Not meant to be a pitch actually. I can understand that it might look that way though since I work for an SDHV company. Im basing it on recent studies by NREL, FSEC, IBACOS, NIST, and a many of our customers. The latest is FSEC at https://www.osti.gov/biblio/1421385-evaluating-moisture-control-variable-capacity-heat-pumps-mechanically-ventilated-low-load-homes-climate-zone. I would be happy to share what I’ve read.

There’s a misunderstanding about blower power. SDHV is rated at 300 com/ton at 1.2 IWC with an ec motor. The confusion is because people talk about the airflow per nominal (nameplate) ton of the outdoor unit. SDHV uses the same outdoor unit as conventional and the SDHV rated capacity is typically 15 to 20 less than the nominal. We rate at 250 cfm/nominal ton. The minimum for good operation before you start to have problems is 200 cfm/nominal ton and we don’t encourage this. Anyway, you get the idea.

Measured Fan efficacy at rated conditions is 0.45 w/cfm compared to 0.20 w/cfm for a 400 cfm/ton conventional system at 0.2 IWC (and it is actually higher in practice because static pressure is always higher than the rated value. DOE is correcting this to be 0.50 in 2021). Using the DOE value, conventional fan efficacy for ec motors is more like 0.25 w/cfm. Translated it is 135w/ton vs 100w/ton. In a low load home at 1500 sqft/ton, that isn’t much difference at all. Starting in 2015, the minimum required SDHV seer is 12 and now you can get 13 or 15 Seer with an inverter outdoor unit.

I agree if all ducts are in conditioned space there is no energy benefit to either system from the ducts but it is easier to find room for smaller ducts. So, there is a construction benefit.

Low load homes require smaller systems (naturally) and this means less airflow is available. IBACOS found that high velocity jets did the best job of air distribution for low airflow systems. They developed their own system using PVC for ducting and a conventional AHU. Not quite SDHV but very similar.

The loads are so low in these houses that the internal loads are more significant then the envelope. This means humidity from showers, cooking and breathing impart a significant latent load. Maybe in a dry cool climate you could introduce an economizer function but most climates don’t allow that. Thus, space shr is typically 0.75 even in climates you might think are dry so you may need to run at normal cfm/ton.

As you point out, it would be great to run at 500cfm/t when no latent load and 300cfm/t when you do. This would be ideal. Unfortunately the rating methods don’t consider this so you wind up with over reporting efficiency (conventional systems running at lower airflow) or under reporting efficiency (SDHV). The truth is in the middle. It’s up to us engineers to figure that out.

I’m not saying SDHV is always better but certainly it isn’t always worse either. Bottom line, use the system that is best for the application factoring performance, comfort, cost and aesthetics. You will find that the two systems are not that much different, especially for low load homes.

Anyway, this is a good discussion and I welcome your thoughts especially for your climate which is probably the least favorable for SDHV although not terrible either.

@Craig, I have to push back a
@Craig, I have to push back a bit on your pitch. I specialize in designing mechanical systems for purpose-built low-load homes and I can’t imagine using a high velocity system in that niche market. First of all, almost by definition, the ducts will already be inside the envelope, so comparing high velocity favorably with systems that have attic ducts doesn’t make sense in this context. There are multiple ways to get the ducts inside without resorting to high velocity distribution.

Beyond that, the blower energy penalty for high velocity is not insignificant, and looms even larger as we ratchet down a home’s overall energy profile, as is always the case with clients who build low-load homes.

But the biggest drawback of high velocity in my opinion is the degree to which sensible efficiency suffers when you run supply air in the low 50’s. You deftly fashion that handicap as a selling point but in my world, enhanced dehumidification should only be done on-demand. Otherwise the system wastes energy as it reduces RH lower than necessary to maintain optimal comfort.

