HVAC Service

The HVAC service tips on this page specifically cover air conditioning and refrigeration equipment since that's mostly what I've worked on, but I think the principles and concepts will apply to heating equipment too.

These are some general HVAC service techniques I've picked up over the years, and I've written this page hoping these tips will help technicians who are just starting out in our trade.

If you're looking for a more detailed, step by step troubleshooting guide, you might want to take a look at some of the other pages on the site.

Start out by getting focused.

I think almost everyone would agree that sharp, clear mental focus improves HVAC service and troubleshooting efficiency, and I think that the types of questions you ask yourself while troubleshooting can affect your mental focus.

Open ended questions like "Why isn't this thing running?", and "I wonder what the problem is?" don't do much to switch on the analytical power of the mind.

They're too general, and allow open-ended and un-focused answers like "Something's broken."

On the other hand, if you start out your HVAC service calls by asking a question like "Exactly what kind of equipment is it, and what is it supposed to do?" your mind will automatically start focusing on how the machinery is put together, and it will start trying to build a mental picture of how the equipment is supposed to run when it's operating normally.

The next two questions to ask yourself on HVAC service and troubleshooting jobs would be:

"What is this unit supposed to be doing that it isn't doing, and are there any components not operating when they're supposed to, or operating incorrectly?"


"What is this unit doing that it isn't supposed to be doing, and are there any components operating when they're not supposed to, or operating incorrectly?"

These questions will automatically start narrowing the focus of your mental picture from a 'wide-angle' general picture of the equipment to 'close-up' views of the different sections of the machinery, and the individual components in those sections.

While you're answering these questions and building these mental pictures of the equipment, you'll start opening the panels to see what components are inside and start evaluating the condition of those components.

While you're inspecting the components you should start asking yourself even more tightly focused questions like "What is this?", "What does it do?", "When is it supposed to run?", and "Should it be energized and running right now?".

The schematics in the equipment, the owners manual, and factory HVAC service manuals will help you identify the components and figure out how they're supposed to work; and it's always a good idea to keep a good HVAC book in your service truck for reference.

Technicians with a lot of HVAC service experience go through the process of answering these questions without even thinking about them, but for newer techs on the job, taking a few seconds to consciously ask yourselves these questions is a good alternative to asking "Where should I start troubleshooting this equipment?".

When you open the panels you might find oil from a leak; broken piping; wires that are burned or chewed to pieces by a rat; a dead snake wound around a fan or blower shaft; a lizard or insect fried across contactor contacts; or coils and filters that are totally plugged with dirt.

You never know what you might find; and sometimes you'll even get lucky and the problem will seem to pop right out at you as soon as you open a panel.

HVAC service call tips they might not have covered in school.

If you use your imagination, you might get a few laughs out of this section, because I, and guys I've worked with, learned some of these lessons the hard way, if you know what I mean.

First, at the start of every HVAC service call, make sure you're headed to the correct customer and address.

When you arrive, verify that you're at the correct customer and address before you start working on the equipment.

If you're working on a roof or in a machine room and there are multiple remote units, make absolutely sure you're working on the right one.

You might find a condensing unit with a problem, but it might be something the owner doesn't need repaired immediately, and your service call might be for another unit.

If you go on an HVAC service call that's only supposed to be for scheduled maintenance or preventive maintenance, and the equipment isn't running when you arrive, have the customer turn it on, and make sure it runs normally before you touch it.

When you troubleshoot a unit that's failed and isn't running, personally make sure that the thermostat, or main control switch, is 'on', and set to a temperature and/or cycle setting that will turn the unit on.

Don't take the customer's word for it; visually verify it yourself.

Then verify that the breakers and disconnects are all on, and that all equipment has the correct power supply.

After a maintenance call, before you drive away, make sure the breakers and disconnects are on and that the unit will run after you've left the site.

If you're on an HVAC service call for a unit that isn't running; if you suspect the system has a leak, verify that there's pressure in the system before you start using your soap solution, halide torch, electronic "sniffer", or ultra sonic leak detector.

If you've recovered the refrigerant from a system, and are getting ready to do some brazing, pressurize the system to atmospheric pressure with nitrogen before sweating off any old parts.

Leak Testing and Evacuation

I'd like to add a couple more HVAC Service tips here about leak testing and evacuating after repairing a system.

The best way I've found to leak test a system is to pressurize it with nitrogen to about 125 psi, check the system with a bubble type leak detector, then leave it pressurized overnight.

I'll leave a pressure gauge connected to the system so I can check pressures the next day without having to mess around with any schraders or isolation valves.

I'll connect the gauge with brass fittings and copper tubing, but I won't use the hoses from a gauge manifold set.

If the nitrogen pressure holds overnight, I can be pretty sure there's no leak in the system.

After that I'll vent the nitrogen, and hook up a vacuum pump.

I use copper tubing, put an isolation valve on the vacuum pump and at both access ports, and I find one more location to attach my micron gauge, also with an isolation valve.

Then I vacuum the system until the micron gauge reads below 500 microns.

What I do is leave the isolation valve on the micron gauge closed during the evacuation, except for when I want to check the micron level.

When I want to check the micron reading I open the isolation valve and wait for about a minute, then close the isolation valves at the vacuum pump and access ports, and turn off the vacuum pump.

Depending on the size of the system you're repairing on your HVAC service call, it might take a couple of minutes for the micron reading to steady out.

