Replacing AC unit(s)

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The 2.5 is 30 years old

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I don't have anything to contribute other than to say that that's impressive.

I think the best advice I can offer after studying this in depth the past few years is to place your emphasize on finding the best installer, however you are able do this, word of mouth, references, etc. with manufacturer/brand name being secondary. That said I’d be comfortable with all major name brands - Trane/American Standard, Carrier/Byrant, Rheem/Ruud, etc.

By and large there is not major differences in equipment quality between HVAC manufacturers, most use common parts manufactured by third party companies, but certainly with some components, like condenser coils, air handler and condenser cabinets, or in a few cases compressors being proprietary and manufactured by the companies. There are more differences between lines/models of equipment made by a single manufacturer than between manufacturers.

As you know HVAC are not plug and play like refrigerators, washers/driers, etc. There is a great deal assembly of all the HVAC components in the field (your house), cutting, brazing, pressure testing, vacuuming, installation of filters/driers, adjusting and dialing in refrigerant charges, etc., testing static pressure and adjusting factory set blower speeds if needed, etc. and the little details are so important as to reliability and longevity of the system, and as in other occupations not all HVAC techs are created equal. For example, having to replace 5 or 6 evaporator coils in that one unit, even though evaporator coils are problematic, indicates other issues with that system causing the evaporators to fail prematurely, that the HVAC tech did not/could not identify and repair.

Larger companies often use separate crews for installation and often older, experienced techs for service and repairs. I’d personally see if you can find a company where the installer of the new equipment are the same guys will likely be sent out if a problem arises and a repair is needed.

Ask if the company does a commissioning report after the install, given to you, that shows they have completely tested the new system and verified it is operating within all parameters as recommended by the equipment manufacturer.

As far as SEER, single stage 14 or 16 SEER systems are the simplest less expensive units and easiest to diagnosis and repair if and when problems arise. Higher SEER units are more expensive, but use less energy and at worst you should break-even on the payback period before the unit needs to be replaced. Highest SEER 17 to 21, two stage or multistage (inverter unit) units jump up in price considerably, and are more expensive to repair, and require experienced, knowledgeable HVAC techs trained on that equipment to diagnosis and make repairs. House better be designed and constructed just right to take advantage of those very high SEER, multi-stage units, and think of it as you are buying comfort, not saving $ with lower operating costs, b/c by the time you break even, it will be the time the units needed to be replaced.

Lastly, have a manual J and ideally manual D performed to determine the proper tonnage of the replacement units and to determine if your duct sizing is correct. Don’t let them go by rules of thumb for sizing, and maybe you’ve made energy improvements to your house over the years whereby less tonnage is required. HVAC is one area where you don’t oversized equipment. Most experienced HVAC techs state the most common problem they see with clients with home comfort problems are over-sized HVAC units and undersized ductwork.

Anyway, my thoughts. and I typed to damn much - didn’t mean to.

you sure you dont have chinese sheetrock in your home? you arent getting corrosion on wiring too in the home are you?

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If you keep having to replace evaporator coils check your return and make sure it is properly sized. You should have 200 square inches per ton. If not, the air will not remove enough heat from the coil land it makes the metal brittle and fail.

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I went through multiple companies trying to figure line out until a local self-employed tech came and looked up my return (mine is low in the wall). Had the right size return but there was only an 8 inch hole cut at the top so I was severely undersized for air flow across the coil. My grandpa also recently went through the same thing, called my new ac guy and he replaced his and added another 6 inches to his return.

