I would hazard a guess than 20% of systems in new homes are within 25% on cooling capacity. That’s impressive progress since Energy Star first starting pushing proper sizing to builders in 2006. And distributors in some markets are telling me they’ve experienced and responded to increased demand for smaller equipment compared to 10 years ago.

Indeed, demand for mini-splits has skyrocketed over the last decade and US manufacturers are scrambling to participate through branding partnerships and even acquisition (Daikin/Goodman). That said, US manufacturers have been reticent to compete with Asian mini-split mfrs in product development in sizes less than 2 tons.

But where the equipment is being developed and manufactured isn’t what’s constraining the high performance home market. It’s the difficulty of getting hvac contractors on board with proper design, and/or getting home buyers & builders to see the value of 3rd party design. That’s is why we’re still seeing markets where smaller equipment isn’t stocked.

Change is hard…. which is why I see the progress that HAS been made as a glass half full rather than half empty.

There’s still one area where manufacturers could do better. We need smaller furnaces! I believe Dettson is the only manufacturer that offers furnaces with less than 40k capacity (it’s smallest model modulates from 6k to 15k). I don’t think many contractors are even aware of Dettson, but more than that, it’s difficult to get folks to break out of what they’ve always done.

The cost difference between sizes is relatively small, and to truly get it right, the duct system may need to cost more. But system cost isn’t the only consideration driving this…

There’s a perception among builders, reinforced by hvac contractors, that bigger is better, and the corollary fear that smaller may lead to comfort complaints. The reality is that comfort complaints are rarely due to under-sizing, and in fact, comfort is enhanced by proper design.

There’s also the perception that design costs associated with right-sizing exceeds equipment savings. While that’s often true in the narrowest sense, there are larger economies to be had from proper design when the entire system (i.e., the house) is on the table. My clients nearly always spend less on their homes than what they would have spent had they followed the status quo approach.

What’s the cost difference? I have no idea.

Availability of small enough equipment is mainly a problem for super-insulated homes and multi-unit buildings. For typical homes in most of the country, 1.5-2 tons is not grossly oversized.

There is a company in Canada, Dettson, that sells smaller split systems. I don’t know why they don’t get more attention. I didn’t choose them due to concerns about availability of local contractors to service them. Do contractors that participate on this forum have reasons for not offering them?

Humidity is DEFINITELY a huge factor. Installed a new, smaller sized split system in a semi leaky house, and immediately noticed that a) my condensate water collection has about doubled; b) indoor air temp is very comfortable at 77 vs 72-73 with the old system; c) the house must be drying out as after 20 years there are nail pops in places they never were, which I don’t quite understand because I would think the low winter humidity would be lower than the summer conditioned humidity…

Not even close. My comment (re: pressurizing in a cold climate) is basic building science. I would say your approach would is more akin to a science experiment. Good luck with that. See caveat noted in my previous comment.

We seem to be running across a lot of homes with air quality problems. Since most homeowners can’t completely gut and abate their homes, our approach has been to design solutions that fit client problems and budget. This means abate where feasible, tighten the snot out of the house, and pressurize. We really like to pressurize with a ventilating dehumidifier as it allows warmer comfort temperatures which in turn means *warmer surface temperatures* (and, of less import, nice energy savings).

In tracking relative humidity to health events, there appears to be correlation to adverse events when humidity levels above 50%, even when pressurizing.

We like the idea of pressurizing with dehumidified air as we suspect this increases drying potential to building components that these houses may never have previously seen. This is quite possibly due to oversized air conditioning and wintertime humidification.

Yes, spot ventilation is critically important. The observation about extended exhaust I thought might be helpful as it is one most people are unlikely to run across. After all, how many people here are continuously monitoring client home temperature, rh, indoor air quality, and health events?

🙂 –

Regarding your hypothetical “generally bad idea” – If you let every project become a science project, the search for perfection can become enemy of “great,” and nothing ever gets done.

Rule of thumb prescriptive hypotheticals from a building science book are good to understand so you don’t make catastrophically dumb mistakes, but you simply can’t remove 100% of all risk, unless all you do is armchair hypotheses. I know retrofit sales and design are not your wheelhouse, so you may have to just take my word on this.

And since we’ve started tracking indoor metrics, it’s become obvious these houses are too wet. Lots of them, too wet. Full Stop.

And even properly sizing the air conditioning, tightening the houses, and blocking moisture in the basements doesn’t solve the problem a lot of the time for some, and all of the time for any. Humid climates often have latent load with no sensible load, they even sometimes have latent cooling needs and heating load!

And in one recent situation the health problems had the client worried they were on the path to financial ruin. Those problems appear to be solved. They are ecstatic.

So it’s becoming clear that having an operating dashboard with logging is crucial. The technology is available so we no longer have to operate our homes accidentally, it’s time to take the blindfolds off.

The barriers for us are: Finding experienced installers, our lack of experience, lack of local resources, and lack of evidence there is any beneficial reason to use them.

Do either of you have case studies on how your installs have worked out, including why they were a better option than a standard high quality split systems?

To the extent that IAQ issues are internally generated (odor, moisture, pet dander, dust, smoke, cooking, voc’s, etc.), exhaust/negative p is a good thing, no?

Also, maintaining a positive pressure balance in a cold climate is generally a bad idea. I hope the homes where you recommend a positive pressure balance have continuous insulation outboard of the sheathing.