Apparently, I haven't written a single article this summer about oversized air conditioners. I've written several in previous summers, mostly about how HVAC contractors bypass the HVAC design process by using rules of thumb and how they mess up the Manual J load calculations when they do go the preferred route. I've never written an article focusing on the reasons to size an air conditioner properly, though, so here it is. When an air conditioner runs, it does two jobs. It lowers the temperature of the air, and it removes moisture from the air. To do the second one, the AC has to run for a while. As the air passes over the evaporator coil, it encounters a very cold surface. Ideally, when the air passes over that cold surface, the air temperature drops about 20° F. In places where we have higher relative humidity in the summer, the other important process that happens when the air hits the coil is that the temperature of the coil is below the dew point of the air. As a result, water vapor condenses on the coil.

Here's the thing, though. Water vapor condenses on coils in oversized air conditioners, too, but you have to get enough condensation on the coil for the water to start dripping down into the pan below the coil. Even then, you're not there yet. You still have to get enough water in the pan for it drain to the outside. Until the water that condenses actually makes it to the outside, you haven't really dehumidified the air. Because that water on the coil can evaporate and get back into the air in the home. If you make the mistake of leaving your thermostat in the Fan-On position instead of Auto (Don't do that!), the water on the coil gets back into the air even quicker. Oversized air conditioners don't run for a long time because they satisfy the cooling load quickly and then shut off. Properly sized air conditioners run longer, so if you want your air conditioner to dehumidify your home as well as cool it, don't let the HVAC contractor oversize it. If you live in the desert, what I just said above doesn't matter to you.

There's no water vapor to condense on the coil, and if there were, your dew point is probably close to absolute zero. (Well, OK, that might be a bit of an exaggeration.) The second reason definitely should matter to you, however. The thing that wears equipment out is starting up and shutting down. The more it happens, the shorter the life of the equipment. When an air conditioner is oversized, it starts up and shuts down a lot more because it runs for only a short time to meet the thermostat setpoint. Then a few minutes later, it comes on again and runs for a short time. Over the course of a day, an oversized air conditioner can have a lot more start-ups and shut-downs than a properly sized air conditioner. That means you'll probably be repairing it more often and replacing it sooner. People in the industry used to think you'd save money on your air conditioning bills with a properly sized air conditioner, but that thinking has changed. John Proctor wrote an article for Home Energy Magazine that shows only a small savings for the homeowner (but the utilities benefit by lower peak loads).

You should save money on the upfront cost, though, because you're putting in a smaller AC. (As Proctor points out in his article, oversizing isn't always the worst problem, though. You have to be a subscriber to Home Energy Magazine to read the article, but it's well worth the read if you can get it.) For new homes, use the full HVAC design process, which starts with Manual J, the heating and cooling load calculation protocol from the Air Conditioning Contractors of America, the trade association for AC contractors. split ac two indoor units indiaFor existing homes, the best way to do it is to see how long your current air conditioner runs when it's at the design conditions. 2 1 2 ton trane air conditionerOf course, you want to do this while your AC is still in good shape, not when it's on its last legs. car ac repair tomball tx

If it runs for only 5 to 10 minutes before shutting off, it's definitely oversized. If your runtimes are over half an hour at design conditions, it's probably sized close to the actual cooling load. If you're building a new home, remodeling an existing home, or just getting a new air conditioner in an existing home, ask your builder, remodeler, or HVAC contractor how they're planning to size the air conditioner. If they tell you they're basing it on the size of your house, don't let them do it. There's a lot more to it than square footage of conditioned floor area, and of course, I haven't touched at all on the capabilities of different types of equipment (single speed, fixed capacity, multi-stage, mini-splits...). More on that in future articles. Oversized AC, Screwed-up Manual J, ENERGY STAR HVAC Tirade!Don't Assume It's Correct. We Are the 99% — Design Temperatures & Oversized HVAC Systems An Oversized Cooling System Isn't Always a Bad Thing: Creating default object from empty value in on line

