Summer has arrived and you’ve probably already used your air conditioner. But how do you know it’s working at maximum efficiency?
Unfortunately, it’s not easy to tell if your AC is running at full spec. Even many professionals have difficulty squeezing out every last percent of efficiency, but you can certainly figure out whether there are big problems with your system.
- Start with the easy stuff and replace the air filter(s) if they’re not really clean. The more restriction to the air flow, the less efficiently the system will operate.
- Make sure you have an air tight cover on your filter port if the filter is one of the types that slide into your air handler’s housing. Why? If your air handler is in the attic and you leave that open, you can be losing 30% or more of your efficiency because of all the hot air that it sucks in through that opening.
Evaluating your cooling system performance
- Measure the air speed and temperature at each of the vents, placing the unit right on the air register. The main thing is to be consistent in placement. Let it measure until the temperature stabilizes.
- Also measure the air temperature going into your system. If you have a central air return, this is easy. You just need a general measurement of temperature. Usually this will be around 75F.
- For each air output, with a typical system, you should measure the temperature of the cool air at about 20 degrees cooler than the air going into the system. That’s called the “temperature drop.” This number varies a fair amount, but if it’s less than 15F or greater than 25F, you probably have a problem with the system.
- Get a feel for the amount of air coming out of each of the register. It should be a pretty good flow. If you find any that are much less flow than others, make a note of that. That could mean that a duct is detached or it might be shut off with a damper.
- It’s also a good idea to note any rooms that don’t seem to be getting cool. There are a number of things that could cause this – insufficient cold air flow, poor insulation, solar heating through windows, hot air leaking into the room and so on. Try to determine the cause and make a note of this. For example, if the room isn’t on the sunny side of the house and the cool air flow seems good, you’ve probably have bad insulation or a big leak of hot air from the attic.
Check for hidden duct leaks
If you’re lucky, you can check your house for hidden duct leaks. It’s pretty simple.
- Make sure all the windows and doors are closed tight and latched.
- Check fireplace dampers to ensure that they are closed too.
- Turn on all the bath fans, if you have any
- Turn on the kitchen or range fan if it vents to the outside. If you’re lucky enough to have a Jenn-air range with blower, you’ve got access to a really great blower that’s perfect for this test!
- Turn OFF the air conditioner and its fan. It may take several minutes for the blower to turn off.
- Just as you did before, check all the air conditioner vents. Since the AC is turned off, there should be no air coming out of them.
- If there is any air coming in the vent, it means the duct system is leaking air to the outside.
- Often, upstairs ductwork runs through the attic, and any air leaks there will pull very hot attic air into the house. These will be the easiest to find.
- Make note of any vents that seem to be blowing hot air. The more air that comes in the vent, the closer the vent is to the leak.
- If you find a particularly leaky vent, you might want to remove the vent and look in. I have found many that aren’t connected to the ceiling so you can see right into the attic! Or, the duct might have fallen off. A little detective work can go a long way.
What about the main unit?
So far, we’ve done things that you can do yourself to track down problems. But some things need to be done by a pro.
In general, if the system is delivering sufficient air to all the rooms and it’s about 20 degrees cooler than the air in the house, you’re probably in good shape. It still wouldn’t hurt to have the system checked out yearly, but honestly, what you’ve done so far enough to find the significant issues.
If you do find that the system isn’t cooling the air properly and you’re sure the air flow is good, then you should probably get the system checked out. But beware! If you have an AC contractor come out and he (they’re almost always male) says he need to “top it up with a pound or two of freon”, send him packing! AC systems never have to be “topped off” – they are “closed systems” so they should never use up their refrigerant (i.e. freon). Worse, these hacks can cost you hundreds or thousands of dollars in unnecessary repairs because adding too much refrigerant will damage your compressor and cause your system to run inefficiently.
One of the signs of an over-charged system is insufficient cooling of the air. So, in fact, I trust a technician far more if he says that the system is over-charged. But really, the proper way to evaluate the system is by doing a set of measurements on the refrigerant pressures and temperatures. With proper tests, they should be able to tell if there’s a problem with the system and, if so, what it is.
If your system does actually have a leak, it has to be fixed. You should not just add refrigerant because that will leak out and, if the system loses too much, you could end up having to replace the compressor. So make sure they track down the leak and fix it properly! With the proper equipment, most leaks are very easy to find.
This should get you started with your summer preparation. Stay cool!
Ted's Energy Tips 
I like how your suggestions were itemised, and they were really detailed. I think someone with an AC unit could easily follow your tips and identify a leak themselves, as the last thing they’d want is to be stranded without a unit during the hottest months of the year.