Interesting Florida Solar Energy Center study
This discusses 4 homes they replaced the AC units in that were oversized with a
smaller unit (sized by the manual J method.)
They discuss reasons why they think 3 ended up costing more in cooling costs.
I seen that before.They state that they sized by Manual J,but Man. J gives you the cfms needed and they ignored that completely.
So they ended up with poor dehumidification and the homeowners likely ran the systems at lower temps to try and get comfortable,which caused longer run times.
Interesting reading. Anyone know the link to the wisconsin study they mentioned?
When all was said and done in the Florida homes the less than impressive energy savings coupled with the increases in humidity brings one thing to the front of the class......
It's all about the installation!!!!!
The consistent reason they figured for the lack of a more impressive energy savings and the increases in indoor humidity was the original ductwork. They speculated that the duct leakage was introducing more outdoor humidity into the home due to longer run times and these same longer run times were exposing conditioned air to picking up heat from the infiltration and possible lack of sufficient insulation on the ducts.
Guess the lesson here is: If you don't do a complete reinstallation (or at a minimum sealing and insulating ductwork) a new higher SEER system sized correctly may generate complaints that it doesn't perform as good as the old one and the big savings on the electric bill aren't showing up.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.
That and their data was misconstrued......
When I first saw that study last year, my first reaction was "DUH!". Could have saved them some trouble and told them from the start that is what would happen.
Originally Posted by firecontrol
Another loss with over sized ducts is from increased heat gain in the ducts due to the low velocity air, and increased surface area of the ductwork.
It is the same reason very many people who buy 2 stage systems don't see anywhere near the energy savings or comfort they were expecting.
> Another loss with over sized ducts is from increased heat gain in the ducts due to the low velocity a
Didn't they say in the study, the CFM was up compared to the original system.
This would make the velocity higher, not lower.
So is there a takeaway from this study wrt replacing an air handler/ac/heat pump with a smaller set ? (Besides the obvious make sure the ducts are sealed...)
[ Do you guys take off the existing insulation to seal all the ductwork and then put it back on, or just seal the ends where they attach to things ? ]
They state relative air flow was higher,based on cfms per ton,total cfms was less ,so in the same ducts ,lower velocity.
They did not follow Man. J,first by setting the air flow at M-J required cfms,and second they only looked at total btus at standard conditions,not sensible and latent at the dsign or actual conditions.
I also find it odd the the pre-changeout air flow was as high as they measured,seldom the case here in Florida.
I searched for one yesterday, no luck. Perhaps that study was flawed too and they decided not to publish it.
Originally Posted by firecontrol