If the reading steadies out above 500 microns, I close the isolation valve and continue evacuating.

If the reading steadies out below 500 microns, I close the isolation valve, wait for 10 or 15 minutes, then open the isolation valve again and check the reading.

If the reading has stayed the same, I consider the evacuation complete, and start the re-charging procedure.

If the reading has risen above 500 microns, I close the isolation valve and continue ebvacuating until the micron reading will stay below 500.

Occasionally an isolation valve, flare nut connection, or micron gauge will go bad and start leaking during an HVAC service call, so keep this in mind, and leak test the equipment on a regular basis.

If your system held 125 psi of nitrogen overnight but won't vacuum down to below 500 microns, carefully check your vacuum pump and all evacuation connections to the system.

What do I do when I'm on an HVAC service call and I start feeling stumped?

Every once in a while I'll be on a roof or in a machine room troubleshooting a piece of equipment, and I'll get to a point where I'm scratching my head and asking myself questions like "What the heck is wrong with this thing?".

When that happens, I ask myself "What would Johnny do if he were here?".

Johnny's a friend that I often worked with at a previous job, and he's the technician who taught me how to work on chillers.

He's probably the best HVAC troubleshooter I've ever known, and I've found that when I ask myself "What would Johnny do if he were here working on the equipment?"; I start feeling calmer, and I'll notice something about the machinery or schematic that I know he'd want to check out.

Whatever it was that I noticed, I'll check it whether I did it already or not; and even if it turns out not to be the source of the trouble, the confusion seems to clear away, and the rest of the service call always seems to feel easier.

If you've worked with an HVAC service technician who's a great troubleshooter; next time you feel stumped, ask yourself "What would he do if he were here?".

What operating characteristics do we look for on HVAC service calls?

Once you've started opening the panels and inspecting the equipment, what is it that's going to tell you how the unit is supposed to run?

For basic air conditioning, reach in, and walk in refrigeration equipment; the wiring diagrams on the equipment, the owners manuals, and the installation/start up manuals provide good information about how the machinery should run.

For ice machines, ice cream machines, transport refrigeration and other complicated equipment like chillers; if you want to be efficient on the job, get the service manuals.

How will you know what pressures or temperatures to expect?

Our Air Conditioning and Refrigeration System Evaluation Manual provides guidance on evaluating operating pressures, temperatures, superheat, and subcooling on air conditioning and refrigeration equipment, and there's also a cycle diagram you might find useful.

It also has service and troubleshooting information and cycle diagrams for air-cooled and water-cooled chillers.

Once again, for ice machines, ice cream machines, transport refrigeration and other specialty equipment, the best way to troubleshoot is with the factory HVAC service manuals.

Tracing out complicated wiring diagrams.

When I have an HVAC service call on a piece of complicated equipment that I'm not familiar with, and I'm starting the process of tracing through the electrical schematics; if they're not drawn in a way that takes me straight to the control I think I need to find, I'll go ahead and pick a couple of lines out of the schematic, (which ones doesn't matter), relax, and identify and locate everything on those lines just to start getting familiar with how the drawings are organized.

I've found that after I trace out these first two circuits, the rest of the troubleshooting procedure seems a lot easier.

A lot of times, while I'm tracing out the first two circuits, I'll find the circuit I was looking for in the first place, so I'll go ahead and start tracing it to the control I originally wanted to check.

Try this yourself, and see if it helps.

Electrical troubleshooting is an evaluation of four basic things:

Power quality,
Switch contacts,
And loads.

Power quality means the correct voltage and frequency for the equipment.

The conductors, of course, are the wires.

The switch contacts include every switching device in the unit: line voltage and low voltage, manual and automatic, from the breakers to the disconnect, through the thermostat, safeties, contactor auxilliaries, compressor overloads, start relays, etc.

A load is anything in the unit that uses electricity to do something: the transformers are loads; the control relay coils are loads; the fans are loads; the compressors are loads; the solenoids are loads; the water pumps are loads; etc.

Loads are controlled by switch contacts.

The job of switch contacts is to control loads.

For example:

The compressor is a line voltage load,
Controlled by a set of contactor contacts,
Which are pulled in by the contactor coil which is itself a load,
Which is controlled by a series of switch contacts that might include the thermostat, low and high pressure safeties, an oil safety switch, a time delay relay, etc.,
And these switches are probably in a low voltage circuit supplied by a step down transformer, which is itself another line voltage load.

So the basic electrical troubleshooting procedure on HVAC service calls is:

If a load is not energized, check at its terminals for correct voltage.

If the correct voltage is being supplied but the load isn't energized, or isn't working properly, it has failed.

Otherwise, if the correct voltage is not being supplied to the terminals of the load, trace through its control circuit to find an open conductor or set of contacts.

If you find an open conductor, repair it.

If you find an open set of switch contacts; determine why they're still open, and take the appropriate corrective action.

Repeat this procedure until all the controls are satisfied and the necessary loads are energized for normal operation.

And don't forget to start by making sure of the following:

The thermostat, or main control switch, is 'on', and set to a temperature and/or cycle setting that will turn the unit on.

The breakers and disconnects are all on, and all equipment has the correct power supply.

I hope this page helps, and please, feel free to contact us with any specific HVAC service questions you might have; including questions about air conditioning on Guam, or refrigeration on Guam.

Are you learning the HVAC Trade "on the job"?
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