6 Ways Your Oversized HVAC System Makes Things Worse
Whether you live in a house, an apartment, or a condo, your HVAC system is probably oversized. In fact, it’s not uncommon for residential systems to be 2 or even 3 times larger than they should be (although they don’t typically tower over the house like the one above). I’ve covered this problem here for 16 years now, but today I’m going to lay out 6 problems with oversizing. Before scrolling down, how many can you think of?
Higher upfront cost
This may be the most obvious problem with an oversized HVAC system. Bigger costs more. A 4-ton heat pump doesn’t cost twice as much as a 2-ton heat pump, but it’s definitely gonna cost you more.
What do you get for that extra cost? Heating and cooling capacity you’ll never use. That’s called throwing money away. But in the new homes market, that money got thrown away before you came onto the scene.
Oh, and the additional cost of the equipment isn’t the only extra money you have to pay. There’s more down below, so read on.
Poor humidity control
When you run the air conditioner in a humid climate, you’re looking for two results: cooling and dehumidification. Dropping the temperature of the air is the easy part. An oversized HVAC system helps you do that even faster, but at the cost of worse dehumidification.
Why is that, you ask? Because dehumidification occurs when the air passes over a cold coil…and then does it again and again and again. You need a lot of runtime to wring that moisture out of the air. And long runtimes are NOT something you get from systems that are oversized.
Comfort problems
Poor humidity control creates one of the comfort problems I’m referring to here. When the humidity won’t drop, people often lower the thermostat. That often has the result of making the house cool and clammy, not comfortable.
But the other side of the comfort issue has to do with runtime in a different way. If a system is twice as large as it needs to be, it’ll run half the time when the house is at its design conditions. And it’s blasting the house with twice as much air flow and twice as much heating or cooling as it needs.
The result is short cycling. The system kicks on and blasts you with warm or cold air. Then it goes off. In a house with lots of air leakage, insufficient insulation, and bad windows, the house starts cooling off in winter or heating up in summer as soon as the system goes off. It doesn’t take long before you notice.
Bigger ducts needed
The duct problem is a really interesting one. A larger HVAC system has a higher air flow rate. That means it needs bigger ducts. OK, so do you have space for large enough ducts? And are they designed properly to move the right amount of air?
If your ducts are big enough, that’s the second area where you pay more. Bigger duct systems not only have larger individual ducts but also they probably have more fittings. Every extra piece of ductwork means more cost for materials and for labor.
Unfortunately, most of those homes with oversized HVAC systems did NOT get properly sized and designed duct systems. Actually, that’s one reason so many contractors oversize the systems. They need a bigger blower to move the air through ducts that are too restrictive because they’re too small.
This one’s a damned if you do, damned if you don’t scenario.
More registers and grilles
Related to ducts needing to be larger, you may also end up with more and larger grilles and registers throughout the house. In our HVAC design work, we generally add another register to a room is the air flow needed is greater than about 150 cubic feet minute (cfm). (But it depends on the type of register and other factors.)
I’ve been in some homes that have far too many supply registers. How many do you have?
Noise
When you have a bigger system and thus a bigger blower, you’re likely to have more noise, too. Do you have to turn up the TV every time the heating or cooling comes on? Do parts of your house rattle when the oversized HVAC system kicks on?
Sometimes we get used to things like that because it seems like that’s just how it is. But no, it doesn’t have to be like that. A right-sized heating and cooling system with a well-designed duct system should be quiet.
In some areas, bigger is better. You’ve got a much better chance of making it to the NBA if you’re 6’6″ rather than 5’6″. And if you’re chasing a 25 foot great white shark, “you’re gonna need a bigger boat.” But when it comes to heating and cooling, an oversized HVAC system just isn’t worth it.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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This problem is so common. And yet: 80 – 90% of homeowners replace systems only when they stop working. There are at least four big problems with this: (1) They use the first company who will show up, and if more than one show up, they default to the cheapest option; (2) There is no real code oversight for replacing units in existing buildings, so system size is assumed without Manual J calcs to be like for like, embedding the oversizing problem for the next 15 – 20 years; (3) There is a missed opportunity to improve the building enclosure (which is the baseline condition for system efficiency and sizing) — and the problem gets worse and more complicated if they do building enclosure improvements after installing a new, oversized system; (4) 99% of the contractors I’ve worked with will ignore the V in HVAC, in terms of potential filtration improvements, combustion equipment efficiency and indoor air pollutants, or fresh air ventilation. These issues are so frustrating — and don’t get me started on how much worse they can be for small commercial buildings, where HVAC is designed for maximum building occupancy rather than actual occupancy along with the above issues.
Monica: Some other aspects of the system replacement scenario can make it even worse. First, some contractors assume that the existing system is too small so they put in a larger, even more oversized system. Second, even with the same nominal system size, they may increase the amount of oversizing by not considering actual capacities. A 3-ton 14 SEER system, for example, probably has more capacity than a 3-ton 10 SEER system.
