January 23, 2024 | By Ian McTeer
Part of every HVAC/R tech's list of skills includes finding and repairing those elusive leaks.
In the grand global comedy of life, there exists a peculiar quest that’s as elusive as a unicorn at a magic show: the hunt for leaks.
Whether it’s water gushing from a pipe, refrigerant making a surreptitious escape, gas slipping through uneven threads, or steam staging its own great vanishing act, finding and repairing these elusive culprits is an everyday activity for HVAC/R technicians—the job is literally about playing hide-and-seek with invisible actors who are masters of the art of ‘fix me if you can.’
It’s a task that leaves us wondering if we’re actually chasing leaks or just training for the world’s most bizarre detective agency.
So, grab your soap, leak detector, nitrogen bottle, bright flashlight and prepare to embark on a journey into the leaky labyrinth that tests the wit and patience of those who dare to tread in the realm of the watery, chilly, gaseous, and steamy unknown.
Let’s face it right off the bat, leaks are inevitable for a variety of reasons. Liquid or gaseous commodities, especially those contained in pressure vessels, will eventually leak out mainly because no known metals or plastics will last forever.
If it weren’t for leaks of water/condensate, steam, gases, and even heat—or stretch the concept to electricity leaking from energized components to ground—an HVAC/R technician’s life would be all peaches and cream every day; well, maybe that’s taking it a bit too far!
And the trouble is, not only do leaks of these commodities cause equipment breakdowns and poor performance; leaks can too often be life-threatening at worst, and/or at least cause significant property damage.
Thus, our job is clear, we must design and manufacture products less likely to leak, and we need to install them in such a way that leaks are nearly impossible during the equipment’s lifespan.
Water and Condensate
The images in Figure 1 (below) show examples of ruinous damage to gas furnaces from acidic condensate leaking from defective heat exchangers, draft inducer connections and overflowing cooling drain pans.
The value of yearly servicing cannot be overemphasized as many of these leaks can be detected and repaired before an expensive repair or equipment changeout is necessary.
The images in Figure 2 (below) illustrate the type of property damage overflowing condensate can do. In this case, condensate from a gas furnace was pumped to a small wash basin near the furnace. The basin drain plug was inadvertently closed causing condensate to overflow the basin and run underneath the partition wall into a finished adjoining room.
Interestingly, an infrared camera was used to detect the extent of the water damage. It should be noted that an infrared camera is not intended to detect water leaks, but it can be useful since the wet or water-damaged materials have a different emissivity rate causing a temperature difference that the camera can detect.
Condensate soaked into the framing material, drywall, baseboard, and carpet.
The image in Figure 3 (below) is just a reminder that small leaks can become larger, more worrisome, leaks.
And don’t forget leaks in piping systems and components related to fossil fuels such as natural gas, liquid propane (LP) and even fuel oil. In most residential buildings, natural gas, piped in at 7-in. w.c. (just 0.25 psi) can still leak through a poorly made flare, an uneven pipe thread, or pin holes in fittings; I found the older lubricated gas cocks would often leak a bit after using it to isolate a gas fired appliance.
Detection methods vary, however, soap bubbles worked well for me over the years. Of course, gas or fuel leak complaints must be detected by the best means available and repaired without delay.
There are many ways a building can leak more heat than the designers or even building codes call for, often related to construction inefficiencies. Recently, I was amazed by a video I watched in which a passive house under construction, but replete with doors, windows, vapour barrier, taped joints and caulking everywhere (but not yet drywall boarded), failed a test in which white smoke pumped into the building leaked out with reckless abandon from dozens of small holes here and there.
And what about pipe insulation?
Figure 4 (below) shows a house with a recently completed mini split heat pump system installed with indoor heads connected by insulated copper tubing running outside of the building.
I snapped an infrared image of the cladding, and it seems the piping system is losing too much heat.
It would have been better to run the line set inside the house, obviously at greater cost and complication, or perhaps a second smaller unit could have been used to reduce the overall line length and resultant heat loss.
Is this a source of heat loss we should be paying more attention to?
I have written in the past, back in the day when “venting to atmosphere” was the accepted practice for removing refrigerants from a system, that too many HVAC service personnel would automatically add refrigerant to a system that demonstrated poor performance related to low suction pressure.
Connect the gauges first thing and, on an R-22 system for example, read a suction pressure of 58 psig. This condition was often a signal to the poorly trained servicer that the system needs more gas; the pressure should be higher!
Well, yes the pressure should be higher, but no, the first step in diagnosing poor performance is to inspect the air handling side (look for duct system air leaks too) using your senses: listen for odd sounds, view obvious symptoms, and feel for obstructions. A bright flashlight is very helpful too.
Be sure the system has a leak! Look for a plugged air filter, dirty evaporator coil.
What’s the condition of the blower wheel? Are the zone dampers working properly? Ductwork restrictions or blocked supply or return registers and grilles?
