One of the toughest issues to troubleshoot is an intermittent failure. When the technician arrives, the equipment is working fine. And then when the technician is not there, the equipment does not work properly. An example of this is when a rooftop HVAC unit in winter heats a building fine during the day. However, when the building occupants arrive in the morning, they find the space cooler than the thermostat setpoint. A technician comes out during the day and the RTU furnace checks out fine. But then the next morning, the occupants again find the building space not adequately heated. What might be going on in this situation? We may have a RTU Gas Regulator Intermittent Failure.

Accurate Heating & Cooling is an established HVAC contractor. We have proudly served our commercial customers in the greater Omaha metro area for over 20 years. Call us today at 402-238-2425 and set up an appointment. Check out our A+ rating with the Better Business Bureau. Also check out what our customers are saying about us at our customer Testimonials page. We also service residential heating and cooling customers as well as geothermal units. Call us for your minisplit unit issues as well.

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RTU Supplying Furnace Heating to Building Spaces

Roof top units (RTUs) provide heating and cooling functions for commercial buildings. There are many manufacturers of RTUs. Examples of RTUs are shown in the photo below. The tan RTU in the middle of the photo is of an older Lennox RTU. The grey RTU in the upper right of the photo in the background is of a newer Trane RTU.

Older Roof Top Unit image on a commercial building

RTU Furnace Not Heating at Night But Heating Fine During the Day

In the colder months, the RTU furnace operates to heat the building spaces. The furnace turns on when the room thermostat calls for heat. This happens for example, when the thermostat setpoint is at 75F but the actual room temperature drops to say 74F. The thermostat sends a signal to the RTU furnace to start and add heat to the building. The furnace continues to supply heat to the room until the room temperature raises to 75F. At this point, the thermostat setpoint temperature is reached. The call for heat has been satisfied and the thermostat sends a signal to the RTU furnace to shut off. This cycle continues as the room temperature is maintained.

We responded to a service call for a commercial customer in the cold season. The staff reported that the indoor spaces in one area of the building were cool in the morning. The thermostat setpoint was at 75F. And in the morning when staff arrived, the actual temperature indicated on the thermostat was 65F. Yet, as the day progressed the RTU furnace started up and heated the spaces. By the late morning, the spaces were at the set temperature of 75F. When we arrived for the service call, all of the RTU furnace systems were operating normally. The RTU furnace fired up when it received a call for heat. The furnace shut down when the call for heat from the thermostat was satisfied. All other furnace parameters were operating normally.

Servicing equipment where there is an intermittent problem can be challenging. The issue turned out to be with the natural gas regulator supplying that RTU.

Natural Gas Pressure Regulator Supplies RTU Furnace

The photo below shows the natural gas line supplying a natural gas pressure regulator for a RTU gas furnace. The vertically oriented J shaped copper tubing connected to the top of the regulator is a vent line. A vent is necessary to vent the space above the regulator diaphragm.

RTU Gas Regulator Intermittent Failure from moisture intrusion

Moisture Intrusion Into Top Housing of Natural Gas Pressure Regulator Causing Issues

Some moisture had accumulated inside the top housing of the regulator above the diaphragm. During the overnight hours when it was very cold (below freezing), this accumulated moisture would freeze. Frozen water would prevent the diaphragm from operating correctly. The furnace would not run as a result of low natural gas supply pressure. The regulator was not passing the correct amount of gas pressure for the furnace to run.

A photo below shows a closer shot of the top of the natural gas regulator.

Close up image of RTU Gas Regulator Intermittent Failure from moisture intrusion

Internal Arrangement of Natural Gas Pressure Regulator Supplying RTU Furnace

The diagram below shows the internal arrangement of a typical natural gas regulator. Moisture can enter the top housing as air is exchanged through the regulator vent. It is essential to the proper operation of the regulator that the vent be open to the atmosphere. Omaha metro area winters can bring snow. It is important that the vent tubing be long enough to keep the vent protector out of accumulated snow. The vent protector is attached to the end of the vent line to protect the breather hole. The vent protector helps to keep rain, snow, foreign particles, and insects out of the regulator top housing.

Diagram of internal components of Maxitrol RTU natural gas regulator

If the above diagram is hard to read, the components are: 1) seal cap, 2) stack, 3) top housing, 4) rubber valve, 5) valve seat, 6) seal cap gasket, 7) adjusting screw, 8) spring, 9) vent connection, 10) diaphragm, 11) diaphragm plates, and 12) bottom housing. Or refer to the attached Maxitrol pdf further down in this blog.

We can get high humidity in the Omaha metro area in the warmer months. It is possible for some moisture in the air to condense inside the top housing of the regulator. A properly designed vent tube and vent protector will keep rain or snow from entering the vent. This condensed humidity from the air over time is the likely source of moisture inside the top housing leading to the RTU Gas Regulator Intermittent Failure.