Bryan Orr wrote about a case where this happened and the house got way worse. It’s been a few years, so I don’t have the link.
Yes! We’ve seen this happen, as well. And then there is the issue of an older, lower SEER system being better at dehumidification (capacity being part of that picture). Bigger is not better. Wasn’t it you who said “The most expensive single stage system is an oversized variable speed system”? Can’t find that link — maybe it was on LinkedIn. And then there is ductwork leakage / configurations … and shiny tech … and the perils of homeowner google search results … and more. We need a building science to become the normal first move for any building owner looking to spend $$ on home equipment, envelopes, renovations or additions.
Monica: I’ve been spreading that message for a long time but in different words. Adam Mufich of the National Comfort Institute is the one who put it like that, and I put that quote in another recent article:
18 HVAC Design Mistakes in Custom Homes
https://www.energyvanguard.com/blog/18-hvac-design-mistakes-in-custom-homes/
Allison,
As an Atlanta resident I wanted to ask if you remember whether it would is cost effective to convert a 3-ton gas furnace to an equivalent sized heat pump using the existing duct work. I’m having to replace the gas regulator on our 13 yr old furnace and the HVAC system (R410?) is +/- 3 yrs. Has the pricing between heat pump vs nat gas w/AC converged?
Monica, You needn’t be concerned about “embedding the oversizing problem for the next 15 – 20 years” (sarcasm). The poor customer will be lucky to get 9-12 years with oversized equipment coupled with undersized ducts (Don’t ask how I know this). I have lived it personally and observed it far too often for other people. Live in that dwelling long enough under those conditions and you will effectively buy an extra one or more whole new systems. That gets very expensive over time.
Any advice for those with an oversized HVAC system on how to run it more efficiently? Speaking as someone with an oversized single stage heat pump.
Robert: For humidity control in summer, you need runtime. For runtime, you need cooling load. So do lots of baking, open the windows, use incandescent lights… Just kidding.
You don’t have a lot of options. You can play around with setbacks while you’re away to get more runtime, but that can help only so much. For cooling, you may need a dehumidifier for the part-load days.
And it depends on what problem you’re trying to solve. For noise, you can make things quieter with duct modifications.
I’ve had success with using a thermostat that gave me control of the temperature delta between set temp and ambient temp. I went with an Ecobee, which at the time was one of the few with this amount of user control.
If the set temp was 72 with a 2 degree delta setting, then the system would not kick on until the ambient was 74, then it would run until it was 72 for a longer stretch, resulting in better dehumidification and improved overall comfort. I think most thermostats have a default of .5 degree delta before they kick the system on, a lot of the short cycling comes from this.
Air volume makes a big difference. I just replaced my 5-ton system (after 12 years) with a new 5-ton system from a different manufacturer. The old system was whisper quiet but in the winter time the bathrooms were always cold, and wife and daughter used small electric heaters to feel cozy. With the new system the bathrooms are warm, and I have power savings (no elec. heaters) but when the heat comes on, the air roars just like that garbage truck driving by the house on Tuesday morning. How can there be such a huge difference between two 5-ton systems on the same ductwork? For this new system I got quotes from 4 different vendors, each vendor quoting a different HVAC brand, but details about blower size and resulting air volume are nowhere specified. How can a homeowner find out about what he is going to have in his crawlspace? Is it going to be a purring kitten or an accelerating diesel truck?
Edmund, One possibility is that the replacement system is running heat with a much higher blower speed than your prior system. That would account for better bathroom heating accompanied by a higher noise level. While a lower fan speed would reduce noise you are limited to keeping the temperature rise within manufacturer specifications to avoid system damage such as with the heat exchanger. Talk with the installing company about your situation. My own system had three possible in spec choices for heating speed and the builder’s HVAC company had set the highest speed which was ridiculously noisy which of course the builder could not hear (over the noise?). After fighting to get the oversized system replaced with something closer to real need, I was lucky with having the latitude to lower the fan speed. As you noted though, air volume matters and even if it is safe to lower your fan speed you could lose sufficient heating to the bathrooms. That brings us back around to the other demon of potentially undersized duct work. Even if the HVAC guys recognized undersized ductwork, they are usually loath to say antything about it for fear of losing the job to a competitor who does not care about it. Also, I don’t know whether you have duct dampers and whether those settings were altered during your HVAC replacement. It is complicated which is why some feel that HVAC is the most complex home system to properly design. There is no real excuse for bad design given the multiple software solutions out there to support good design but the HVAC companies around me vastly underutilize those tools even when they own them. My builder’s HVAC company only used good software to be able to submit municipality required Manuals J, S and D. The so called designs for my house were a nice work of entertaining fiction produced by that old computer adage of garbage in garbage out (GIGO). A sad state of affairs in the industry which is all too common. Sorry to say that only professional help can address your situation if that work is outside your skill set.