Temperature Rise

In heating systems where airflow through the cabinet appears to be a problem, it can be difficult to determine the exact air volume that is moving through the system. In order to test this, you will first need to have verified that the manifold pressure is correct, and that the gas meter has been clocked and found to be accurate.

  1. Start the furnace on high fire and let it run for at least five minutes
  2. Take a temperature reading at the return plenum
  3. Take a temperature reading at the supply plenum
  4. Determine the difference between the two
  5. Verify the input BTU’s listed on the data tag located on the equipment
  6. Follow this formula to determine CFM’s

CFM= (BTUH Input X Efficiency) / (Temperature Difference X 1.08)

The temperature difference is found to be 40 degrees. Our BTUH input is 80,000, and our efficiency is 80%. Given our above formula, we multiply BTUH input by efficiency: 80,000 X .80 = 64,000 Temperature difference of 40 X 1.08 = 43.2. Next we divide the numbers: 64,000 / 43.2 = 1481. So our CFM’s are 1481.

Charging a System with an Indoor TXV

Charging a system with an indoor TXV is different from charging a system with a fixed metering device. TXV’s merely require a full line of liquid in the liquid line (or “subcooling”) to perform. The TXV will then factor in the current system superheat and adjust refrigerant flow accordingly.

We are regularly asked, “What should the superheat read when using an indoor TXV?” Checking the system superheat on a system with a TXV is important, but it is not a way to check or adjust the system charge. TXV’s typically run between 12 and 20 degrees superheat. The system charge must be adjusted based on the system subcooling to verify that the TXV has a charge that will allow proper operation.

A high superheat reading on a TXV system can indicate low charge, but it can also indicate airflow problems, or restrictions. Low system superheat can indicate an overcharged system, restricted airflow, or a loose/poorly insulated sensing bulb. Both low and high superheat can also indicate a failed valve, but checking the system subcooling and airflow will confirm the diagnosis.

The system subcooling is typically listed on either the data tag on the unit, or in the installation manual.

Jeff Rosenblum
Technical Support
15 Years Industry Experience
Cell (330) 962-2491

POE vs. Mineral Oil

As many of you have already noticed, all Copeland, R22 compressors are now being shipped with POE oil instead of the standard mineral oil. This has made many question installation practices for new compressors, and refrigerant usage for dry charge units.

When installing a new compressor in an existing unit that was previously using mineral oil, the entire system (outdoor coil, line set, indoor coil) must be flushed. There must be NO mineral oil remaining in the system. We recommend that you follow the same flushing procedure outlined in our installation manuals.

Even with the use of POE oil in R22 compressors, allied does not recommend or allow the use of any “drop-in” R22 replacement refrigerants. Use of any refrigerant other than R22 will affect the system capacity and efficiency, and will void the manufacturer’s warranty.

Check with the manufacturer to distinguish between compressors and units with POE and mineral oil.

Jeff Rosenblum
Technical Support
15 Years Industry Experience
Cell (330) 962-2491

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