Disagreement on sizing for new AC system

AlexT · March 2016

I recently bought a new home with hot water heat. The previous owner ran flex duct for AC in the 2nd and 3rd floors but did not include an air handler or other equipment. Apparently the duct wasn't installed particularly well. For example it's all six inch duct instead of varying the size depending on the purpose. The contractors think it can be made to work with tuning. I'm not expecting a perfectly even system and being in New England I won't be using it a ton so this is acceptable.

I've had three well regarded HVAC companies out for a quote, and I'm not sure what to make of what they said. The first company came out, measured the rooms, ran a manual J and proposed a three-ton single stage system. The second guy didn't measure rooms and said there is an application that lets you measure the conditioned space using an aerial view of the house. He believes that due to the ducts all being flex that he can only get two tons of cooling out of the system. He also suggested a two stage system for more comfort. The third guy, measured, and also suggested a three-ton system and downplayed the flex duct loss. Eventually he told me he declined to bid as he doesn't like installing into existing systems.

I don't have time to get another quote as I have a pending insulation contractor coming next month. Any ideas on which route I should take? I've read that a smaller system will be more comfortable, but i don't want it undersized either.

ChrisJ · March 2016

I'm not a pro, and I don't know what to make of it either other than this aerial view stuff sounds like BS to me. It's hard enough to do a heatloss one room at a time manually and get it fairly accurate.

Doing it off of a picture from a satellite image? Come on.

Just out of curiosity, how many square feet is the house, how high are the ceilings, are there a lot of windows and how well is the place insulated overall?

Bob Bona_4 · March 2016

I would size the equipment to manual j and deal with the distribution system as needed. Man J will say what size ducts needed to each room.

I would not size the equipment to the distribution system. It would likely be an error following another.

You can stop the project to correct things, you can't easily do that once everything is closed up.

John Mills_5 · March 2016

I've seen an aerial view website. They said my 970 sq ft house was 1600 sq ft. Guess how accurate using that service would be.

The guy that ran a manual J and came up with 3 tons is likely right. Then he needs to see if you have 1200 CFM worth of ductwork. If not, he will have to correct that.

In a mild climate with a system that won't be used that much, a basic 13 SEER system is all that makes economical sense. If you were in Florida or a similar steamy climate, 16 SEER 2 stage may be a wiser move.

AlexT · March 2016

The 2nd and 3rd floor are approximately 1200 sq/ft total. The 3rd floor is a converted attic with high ceilings and skylights and presumably would get hot in the summer. The first guy quoted a Lenox 14 SEER. The other one was 17 SEER Carrier Infinity, which I know is the top end of the Carrier stuff.

Bob Bona_4 · March 2016

I'm a contractor in SW CT. A popular option is a variable speed air handler and 14 SEER condensers. Bumps the rated SEER to about 15.5, qualifies the client for Eversource rebates.

Is this to be a zoned system?

Mike · March 2016

I agree with John, the manual J is the way to go. I'm curious about the returns? The locations, number of and sizes. Are they already roughed in? The Infinity is a great system for comfort. It's made to be used all the time. Not really economical. But it does have the benefits to dehumidifiers and overcome SOME duct flaws. I'm in jersey, and the humidity beats the hell out of me. I was in San Diego, on a 95 plus degee day. No a/c and I comfortable. Low humidity. So talk to your contractors about a dehumidifiers control.

AlexT · March 2016

No, it won't be zoned. The 14 seer isn't eligible for the zero interest loan or the rebates, but still doesn't make a ton of economic sense going to the more efficient unit.

Can anyone speak to flex duct resistance versus metal duct resistance? The second guy strongly felt flex duct won't in practice allow the rated CFM that was installed.

aircooled81 · March 2016

So, bob bona was right, dont compound your problem.
If in fact the ductwork is undersized, your new expensive equipment will not operate correctly.
Now a 6" duct to a single register is not uncommon at all. But a 6" main feeding many 6" registers is fairly uncommon. Sometimes carpenters know what fits in a wall cavity, but not how much air fits in a duct.
So a 6" duct (wireflex, aluma flex, kd snap lock) will move about 160cfm @.2"wc static, and about 560cfm @2.0"wc duct static. Not likely youll find an airhandler on the residential side able to create more than 2" duct static. So if in fact all of your ducts are only 6", you will not have enough air volume for the equipment to operate properly and cool the load.
So, expect some main ducts to be upsized.
That being said, a 1200sqft poorly insulated house will need atleast 1cfm per sqft. Your 3ton ac sound ok, but you need to move 1200cfm through it. Out here, shooting from the hip 1ton of cooling for every 400 sqft of floor space to be conditioned. (Other factors can change this, but loose rule of thumb here folks).
The two ton system may be more suitable for your undersized ducts, but you may find on days hotter than 78*f it runs all day and never achieves your ideal temp. Do you prefer some cooling, or the cooling required to condition the space?
Two stage is very nice upgrade, little bit of energy savings there. If you live in a humid climate, make sure the fan speed will drop significantly - or your stat operates off humidity, as two stage cooling with one fan speed doesnt ring out the moisture as well. Two stage cooling with proper speed changing fan can certainly work well.

MikeJ · March 2016

Using a duct sizing calculator. 6" round metal at a .10 friction rate will do 110 cfm, wire helix flexible duct at .10 will do 80 cfm

Big Ed_4 · March 2016

Just to add...Depends where you live , but what you mentioned New England and you would not be using AC an ton. , must be an pun .. First in the North East humidity gives us our discomfort by sweating . Smaller systems are more efficient if you can get set temperatures out of your head .We should use humidistats rather then thermostats ....The engineers recommend designing high efficient system from our heat gain calculation at 12* delta T .. Meaning at outside 82* you can bring the inside temp down to 70* ... And if it's 92* outside you can get it down to 80* inside ,Yes this is an hard sell, the smaller system will use less juice but run longer and pull more moisture out of the house .. 80* and dry is very comfortable ,to state my point , say Aruba .. I normally explain this to the potential customer before I design an system . But everyone especially men want an meat locker

. And settle on an 20* delta T .

The previous home owner under construction just asked for ducts .And they throw in all 6" with out an design .. As I am sure it was explained to you too large of an system and not enough duct ,return and supply will freeze up that condensate in the cooling coil and will increase static pressure .. Noise and discomfort from the force of the cool air blowing against you, too much throw... . Too large of an supply duct it will not mix up the air in the room , too little throw. ( return should be 120% larger then supply , with the proper size filter return box) ..

As you can see that third contractor that walked away had the most experience ... Good luck with your decision ..

unclejohn · May 2016

You can always just see what the neighbor has in his house.

Disagreement on sizing for new AC system

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