Where is the house located? I good HVAC contractor will do a load calculation to determine the size of the heat pump. The house size and climate helps people make a gross estimate but it is no substitute for a load calculation.

Air handlers have speed settings which regulate how much air will flow. A 5 ton air handler means it can provide up to a maximum of 2000 CFM but could be adjusted to operate at a lower rate. An AC condenser needs about 400 CFM per ton, so your 5 ton air handler is probably set to push 1600 CFM of air. Paying for a larger 5 ton air handler is probably not going to have much bearing on how your heat pump will perform. In some cases moving up from a 4 to a 5 ton air handler may provide a better SEER rating, but the difference is probably small.

The question you should be asking is what is the cooling load. The load numbers are used to select the appropriate size heat pump with a matching air handler. Heat pumps have to be sized for the cooling load. Any deficiency in the heating load is made up with the auxiliary heat.

Your plenum is a little undersized for a 4 ton (1600 CFM) heat pump. If you could make it 25x10 or 20x12 without too much trouble then I suggest spending the extra money.

I good HVAC contractor will do a load calculation to determine the size of the heat pump.

But yet you will look at the insulation in the attic and guess at the R rated value?

Will you open the walls up to see the R rated value of the insulation in the walls or will this be another guess?

You will measure the windows, but you will guess yet again as to what type of window they are. Same thing with doors.

A manual J is a tool, but at the end of the day it is often still just a more elaborate guess.

If the customer wants a 5 ton, I could make the system a 5 ton with a flick of my manual J wrist... just as much as I could make it a 4 ton.

Buyer Beware.

Thank you to everyone for commenting. But I suppose these responses has raised another question. After reading up on cooling load calculations (and determined no companies did that during their estimates) I realized that by following the "rule of thumb" of approx. 400CMFs per ton, my suggested size heat pump should be closer to 6 tons (2283sqft / 400 = 5.7)?! Obviously this seems excessive/inaccurate, but maybe my condenser is undersized?

By the way, I live in northern Florida (Jacksonville). Thank you!

Not really. You definitely don't want to oversize a heatpump in a humid climate.

Not knowing the exact values of the R values of walls and attics is in my opinion is just an excuse for not doing a load calculation. You can climb into the attic to see what is there and make an educated guess of what the walls have. Yes you can enter garbage into the calculation and garbage will come out. It may be not be precise but it is still better than using rules of thumb based on the area of the house.

Where did you come up with this formula?

2283sqft / 400 = 5.7

This is exactly the types of rule of thumb that in most cases result in oversized equipment. You live in a humid climate so the last thing you want is an oversized AC. Sizing the AC correctly will make you more comfortable, cost less money to operate, and potentially extends the life of the equipment.

Ok Mike, that must be why on occasion I get a call from a customer that has multiple manual J's done with each of them coming up different from one another.

So after all that what do you do?

Put them in a magic hat and pick one?

My point is: Pick an experienced contractor and don't look back.

400CMFs per ton, my suggested size heat pump should be closer to 6 tons (2283sqft / 400 = 5.7)?! Obviously this seems excessive/inaccurate, but maybe my condenser is undersized?

400 CFM's mean 400 cubic feet of air per minute per ton of air conditioning to meet the rated capacity of the machine. This equation should never be used to size an AC to the square footage of a home.

As an example if you have a 4 ton AC and supply air is capable of delivering 1200 CFM's the AC capacity is limited to roughly 3 tons of air conditioning even though the unit's rated capacity is 4 tons.

There is more to sizing than just hooking up boxes. Sure the boxes are magical to a degree, but not that much.

The 'easiest' way to determine if your equipment size is adequate is to look at your light bill and study it. If your utility bill is out of alignment in consideration of what equipment you currently have only then would I look at sizing issues.

This takes guessing out of the hands of the ignorant, and puts it back in your hands. If you oversize equipment and it's wrong, the cure is to rip it out and start over.

If you undersize the equipment, the cure is to go in and re-insulate.

Some manufacturer's use a 5 ton rated evaporator coil for both 4 ton and 5 ton condensers. The discrepancy you have doesn't necessarily mean that much.

"As an example if you have a 4 ton AC and supply air is capable of delivering 1200 CFM's the AC capacity is limited to roughly 3 tons of air conditioning even though the unit's rated capacity is 4 tons."

Where does this come from, and how does it relate to latent vs sensible cooling capacity?

An air flow of 1200CFM for a 4 ton AC is probably going to result in a frozen coil.

Where does this come from, and how does it relate to latent vs sensible cooling capacity?

In our opening statement we quoted a rule-of-thumb about air conditioning requiring 400 CFM per ton. This value is usually the basis for rating self-contained air conditioners. Thus, we have a 2-ton unit handling 800 CFM, a 3-ton 1,200, a 5-ton 2,000, a 10-ton 4,000, etc. With 80° F DB 50% RH (77 gr/lb) air entering, these units usually will do about 70 percent sensible cooling and 30 percent latent cooling. On this basis, a 3-ton unit would do 36,000 Btuh total cooling, 25,200 Btuh sensible cooling and 10,800 Btuh latent cooling. With 400 CFM per ton, air would be cooled according to: T2 - T1 = 25,200/1,200 x 1.08 = 19.4° F and would leave the coil at 80° F - 19.4° F = 60.6° F. Air would be dehumidified according to: G2 - G1 = 10,800/1,200 x 0.68 = 13.2 grains and would leave the coil at 77 - 13.2 = 63.8 grains or 57° F WB.

