One or two units for 4500 sq. ft. home?

[HDNord]

Home I'm having built is approximately 4500 sq. ft. with 2250 on the daylight basement and 2250 on the main floor above. I'm in the process of getting bids for HVAC and have 3 bids so far. One bid, $11K, was sized for one 5 ton A/C unit zoned for each floor. One bid, $13K, was for a 2 ton A/C in the basement and a 3 ton unit for the main floor. The other bid was $14K, 2 1/2 and 3 1/2 ton units.

What are the advantages/disadvantages in having one unit that is zoned compared to having two separate units for this type of home? Do the bid prices seem reasonable? Heaters are 90%+ efficient and A/C's are 12 SEER.

All bids include vent ducting for 4 baths and range; and natural gas lines ran to furnace(s), range, water heater, dryer, two fireplaces, and one outside gas line for a BBQ. One bid also has as an option for a 36 inch fireplace with vent kit for $895, $1095 for a 42" fireplace.

[rabadger]

Lets put first things first. The only way you will be able to tell what equipment is correct is by doing a set of load calculations to determine the BTUH needed and required air volume for the system. You can't size by square footage. Sizing by square footage is just guess work. Without knowing where you are building I can't answer. There are just to many variables to consider. You called three contractors and not two sizes were the same.

I had a home that had two systems in it. I now have a home twice the size with one system and 3 zones. Its less costly to operate, keep up, and more comfortable.

I get the gut fealing that none of the quotes were prepared by sizing the system properly. I hope I am wrong. Did any of these contractors do the load calculations on the home and explain them to you?

Go to my web site and visit the links page. Spend some time reading the consumer info on the ASHRAE and ACCA sites.

[HDNord]

Perhaps my post was not very clear but what I'm meaning to ask is the advantages/disadvantages/preferences of having one larger A/C heating system for the entire two story home compared to having two smaller systems.

Proper sizing is another issue although two of the three bids so far do have the same total BTU's for cooling. The bids I have received have been faxed to me and the bid with one system for the entire home does have load calculations. I have not followed up with the other bidders to determine how they sized the units.

I'm waiting for another bid before I follow up with the bids and last bidder indicated he would also would be doing load calculations and I suspect he also will be designing the system with one large zoned unit.

I have spent time on your web site and the links and unless I missed it have not seen any information on the question I'm asking.

[rabadger]

I had a home that had two systems in it. I now have a home twice the size with one system and 3 zones. Its less costly to operate, keep up, and more comfortable.

[GiantScale]

Id lean toward two. You do have twice the maintenance. But one plus is that you would always have back up heat just in case. Main thing I dont like about zoned systems is that you need a dump zone if only one zone is calling. Unless the system is 2 stage. Dampers will break and sometimes whistle. Then ask them if they used manual J for load calculations. Bet none of them do. But its the most accurate. I have a 1970 2600 sq foot home and removed a 4 ton and installed a 2 1/2 ton condenser on a 3 1/2 ton coil In Iowa. It runs alot. In fact, I set the stat to 50 in the a.m. and let it run nonstop during the day when its 85+. It cycles one time a day. When I turn it off. It may take a day to get it down to 65. But its drier than a pop corn fart. Very comfortable and pulls 9 amps instead of almost 20 with the old 4 ton.

What Im saying is dont oversize. If it cycles allot (more than one an hour) its over sized and wont pull the humidity.

Meet the "crews" who would do the work. Then go with the group who you feel most comfortable with gut feeling.

[ODDJOB]

Two units on a multiple floor house will allow better temp control and independent to both areas

[rabadger]

All two units do for you is create higher upkeep costs. I would use two stage equipment and 24V zoning and variable speed drive equipment. Much better humidity control.

Using two units results in over sized equipment and conditioned air rising up and falling down stairways without any conctrol of recirculation. If you want just air movement you have to run two fan motors not one.

Of course the contractor installing the one unit and zoning system must know about duct sizing and stay away from the cheap way of doing things. Dump zones are not needed if done correctly, with a counter bypass static pressure damper.


But if you have the load calculations and duct design done in advance by a third party that has your goals in mind you would come out ahead on the whole deal. The bidding contractors would be forced to quote on the same system then give alternitives at deducts.