What’s so wrong about oversizing HVAC?
We’re building a new home.
Yes, I’ve heard the advocates of right-sizing the A/C to keep the unit as small as possible. Take into account everything — windows, ceiling ht, wall types (in our case, SIP with 6″ foam, etc. Size system for max summer temps (in our case, 105 degrees a dozen or so days). etc. That way humidity is better controlled with a system that is constantly going during those maximum days.
An oversized system, on the other hand, costs more to buy and operate and won’t control humidity as well since it’s off much of the time, one needs a dehumidifier — for one thing.
Have I missed anything? Of course I must have because so many systems are installed by Joe Bubba who convinces the homeowner that if the computer misses something in its calculation then their house will not be cool enough during those hot days — therefore simply add a ton for good measure.
Okay, what’s so wrong with that. Is the cost of a dehumidifier and its running cost all that great? Is that the only downside? Has anyone put a pencil to both sides of this issue?
TK
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Replies
Here you go… Knock yourself out: all from the GBA
1. https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
2. https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-2
3. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-cooling-loads
4. https://www.greenbuildingadvisor.com/blogs/dept/building-science/ac-sizing-benchmark-high-performance-homes
Humidifiers do use a bit of E, yes sir, mine does anyway.
A big part of all this is... personal too. Many of us want the AC to run at real low temps, like 68-72 degrees. The green temp recommended though is to set temp at 78 degrees. The idea is to keep an AC home set constant 24/7 but many again like to turn off the AC but then expect the system to be able to blast a home from 100 degrees down to 68 degrees in five minutes.
So to give us all what we want, the AC guys..... give us what we want. Big a** systems.
What world is yours, big hats and trucks and free markets... or smart tstats run by others to control your lack there of?
Leonard,
You're on the right track. For years, energy experts have been preaching against equipment oversizing, but the pendulum is beginning to swing the other way. It turns out the equipment oversizing isn't an egregious error after all. The main disadvantage is that you are spending more than necessary for your equipment.
The humidity control problem has been exaggerated. Most oversized air conditioners work just fine.
Only rarely (and only in a few climates) is a dehumidifier necessary. In general, the classic example of a house that may need a dehumidifier is a very well insulated, airtight house in Houston with very good windows. Because the air conditioner runs for fewer hours in such a house, it may need a dehumidifier during the swing seasons.
Regarding dehumidification in oversized systems - I suspect that humidity control hasn't been such an issue for these systems since it's also VERY common to have reduced airflow from undersized ducts and poorly designed ducting. Reduced airflow = colder coil = more demudification, unintentionally. I'll see if I can dig up some hard data on this.
Another point that's often made against over-sized systems is increased wear and tear on the systems' various motors due to short-cycling. Again, I'll see if I can find some data on this.
The climate in which you live and the efficiency of the building envelope should be considered. A cold climate home with average insulation needs a lot of heating capacity if you choose a nightly thermostat setback. With a multi stage, forced air furnace, I would recommend twice the design temperature heat loss as the maximum Btu output. Once the setpoint is reached in the morning the furnace will mostly run at the lowest blower speed, quietly and efficiently.
As for AC, I would size accurately, just right makes sense for proper dehumidification.
Hi ya'll and thanks for all those well-considered thoughts. This is one of the biggest decisions in our project, the house is mostly designed and I'll be sitting by the HVAC man (my wife's nephew and owner/manager of a thirty man crew) as he enters the data in the computer.
TK
Dehumidifiers generally SUCK in a cooling dominated climate, since they just convert a latent load into a sensible load, while adding a sensible load of their own (the heat dissipated in the motor & compressor.) An air conditioner dumps both the sensible & latent heat outside where it belongs!
During the shoulder seasons when there isn't a sensible cooling load it's usually possible to dehumidify a tight house by upping the ventilation rate- rare is the climate where foggy 65-70F with 65-70F outdoor dew points are commonplace. Whenever the outdoor dew point is below 60F ventilation alone can do purging interior-generated moisture from the house.
Short cycling is a pretty good reason to not oversize your HVAC euipement.