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Tech Tip: Come On! You Can Just Add a Ton, Can’t You?

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An extra ton? Have you had a customer ask that? I have, and my answer has typically been no. Some of the bad stuff that comes with oversizing is short cycling, lack of moisture removal, and whether the existing duct system can handle the extra air that will/may be required. Consult Manual J – Core Training – Section 2 for more reasons not to oversize. We should always avoid oversizing, but it can be even more critical in humid climates where air conditioners operate with a wet coil. However, occasionally, I said yes. But the customer had to agree to a few things first. 

So, Should We? 

Are there specific circumstances that justify oversizing? I believe there are, but a few specific things must fall into place to technically justify it. 

Let’s start with why they want that extra capacity. If it’s because their brother-in-law Bob has a bigger unit, I’m typically going to explain that more runtimes will produce a system that functions better than an oversized system that will give less-than-desirable results. I usually must do a song and dance to get them to understand (maybe whip out my crayons), but I would say most of the time, I am successful. 

Sometimes It’s Okay 

Here’s a better scenario that you might be more familiar with. I have a customer who has a lot of parties, and the house lends itself to having people congregate in a common area. We’re going to call the common area the kitchen/great room, which is on the first floor of the home. Just for this story’s sake, let’s say they have a footprint of 2,000 square feet and a 2½-ton air conditioner that is correctly sized for this space. When I say correctly sized, an accurate load calculation was performed, and proper equipment was sized and selected where we met the sensible load, the latent load, and didn’t exceed the total by 15%, as required in Manual S (Table N2-1).  

Bigger is Not Better 

Could we simply add more capacity to the existing system? Yes. Would I be content with the performance out of that system? Probably not. A larger than necessary system will perform poorly on a design day and even worse on a part-load day. I want to avoid that. Adding capacity to the current system will create more problems than it fixes. However, there is a path to provide them with the additional capacity they desire without ruining our paddle and raising the dew point in the house. Let’s consider an alternate path.  


Let’s Add Some Equipment 

I’ve done this, and both the occupant and I were happy with the results. It’s only been within the last year or so that I became familiar with the Florida Solar Energy Center study FSEC-RR-646-16 on adding a mini-split to an existing home. The study had a different objective than I had in mind (install a high-efficiency mini-split to reduce the energy consumption of the existing central HVAC system and “do no harm”). In short, the results of the study were positive. I was quite pleased when I read that the research used math instead of opinion. It acknowledged that adding capacity via an additional piece of equipment kept the main living area comfortable and didn’t raise the dew point in the home. This strategy does have flaws: added cost, installation complications (line sets, line voltage, drains, etc.), and another system that requires maintenance. But, if the customer is serious about adding capacity and understands what’s involved, it’s a solid path. Remember to always look at the expanded performance data of your chosen equipment to ensure the system will have the moisture removal capability you expect. Don’t rely on hope; rely on math. If you are unfamiliar with this study, I recommend reading it. 

And That’s the Way I See It 

In my opinion, selecting a system with additional capacity that would be spread out amongst the entire first floor would be a mistake. Adding additional capacity with a separate piece of equipment that targets the area where those extra bodies will congregate is something I know works.  

In Conclusion 

If you run into a situation where somebody really wants more capacity because they allow too many people into their house (or whatever their reason is), this is a strategy that I would strongly consider. I am a fan of addressing the homeowners’ concerns as long as we keep controlling humidity in mind.  

Ed Janowiak
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Posted In: ACCA Now, Print Edition, Technical Tips

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