For those who aren’t aware, high velocity systems operate at an extremely low CFM-per ton (250 or even less!). This is done so blower energy doesn’t eat your lunch. At rated conditions, this, plus the higher blower energy is what reduces the SEER (federal minimum for small-duct, high-velocity carve-out is 11 SEER, 6.8 HSPF).

In my climate (desert southwest), I typically design to 500 to 550 CFM per ton. This increases system capacity and efficiency beyond rated values — but only if ducts are low-static (i.e., low velocity). On the other hand, high velocity will operate at less than its already low rated efficiency when there’s no latent load to speak of.

That said, the majority of my clients over the years are in humid climates where there may be a need for enhanced dehumidification (DH) from time to time. However, a well-sealed house that’s not over-ventilated should only experience occasional DH calls. Conventional variable capacity systems are designed to reduce blower speed on demand to address DH calls. That makes MUCH more sense than running the system at low speed (cold coil) throughout the cooling season!

Don’t get me wrong… high velocity distribution has it’s place — for example, for retrofitting A/C in an older building that doesn’t have a central duct system and no other way install conventional ducts. But I would argue that it has no place in high performance, low-load homes. At least not that I’ve ever encountered.

It’s been available for years, known as small-duct high-velocity. Sometimes people get hung up on the lower SEER but that’s an inherent aspect of the slightly extra fan power needed to create the jets and make the air colder. Interestingly, it’s not much of a penalty, especially for low load homes and, in many cases, no penalty at all if you factor duct efficiency. The ducts fit inside the conditioned envelope and are extremely tight.

I’ve seen studies that use smaller ducts with nozzles with conventional blowers but you still get a fan penalty because nozzles have more pressure drop than grilles in order to create the jets. You can reduce the air friction using PVC but this doesn’t pass fire codes. And hard pipe is not as easy to install as flexible ducts. In addition, conventional blowers don’t provide cold enough air unless you use less than rated airflow — which then makes it similar to an SDHV system except it wasn’t designed for lower airflow.

You might also want to know that we have a 1.5-ton inverter heat pump with a really small air handler (and it can be zoned).

@Curt, I’m totally familiar
@Curt, I’m totally familiar with the specs for that model. I realize the compressor is a de-tuned 2-ton model.

300 CFM minimum works for my situation since my design CFM is 550 CFM/ton (there’s no actionable latent load). The stage-1 capacity @ 95F/75F and 57F EWB is 6,420 BTU/h @ 300 CFM, which works out to 560/ton. I don’t think the 2-ton model would work because of the way the Infinity Zone control is programmed when matched with a two ton outdoor unit.

I’m not sure you should get
I’m not sure you should get too excited about the apparent unique characteristics of Carrier’s “1 ton” 5 stage split heat pump system. A 15 minute browse of Carrier product data suggests that the 1 ton is simply a factory de-tuned 2 ton. I base that assertion on Product Data showing the 5 stages of cooling capacity percentage: 1 ton: 58,72,81,90,100 2 ton: 35,56,69,87,100

The min speed for 1 and 2 ton systems are both set at 1500 RPM, likely for oil return concerns.

The smallest air handler available is a 2 ton, and the lowest allowed air flow for the one ton system is 300 CFM…not much turndown.

A load matching system can solve a number of problems outside of that of structure sizing concerns. For one it can reduce the number of pieces a manufacture has to make, the number of parts needed to repair it when it breaks. Everyone knows when it come to sprockets and gizmo’s the less you have to stock the more profitable you become in that the less parts you must make to sit on a shelf until the day it may or may not be needed.

So what is a load matching system? Inverter is the name of the game. They already exist but they are all high end machines when it comes to central duct type systems. While you can buy them in mini split configuration these options tend to be even more pricey than a central ducted system as mini splits are mostly designed for small structure spot cooling and heating applications.

The larger the structure the less likely a mini split will adequately cover the structure without additional more costly equipment and more maintenance and eventual breakage.

