The Most Expensive Fixed-Capacity HVAC System
It didn’t come naturally for me to use my hands when I talk. I had to teach myself to do that, and it has come in quite handy. (Did I really do that?) When I’ve spoken about the mismatch between heat pump capacity and house load, for example, I’ve often used my hands to demonstrate it. Now, I’ve made a diagram to illustrate this mismatch. Before we take a look, though, let’s go over the basics of how a variable capacity system can become a fixed-capacity HVAC system.
I’ve written and spoken about this issue a lot. Probably the best article to read on this topic is Can You Oversize a Mini-Split Heat Pump? I titled the article that way because a whole lot of people who ought to know better say, “You can’t oversize a mini-split. It has variable capacity so it can throttle down to meet the load.”
But how true is that? Now we’re ready to look at the diagram.
Load and capacity mismatch
The diagram below shows the problem. Let’s say you install a 6,000 BTU/hr mini-split in a bedroom. It can operate at its full capacity of 6,000 BTU/hr and can throttle down to a minimum capacity of 1,500 BTU/hr. That’s the beige part of the diagram. But that heat pump serves only one bedroom, and that bedroom doesn’t have much load. Even in extreme weather, it needs only 500 BTU/hr of heating or cooling.
See the problem? If the most that room ever needs is 500 BTU/hr and the heat pump that serves can never provide less than 1,500 BTU/hr, you get no benefit from the variable capacity. Because it never varies! It will always be bottomed out. It may have the capability to operate between 1,500 and 6,000 BTU/hr, but it never uses that capability because it’s stuck sending 1,500 BTU/hr to a room that always needs between 0 and 500 BTU/hr. That’s why the diagram shows what I called the No-Go Zone.
What do you call variable capacity that doesn’t vary?
Huh! So, you paid extra for a variable capacity system that behaves like a fixed-capacity HVAC system. Adam Mufich of the National Comfort Institute put it better than I ever have. Here’s how he framed it:
Sizing matters! I’ve been hitting this topic pretty hard lately because the more I do this kind work, the better I understand how oversized so many systems are and how much better homes perform with properly sized heating and cooling equipment.
The good news is that you don’t have to depend on the HVAC contractor to size your system. Yes, some do a great job with HVAC design. The majority, however, do not. But third-party HVAC design is becoming a lot more accepted for homes. We do it at Energy Vanguard, and we cover the whole United States. Other companies are also doing it. A few are Positive Energy and OneThirty in Texas, TE2 Engineering in Massachusetts, and Judy Rachel in California.
I hope the diagram above helped make this concept clear. I’d love to hear any feedback you have about it because I’m always looking for better ways to explain things.
And with this, now I don’t have to wave my arms around to try to make the point…although I’ll still do that, too.
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.
Note: I used the Claude AI to make the diagram.
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It’s Hard to Undersize a Heating System
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So, I’m running four zones on four levels through a branch box, powered by a 4 ton cold climate inverter compressor – three wall heads and a 2.5 ton airhandler. The minimum on the compressor is 2 tons. the inside units are all variable capacity. if only a half-ton wall head is asking for heat or cooling, what happens to the excess capacity of the compressor?
The outdoor unit will run for a short period at its minimum capacity, then cycle off once the load is satisfied. This will repeat itself (short cycling) until there is no further call for heat or until other loads appear which are sufficient to equal the minimum capacity of the outdoor unit which allows the system to operate continuously. The short cycling results in inefficient operation of the system and increases wear and tear on the compressor.
So the only way around this would be multiple smaller compressors.
Perhaps. If that 1/2 ton load needs heating a lot by itself before the other zones call for heat, it might have made sense to put that zone on its own small variable capacity unit. Single zone ductless systems are generally more efficient and have better turndown than larger multizone systems. BUT……..the first cost of that approach is higher. You have to weigh the first cost savings of your approach against the energy penalty cost (which may not be easily quantifiable). The contractors don’t typically install the most efficient systems. They are usually competing on the lowest cost system, which is often not the most efficient.
Of course, during the course of a year, all variable mini-splits are in the no-go zone some portion of the time, especially in the shoulder months. Proper sizing will eliminate some of this, but it is still there. The trick is to not oversize, so less time is in the no-go zone, and not undersize, so peak demand can be handled without sacrificing too much comfort.
My biggest peeve in HVAC design is systems not being available small enough. 9k in a minisplit is way too much for many projects. Outdoor air requirements on a 200 square foot office with a 2 MBH load only makes it worse. The LED lighting, energy code envelope, and efficient construction has made loads drop to the point of challenging equipment selection.
In our home office we have my PC and the better half running hers playing games when she is between trips. In the winter I have to open the window because it gets toasty in that room.
Her PC and multiple monitors and mine as well they are both dumping around 3000BTU/hr each system. Both computers/monitors are sucking down allot of power and computers are pulling more power every generation which means more heat. The BTUs are from doing the math of electric usage of each unit.
I have ended up moving my office to the shop as it gets too hot in the office in the summer and in the process of adding a second minisplit out there. If we don’t end up moving in the near future we will be moving the office to a larger bedroom and putting in a minisplit. More people have home offices in a bedroom however no one is doing anything in the cooling systems to accommodate the heat produced by them as they still treat them as bedrooms.
I’m in TX and does oversizing exist, yeah. Is it an issue in our harsh environment not really. If you are down on the coast or other more temperate temps but high humidity then it’s more of an issue. Here where it’s 100*+ during the day 90s at midnight and just barely hits 80* as the sun is coming up there is no such thing as too much cooling capacity.
And those shoulder months of october through feb we are running heat. We have a little summer on either side of misery season and the cooling efficiency of those times don’t matter in the whole scope. Other parts of the country keep on about what about the shoulder seasons. Well we don’t have the same kind you guys do.
I disagree – the most expensive fixed capacity system is more likely an oversized conventionally ducted central variable capacity heat pump such as Carrier Greenspeed or Trane XV.
Those systems’ installed costs are often even higher than variable mini / multisplits, and they often have lower turndown ratios
A timely blog post. As a HERS rater, I don’t think I have ever seen a properly sized mini-split. Yesterday I was rating an 8 apartment building sandwiched between two buildings. There were few walls with ambient exposure. Each apartment is less than 800 SF. They all had two heads totaling 19k Btu.
Builders frequently say they can’t sell anything with electric heat. I hear many complaints of excessive electric bills using mini-splits. Does anyone have experience with properly designed and sized equipment. I am in Massachusetts, climate zone 5. I would like to know if there is any basis for complaints with properly sized systems.
Most HVAC contractors don’t do actual load calcs (man J). They size stuff by rule of thumb (eg 25 BTU/SF). They generally oversize to avoid call backs. They don’t pay the utility bills. Then if the owner complains about the bill, they say “I told you this was a bad decision. You should have gone with gas like I told you”. As an energy coach (and retired HvAC engineer), I’ve seen plenty of bad installations, especially ducted system converted from AC. But I’ve also seen systems that work well. The problem is, it’s got to be engineered properly starting with load calcs, proper equipment selection, and attention to detail on the installation. Unfortunately, that’s a lot to ask for from the run of the mill contractor who just wants to move his crew on to the next job.
what do you charge to review a heating and ac system plus erv system for new construction? mark
Marc
I don’t do this professionally anymore. If you want a professional review, hire a HERS rater. If you would like an informal review, I’d be happy to review and comment at no charge
mark: We tried to send an email to you with some info, but it bounced. If you go to our HVAC design pages at the link below, you can fill in the form and we’ll get in touch with you.
https://www.energyvanguard.com/hvac-design/