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Community and Q&A

Geothermal and SDHV in new ICF home

RhodyHome | Posted in Mechanicals on

We are in the process of building a new home in Climate Zone 5. It is an ICF [insulated concrete form] construction from basement foundation to gable.

I am intending to use a SDHV [small duct, high velocity] system for forced air and heating in conjunction with a closed loop ground source heat pump.

What I am trying to understand is the impact to efficiency caused by my desire for SDHV on the heat pump options side of life.

From my research, integrated HP/air handler geothermal systems offer the highest efficiencies, packaged split systems are a little less efficient and the least efficient geothermal heat pump is a water to water system. As I understand it the only option to connect SDHV to a geothermal heat pump is is to use a water to water heat pump.

Is it possible to hook the GSHP from a split system to the air handler of a SDHV?

Is SDHV materially better than the air handler with traditional ducting/velocity to warrant the loss in efficiency of the water to water?


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  1. charlie_sullivan | | #1

    The only advantage of SDHV is the SD part. In new construction, you can plan to have enough room for full size ducts, and there's no reason to use SDHV. Also, if you used a SDHV fan coil with a water-water heat pump, you'd need to make sure it was rated to work with the low water temperature of the water-water heat pump.

    But what advantage did you hope to get from SDHV? Perhaps you can get all you hoped for and more with mini-splits, or with a chiltrix system.

  2. RhodyHome | | #2

    Thank you for the response. It is my understanding that there is incremental dehumidification from SDHV as well it tends to do a better job than low velocity systems in rooms with high ceilings. We will have 9 and 10 foot ceilings through out the house.

    While I appreciate the benefits that mini split systems can provide, they are a non-starter conversation with my wife on aesthetic grounds. As such ducted it will be.

  3. Reid Baldwin | | #3

    I watched a DOE ZERH (Dept. of Energy Zero Energy Ready Home) webinar a few days ago advocating SDHV for low load homes. Specifically, they advocated high wall mounted supply ducts on interior walls resulting in a compact duct layout. The primary motivation given was improved air mixing in each room. They also said there is less temperature change on the way to distant rooms.

  4. SwitchgrassFarmer | | #4

    We have SDHV and water to water geo. However the SDHV is only used for cooling, it does that quite well. (We have thick slab radiant heat ground floor, thin slab 1st floor, and panel radiators on the second floor.)

    As another commenter pointed out, I too would worry about the geo system being able to generate hot enough water to use the SDHV air handler for heating. It's nice having a cool breeze move the air on a hot day. It's not so nice having a lukewarm breeze hit you on a cold day.

    Lastly, in response to the concern of efficiency for water to water geo, actually water is an excellent way to heat or cool a distant location. For that reason hot water, steam, condenser, or chilled water are often the methods of choice for large buildings. However, water loses dollar efficiency in the residential arena because it's uncommon, generally one off, custom, complicated etc. Tough to compete price wise with a simple package furnace unit with an A coil sitting on top of it.

  5. user-626934 | | #5

    Esteban, you may want to look into DUCTED mini-splits - such as:

  6. GBA Editor
    Martin Holladay | | #6

    I think that Charlie Sullivan gave you good advice. Large ducts almost always perform better than small ducts. While manufacturers of systems that use small diameter, high velocity ducts have figured out how to make these systems work (in order to snake small diameter ducts through the cavities of older homes in retrofit jobs), there really is no advantage in a new construction job.

    You want generously sized ducts with low static pressure.

  7. Reid Baldwin | | #7

    Not everyone agrees that large ducts always perform better than small ducts and that the only advantage of SDHV is the ability to handle difficult retrofit situations. See for example this webinar from the Dept. or Energy: Also see this Building America report: and this paper:

    The authors cited above come to SDHV from a different direction than some of the vendors of the SDHV systems. Some purveyors of SDHV systems start with the idea that they need small diameters in order to get ducts in confined spaces. Then, given the need for a particular volume of air, small ducts lead to the need for high velocity. The authors above start from the fact that the air flow requirement in a low heating load home is substantially less than in a traditional home. Proper mixing requires a high register throw which requires a high enough velocity. Getting the higher velocity at reduced volumetric flow leads to small duct sizes.

