Proper Sizing of HVAC Systems

Proper Sizing of Heating and Cooling equipment is essential to indoor air quality, comfort and energy conservation.  What does this mean?

First let’s get a few acronyms out of the way. A unit of energy is commonly referred to or defined in terms of British Thermal Units, or BTU’s. Another common term used to define the systems that maintain a home’s living space temperature is Heating, Ventilating and Air Conditioning, or HVAC. Now that we have that out of the way, let’s get to why size matters.

For every 12,000 BTU’s needed to maintain your home’s indoor temperature, you need 1 ton of air conditioning (AC). So, if you live in a home that has one AC unit that is 5 tons, then theoretically an HVAC professional calculated that your home needed 60,000 BTU’s to maintain comfort. The million-dollar question is whether that professional took the time to really calculate the load, or did they simply use a rule of thumb approach. Worse yet, did they upsize your unit by one or even two tons to give you some “extra capacity” to ensure you would be able to stay cool. Since every ton more typically costs about $1500, extra capacity equals extra profit.

A study done by Pacific Gas and Electric Company, the large utility provider found that 53% of air conditioning units examined were a full ton oversized. So, over half the time the HVAC professional put in too much and the buyer paid for it. The cost factor is one problem, but it’s more than that.

When AC units are oversized, they run in shorter cycles, as it doesn’t take as long for the unit to cool the home until the thermostat turns the unit back off. This is known as Short Cycling. The problem with short cycle times is that one of the primary jobs of the AC unit is to dehumidify the home.   Air conditioners are very inefficient when they first start operation. It is far better for the air conditioner to run longer cycles than shorter ones. The efficiency of the typical air conditioner increases the longer it runs.

The ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain.

For short cycles, the coil does not have time to operate at the low temperature and when the unit stops, the moisture on the coil evaporates back into the indoor air. Thus, in humid climates, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units. An oversized air conditioner cannot remove enough moisture from the air and a home can become cold and clammy.

In addition, mold needs water to grow. Humidity provides water in the air and the more humidity you have in a home the more mold can prosper. When a home is over 65% relative humidity or above mold has an environment to grow. As the humidity rises the environment only improves for mold growth. So, not having an AC unit run for longer cycles is more than just about energy or comfort. It’s also about health.

Given all that we know about indoor air quality and comfort, it’s amazing that the problem of over sizing AC equipment is so pervasive.   The Air Conditioning Contractors of America (ACCA) have published the standard for performing load calculations, known as Manual J. When you consider that every house built is unique and that every choice in the house affect efficiency and BTU load, you can imagine how challenging such a calculation can be. As a result, it’s not surprising how many contractors use the rule of thumb approach. There are a number of software programs that have been created to make this potentially complicated process much easier for HVAC contractors. However, as demonstrated in the PG&E study, these tools are all too often not used and the homeowner pays the price in energy costs, installation costs, comfort and health.

This problem only grows as standard construction requirements of our building codes continue to emphasize more efficient structures. Today the wall and ceiling insulation minimums and minimum efficiency ratings on HVAC equipment have all been increased. In just the last 10 years the increases in the building codes alone have stepped up minimum efficiency ratings of AC units by 30%. Add to this the emphasis in the market place and the tax credits for programs like Energy Star, and the problem only grows further, unless those builders and HVAC contractors are keeping up with their expertise on how to size their equipment.

In our homes we use third party engineers to calculate our loads. Since they are separate from our HVAC contractors, there is no conflict of interest. That’s a good way to go. In fact we recently had a quote come in a full 2 tons more than required. It was a Geothermal home with full foam insulation and a number of other upgrades. We told all of this to the contractor but still the quote was 2 tons high. Since it was a Geothermal HVAC system (which are about 2 to 3 times more in upfront cost than standard equipment), the savings to the customer of proper sizing was $8,000 in upfront installation cost. Since the quoted unit was 6 tons and the actual requirement was 4 tons, the savings long-term on monthly operation is also significant.

A little knowledge goes along way. If you are considering building a new home, ask the right questions. It isn’t just about how pretty the finish is. If you are going to make one of the largest investments of your life, shouldn’t it be more than just pretty? Shouldn’t it operate as good as it looks?