Rough Pricing for Budget Purposes Indoor Packaged HRU w/Controls Air Cooled Scroll Chiller 10-190 Tons Air Cooled Screw Chiller 175-530 Tons Air Cooled Condensing Unit 6-140 Tons - Scroll 30-200 Tons - Screw 130-190 Tons - Centrifugal 150-3000 Tons - Magnetic Bearing 150-550 Ton - Standard Efficiency 3-35 Tons Applied Rooftop 15-135 Tons Unit Ventilators - Vertical Floor Mounted - 750 – 1500 CFM w/o controls - 750 – 1500 CFM with controls Unit Ventilators - Horizontal Ceiling Mounted Water Source Heat Pump – Water to Air - Heat Pump Systems 3 – 20 tons - Heat Recovery Systems Please note that the above prices are for quick budget purposes only and will not be considered contract pricing. Please request a quotation for more specific pricing on your project.Of course, there are more than 3 reasons that your 3 ton air conditioner isn't really 3 tons, starting with the fact that it's not 3 tons in weight.

That unit refers to cooling capacity and harkens back to the days of ice. I'm also not talking about any of the multitude of reasons having to do with improper design, faulty installations, or lack of maintenance, topics that I discuss plenty in this space. No, today I'm going to tell you that your 3 ton (or 2 ton or whatever size you have) air conditioner may not be what you think it is even when everything's designed, installed, commissioned, and maintained perfectly. David Butler wrote about two of these reasons in a guest post on ACCA's Manual S protocol for selecting HVAC equipment two years ago, and that's a great article for understanding some of the subtleties. So, what are these 3 reasons? The first reason is that when we talk about air conditioner capacity, we're usually giving the nominal size. A 3 ton air conditioner has a nominal capacity of 36,000 BTU per hour, but the actual rating using the operating conditions specified by AHRI is rarely the same as the nominal capacity.

For example, the AC shown in the AHRI certificate below is a 3 ton air conditioner (36,000 BTU/hr) with an actual capacity of 2.8 tons (34,000 BTU/hr). As David Butler discussed in his article on Manual S, AHRI ratings are done for an indoor dry bulb temperature of 80° F and indoor wet bulb temperature of 67° F. ACCA recommends using an indoor design temperature (dry bulb) of 75° F and relative humidity of 50%. That's closer to the actual conditions that most homes actually operate at than AHRI's conditions. Let's think about the temperature difference and see what effect that might have on the cooling capacity. Which way do you think it would go if we bring cooler air into the air conditioner than it was rated for? Well, let's frame that a little differently. Is it harder to cool cooler air or warmer air? The answer is the former. The lower the temperature goes, the harder it is to remove more heat from it. Just ask the folks at the Microkelvin Laboratory at the University of Florida, where they get about as close to absolute zero as is possible.

If it's harder to cool air at 75° F than air at 80° F, then that means the 2.8 ton air conditioner above isn't even going to be 2.8 tons. To find the answer here, you have to factor in the humidity levels, too. As David wrote, it's a moving target, but the net result of AHRI's operating conditions is that your air conditioner's capacity is lower than it's rated (unless you keep the thermostat at 80° F or higher). AHRI uses 95° F as its outdoor test temperature, so if your outdoor cooling design temperature differs from that, your AC capacity will again vary from the AHRI rated capacity. In this case, we get a little of that lost capacity back here in Atlanta. Our design temperature is 92° F, which means that the air conditoner has an easier job of dumping heat into the outside air than it would if it had to dump the heat into 95° F air. If you live in Tucson, Arizona, with a design temperature of 103° F, however, your 3 ton air conditioner has now dropped in capacity again.

It's just harder for that refrigerant to give up those BTUs to air that's hotter. It's important to remember that the three reasons above don't have anything to do with poor design, installation, commissioning, or maintenance. Plenty of other factors related to those issues also affect capacity: These are not good reasons to oversize an air conditioner! The main thing to be aware of is that you need to know more than just the result of the Manual J cooling load calculation. A Manual J report may say you need a 3 ton air conditoner, but for the three reasons above, you might really need to install a 3.5 ton unit. That's why Manual S, the equipment selection protocol, is so important. The thing is, though, that even with these issues that mostly reduce the capacity of your your air conditioner, most air conditioners still end up oversized. I recently heard someone say that thumbs are great things, but it's good to recognize their limitations. Having opposable thumbs allows us to write a letter, examine a mulberry, and hold a glass of beer, but no matter how great they are, they can't design HVAC systems.