    The different motivations lead to different recommendation regarding aspects other than duct diameter. The purveyors of retrofit systems may advocate putting trunks in an unconditioned attic, whereas the authors here emphasize keeping all ducts in the conditioned space. The authors above emphasize designing the trunks to provide short runs while the purveyors of retrofit systems don't seem to care about that. These authors also discuss the advantage of having the supply diffuser high on a wall so that it blows across the ceiling instead of on the people and is not blocked by furniture.

    I am debating which direction to go in the house I am building. If there are people in this forum who live in a house with an SDHV system, I would appreciate their observations.

  8. charlie_sullivan | | #8

    Reid, you convinced me to look at those reports. Thanks for pointing them out. They are primarily recommending:
    1. If you install a 30 kBTU/h furnace, you should design a duct system to match, rather than the same duct system you previously had for an 80 kBTU/h furnace, or the same system you usually install for an 80 kBTU/h furnace, and
    2. Use a compact layout with shorter runs--compact in length.
    I generally agree with both of those recommendations.

    Included in 1. is that idea that with lower airflow, you might need high velocity registers to get sufficient throw for mixing, and that it might make sense to have high velocity ducts to match the high-velocity registers. I think that concern is misplaced, because I think that they have neglected the fact that a better insulated and air-sealed building is less likely to have stratification. If the heat can't leave the room, it tends to bounce around the room until the room is evenly warmed, even if you aren't actively blowing it around. To the extent that the air circulation induced by a high velocity register blows air on people, it will also help summer comfort and hurt winter comfort. I think it's a better idea to add separately controlled fans if you want that effect, so you can get it in the summer and not in the winter, and so you can turn it off in empty rooms.

    It's true that you could buy ducts meant as SDHV ducts and connect them to a small heating system and end up with a SDLV system. At some point it becomes question of how you categorize systems rather than how you design them, which is a less interesting question to me. So if we come back to the original question here, if Esteban goes with a GSHP, I see no reason not to use the GSHP's own air handler and to design a duct system to work well with its flow and pressure capabilities, using runs as short as feasible, located within the building envelope.

  9. RhodyHome | | #9

    I did not mean to imply water was a poor conductor, just that the efficiencies of the systems are materially different. A top of the line packed system has EER/COP over 40/5, split systems are a little worse, while using a mix and match approach it is mid high teens to barely over 20 EERs and 2.7 to 3.4 COP. That is a stark contrast in efficiency.

    The rest of your post has me intrigued as we initially wanted to do radiant heating but that meant duplicate systems so we started to steer away from it and because of the large volume rooms wanted to do SDHV only to find out about the water to water limitation on SDHV. I might end up back at radiant heat and ducted cooling, still to determine HV vs LV.

    In regards to mini splits of any kind, no wall or window mounted items are permitted and I am actually trying to keep all equipment hidden, even condensers/compressors that would ordinarily go outside.

    As to air mixing in tight buildings, I disagree with your premise although I admittedly have no studies to point to. I will use my own lay men's logic, oil and vinegar like hot and cold do not really want to mix yet they can be forced to given enough force applied to the overall system. There is a minimum force that will cause the layers to mix and the larger the volume of the space the layers occupy the larger that force needs to be. As such I am still leaning HV, although not as convinced as I once was.

    Lastly, with regards to shorter runs, I am trying to minimize the locations of equipment to just the basement, which will require longer runs. While I know that I can super insulate LD systems, this is another area where SD, to me, are superior...

  10. Reid Baldwin | | #10

    The reports I cited don't use the term SDHV. They are not advocating higher velocity than normal ducts with normal volumetric air flow. They are advocating higher velocity than you would get with a right sized air handler for a low load using a duct system sized according to home square footage. SDMV (medium velocity) would be a better term.

    The systems I am aware of are designed to operate at between 1.0 and 1.5 static pressure, as opposed to less than 0.5 for which most conventional air handlers are designed. From what I can tell, most of that pressure drop is in the 2 1/2 inch flex duct rather than in the trunk lines. I wonder whether using Comfotube from Zehnder instead of regular flex duct would improve on that.

