After more than a decade spent exploring Atlanta’s residential HVAC scene, we’ve seen just about every problem you can imagine. Big problems, small problems, unusual problems, trivial problems, expensive problems… You name it, we’ve seen it.
And oversized systems are among the most common HVAC problems in our area.
In fact, it’s unusual that a week goes by that we don’t encounter a system that’s too big for the house it’s tasked to cool and heat. The problem is that widespread.
But wait. Isn’t a bigger HVAC more powerful and, therefore… better?
That’s a common misconception. No, a bigger HVAC system isn’t better. There are actually proper tonnage (air conditioning) and BTU (heating) sizes for every home. Bigger HVAC equipment isn’t better because…
- Oversized air conditioners don’t dehumidify very well. All that extra tonnage cools your house down very quickly, so the AC cycles are short. Longer AC cycles are more effective at clearing out humidity. Shorter ones aren’t. So even though that oversized AC brings your home down to 73 degrees really quickly, it has a hard time keeping your relative humidity below 60%. The bottom line? You’re hot, sweaty, and uncomfortable – even though the thermostat says you shouldn’t be. Oh, and subpar dehumidification leads to subpar indoor air quality.
- Oversized furnaces turn your home into a scorch chamber. In winter, you want consistent heat throughout your home. A properly sized furnace can give you that. An oversized one blasts you with scorching hot air and satisfies the thermostat right away. As a result, you have hot and cold spots throughout your home. And if your bed is right under a vent, you feel like you’re sleeping inside a volcano.
That’s why oversized HVAC systems are best avoided. They actually make you less comfortable – not more.
So, why are so many systems oversized in spite of these crippling problems? Let’s explore the most common reasons.
Most HVAC salespeople don’t understand HVAC design.
Being good at selling HVAC equipment doesn’t make you good at selling the right HVAC equipment.
Here’s how things usually go down. An HVAC salesperson carries a price book from an equipment supplier. The book includes matchups for different system components – ACs, furnaces, and coils. If you consult the price book, you might find that a 3-ton AC pairs with an 80,000 BTU furnace.
Ok, fine. The thing is, that same 3-ton AC might also pair with a 60,000 BTU furnace. You can go either way, but the option isn’t always reflected in a supplier’s price book.
What furnace size makes the most sense? Well, the HVAC company should perform a load calculation to determine what size is best for your home. This calculation is called a “Manual J.” Salespeople don’t usually do load calculations, though. They just replace what you’ve got based on rules of thumb (more on those in just a sec) or they look at the price book and say, “Ok, an 80,000 BTU furnace it is.”
So in the Atlanta area – and you can file this under “nice to know” – most 3-ton air conditioners don’t need to be paired with an 80,000 BTU furnace. Unless you live in a super-leaky house with no insulation, typical heating loads don’t require that much power.
And yet, it’s what a lot of people have. Why? Because of the matchup in the price book. Or because the salesperson thinks you should replace what’s already there.
So, how should the salesperson go about system selection? Here’s what we do:
- Perform a Manual J load calculation. This is a fairly intense process and involves measuring several things around your house – square footage, orientation, number of windows, insulation levels, and more. For accuracy in our Manual J calculations, we usually perform a blower door test as well. After we’ve gathered the data, we calculate the perfect tonnage/BTU pairing for your home.
- Use Manual S to determine the best matchup. Depending on the load requirements, one matchup will be more advantageous than the others. Manual S shows which one makes the most sense. There might still be sizing issues, but they won’t be as severe. Besides, we can usually eliminate them when we…
- Present you with options that mitigate sizing issues. For the hypothetical 3-ton system we discussed, Manual J might show us that even a 60K BTU furnace is overkill. But the 60K BTU pairing is the smallest furnace that works in a 3-ton matchup! Thankfully, it’s not that big of a problem. When this happens, we can suggest other options – like a two-stage gas valve – that ensure the system doesn’t run at full capacity when it doesn’t need to. That way, you avoid most problems associated with oversizing.
Easy, right? It actually is. This approach should be the foundation upon which honest, effective, reliable HVAC service is built. Sadly, it’s the exception rather than the norm.
There are lots of 3-ton/80K BTU systems in Atlanta homes. Maybe you’ve got one?
Adding insulation saves energy because it helps your home retain (and avoid gaining) heat depending on the season. The same goes for crawlspace encapsulation, air sealing, and, to a lesser extent, window replacement. These projects help you save energy, and that’s a good thing.
Until it isn’t. Perform enough of these upgrades, and your home won’t require as large of an HVAC system. That might sound good, but it can lead to problems: