View Full Version : two story residential system
07-16-2008, 02:33 PM
I'm buying a new two story home 2855 sf in florida. all windows are low e and not too many windows. the house has minimal insulation per code on walls, etc.w/ concrete block first floor and wood frame second floor. this model home use to come with 2 separate ac units, one for each floor but the builder decided to install a single unit system. the builder also included a damper system to have two zones (1st and 2nd floor separated) with one thermostat on each floor. the builder first installed a Carrier 4 ton system. however, after talking to a friend i found the house should have at least a 5ton system. So i battled with the builder and he agreed to install a 5 ton system. i'm still skeptical about performance since it is hard to tell the difference between the 4 and 5 ton systems by just hanging at the house. the units he installed are Carrier ahu - fy4anb060tco and condenser 25hba360a320. is this truly a 5 ton system? what SEER #? is a 5 ton sized correctly? also when both floors are in cooling demand very minimal air comes out of some diffusers upstairs. the builder also says that it cannot cool less than 72 deg or the unit will freeze. Please advice. Your help will be greatly appreciated. Have a great day!
07-16-2008, 04:35 PM
Tough question. I would have gone with the two separate systems sized properly from an accurate heat load analysis. I encourage people to have the house energy modeled before construction that way you can make informed decisions about construction details. If you talked your builder into increasing the AC size he probably doesn't know what size should be installed. Freezing coils is an airflow issue if the unit is charged properly but you shouldn't need to go below 72 IMO. Correct air flow can only be determine by a room by room load calc and then testing the system to verify performance. Was this done? The #'s look like a 5 ton 13 SEER builder grade heat pump.
07-16-2008, 05:03 PM
Thank you for the quick response. The AC Sub said he had run calculations initially and results came at 4.3 tons. That's why I said why not rouding up instead of down. Then they agreed on installing a 5 ton since they don't make 4 1/2. They are supposed to give me a copy of the calculations. I don't know if they did a room by room calc (which i doubt) but i will ask. That is a good point. I also spoke with a service tech at a big HVAC company who stated that they normally go with approx 1 ton for every 500 sf in Forida as a rule of thumb. Does a 5 ton unit for this scenario sound reasonable or does it look like it needs more? For some reason i did not feel much of a change when they changed from 4 to 5 tons, unless they have not set it up correctly or need to re-balance. thanks again for the advice.
07-16-2008, 05:41 PM
There is nothing wrong with a zone system as long as the ducts are properly sized and it has a by-pass in place with a freeze stat. It should work very good.
Maby you should have let the HVAC company size and do the house as they were doing before you talked to your "friend" and resized it for them. What kind of difference were you looking for between a 4 and 5 ton system. Were the ducts installed before you made this change. How did your "friend" size the system?
The only way to properly size a system is with a manual J. So much tonnage per square foot is a bad way to try to size things and will most likely be way oversized. Most manual J calculations are designed with the indoor temperature at 75 degrees for the summer. Why would you want to run it below 72 degrees? Most likely its not designed for that.
07-16-2008, 09:51 PM
Thank you for the response. The duct work was already installed when the unit got changed from a 4 to a 5 ton. The house was at punch list stage. The friend that did the calculations happens to be a mechanical engineer who designs HVAC systems for living. He did a manual J based on the house info I gave him and his result was approx 4.6 tons. I understand the fact that the unit may not be designed to go under 72 (which i personally don't like) deg but the fact is that we like it around 72-70 degress every now and then and the unit seems to be having a hard time getting to 72-73 during the day and even during the evening. We even installed a radiant barrier and low e windows and the unit is not cooling some rooms of the house as well as the rest of the house. I'm wondering if the problem may be a balancing issue.
I do appreciate your comments and especially your time. By the way I am a civil engineer and have worked managing big construction projects for 15 years but don't have much knowledge about residential AC systems until i started digging, reading and now I found this web site which i think is a great place to look for expert advice. thanks again.
07-16-2008, 11:08 PM
Just a sign of the times but the builder and a/c contractor caved. Throw out any rule of thumb. The top floor reduces the load on the ground floor while the top floor is under attic.
You or your engineer friend should have had to sign off on the up size. If the ducts and zoning system were sized for 4 ton and all they did was swap out equipment you have duct issues.
Zoning is a great way to get even temperature in a 2 story home.
72 at night shouldn't be a problem. Our indoor design is 75 degrees DB for cooling. But if your engineer used the 72 degrees that maybe where he got the 4.6 tons from. A dollar to a donut your system is oversized, b ut there is a silver lining. The restricted duct will cause a reduction in air flow which will lower the indoor coil temperature which in turn will increase the units SHR capability. As long as you don't get carried away with extended periods of low temperature operation it will proably take a year or so for the supply plenum interior to be covered in mold.
07-17-2008, 07:45 AM
Thank you for your sincere advice.
07-17-2008, 08:58 AM
Since the ductwork was already installed it is most likely undersized for the 5 ton system. This can cause many problems. If you don't have enough return then the supply will not work right. Is there a by-pass damper on the zone system?
Changing this in mid stream was not a good idea. I would have the HVAC company run some static pressure tests on the system to determine just how undersized the duct is and what options there are at this point to repair.
Around here mechanical engineers oversize everything to CYA. They do mostly commercial and usually do not do residential properly. Had one the other day size a 1400 square ft townhouse (middle unit) at 5 tons of cooling. Its a city job so that is what they want installed.
I don't know about your area but around here a zone system rarely calls for both zones at the same time or programmable thermostats are used to set back between bedroom areas and main house.
07-17-2008, 09:41 AM
Another point about a system that was not designed, installed, and commissioned properly is the extra energy cost due to the reduced capacity. Your system may only operate at 10 SEER or less, costing you more money to do the same cooling. I had a system last week that operated at 54% of capacity due to poor air flow, after corrections it was at 92% (measured). If you don't test it's just a guess.
07-17-2008, 09:58 AM
Great pointers. They did increase the return. I don't know if they installed the bypass but I recall it being mentioned. I will inquire about the bypass and getting some static pressure readings.
thanks. i truly appreciate the help and the very quick responses.
07-17-2008, 10:04 AM
Your supplies are under sized.
Just increasing the return isn't enough. Thats why they said it will freeze if you set it under 72.
You have a 5 ton condenser and fan coil. But you won't get 5 tons of cooling.
07-17-2008, 05:29 PM
Ok, so the builder confirmed today that his ac sub increased several supply ducts as well. I saw some of the work myself. They will provide me with detailed information on the adjustments they made. I asked the builder to find out about the freeze stat and the damper by pass. They will let me know probably next week since he is taking the day off tomorrow. I also spent some time at the house today and it felt much better, with more air flow at the rooms that used to be considerable warmer and I can really tell the difference now. they will do some checks per all the suggestion you all have made. Your advice has been very very helpful. Thanks again and god bless!
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