Any condensed moisture inside the top housing will collect on the regulator diaphragm. Over night when the temperatures drop, any moisture inside the top housing may freeze. This will prevent the diaphragm from operating correctly. And gas pressure on the outlet of the regulator may be low enough to prevent the RTU furnace from operating.

Radiant Energy From the Sun Melts Ice on Regulator Diaphragm

In the morning, when the sun comes up, any ice in the regulator top housing will melt. Then the regulator will work well enough for the RTU furnace to operate normally. This can happen even though air temperature on the roof may still be below freezing. The radiant energy from the sun can melt the ice around the diaphragm.

But when the sun goes down in the evening, the ice reforms. The gas regulator malfunctions and the RTU furnace shuts off. The building spaces supplied by this RTU drop in temperature.

WARNING: Accessing components in the natural gas line of a furnace pose many hazards. Hazards may include the risk of fire or explosion and also the possibility of asphyxiation. Follow all safe gas line work practices and ensure that only qualified, trained service technicians perform repair activities. Follow proper lockout/tagout procedures.

Diagnosing RTU Gas Regulator Intermittent Failure

The best way to diagnose a RTU Gas Regulator Intermittent Failure is to be available in the evening. Measure natural gas pressure at the inlet to the RTU furnace gas valve. (The gas valve in the RTU furnace, not the gas regulator supplying the RTU.) This might not always be practical though. It is usually possible to remove the copper vent tubing from the threaded vent connection. A flashlight can be used while looking down through the breather hole in the vent connection. Liquid water inside the top housing can be confirmed visually looking through the breather hole.

Replace the Failing RTU Gas Regulator

Once it is diagnosed that the gas regulator is the problem, the recommended action is to replace the regulator. Taking the regulator apart and trying to repair the regulator is not recommended. Many gas regulators are constructed with the top housing riveted onto the bottom housing.

The photo below shows a close up of a typical natural gas regulator supplying a RTU. The top housing is riveted onto the bottom housing.

Close up image of RTU Gas Regulator showing rivets holding top housing to bottom housing

In addition, there may be significant labor hours involved. This would involve removing the regulator from the system. And it would require taking the regulator apart and trying to remove all the water from the top housing. The spring and adjusting screw may need to be removed during this activity. They would need to be adjusted correctly upon reassembly. All this additional labor cost would be better applied to a new regulator. Plus, the regulator manufacturer would likely advise against repairing a regulator in this manner.

Avoiding Call Backs

Other technicians that have attempted these regulator repairs report that the repaired regulator just does not operate that reliably afterwards. It could be that the ice formation on the diaphragm weakens the diaphragm. Whatever the cause, other technicians report unfavorable operation following repair of an older gas regulator. And it would certainly be considered best practice to replace the regulator and not risk a call back.

Occam’s Razor Applied to HVAC Troubleshooting

Finding a gas regulator with this mode of failure is a very rare occurrence. We might see one such instance in a hand full or two of years servicing commercial RTUs. It would be useful to keep in mind the problem solving principle of Occam’s razor when diagnosing intermittent RTU issues. This idea is attributed to English Franciscan friar William of Ockham. A variation of Occam’s razor useful for troubleshooting is that when one hears hoof beats, think horses not zebras. So an intermittent gas regulator failure should be about the last thing checked when troubleshooting a RTU furnace issue.

Image of Occam’s razor principle when diagnosing HVAC issues

For technical information on the Maxitrol regulator commonly used to supply RTU furnaces, refer to this Maxitrol Gas Regulator pdf.

Additional information about the Maxitrol gas regulator can be found at the Maxitrol website.

Additional Information

Check out our Home Page here to learn more about us. And for information on the maintenance items homeowners can perform to keep their HVAC system operating efficiently, refer to this blog.

For helpful hints when troubleshooting a failed gas valve in either a residential furnace or a commercial building roof top unit, refer to the blog for troubleshooting a failed gas valve. Once in a while we run across a RTU that uses electric heat instead of gas furnace heating. Check out this blog for an example of a York Electric Heat RTU.

Check out this blog for proper low voltage in RTUs.

There is also a blog for helping to decode the complicated meaning behind the Trane roof top unit model number. Similarly, there is also a blog to help troubleshoot the model numbers for a Trane residential furnace. For a description of Armstrong furnace model numbers, refer to this blog.

For a description of what the MERV rating on air filters means, refer to this blog.

And for a good discussion on what air filter would be best, refer to this blog.

For a description on how to control a furnace second stage heat using a time delay relay, refer to this blog.

Accurate Heating & Cooling proudly services, repairs, and installs Rooftop HVAC Units. Refer to our Commercial page for more information. Call us today at 402-238-2425 to schedule an appointment. We have served the greater Omaha metro area for more than 20 years.

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