I need some help from you HVAC/electrical guys out there with a hypothetical question:
If money were no object, what would be the most efficient way to cool a small, say 1800 square foot house–and I mean efficient in kilowatt hours. I know it’s apples to oranges to compare window units to central to ductless systems—but if it came down to BTU for the buck, how do they compare. Or can you even do a BTU for the buck comparison?
More specifically I’m comparing an 18 SEER state of the art central system, very expensive, with having a new very efficient window unit it each bedroom and living space (ugly I know, but I’m trying to look beyond that), not so expensive, with maybe a three zone ductless mistubishi (only rated at 10 SEER) with one zone of every floor, middle of the road expensive.
I know there are other variables, like layout (specifically it’s an old victorian, not an “open” floor plan), looks, noise, etc. but if your goal was to be as conservative with energy as possible, what’s the best option?
Thanks All,
Michael
Replies
money were no object, what would be the most efficient way to cool
Obvious answer here is GSAC (ground sink air conditioner). In the right conditions, you could simply circulate cold water from one well to another and not even have a vapor compression cycle cooler, only kW-hrs would be for circulating water thru a lotta pipes in the house.
that's the inverse of ground source heat pump, right?is there any way of using it if you don't have ducts? can you run the cold water through and existing radiant floor system?
That would give you cold floors and not much else.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
You forgot also wet. If you have high humidity.
There is a system that uses small-diameter high-pressure ducts for AC. A good approach in many cases where retrofitting is done, since the ducts can be snaked like cable.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
Yeah, a ground or water source AC is more efficient than an air source unit in most parts of the country. (There are probably a few areas where the ground is too dry for good heat transfer.)
Another option is an evaporative cooler, either a simple "swamp cooler", in drier parts of the country, or a commercial style hybrid evaporative/heat pump design. This last, while quite expensive, is probably the most efficient of all, save for a place where you have a good source of cold water for a water source unit.
The biggest "bang for the buck?" Plant a tree. Keep the place in shade (so it doesn't get hot to begin with).
The second? Add a roof. That is, lay another roof atop the existing one, with a few inches of space to allow plenty of air circulation. This has the effect of keeping your house in shade.
Never underestimate the value of opening windows, and using fans to circulate air.
Still not enough? Still want to burn oil? OK.
In humid areas, a de-humidifier greatly increases your comfort. In dry areas, a swamp cooler is usually enough. Both consume a fraction of the energy of air conditioning.
If you simply must use air conditioning, the basic unit is nowhere near as important as the controls. Most homes have on thermostat, one blower, and lots of ducts. Too bad your home doesn't heat up evenly. There should be a thermostat, and separately controlled duct, for each area of the house. That is,rooms that face the sun ought not be on the same control as the basement. It might even make sense to have a few smaller units, rather than a single huge one.
My limited experience tells me that a GSHP (ground source heat pump) will outperform your 18 SEER unit.
The one I had installed about 10 years ago, claimed cooling efficiency of above 19 SEER. A 4500 sq ft. house gave total monthly electrical bills of about $100. All electric house, including the well pump.
I moved away 9 years ago.
From a different perspective...
I had a GSHP in my last house - it was great, and kept things cool.
This house is smaller (1700s.f.), built of ICFs and r50 in the ceilings. We put in a $350 sams club AC unit in the wall yesterday, and it does a great job cooling the place. I saved $7,000 (and a week excavating & hooking up ductwork) in the process - its only 8.5 seer, however.
I couldn't justify state of the art w/the price difference between.
Move north in the summer.
Move south in the winter.
Lennox has a 21 SEER unit available now. It has a two speed compressor of course.
In general, the more zones, the cheaper, assuming you turn off the units when nobody is in the room. So, I'd go for the ugly window units. The nice part is if energy costs go through the roof, you can all retire to a single room, and turn off all the other units. But there are, as you've seen, a huge number of variables. Where is the house located?
The most efficient system would be a system that is designed and installed to manufacturers specifications.
Yes, the ground source heat pump stuff is probably the best, but it is awful expensive.
Two compressor condensers are pretty "efficient", also.
The SEER rating is a ratio that includes things like duct leakage. The only true way to compare the efficiency of condensers is with the EER rating (basically how many btu's you get for each kWh).
You must also consider the payback horizon on the high SEER stuff.
I'm in Ma and there is no way in the world an 18 seer could pay for itself in a reasonable amount of time. I think national code is now 13 seer for central ac and I would go to 14 if the price difference was small but the price steps up too much for the higher stuff to justify the purchase. The high stuff only makes sense down south where the warm season is much longer.