Energy Penalty of Ducted Air Conditioning Systems
HarryVoorhees
| Posted in Energy Efficiency and Durability on
Hi, I have a question about the energy penalty of high-velocity vs. traditional ducted vs. ductless A/C systems.
We’re planning to have central A/C installed in our circa 1890, ~2600sf house (which includes 600sf of attic being converted to conditioned space) in Andover, MA and are weighing two options:
1. A single, high-velocity A/C system (Unico) that reaches most every room via ~18 long and thin ducts from a plenum in the attic, and
2. A traditional A/C system in the attic that reaches the second floor rooms through ~7″ ducts in the ceiling, plus a 2-headed ductless mini-split serving the first floor.
In either case we’d use a ~18 SEER Bosch air-source heat pump for the A/C system. In theory it could be used for fossil-free heating, although the intended use for now is cooling.
We’re not considering mini-splits on the second floor due to aesthetics and having several small, closed rooms. In fact, my wife doesn’t want them on the first floor either (hence option 1). I was trying to guilt her into option 2, based on the energy/carbon savings, to which she asked, “how much?”
Good question! The installers can’t answer (I can’t even get them to do a Manual J :). I assume that if the ducts are in conditioned space, then the difference in efficiency is mainly due to the energy required to move air through ducts, and it could be calculated somehow.
But can anyone offer any general quantitative comparison of the energy consumption difference between these three types of systems?
Also, are there any rules of thumb for cooling vs. heating load is this climate? (Our heat loss 0F is 45K BTU). Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
>Also, are there any rules of thumb for cooling vs. heating load is this climate?
First thing is to look up the weather stats for your climate. Look at: https://www.captiveaire.com/CATALOGCONTENT/FANS/SUP_MPU/doc/Winter_Summer_Design_Temps_US.pdf
I think Lawrence is the closest location to you. The 1% temperature (99% of the time it's warmer than that) is 9.3F. The 99% temperature is 87.8F. The heat flow through a wall is assumed to be proportional to the temperature difference between the inside and the outside. If your house loses 45K BTU/hr at 0F, let's assume an interior temp of 72F so that's a difference of 72 degrees, or 625 BTU/degree. In the summer the thermostat is typically set a little higher, let's say 76 for a difference of 12F on a 88F day. With that same 625 BTU/degree you get 7.5K BTU/hr. Now you also have to account for occupant generated heat, dehumidification and the effect of solar energy coming in the windows. Those are highly dependent on the layout of the house, which is why rules of thumb are perilous, but I'll throw caution to the wind and say together they're about the same magnitude as the heat coming through the walls. So order of magnitude 15K BTU for everything.
The heating degree days are 6092 and the cooling degree days are 636. So you're going to need roughly ten times as much heating as cooling over the course of the year.
It's going to be hard to get a small enough cooling system. You are in a very humid climate and you're going to want dehumidification as much as cooling. An oversized cooling system tends to do a poor job at removing humidity.
Conditioning a flat floored attic has a huge energy penalty in that double the surface area and are almost forced into using spray foam insulation that cost so much per square foot you end up skimping on the R value to keep the costs affordable.
Putting in the half story in the attic is a recipe for an energy nightmare. I say blow off the roof and put in a full second story you will have real usable space not 7 foot ceilings sloping down to 3 foot knee walls for 70% of the room.
Rules of thumb are for people to lazy to do a manual J calculation but 700 BTUs per sqf is my local number.
Walta
700*2600=1.8 million. Are you sure it's not 7? That would give 18K.
Walta I do not understand. Its cheaper to add a second story instead of closed cell spraying the underside of the existing roof deck? IIRC R40 closed cell on the underside of my roof deck was about 5K (including intumescent paint).
you are correct the local rule of thumb is 700 per 100 sqf
Walta
Thanks for the responses. I just did a whole-house Manual J using loadcalc.net and got 24K BTU of cooling (22K sensible + 2K latent load), which is half my heating load. This was using a 15°F temperature drop (say 90°to 75°) and assuming all ductwork in conditioned space. Even with more conservative assumptions about ductwork and a 20° design drop, the system size is only 30K BTU. This works out to about a ton per 1000 sf, and is half the size the installers proposed, so the model result seems plausible. :)
No one commented on my question re the energy penalty for blowing air through ducts. Maybe I should ask that in Mechanicals?
I don't know the answer, but a ducted system is a long term asset. The equipment attached is longer lasting, and the blower should last 2x the heat pump. If saving $100 in energy is a primary goal then selling the 130 year old house may be the way to go.
People in cold climates do ductless because opening the walls is not realistic. I agree with your wife that ductless wall units are unacceptable in an old house in Andover. "Ducted" ductless systems can work well in an old house when the air handler can be centrally located at the top of a closet.
There is a bit, but generally not much. The reason you see higher SEER numbers for some wall mounts is they are sometimes paired with a matched outdoor unit that is not used on other series.
For example the midea 18k slim ducted and 18k wallmount have similar efficiency:
https://ashp.neep.org/#!/product/47330
https://ashp.neep.org/#!/product/47509
Wall mount has slightly higher SEER (which is common) but surprisingly a slightly lower HSPF.
Overall, at full load a ducted unit would add between 50W (low static slim unit) to 300W (high static furnace style air handler) to your consumption. Since these are EC blowers, the extra power consumption proportional to your external static pressure, so you can get it close to the same as a wall mount with oversized ducting.
You may find this article interesting.
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die/
My wild guess are a standard induction blower uses about 10% of the AC energy high velocity 13%, maybe 7% for a standard ECM motor and back to 10% high velocity. 5% for a ducted mini still less ductless mini.
Walta