4 ton-5 ton

[Deltarat]

My AC in my house is not quite enough when the temps go over 100*. I have a 4 ton unit. The inside part is pretty old, but the outside unit is newer. A heat and air guy said since the outside was fairly new, I could replace the inside since the cabinet is starting to rust, with a 5 ton with a 4 1/2 ton orifice-that maybe the wrong term. He said it would give me the equivalent of 4 1/2 tons. Then at a later date replace the outside unit with a 5 ton when it goes bad.
Do you heat and air guys think this a good approach?

 

[philjafo]

How new is fairly new? It's possible that a 4 ton is the correct size and the old mixed/matched unit may not be performing at 4 tons. If you replace the indoor portion be aware that new equipment uses r410a and old stuff uses r22 (started in 2010) the two are not compatible. While you can still get r22 air handlers and dry condensers (empty of refrigerant) who knows how long you will be able to. Then you would be stuck with a newer air handler and a broken compressor that you can't repair. I would suggest replace it all now and use a company that does a heat gain calc on the house, not just trusting that whoever installed the first system put in the right size. It's a bigger investment now to replace the whole thing but can you afford to do it twice and live with a system that may or may not be right for the house.
My favorite thing that my great grandad always used to say sometimes "If you don't have time to do it right the first time, how are you going to find time to fix it."

 

[danski0224]

My AC in my house is not quite enough when the temps go over 100*. I have a 4 ton unit. The inside part is pretty old, but the outside unit is newer. A heat and air guy said since the outside was fairly new, I could replace the inside since the cabinet is starting to rust, with a 5 ton with a 4 1/2 ton orifice-that maybe the wrong term. He said it would give me the equivalent of 4 1/2 tons. Then at a later date replace the outside unit with a 5 ton when it goes bad.
Do you heat and air guys think this a good approach?

No.

You need to have a load calculation done to determine the proper size equipment.

You do not mention the indoor temperature that you are trying to maintain. For what it's worth, residential air conditioning combinations are rated at an indoor temp of 80*F. Once you go lower than that, some other stuff needs to be factored in.

Once the building load and your personal indoor temperatures are determined, then the equipment size can be determined.

Then, the ductwork needs to be evaluated to see if it is properly sized to the airflow requirements of the proposed equipment.

If you do not have ductwork capable of passing "5 tons" of air, if you really need "5 tons" of cooling, you will NEVER get "5 tons" of cooling out of the system... even though you are paying to move "5 tons" of fan and compressor.

That's the right way to do it.

The hack installer or cheap customer will just slap in a bigger unit and see what happens.

Good luck.

 

[Deltarat]

I have a large truck line in the attic-16'x20-24" that runs the length of the house with what looks like 8" drops to each register for the rooms if that means anything. The guy has been in the business for 40 years and does a lot of residential and commercial systems, for what that is worth. Whether he can do the calculations you mentions, I do not know. I guess he should be able to, to size new equipment. I will check next time I talk to him.
If the ductwork will support it, would this work?

 

[danski0224]

If the static pressure losses from the duct (supply and return) + A/C coil + filter is less than or equal to the static pressure capacity of your fan at the desired CFM, then yes, it will work, if the duct system is air tight.

If it is sized right, but the ductwork isn't sealed, then it still won't work (1) as well as it could to (2) no real improvement. A "duct blaster" test or duct leakage to outdoors test will tell you how good/bad your system is.

 

[JimL]

Miss matched equipment sometimes doesn't work at its best. How much insulation is on the ductwork in the attic? Could be a big part of it. Flex duct alone is only R4-R8 at best. Needs covered with bat insulation.

 

[danski0224]

Depending on the climate zone, adding another layer of insulation (batt, wrap or blown-in) *over* existing ductwork insulation with a vapor barrier can be a very bad idea. Typical insulated flex duct has a vapor barrier.

In general, duct in an attic ***** from the start, unless the attic is part of the building conditioned space envelope.

And yes, mismatched equipment, like installing a high SEER condenser on an existing low SEER indoor coil will result in very low SEER operation. There are several industry white papers out on this, basically 13 + 10 = 8.

Unfortunately to the typical homeowner, the system appears to cool.

 

[Deltarat]

And yes, mismatched equipment, like installing a high SEER condenser on an existing low SEER indoor coil will result in very low SEER operation. There are several industry white papers out on this, basically 13 + 10 = 8.
Would the reverse be true also? A new indoor coil and an older compressor?
I went around and sealed any leaking air from my ducts that I could find.

 

[danski0224]

You can put a new A coil on an older system.

Trouble is, there are no new drop-in R22 coils being made available for purchase, at least within countries that are honoring the Montral Protocol.

The "dry R22" condensers are exploiting a loophole and the recent runup of R22 refrigerant pricing partially due to these units that were not factored in.

The other issue with a new coil on an old system is the time and money invested in repairing something that is usually old. Then, the other part breaks.

 

[Mike007]

My AC in my house is not quite enough when the temps go over 100*. I have a 4 ton unit. The inside part is pretty old, but the outside unit is newer. A heat and air guy said since the outside was fairly new, I could replace the inside since the cabinet is starting to rust, with a 5 ton with a 4 1/2 ton orifice-that maybe the wrong term. He said it would give me the equivalent of 4 1/2 tons. Then at a later date replace the outside unit with a 5 ton when it goes bad.
Do you heat and air guys think this a good approach?

I see this scenario play out quite often: 3 ton unit installed in a 2-1/2 or 3 ton house not cooling the house due to ductwork issues. "Contractor" comes in, never even considers the ductwork, and installs a 4 or 5 ton unit to solve the problem and makes the problem worse. The H.O. has just thrown away several thousand dollars that could have been used to address the true problem.

 

[DPelletier]

some good advice here. A couple of thoughts;

- a good refrigeration tech can tell you if the unit is functioning properly or not.

- While there may be some merit in the concept of using a slightly larger evaporator coil, the mismatch of the refrigerant can cause problems.

- to echo what some of the other posters have said; everyone forgets that these are all parts of a system and you can't just replace the condensing unit and evap coil and call it good without ensuring adequate airflow which means the blower and ductwork have to be able to support the added capacity. Figure on 400 - 500 CFM per ton of cooling capacity. Residential ductwork is usually designed at 0.1" static pressure and too much more than that will likely increase the external static on the blower until you can't maintain adequate airflow and the coil freezes.

It's sorta like throwing a 454 in a Chevette and then being surprised when the transmission can't take the abuse!

Dave

 

[DPelletier]

Depending on the climate zone, adding another layer of insulation (batt, wrap or blown-in) *over* existing ductwork insulation with a vapor barrier can be a very bad idea. Typical insulated flex duct has a vapor barrier.

In general, duct in an attic ***** from the start, unless the attic is part of the building conditioned space envelope.

Just to clarify; as long as there is a vapor barrier on the insulated ductwork installed in an unconditioned space, it is fine and the more insulation the better. I actually prefer to run ducts in attic spaces rather than have ugly, inaccessible drywalled bulkheads in the ceiling spaces. I use a minimum of 1" insulation with integral vapor barrier in attics and other unconditioned spaces and a minimum of 2" exposed to the outdoors. My crews just installed some 78" x 36" duct on a rooftop last week. It had 2" of internal insulation.

Dave

 

[Deltarat]

Does the vapor barrier go next to the duct or on the exposed side of the insulation?

 

[DPelletier]

on the exposed side. Typical is one inch flexible thermal insulation with integral vapor barrier that is reinforced foil. The silver foil is exposed and that's what you see.

Dave