High Velocity System for HVAC
Hello GBA community, I’m considering installing a high velocity system as part of the HVAC for our new residential home. This is recommended by our builder’s HVAC vendor. The vendor states that it has been used in commercial applications for many years, and of late is also being installed in many residential applications. Does anyone have direct experience or technical background such that you share with me the advantages and disadvantages, including energy efficiency?
For context we are building in Climate Zone 7A. The high velocity system would be part of an air handler/fan coil unit. The primary heating and cooling will be a heat pump. Auxiliary heating for cold days below the limit of the heat pump will be provided by a boiler that is part of the basement hydronic system.
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> Does anyone have direct experience or technical background such that you share with me the advantages and disadvantages, including energy efficiency?
I had a a Unico high velocity system in my home (1916 build, climate zone 4A) before I did any major energy improvements. It was expensive and in some ways a waste given how much the house was renovated after the fact, but it provided substantial summer time comfort while it was in operation.
Benefits:
- Easy to get ducts where normal ducts can't go
Drawbacks:
- Cost
- Noise. Not objectionable. Not a distraction. But noticeable.
- The physics of it is clear: moving air like this is not energy efficient.
> The high velocity system would be part of an air handler/fan coil unit. The primary heating and cooling will be a heat pump. Auxiliary heating for cold days below the limit of the heat pump will be provided by a boiler that is part of the basement hydronic system.
This is where I'm slightly unclear. Hydronics are expensive. I can't see installing a hydronic system that is not the primary heating system. But maybe saying "basement" hydronic system means you're expecting basement heat to leak into the rest of house as back up?
Either way, hydronics are expensive. :)
Thanks Patrick, and yes hydronic systems are expensive. The decision to include one for the basement in our new build is based up the comfort we get from the hydronic system in our existing home. With a large family and a basement well-used as the TV/Rec room and spare bedroom, the comfort is well worth it if your budget allows for homes in Northern climates.
The hydronic system in our basement is primary for heating the basement. Certainly as the heat will rise it does help heating a bit on the main floor, but the main floor and second heating will be provided from the heat pump (when not cold).
High velocity systems are rare in new construction. They're usually used in retrofits, where the big advantage is the piping can be drilled through joists so they can be used in places where there's no room for ductwork. In new construction I think you're better off designing in space for ducts.
The branch lines for high velcoity are small, the main trunk is smaller but not that small, they still take up space.
In new construction, the branch lines are mostly through the floor joist space, so size doesn't matter. High velocity or not you would still have to find space for the supply and return trunks, these are the ones that take up real space and very hard to hide if not part of the design.
I guess if the cost is the same. From my experience high velocity is much more expensive. All those bits really add up and none are cheap.
If you do go for high velocity, I would aim for medium pressure side closer to a normal furnace, pumping air at high pressure uses much more power, for something that will run a lot it can add up to. Generally a high velocity unit running at rated pressure uses about 2x the power as a comparable standard ECM air handler.
I've owned several of these units, in houses in zones 4 and 6. My zone 4 house used two systems with fan coils for heat and refrigerant cooling and was retrofit into a 100+ year old house as part of a gut/rehab project. The zone 6 system was for cooling only and was installed during original construction.
I'll add to Patricks notes above:
Pros:
Very good at dehumidifying along with cooling. I never really dug into the physics, but I'm told that the coils can be run cooler, improving dehumidifying performance. Whatever the process, it worked in very humid east-coast weather to the degree that we could comfortably keep the thermostat at 76 degrees, hopefully saving a bit of energy that was lost with the higher fan pressures.
Even when the emitters are not aimed at you (they shouldn't be), the system seems to induce greater air movement and mixing in the rooms. This is not a quantified thing, but it definitely helped with A/C comfort.
Cons:
Because the room air moves faster, it is less comfortable in heating than something with still air like hydronic radiators or baseboards, or radiant floor heat.
You must consider the location of emitters. Duct routing forced us to use linear emitters along one high gable wall of a cathedral ceiling space. Every time the system came on in cooling, air shooting across the cathedral ceiling would entrain the hot air up there and mix it down into the living space. This happened for just a minute or two but was very noticeable. I've also seen several of these systems where the emitters were placed in locations where the wind was not a good thing, like near kitchen ranges, over doorways, etc.
This is probably the most important con for you. I would not recommend these systems for heating in a very cold climate. When using fan coils, the air coming from the emitters is barely warmer than the room. Coupled with the higher air velocity, the moving air can be uncomfortable. We had very experienced designers/installers and this was something that we were unable to improve much. Just a nuisance in zone 4 but possibly a deal killer in zone 7.
Others have mentioned energy use. They use a LOT of fan energy. Basically, it is the opposite of what our current guidelines are for efficient space heating. Systems are more efficient when they run at a low fan speed for as long a cycle as can be maintained. This is what minisplits do when modulating during average heating and cooling conditions. High velocity runs at a very high fan speed/pressure briefly, then shuts down until the next call from the thermostat.
Sorry to burst the bubble, but I don't think you would be happy/comfortable with high velocity air for heating in your climate.
Thanks for the intel Peter, and the feedback from everyone. Decision is yet to be made, but I now feel better equipped with questions to ask the vendor.
Note that comfort benefits of hydronic systems are often related to mitigating deficiencies in the envelope. With a great envelope, you can achieve similar comfort with any heat source.
There are two perceived comfort benefits from hydronic heat. One is being able to sit close to a warm object. The other is not having the air so dry.
Both of these go away when the house is tight. In a tight house you don't have drafts and cold spots. The reason forced air dries the house out more has nothing to do with the technology as such, it's that forced air pressurizes part of the house and depressurizes another part, which makes the house leak more. In a leaky house that has a noticeable impact on indoor humidity.
In a tight house, the temperature is much more even throughout.
Would it be possible for us to talk you out of the hydronic boiler?
Yes, hydronic systems are very comfortable in old homes The old windows lose much more heat and leak more air than modern windows do. The old walls had little or no insulation and they left gaps so the house could breathe so that it leaked air on purpose. The same for that ceiling and attic. Today we are no longer allowed by law to build that bad of a house. The old house lost heat so fast the floor needed to be over 90° to keep the room at 68°. Today you build the code minimum house and the floors will be more like 78° and when you walk barefoot on the ceramic tile it will feel cool if not cold.
I have fond memories of my grandparents’ hydronic floors but even when energy was much less expensive the gas bill was painful.
Seeing how the contractors are needlessly pushing the most expensive cool system possible on top of your request for the most expensive heating system has me concerned that they little regard for your budget. Cost plus % contract?
Walt