Evaporator coil leaks are a fact of life in the industry. (We could have a long and lively discussion about how evap coils aren’t made like they used to be, but that’s for another day.) When you have an evaporator leak on a system, you’ll have two choices to fix the problem: replace the coil or patch the leak. The route you choose will depend on your company’s policies, your skill level, and the customer’s choice.

An evaporator coil replacement is a pretty straightforward procedure, but it does have a few special considerations. Patching is also not too complicated, but it requires practice and skill. We’ll take a step-by-step approach to the topic.

Before You Make the Repair

We don’t just show up to the job site, remove the evaporator, and put a new one in. Replacing an evaporator coil is a major repair and can be quite expensive. We need to make sure that we solve the right problem and take all the necessary preparatory steps to make sure we don’t get called back out in a few weeks.

Confirm the Leak

Confirming the leak on-site is easy if you’re the person who took the initial service call and performed the leak detection. You saw the leak detector go off at a particular point; all you have to do is confirm that the same thing happens in the same spot.

This step can be trickier if another technician diagnosed the leak, especially if they weren’t detailed in their service notes. You must pinpoint the leak so that you can be positive that the coil is the problem.

Even if you found a leak at the coil, check the rest of the system for signs of leakage. Potential leak points are typically easy to spot on the high side of a system because the refrigerant and oil are under pressure. Oil spots will be easier to see, and the charge will deplete faster. Be sure to check brazed joints, fittings, and valves, as those are all common leak points. Failing to notice and resolve these will lead to a frustrating callback.

Pump Down and Recover

Just like whenever we cut anything out or put anything new in, we don’t want refrigerant to travel through the lines. The best thing to do is pump down as far as the compressor will allow (you can generally pump down more on reciprocating compressors than scrolls). Then, you can recover the rest.

Recovery comes with losses and the risk of mixing, so we want to minimize those risks as much as possible by letting a pump-down do the heavy lifting.

To do that, start by closing the liquid line service valve. You can usually do this by popping the top off and using your service wrench with a hex key or adapter to close the valve. Then, pump down and watch the pressures drop. When they get to about 10 PSI, shut the system off at the disconnect, close the suction line service valve, and recover whatever is left in the line set. Keep at least a little bit of positive pressure on the system; negative pressure will allow air to come in, and that can introduce non-condensables and moisture.

Considerations for Factory-Installed Filter-Driers Inside Condensers

Because moisture is such an issue in climates like ours (Florida), we usually like to cut out any outdoor liquid line filter-driers, replace them with straight pipe outdoors, and braze new ones in near the indoor air handler. That’s a pretty straightforward process.

However, some HVAC systems come with liquid line filter-driers installed inside the condensing unit. When it’s installed before the liquid line service valve, we can’t remove it without losing the refrigerant in the system. In those cases, recovering the entire charge is best. Then, we can cut out the filter-drier, straight-pipe it, and install a new one indoors.

Hooking Up Core Removal Tools and Flowing Nitrogen

Once you get down to around 1 PSI while recovering, you can hook up your core removal tools, remove the cores, and attach a nitrogen tank. Flowing nitrogen while you take the evaporator out keeps air and oxygen from coming in. 

Oxygen turns into black scale (cupric oxide) at high heat, which it will encounter when you braze in the new evaporator. Air consists of nitrogen and oxygen but also water vapor, which can contaminate refrigerant oil as it moves through the lines. Flowing nitrogen at the same rate you’d use for brazing (3–5 SCFH) should be plenty. 

Replacing the Coil

After taking care of the refrigerant, we can remove the old evaporator coil. As with other major repairs, like the compressor, it’s best to cut the evaporator out. Cutting is safer than unsweating because oil residue can catch fire. 

You can pull the coil straight out once you’ve cut it off from the line sets. Since TXVs and drain pans can break during the replacement process, our techs at Kalos usually just replace those along with the coil. When you attach the drain line to the pan, use a quality pipe dope on the tubing threads; we like Nylog White by Refrigeration Technologies.

When you slide the new coil into place, use a level to make sure it’s in there properly. If there are any retaining clips, make sure those are in place. Once the coil is in place, you can braze it in, along with a new liquid line filter-drier (and any straight pieces of copper tubing, if needed). Follow all the usual brazing best practices:

• Purge air from the lines and then flow nitrogen
• Cover the TXV and other heat-sensitive parts with a wet rag or WetRag
• Heat the base metal and draw the alloy all the way into the joint

What About Patching the Coil?

Patching the coil is a somewhat controversial alternative to replacing the whole evaporator coil. For the record, patching is okay under certain circumstances. You just have to exercise good judgment, be careful, and make sure the customer understands that there’s a chance it won’t be successful (or last a very long time). 

Patching has the lowest risks on the low side of the system (which is where the evaporator is, at least on a straight cool system), as there will be smaller fluctuations in temperature and less vibration than on the high side. You just have to be careful not to burn through the tubing (since the thickness and metal type are different than the typical copper tubing we’re used to brazing), use a ductile alloy (like 15% silver solder), and avoid drawing the filler metal into the tubing.

You can solder aluminum on a U-bend or feeder tube, or you can even cut some of the fins away to get to the tubing within the coil to help an evaporator last longer. We have a tech tip on the indirect heating technique with a 9-panel diagram.

Testing and Wrapping Up

Then, we end with the typical service procedures: nitrogen pressure test (with a liquid leak reactant on the joints), pull a deep vacuum, and perform a decay test. Since this is a repair, pulling below 500 microns and making sure the system doesn’t exceed 1000 microns within 10 minutes will suffice.

Once the decay test has been passed, release the charge and let the system run in cooling mode for 15–20 minutes. Take all of your typical readings:

• Superheat
• Subcooling
• Suction pressure
• Head pressure
• Delta T (temperature split)
• Static pressure
• Delivered capacity in BTU/h (if you can)

If you have measureQuick, that process is very easy. It’s also a good idea to clean the drain and make sure the customer is left with a system that’s running well and should continue to do so for a long time.

Clean everything up, finish your paperwork, make sure you didn’t leave anything behind, have a closing conversation with the customer, and go on your merry way. That’s how you repair or replace an evaporator coil.