How (not) to size an air conditioner


Just about done; properly sized and correctly installed – a good installation is a source of pride for the contractor and long-lasting comfort for the home’s occupants.

By Glenn Mellors

How not to size an air conditioner?

Why for every 1000 square feet you need one ton of cooling – any fool knows that! If I had a dollar for every time, I heard that I would be rich, and if it were true, I would likely be out of a job!

If you are like me, you find it very frustrating when HVAC professionals have no idea how to properly size an air conditioner. Yes, I said it! At a meeting not that long ago hosted by Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI) and attended by some larger HVAC contractors, I was shocked at the number of them that said that their sales representatives did not perform or worse did not know how to calculate the proper size air conditioner for a building. Even worse, some didn’t even care.

This was at a time when government rebates are built around performance and energy savings. The proof became evident that after several attempts to improve efficiencies, grid load has not lowered one bit, meaning little to no energy saving has been had and millions have been handed out in rebates to consumers. Fail!

If you are like me, you find it very frustrating when HVAC professionals have no idea how to properly size an air conditioner.

One wise man shared with me a theory on evolution and indulgence. He said that if people were convinced that what they did was more cost effective that they would do it more often because it was cheaper. How that plays out in our industry is the same – when people try to conserve, they will tolerate cooler or warmer temperatures depending on the season, but if told that their new appliance will use half the energy, then their tolerance for uncomfortable conditions becomes less: “Honey, the new air conditioner uses half as much energy so there is no need to turn it off at night.” Couple this with the improper sizing of the equipment and we have….no energy savings.

A heat loss calculation provides a solid starting point.

In this article I am going to talk about factors that determine sizing and proper efficiencies and I will let organizations like HRAI teach the actual headload certification courses. Today we have many computer programs to do load calculations for us and many times a simple shell load is all that is required

The perils of “rule of thumb”

A few years ago, I had a group of comfort advisor trainees on a field trip to the Toronto Home Show, a place where they could experience first hand some of the finest sales techniques in our industry. They were instructed to act as a homeowner and to inquire about cooling their cottage up North (anywhere). The cottage was four seasons and was approximately 1,400 square feet. If more information was needed, it has an east/west exposure, still more information if asked, chalet style, glass floor to ceiling.

The outcome – several quotes on 18-24k Btu air conditioners, some with comfort guarantees and money back promises. None asked more than the square footage. One refused to quote on the spot but did offer an obligation free on-site evaluation and quotation. After the day at the zoo, as we called it, we did a manual J heat load calculation… 3.5 tons of cooling required. It grabbed their attention for the need to do a calculation on every site.

Things that go bump in the dark

When we assess a home for air-conditioning, we need to look for the hidden factors. I am not talking about building materials such as brick, siding, windows, two-pane or three, insulated or argon, shaded or not, the direction they face, doors, patio doors, ceiling insulation etc., but the other stuff.

Some of the things that can affect sizing and performance are exhaust fans, HRVs, heat sources such as pilot lights on fireplaces (some get used in the summer too), abundance of appliances, televisions, hot tubs, lighting and additions. Additional bedrooms mean more people load.

The contractor can further fine-tune AC capacity through evaporator choice.

Every bedroom has the potential for two occupants and you should size accordingly. Radiant in-floor spot heating is another environmental condition that can put you in an undersized condition. And finally, ask questions about lifestyle!

By asking questions you will find out if the system you prescribe is capable of meeting the expectations of the homeowner. Many times, when I have been called in to assist in a homeowner complaint, I find that there was a misunderstanding of needs and expectations not clearly explained. You are better to say no before you do an install than yes just to make a sale.

Here’s another fallacy: “I don’t worry too much about sizing an air conditioner as they only come in 6000 Btu increments anyway” Horse pucky! Anyone who knows me knows what that means!

Air conditioning is nothing more than a highly efficient dehumidifier; the byproduct is cool dry air. And no, they are not like car engines where bigger is better! With today’s technology, you can fine tune an air conditioner to deliver almost finite amounts of cooling!

Tuning the AC unit

Once we have selected the condenser unit based on our heat gain calculation, either single stage, dual stage or now variable flow, we can now start fine-tuning the capacity by choosing an evaporator coil. This allows us to narrow down the sizing. Next, we can choose the thermal expansion valve (TXV) or fixed orifice refrigerant meter. I prefer the TXV; it helps to ensure I am getting the maximum dehumidification by altering refrigerant flow based on ambient temperatures. This allows the coil to be equal temperatures over the entire coil surface. The byproduct is more coil surface to extract the moisture out of the passing air.

The last thing that you can tweak to achieve maximum efficiencies is to match the airflow of the system to increase the dehumidification.

Some, if not all, VSM style air handlers can set the fan operation to ramp up the airflow slowly at first to allow the coil to get up to temperature faster. This allows for the evaporation cycle to start earlier in the cycle. The opposite occurs at the end of the cycle to allow evaporation to take place as the coil starts to warm up.

A TXV valve provides precise refrigerant control that adjusts to ambient temperature.

The old days

Remembering back to my childhood, I recall my grandparents saying that air-conditioning makes people sick. They would say that auntie so and so has one and they could hardly breathe when they came out of the house and auntie so and so always had a cold. I now know that auntie so and so had too large of an air conditioner and the house was now a walk-in cooler, low on temperature and high on humidity

Here are a few fast facts to keep in mind:

  • Prescription without diagnosis is malpractice.
  • An air conditioner can be finely sized.
  • It is our responsibility to size and set up accordingly.
  • A home properly equipped will be comfortable at higher temperatures in the summer.
  • Humidity control is the key to indoor air quality.

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