Humming AC

Outdoor AC compressor is about 9 years old. I removed the shutoff switch and the fan stopped, but there is a loud hum to the unit. I have read that this is normal for a heat pump, but this is not a heat pump. When I went inside and turned off the thermostat, the humming stopped. Any thoughts?

Probably the the relay contacts which run on the low voltage wire. Sometimes they’re noisy.

Is this an issue to be referred to an HVAC company, or is it normal operation?

Run relay was pulled in by the thermostat.

Should I recommend evaluation/repair by an HVAC company?

A buzzing noise is typical.

It perhaps is typical, but is likely a symptom of rust or other corrosion buildup on the relay armature (not the contact) which keeps it from closing all the way. This reduces the closing force on the contacts and, over time, will lead to contact failure. When I find that type of condition, I recommend addressing the issue at the next annual service visit. By the way, the problem is not limited to heat pumps as the same type of relay is used on A/C compressors as well.

P.S. - should this happen at your own home, the problem can be resolved by lightly sanding the armature mating surfaces or just replacing the relay - they are pretty cheap anyway.

Unusual noise, recommend evaluation by a licensed mechanical contractor.

Mike do you always kill the shutoff switch?
How long do you leave it off before returning power?

Why the hell would you shut it off???

Guess you don’t check for overheated conductors, line/load or grounding???

I’m not an electrician, just a generalist home inspector.
2.5. Cooling I. The inspector shall inspect:[INDENT] A. the central cooling equipment using normal operating controls.
II. The inspector is not required to: A. determine the uniformity, temperature, flow, balance, distribution, size, capacity, BTU, or supply adequacy of the cooling system.
B. inspect window units, through-wall units, or electronic air filters.
C. operate equipment or systems if exterior temperature is below 60 degrees Fahrenheit, or when other circumstances are not conducive to safe operation, or may damage the equipment.
D. inspect or determine thermostat calibration, cooling anticipation, or automatic setbacks or clocks.
E. examine electrical current, coolant fluids or gases, or coolant leakage.

II. The inspector is not required to:
[INDENT] A. insert any tool, probe or device into the main panelboard, sub-panels, distribution panelboards, or electrical fixtures.
B. operate electrical systems that are shut down.
C. remove panelboard cabinet covers or dead front covers, if they are not readily accessible.
D. operate or reset overcurrent protection devices or overload devices.
E. operate non-accessible smoke detectors.
F. measure or determine the amperage or voltage of the main service equipment, if not visibly labeled.
G. inspect the fire or alarm system and components.
H. inspect the ancillary wiring or remote control devices.
I. activate any electrical systems or branch circuits which are not energized.
J. inspect low-voltage systems, electrical de-icing tapes, swimming pool wiring, or any time-controlled devices.
K. verify the service ground.
L. inspect private or emergency electrical supply sources, including, but not limited to: generators, windmills, photovoltaic solar collectors, or battery or electrical storage facility.
M. inspect spark or lightning arrestors.
N. inspect or test de-icing equipment.
O. conduct voltage drop calculations.
P. determine the accuracy of labeling.
Q. inspect exterior accent lighting.


So your answer would be “NO” and your excuse is that it’s not specifically spelled out in the SOP. Curiously, I don’t even see checking for the presence of a disconnect means in the SOP, aside from the main service disconnect. Does that mean you don’t report if there is no disconnect at all for the AC condenser, electric water heater, furnace/air handler?

Please forgive me, as I use the TREC SOP vs. NACHI so I’m not nearly as well versed in all of the things you are NOT required to do.

I do routinely check for the presence of a disconnect. I also check for equipment ground connections, signs of conductor overheating and line/load orientation. With most enclosures that does require pulling the disconnect handle.

I am not sure it is a good idea to be pulling fuses and tripping mains to see if they are functional as part of a Home Inspection and have never seen anyone around here do so.
Texas may be different .

I tend to be with Linus on this ,though not worded as strongly.
Put this in do so at your own risk as much as pulling breakers out of every panel.I simply raise the flap and check for damage if it is running.

Same here, I don’t pull fuses/breakers. The equipment may not turn back on. What do you do when you shut down an A/C unit on a 90+ degree day and the 90 year old owner is without A/C?? I just dished out $3400 for an A/C unit for my mother-in-law in Florida because she had a $35/visit handyman check it out.

Agreed. That is above and beyond what we should be doing.

If you do not pull fuse blocks at AC disconnects then how would you find things like this?

Is this not a reportable condition?


Electrical Disconnect Aluminum Foil on Fuses.jpg

Nice picture ,but by the same logic do you dig a trench to view the foundation footing?

Do you drill test holes every 16 inches to check for insulation levels?

Do you open each and every wall plate to make sure the conductors are attached properly?


Come on…be realistic 2 of the 3 things you mentioned are destructive testing and you know we do not do that.

Pulling the fuse block (when system is off) and checking fuse capacity and wiring may be beyond your SoP’s but what about standard of care? Do you pull the dead front off a main or sub panel? If yes, then why not on a disconnect panel?


These devices are a means of disconnecting electrical power. They are intended to be operated. Why is operating them such a big liability?

If the 90-year-old lady’s disconnect doesn’t work when you test it, there is a high probability that it may fail or do something like burn the house down!

Exercising a fuse block that fails is simply “failed during testing”.

If you don’t pull it apart, you can’t tell if there’s anything wrong.

You’re required to open electric panels. However you are not required to open a panel that is “hot”! So how do you open the panel if you don’t turn off the breaker?

I know that a lot of people take the dead front cover off the panel without shutting off the main, but just because we do it doesn’t mean it’s right. If you take the dead front panel off and start a fire from a deficiency inside the panel, are you not liable because you didn’t disconnect current to the panel before opening it?

OSHA regulations provide exceptions when the panels can be opened without shutting them down, home inspection in residential settings does not appear to be in their exceptions.

Please enlighten me on this misperception…