Any one or several of these system deficiencies can cause low suction pressure, even low head pressure and low compressor amp draw.
Once put right, system pressures should return to advertised recommended values as determined by the equipment manufacturer.
The Usual Suspects
Sometimes, a refrigerant leak can be spotted easily, often by the presence of compressor oil at the leak site. However, leak locations can still be difficult to pin down: does the presence of multiple oil stains on the coil in Figure 5 (below) mean several refrigerant leaks, or did the coil get contaminated with oil from some other source?
A leak from somewhere in the tube bundle of an evaporator or condenser coil typically requires replacement of the entire coil, although sometimes a repair is possible.
In the case of Figure 5, an electronic leak detector would likely be able to pinpoint the leak location in this coil should one exist. However, in the case of a system completely devoid of refrigerant, a nitrogen test may not produce easily detectable soap bubbles.
Should this coil be replaced, or would it be best to continue searching for leaks elsewhere?
The usual suspects when it comes to leak locations are just about any joint in the system, be it a brazed joint or a mechanical connection.
Field fabricated brazed joints or flare connections are highly susceptible to leaks, and too often the cause is improper preparation of the site. For example, the flare in Figure 6 (below) leaked because the flare is too large, and the tubing was not reamed, leaving a ridge that could not properly mate with its male connection point.
Sometimes refrigerant leaks can be found in brand new equipment at factory-made connections that somehow made it out of the factory.
The equipment might have been damaged in transit or may be suffering from excessive vibration making vulnerable joints more likely to leak.
The refrigerant leak in Figure 7 (below) was discovered during the commissioning of a new unit. This joint will be tricky to repair as it’s near a metering valve, too easily damaged by the misapplication of heat, especially when said repair is left to a poorly trained or novice service person.
Factory caused leaks are rare. However, manufacturers are responsible for their assembly line operators and must be sure to properly train them for the fabrication of any refrigerant bearing equipment.
System access ports, the classic Schrader valve, are meant to seal themselves once the gauge is removed. Sometimes the Schrader valve continues to pass gas, or liquid, even after the hose is disconnected.
In such cases, the valve must be replaced using a valve core tool. Schrader valves are somewhat flimsy and can blow out of the port (happened to me once) or they can start to leak for whatever reason.
Schrader access ports are protected by a valve cap, the brass acorn nut being the gold standard of leak protection, in my opinion.
It is important to understand that brass is ductile and can be easily distorted, so overtightening of the valve cap can ruin the mating surfaces.
Scratched or dented mating surfaces, or caps containing debris, will also not seal properly; perhaps a drop of modern-day sealant on the threads, like Nylog Blue, might help, otherwise, the entire valve assembly will have to be replaced to ensure a proper seal.
Brass caps with rubber gaskets are more likely to leak and should be avoided simply because the rubber will disintegrate over time. Figure 8.
Soap and Water Verification
Techs are still using soap bubbles for leak detection, most often to verify a leak location that was already determined by various electronic devices commonly used today.
It is important to know the limitations of electronic detectors like ultrasonic, heated diode, or infrared devices.
False positive leak indications caused by the presence of other chemicals might lead to unnecessary, expensive repairs. After one’s best efforts, a system leak will often defy soap or electronic detection methods.
In such a case, the tech must double down; for example, with a spilt system, one technique is to isolate the indoor unit from the outdoor unit by cutting the line set outside.
If any charge remains, be sure to weigh it out. By installing an access port into the liquid line of the tubing running indoors and brazing the suction tube closed, the line set, and indoor coil can be pressurized with nitrogen independently of the outdoor unit.
After an hour or so, one side will likely demonstrate a loss of nitrogen pressure allowing the servicer to confine leak search activities to the appropriate area of the system.
Finally, air or other contaminants can leak into a system. Once a refrigeration system passes the crucial nitrogen pressure test, a deep vacuum must be performed ensuring non-condensable gases and moisture are removed to levels that meet industry standards and manufacturer’s instructions.
For example, a manufacturer might require the system to be evacuated to 300 microns, then with the pump isolated, the vacuum must not decay above 500 microns within five minutes.
If 300 microns cannot be achieved, or the vacuum decays quickly beyond 500 microns, this typically means the system is not yet dehydrated, or an undetected leak exists.
Remember, hoses and fittings can also leak. Be sure to use a vacuum rated manifold and hoses; standard test and charge manifolds and hoses are not good enough for deep vacuum.
In the end, any system will leak some or all of its gaseous contents regardless of your best efforts.
Even though metals are typically impermeable to gases, there can be defects, allowing tiny gas molecules to pass through the gaps in the crystal structure of the metal or through any defects such as cracks, holes, or grain boundaries.
However, the rate of leakage can be extremely slow in most cases and won’t be noticed over the entire life cycle of a properly installed and maintained heating or cooling system.
Our industry still needs your best efforts every day even though the invisible actors directing the grand global comedy of life would rather set those molecules free. <>