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This a good example of the importance of a manual J and manual D, as well as testing static pressure on a HVAC system. A manual D (sizing of ducts) would have specified the correct amount and size of supply and return ducts to provide that 200 sq in per ton, or 400 cfm per ton of return air required by HVAC system systems, or after the fact a static pressure reading of return air flow would have likely identified that the HVAC was not getting sufficient return air. Return air and supply air static pressure, and total external static pressure (TESP) meaurements on a HVAC system is a very simple and quick procedure that can help ID HVAC air flow issues, and apparently is seldom performed by HVAC techs in our area, but it should be part of a HVAC commissioning report with installation of a new/replacement system, or done when air flow issues are suspected.

ruger35 could I contact you to get the name and contact info of that HVAC tech in BR? I ID’d a duct work issue in my attic a couple weeks ago that I need to have fixed. Your guy sounds like the type of HVAC tech I’m looking for repairs and troubleshooting - another good example how word of mouth, personal references is one of the best ways to find a good tech in any of the skilled trades.

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Lastly, have a manual J and ideally manual D performed to determine the proper tonnage of the replacement units and to determine if your duct sizing is correct.

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Who does these? How much do they cost?

I've gotten a few quotes, but they are to replace the units with the same tonnage as before.

Thanks. I'm kinda thinking the sizing is right because the original unit with copper coils (4 ton) lasted 20 years and the 2.5 ton is original to the house (30 years). The thing that changed was coil make-up....from copper to aluminum.

Do any of you have opinions about the high SEER units? One of the company's gave me a quote on a 19 SEER variable speed unit. It supposed to keep your house at a cool, dehumidified state using less energy. Of course it costs more up front.

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High seer units usually have a VFD to control compressor speed/envelop. They are usually the high end of units. Almost everyone uses a communicating thermostat (i.e. digital comm bus) that is specific to the brand you want to use.

I have a two speed unit at the house. Most of the time it runs at low speed, which is great for my climate (lowers humidity). It could be a little more problematic if repairs are needed but I do my own. So I would go back with a two-speed in my house unless you want the maximum energy savings and will be in your house for a while (7-10 years).

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All the above comments by UPGDude are spot on. .

And from extensive reading and research I’ve done, though high SEER VFD multi-stage HVACs can be wonderful if set up properly, if your house and duct work is not just right/perfect they can potentially cause issues - for example condensation at the discharge grills - when conditioned cool air is movies slowly through the ducts on a continual basis at the slower compressor speeds. I’ve read some horror stories on the HVAC-Talk AOP forum of people who installed VFD multi-stage systems to replace single-stage systems. I personally would not install a multi-stage VFD HVAC unless I had a spray foamed attic where attic temperatures are 25 to 30 F cooler than conventional blow-in fiberglass insulation (assuming air handler and ducts are in the attic).

Issues of this type would/should not be a issue with a higher seer two-stage HVAC. When I replace my HVAC, hopefully within a year, I’m seriously considering a two-stage system. Higher SEER two stage HVAC systems are not nearly as complex as VFD multistage systems, being more comparable to a single stage system in terms of operation and repair.

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So, I assume all high SEER units are not VFD. If I'm reading right, I could benefit from a two stage unit, but make sure its not VFD.

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I believe your statement is correct. Let me summarize what I think to be generally correct.

Minimum Single Stage Stage HVACs mandated by Feds for the South will be SEER 14 but you can purchase a single-stage HVAC with SEER as high as 16

Two Stage HVAC condensing unit will likely have a Max SEER of 17 and to my knowledge no two stage HVAC has a VFD compressor. Most of the cooling year your HVAC will operate in Stage 1, ~ 65% of max cooling capacity, saving you operating electricity $, and it will only operate in Stage 2, 100% of cooling capacity, during the hottest days of summer.

Multi-Stage VFD HVAC will likely have SEERs that range from 17 to 21, depending on manufacturer and model, think I’ve seen one, Bosch perhaps (?), with max SEER of 24?

Higher SEER, higher initial purchasing price, but less monthly operating cost, so at some point 6, 8, 10 ... years down the road you’ll recoup your higher initial investment - hopefully before your system needs to be replaced. If you plan to live in this house long term, then a higher SEER system may be worth the investment, if you plan to move before you re-coup the higher initial purchase price of the higher SEER system, then purchase a basic single-stage SEER 14 HVAC system.

If you want to play with operating cost and pay back period for different SEER units here is an on-line calculator HVACOPCOST