How a ducted inverter AC works: Typically most HVAC manufacturer’s pull this off by modulation of a variable speed compressor in 5 stages of cooling. So in a space that only needs 1 ton of cooling the 2 ton model would be chosen, due to the ramping of the 2 ton model with 5 stages the system would most likely never hit the 2 ton mode or 5th speed of the compressor in this case. Worst case scenario it would probably hit stage 3 occasionally.

2 tons of cooling = 24,000 BTU / 5 = roughly 4,800 BTU of cooling per step of the compressor. The air flow required is typically run by algorithmic control scheme handled by the communicating thermostat or the control logic of the main board in the furnace.

With that said some manufacturer’s make these ducted inverters full variable. Which means the compressor is almost always running and searching for the optimum amount of cooling needed to maintain the heat / cool load of the structure.

The sizing these AC systems come in currently are 2, 3, 4 and 5 ton sizes.

However, all of this could soon change. There is a Chinese manufacturer that could disrupt all of this. They have a full variable machine that can run with almost any indoor equipment and any basic thermostat. No real special indoor equipment etc.

Basically how they pull it off is that the Inverter controls the compressor via constant Evaporator temperature. There are some requirements but because of the nature of how the compressor is controlled opens up a whole lot of new capabilities with less initial costs in many cases as you don’t need special indoor equipment and communicating thermostat to make it work.

Also, get this the unit comes in 3 ton and 5 ton configurations. If you need a 2 ton you simple convert the 3 ton via a switch of some kind on the inverter. If you need a 4 ton, you convert the 5 ton via a switch of some kind on the inverter. So as you can see, the revolution is coming….

Basements are rare in my area (SE Arizona) but this was non-negotiable for me. In addition to reducing loads, this brings ducts inside and allows me to install a suspended ceiling with lightweight tiles for future easy access to all pipes, wires, ducts, etc.

Above-grade walls are 2×6 24-oc with BIBS (R23 cavities) + 1″ EPS outside (typical stucco system). Roof is ‘flat’ with R32 rigid foam on top. Windows are vinyl with 366 glass (0.27/0.19 for the most part). Furred out basement walls are R13 cavity + R2.5 XPS behind studs, and R4 EPS outside to protect waterproofing system. Floor trusses are hung inside basement wall with 6″ spray foam between trusses. No sub-slab insulation. None needed. Anticipate 1.5 ACH50.

The total cooling load is only 0.9 tons @ 100F outdoor, which works out to 3,500 ft2 per ton. Indoor design is 76F with nighttime setback to 73F. Heat load ‘by the book’ is ~18k @ 28F/69F but I estimate MJ overstates my heat load by 100% given the particulars of the house & locale (e.g., I estimate true design heat load will be less than 9k).

Whole-house ductless isn’t an option in cooling dominated climates, and with only 0.2 ton cooling load for the basement, I gotta have zone control. Unfortunately, I haven’t been impressed with zone control options for ducted mini’s. Given my zone sizes, a single stage 1.5 ton system won’t work, and 2-ton variable speed equipment doesn’t have nearly enough range to handle single zone calls. What to do??

Fortunately, Carrier added a 1 ton model to its Infinity 18VS (5-speed inverter compressor). Given that Carrier’s zone control system is arguably the best on the market, this system is ideal for my project. Normally I’m agnostic when it comes to brands, but to my knowledge, no other manufacturer makes a 1-ton (non mini-split) heat pump. Kudos to Carrier for responding to the market!

To my knowledge, this new manual is not intended as a standard but rather design guidance, so the definition of ‘low load’ has little practical consequence. OTOH, I agree with Thomas that it might get on more folks’ radar had low-load been defined @ 1,000 ft2/ton, especially considering that the 400-600 ft2/ton rules-of-thumb is still prevalent in the market.

More importantly, the idea that ‘low load’ is still a niche market is flat out wrong. Many of the issues being addressed by the task force, in reality, should apply to any home built/verified to comply with today’s energy code.