400 CFM per ton

An air flow of 1200 CFM for a 4 ton AC is going to result in a lack of cooling and higher than normal light bills.

Usually to freeze the coil it will need additional help. Merely dropping air flow across coil will not freeze a coil in most cases. (IE ambient temps are 70 degrees or higher, air filters are clean, refrigerant charge is proper, e-coil is clean... etc.)

Blanket statements do not work in this business.

"On this basis, a 3-ton unit would do 36,000 Btuh total cooling, 25,200 Btuh sensible cooling and 10,800 Btuh latent cooling. With 400 CFM per ton, air would be cooled according to: T2 - T1 = 25,200/1,200 x 1.08 = 19.4° F and would leave the coil at 80° F - 19.4° F = 60.6° F. Air would be dehumidified according to: G2 - G1 = 10,800/1,200 x 0.68 = 13.2 grains and would leave the coil at 77 - 13.2 = 63.8 grains or 57° F WB."

OK, thanks. So if I understand this correctly, you may not have 4 tons of cooling but you'd have more than 3?

"Merely dropping air flow across coil will not freeze a coil in most cases."

I'm sure 3M would love to quote this in their Filtrete brochure! ;)

Well within reason and depending on a list of criteria an arms length long... no air flow will freeze the coil. But it is VERY rare to find a coil that is frozen due to only a plugged up or restrictive filter. This doesn't mean it doesn't happen. However, 99% of the time it's due to a clogged filter plus something else.... typically refrigerant leaks.

You can not have a 4ton, 5 ton, 3 ton AC whatever size it is. If it doesn't get the proper air flow (400 CFM per ton) it will not run at rated tonnage. Exactly what tonnage it would be would be a guess... so you pick the number.

Although it is necessary to have a proper plentum size it is more important to have the right size blower motor in combination with the air condition equipment along with Talk work that is easily airflow able without restriction.

www.heating-repair-chicago.com

Thank you to everyone that replied in this thread. I got a lot of good information from you guys and it made me get a few more estimates from other companies. Unfortunately I don't think I'm any closer to making a decision. With new information from another company, it seems that simply widening the plenum in the "length" direction won't necessarily help the "bottlenecking" of the plenum. He said the only way to truly get better airflow would be to widen the "width" part of the plenum, which is between trusses 10 inches apart. He said that they could cut the restrictive trusses (reinforce them of course) and build a larger plenum box, but told me that this is technically illegal (in the sense that it goes against builder code). Because of this he gave three options: a) leave the plenum as-is, keeping the bottleneck; b) cut the trusses and build out a proper sized plenum for the new air-handler; or c) completely move the air handler from an inside closet to the attic or garage.

So at first I was leaning towards the "cut the trusses" option, but I am a little hesitant even though the guy says it would be verified by their engineer first. And since I'm in Florida I don't really want the air handler in the attic for heat reasons. From what I have been reading the garage isn't exactly ideal either, making the system work harder in the hot months. It seems the ideal setup is to put an air handler in conditioned space, but in my small-plenum scenario would I be better off moving it to the garage?

Thanks again for all the help everyone.

As far as having the handler in conditioned space or in the attic,consider this. If ducts run through attic, figure the total square inches of ductwork surface as well as that of air handler. I think you will discover air handler only has 10 to 15% as much surface area as ducts. If that were true,having the handler in the attic with ducts isn't going to make that much difference. Now if you can have ducts in conditioned space it take's quit a load off the equipment and save's energy. You can easily put additional insulation on the handler if desired. If you make it a diy project think about this. Glue/tape/screw squares of insulation board to handler leaving screws exposed for easy panel removal for service and repair. Don't get bogged down trying to completely cover all of the handler,75% coverage will do fine. I saw one guy who applied spray foam to the handler,looked dreadful but non the less effective.

All things being equal... having your ducts and equipment in the attic doesn't make for high energy bills in and of itself...

'What does this mean?'

It means that an air conditioner's performance is typically decided by the sum of it's parts. Example if you replace the system, duct work and the system is 'Installed Properly'. If you approach it with 'replace a box' mentality then don't expect high marks on efficiency of the system.

I live in Katy, Texas (far west Houston suburb) I have my AC and ductwork installed in the attic, but yet I pay < $35 a month for AC use.

Sum of the parts: 2 stage 18 SEER AC with R8 duct work with 4 zone system. The attic has been re-insulated and is well ventilated (using natural methods... ridge vents, soffit vents etc.

Wade thru my electric bills for 2015: (I can tell you that 2016 wasn't much different I just haven't made the video yet.)

Duct work sitting in an attic that is at 100F degrees will have more loss than duct work located within conditioned space. That loss is added to the load calculation when determining the heat load.

Currently all my ductwork is in the attic, so I suppose moving the air handler to the garage OR attic won't effect the overall efficiency (positively or negatively)? One difference is that the air handler would move from a conditioned space, centrally located in the house with one return, to a new location in the attic or garage with at least two returns on opposite sides of the house.

What prevent's you leaving return as is and adding a second return? With all the jawing about ductwork in conditioned space,maybe run second return in fur down.

The current return is simply the bottom of the air handler, and pulls the air through a louvered door (closet). Not sure how I'd setup another return in that scenario. Anyway, my main issue is the small plenum size (leaving the handler). The talk about returns were secondary to whether or not it's recommended to move the air handler or keep it where it's at.

I said from the onset that it's near impossible to help without seeing the complete set up but how bout this? If there's space between handler and ceiling,go through wall to reach another truss bay where a second supply plenum can handle a couple of existing or additional supply ducts.