View Full Version : 2 units or 1 Zoned Variable Speed Unit
12-21-2006, 02:12 PM
I'm using Wrightsoft for loads on a 2 story 3550 sq ft house in TX - in anticipation of an upgrade.
Currently the house has a 4 ton and a 5 ton, 13 years old ea. (originally 394 sq ft / ton). A bit much to avoid short cycling on a single speed system.
The max cooling & heating loads come in just under 5 tons. I'm looking for feedback on the PROs and CONs of using one zoned variable speed 5 Ton, vs. two units (upstairs & downstairs).
Current thinking is that the new unit(s) will be high efficiency 19 SEER+, so new lines have to be pulled in regardless.
What's better? 1 or 2 units?
12-21-2006, 02:37 PM
Go with the zoning! Look at it this way... you use one system for both floors that acts just like to independent systems. The homeowner will have the same comfort with less energy usage, less maintenance cost, less repair costs and less replacement costs in the future.
If they wanted to use IAQ products then both floors could have the benefit without having double the IAQ investment.
If you were the homeowner would you want to be responsible for two systems or just one? One right? Plus if you're using a 19 SEER unit and variable speed furnace or air handler than the system won't short cycle since it's two-stage (variable speed systems work fine with zoning so don't get worried about that either).
Zoning is so nice... I wish more new home construction projects used it. If I did RNC I wouldn't even give home owners a choice, it would always be included.
Hope this helps!
Go with zoning ,but consider 4 zones instead of two if posssible.You'll have more control,i.e. master bedroom zone and living/entertaining zone.
12-21-2006, 03:34 PM
are both units side by side or 1 attic, 1 basement?
for a house that size you may be better with 2 units, usually don't get the highest efficiency when you go up to a 5 ton unit.
12-21-2006, 06:44 PM
I dont know much about Texas, but in south carolina its nice to have two systems in case one breaks. if it cant be fixed that day sure is nice to lay your head down at night someplace cool. gets mighty sweltering in the south around july. other than that, i do see all the positive ideas with zoned systems.
12-22-2006, 09:26 AM
When you say it comes in just under 5 tons, is that from Manual J alone or did you also do Manual S for equipment selection? Manual J alone doesn't tell you what size equipment you need, it tells you how much capacity you have to actually deliver to the structure.
The rated capacity of air conditioners is for standard conditions of 95 degrees outside and 80 degrees inside. You may be close to 95 for an outdoor design temp there but you lose significant capacity from the rating point when you are at 75 inside instead of 80. For and indoor design temp of 75, you lose 835 BTU * 5 degrees difference * 2.0 (for 2000 CFM on a 5 ton system)... so 8350 BTU less than the manufacturer's capacity rating.
I'm a fan of zoning, especially Infinity/Evolution zoning. If you can actually get enough capacity from a single five ton, that's what I would do. But make sure you really can get enough capacity for your application out of a single system, or else you're going to be running a few degrees warm in the summer!
12-22-2006, 09:49 AM
I'd recommend zoning. In a dual furnace/AC setup the heating/cooling potential is limited to the equipment size. For example, let's say the calcs call for 2 tons AC upstairs and 3 tons downstairs. But then what happens if there is some added heat load to the upstairs and it now need 2.3 tons to keep up? In the dual system the upstairs gets hot. In the zoned system it will have better ability to give 2.3tons upstairs once another zone in the house has met demand.
On a recent install the homeowner would keep one specific room (like a family gathering are) at 70° while the rest of the house at 67° or so. He thought it was the greatest and it was saving him on heating. You can't do that as easily with non-zoned systems. Yes, you can adjust the vents, but the heat load will change as the people go in and out which would require continual adjustment of the vents.
As stated earlier, one thing to watch out for is equipment sizing. If the calcs call for a 5 ton system but the Sensible Heat Ratio is much over 75% (divide the Sensible Heat by the Total Heat to get this ratio) then you would need a blower that puts out more than 5 tons of air; that wouldn't be easy to find in a variable speed.
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