# Air Handler mismatch with compressor

I have a situation where the air handler is 5 ton and the compressor is 3.5 ton (both 2009 models) The HVAC guy says that this was done in order to meet the SEER requirements or something to that effect…

Does this make sense? Can the system be really more efficient under this scenario?

Thanks for you input!

Does this make sense? Absolutely. This is how you obtain a higher SEER rating. Can the system be really more efficient under this scenario? The air handler will be oversized to meet the SEER rating. And yes it will be more efficient.

Why did you not believe the AC contractor?

I smell smoke when you say ahu does that mean refrigerant coil or blower or both.

Standard protocol worst case is to never mismatch more than 1/2 ton over or under between the compressor and the refrigerant coil The CFM of the blower is also calculated to match

As the Master HVAC Mechanic (retired) states - Protocols are such that the mismatch shouldn’t be more than 1/2 ton…

Why didn’t I believe the HVAC contractor? Because I have known them to be wrong and the above stated protocols have been in our toolbox. I haven’t disagreed with the man, I am merely finding out if what he is saying is correct.

If the systems are so much more efficient this way - Why aren’t all systems installed in such a manner?

I am NOT an HVAC professional. I am trying to improve my knowledge base here. Is it not right to call out the mismatch and refer it to a professional?

Would he be able to look them up on the AHRI Directory, Charlie?

Sure if you want to use engineering formulas like this one;-)

6.4 Polynomial Equation. The polynomial equation that shall be used to represent the tabular data is a third degree equation of
ten coefficients in the form of:
X = C1 +C2 · (S) + C3 · D +C4 · (S2
) + C5 · (S·D) + C6 · (D2
) + C7 · (S3
) + C8 · (D·S2
) +C9 · (S·D2
) + C10 · (D3
) 1
where:
C = Equation coefficient, represents compressor performance
S = Suction dew point temperature, F C]
D = Discharge dew point temperature, F C]

That’s true.
Back in the late 70’s and early 80’s that’s how Coleman was achieving their high SEER rating. They were selling them as a package deal.

We installed a lot of 3 ton condensing units matched with 5 ton A coils plus a liquid line shut off solenoid valve and an indoor blower time delay relay in those days.
I had one in my own house until 3 or 4 years ago.
As I recall the SEER was 12.4 or so then.

Of course you can’t (shouldn’t) go the other way and use a smaller A coil.

What was the sweat line on the coil I have a hard time believing that a 3 ton condensing unit can hold enough liquid in its condenser to prevent a 5 ton coil from being starved. I was taught in two different A/C schools that you never over size the A-coil by more than 1/2 ton. To be honest I have never seen a MFG increase the size of the indoor coil to achieve a higher Seer rating but I am not a mechanical Engineer either. My observation has always been to increase the size of the outdoor condenser on the high seer units. Carrier was very famous for this when seer rating became a issue. They increased the size of the outdoor condenser X3 over a standard 12 seer unit, and had this little compressor wrapped in a blanket could barely see it.

BTW Trane also did the same thing monster condensing unit same size coil

I forgot to mention that those units also had an over sized condenser.

I’m trying to remember what Coleman called that particular system, if I remember correctly, the next step down was called DES for Deluxe Energy Saver.

So is a Trane 5 ton outdoor unit ok with a 4 ton air handler?

Geez ya went back a ways to find this thread. Yes the unit would run but not efficiently. I still say 1/2 over 1/2 under was always the rule

Thanks Charley… yes I tried to research it… and found this thread during bing search and most opinions went to what you pointed out. The only time I get these kinds of questions, it seems, is on the weekend when the manufacturer can’t be reached. Thanks again.

Did you get this off the compressor or the condensing unit?
I hope your took it off the compressor.

A basic definition of an air handling unit (AHU) might be “a box with a fan, coils, and filters.” From there it gets considerably more complicated. Proper selection of an air handler requires answering myriad questions ranging from “what capabilities are required?” to “will it fit?” Only after establishing these basic project constraints can the art of evaluating and selecting an AHU begin.
Before starting this process, it’s important to realize that there will not be a “perfect” selection for any AHU as many competing criteria, not the least being cost, will force compromises. It is the engineer’s job to balance and prioritize all of the decisions related to performance, efficiency, maintainability, and space constraints to select a unit that has the lowest lifecycle cost for a given application.

So I have 5 ton Lennox gas furnace and air handler which is really too large for my house and just came upon a brand new 3 ton goodman condenser and coil for great deal. So if I understand this correctly, you CAN put a 3 ton coil and condenser on a 5 ton air handler. I have gotten mixed information with local HVAC guys and need to figure this out. Thanks for your help.

In my opinion (don’t) The 5 ton furnace will have a to high a CFM across the coil for proper heat transfer. If the blower motor was a multiple speed motor and was able to match the CFM for a 3 ton condenser it perhaps might work. But again you might be unhappy with the results.

What is the present size of your condensing unit (tons) and whom made the determination that your AHU is to large. Is your home 2 story and what is the square footage and what is the age of the home, How much insulation in your attic, what type and age are your windows.