"I Received My Load Calculations Back... Should I Upsize The A/C?"

So you got back your load calculations (Manuals J + S) from Johnstone's design department. The designer specifies a 2.5-ton condenser, which shows a capacity covering 110% of the load. But wait, you're an eagle-eyed professional and you notice that this home was modeled with an indoor temp of 75°F in cooling mode. That's ridiculous! Nobody keeps their house at 75. It should be at most 72, right? Maybe you should install a 3-ton A/C instead, just to be safe. Well hold on a minute. Let's dive into this a little deeper and figure out what's going on under the hood.

 

"Why is the indoor temp 75?"

This might seem like a mistake by the designer. Maybe Logan fat-fingered a 5 instead of a 2? Actually, this setting is a product of code. Michigan Energy Code states:

N1101.11 (R302.1) Interior design conditions. The interior design temperatures used for heating and cooling load calculations shall be a maximum of 72 degrees F (22 degrees C) for heating and minimum of 75 degrees F (24 degrees C) for cooling.

In fact, there's an even stricter limitation on initial thermostat settings (although this can be changed later, should the home owner desire)

N1103.1.1 (R403.1.1) Programmable thermostat. Where the primary heating system is a forced-air furnace, at least one thermostat per dwelling unit shall be capable of controlling the heating and cooling system on a daily schedule to maintain different temperature set points at different times of the day. This thermostat shall include the capability to set back or temporarily operate the system to maintain zone temperatures down to 55°F (13°C) or up to 85°F (29°C). The thermostat shall initially be programmed with a heating temperature set point no higher than 70°F (21°C) and a cooling temperature set point no lower than 78°F (26°C).

Michigan’s Residential Mechanical Code also requires HVAC sizing in accordance with ACCA Manuals J and S (i.e., the aforementioned "heating and cooling load calculations").

N1103.6 (R403.6) Equipment sizing and efficiency rating (Mandatory). Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.

 

Therefore, if we're performing these manuals per code requirements—which we at the design department always strive for—75°F is the lowest we are able to model for cooling.

 

"Well should I just upsize the A/C then?"

I'm afraid it's not quite that simple. Code has some things to say about this too—or more accurately, code requires sizing in accordance with Manual S, which has its own specifications. Section N2.3 specifies that single-speed cooling equipment (meaning single-speed compressor; your blower motor does not affect this) have a capacity with a maximum of 115% of the calculated total cooling load (in most cases—there are exceptions to this). This means that 3-ton A/C is going to be outside of Manual S parameters.

 

Now you certainly wouldn't be the first contractor to oversize an A/C, and if the inspector doesn't ask for your Manuals J + S or they don't verify that it's sized in accordance with Manual S specifications, you might get away with your unit being a little oversized. However, this approach is still technically not code, and it wouldn't be without its drawbacks either. Oversizing an air conditioner can come with some notable consequences:

 

1. Airflow
   • A bigger A/C will require more airflow across the coil. More airflow means bigger ductwork, more/bigger diffusers and grilles, and in some cases, force you to a larger air handler with a bigger blower motor.
2. Cost of install
   • Take all the factors from above, increased labor costs to install the duct, and higher cost of the equipment itself, and things can quickly add up to make your bid less competitive or cut into your profit margins.
3. Shorter run times
   • Remember, we only operate at design conditions a few days a year. Most of the cooling season, the actual load is going to be even smaller than the load calculated. Since the A/C is larger than the home needs—and even more so in moderate conditions—the thermostat's call will be satisfied faster than with a smaller unit, causing shorter run times. Short run times can have several adverse effects:
     • Shorter run times mean the evap coil doesn't get as cold, which means less moisture from the air will condense, and we'll have less dehumidification.
     • Because starting a compressor or blower requires more energy than keeping it running, shorter run times can increase overall energy consumption (and the compressor will already draw more energy than a smaller one). Higher energy consumption → higher cost of operation → unhappy homeowners.
     • Short cycles mean more frequent cycles, which means more wear and tear on the equipment and can decrease the lifespan of the A/C and the air handler.

 

"OK, I hear you, but I still need to meet my customer's expectations. What do I do?"

Well, I'm with you 100%. Nobody wants an unhappy homeowner. Your reputation drives your business and you are a top-tier contractor who takes pride in a job well done. We want to enable you to do the best job you can do and to make money doing it—so what are our options? Well, here are two approaches to look into:

 

1 – Consider a condenser with a multi-stage or variable-speed compressor

Remember when I said that the sizing maximum of 115% applies to single-speed cooling equipment? Well, 2-stage equipment gets a higher max ratio. You can size a 2-stage A/C up to 125% of the calculated total cooling load. That might give you enough wiggle room to bump up half a ton and still stay within Manual S parameters. You can now hang in even the most straight-laced jurisdictions. The larger unit will cool to that nice 68°F indoor temp that your customer wants. Is this a good approach? Let's see:

 

Pros:

• Meets code requirements (the requirements of ACCA Manual S, which code requires by reference).
• The unit can stage down when loads aren't as high, increasing run times and decreasing energy consumption.
• You get fast recovery from upsizing the unit, or a little extra power when there's a party going on and the house is packed with people.

Cons:

• You pay a bit more for the equipment.
• The ductwork and blower still have to be sized in accordance with the required airflow at full load.
• You still have a higher maximum amp draw and breaker size.

 

2 – Add a dehumidifier

If we size the A/C for the full load as normal, per Manual S, but then we install a dehumidifier, we can effectively decrease our latent load. In Michigan, latent load can account for 25–30% of the total cooling load. By removing humidity, we are effectively increasing our total system capacity without increasing the size of our A/C. This means we can keep the smaller ductwork and blower, keep our inspector happy, and give our customer greater comfort.

 

Pros:

• Code compliant.
• Prevents increased ductwork sizes.
• Increased humidity control—even if there's not a call for cooling.

 

Cons:

• Takes up some room in the mechanical area.
• If the sensible heat ratio is high (i.e., the home won't have a lot of humidity), it might not be the right fit. This would be a pretty uncommon scenario, though.

 

In summary

Code gives us guardrails for how we can and can't calculate loads and size equipment—but with some creativity and know-how, we can design systems that meet our customers' expectations, meet all the requirements, and set you apart from the competition with quality work and experience. Conversations need to be had with homeowners and your code officials to ensure each project is designed properly.

 

- Logan Dickinson