  11. SwitchgrassFarmer | | #11

    Esteban, kudos, you are definitely doing the right research. However, consider this analogy, when you buy a car you look at the mileage sticker, before you peel it off and put it in the glovebox never to be seen again. Buying an HVAC system is a lot like that. The efficiency rating is very important, but only in the overall context of how you plan to install and use the unit. No sense in buying a Volkswagen Jetta Diesel if you plan to trailer a cabin cruiser. (You might pick up on another hidden meaning within that last sentence, manufacturers game ratings.)

    I would look for an engineer to help you make this comparison. Someone like John Siegenthaler who wrote this article: A full lifetime cost analysis like John has done is what you want. Yes, it's an expense, but I am thinking that if you are building a full ICF home it wouldn't be out of line. (I could go on about ICFs too...I have an ICF foundation.)

  12. user-626934 | | #12

    Reid -

    The DOE and Building America webinars + presentations you linked are right on the mark, except for the part where they starting recommending higher velocity in the duct in order to get good throw at the registers. Throw has NOTHING to do with velocity in the duct, but has everything to do with the size, shape and fin design/orientation of the register itself, along with the volumetric rate (not velocity) of air moving through the register.

    As an example, consider a duct branch that needs to move 80cfm to a particular room. The duct could be a 4" duct at a relative high velocity of 917fpm, or perhaps a 6" duct at a relatively low velocity of 408fpm. Either way, if you attach those ducts to an 8x4 #A618 Hart & Cooley register, for example (via a 4x8x4 boot or a 4x8x6 boot), the throw is the same - 8.5 feet (@75fpm end velocity). If you want a longer throw, then you need a smaller register...not a smaller duct.

  13. Reid Baldwin | | #13

    Thanks John. I think I was associating small registers with small ducts because the systems I have read about follow that pattern. I can see now that those would not have to go together. It would be nice to get the benefits of improved mixing without the higher static pressures.

  14. user-626934 | | #14

    Reid, I often design with the slim-ducted, mini-split heat pumps, most of which require that you design a low pressure system (if you want the correct airflow across the indoor coil). I made one mistake early on where I went overboard on "low static"...the registers (especially the floor registers) were much too large...air just dribbled out...stratification ensued in the summer. Since then, I've focused more on register placement and selection to a point where I feel like I have things dialed in. The main issue now is in getting the installers to actually install the small registers and supply duct boots that I spec. They usually requires a custom order from the distributor, because our local supply houses don't stock 8x4 boots or registers.

  15. RhodyHome | | #15

    Thanks for the info. If I am understanding correctly the size of the register will contribute to the amount of mixing between the temp strata?

    Wouldn't the smaller register on a larger duct create incremental static pressure that needs to be overcome, or am I missing something? Assuming I might have gotten something right, according to my wife a very unusual occurrence, doesn't the lower velocity system have a larger challenge to defeat the pressure created by the shrinking register or am I mixing apples and peanuts?


  16. user-626934 | | #16

    Mixing is a function of room shape/dimensions, register placement, throw and the volumetric rate of air (cfm) being delivered. For a given location, register type and volume of air delivered, a smaller register will have a longer throw and will generally provide better mixing (than a larger register).

    The pressure is not the thing that must be defeated. The pressure is what is REQUIRED in a duct system to move air against "X" resistance. The resistance is caused by things like external heat exchange coils, filters, fittings (elbows, takeoffs, boots, dampers, etc.), straight sections of duct and terminal devices (aka "registers + grilles"). The more resistance there is, the more pressure is required from the fan in order to move the same amount of air through the duct system. More pressure from the fan requires more fan power and more energy use. This is hard to demonstrate with words...easy in a lab/teaching setting with actual ductwork/fittings.

    Yes, smaller registers add more resistance to a system than larger registers, however, one must remember the goal: comfort...ideally with low energy use and some good particulate filtration (ideally). Comfort comes first, therefore we need to spec registers that are appropriate for good mixing. The rest of the system can then be made as low resistance as is practical, which will help ensure low energy use for moving the air around the house.

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