AC question... (Phoenix: house, cost, shop)

[john3232]

The house is 1440 sq ft. and a 3 ton package unit was what the builders installed.

In 2017 the AC was replaced with a Day/Night 3 ton unit. I didn't move to AZ until 2019 at which time I began making sure the filters were changed (rental property) esp. during the summer months.

Last summer the AC stopped working. The AC vendor who came out wrote on the invoice:

After unit ran for 20 minutes it shut off. Found that the outdoor fan motor was not coming on. Run cap. checked out good. Changed 1/4 hp 1075 rpm outdoor fan motor with new run cap.
Unit came on with no problems.

Recently, I got a hold of an AC guy who has been in the business for 30 years. He told me the fan motor shouldn't have gone bad.

I had him come over and inspect the roof top unit. He said the evaporator coils were dirty and he cleaned them. I asked could the dirty coils have caused the AC to stop running and he said it's unlikely. He went on to explain the ductwork is too small for the 14 seer unit. He then recommended I replace the 12 in. ductwork with 20 in. which will improve the airflow.

Not sure what to believe.

Thoughts?

[R J]

Condenser motor went bad?

12" duct for 3 tons is undersized and is best suited to a unit size of about 1.5 tons. You would need 16" duct, at a minimum, to move 1200 CFM of air and still be in the equipment recommended static pressure range.

Undersized duct can cause all sorts of issues. Coils icing over, compressor damage due to refrigerant slugging, water leaks and moisture buildup on the supply registers, mold growth in the ductwork, and evaporator motor failure due to excessive rpm (ECM). If most of your duct was installed with excessive lengths of flex, the restrictions are probably worse due to kinking and sag.

You should have your AC guy perform a Total External Static Pressure test. This value will give you a ballpark actual CFM number flowing through your system. From there, you can base your decisions on whether to replace ductwork with actual evidence.

If you'd like to take this a step further, you can have a professional in your area perform a Manual J on your property along with a blower door test. This will tell you what sized unit is ideal for your space based on actual loads and leakage of your house. You should also perform a Manual D which will give you the optimal duct sizes based on your system. If you've done these, you may actually find the duct isn't the problem but rather the unit is oversized, undersized, or some combination of unit and ductwork.

[DAXhound]

Generally, they say 1 ton per 400 sf in Phoenix. Your sizing sounds correct. Glad I use ductless.

[john3232]

Originally Posted by R J

Condenser motor went bad?

12" duct for 3 tons is undersized and is best suited to a unit size of about 1.5 tons. You would need 16" duct, at a minimum, to move 1200 CFM of air and still be in the equipment recommended static pressure range.

Undersized duct can cause all sorts of issues. Coils icing over, compressor damage due to refrigerant slugging, water leaks and moisture buildup on the supply registers, mold growth in the ductwork, and evaporator motor failure due to excessive rpm (ECM). If most of your duct was installed with excessive lengths of flex, the restrictions are probably worse due to kinking and sag.

You should have your AC guy perform a Total External Static Pressure test. This value will give you a ballpark actual CFM number flowing through your system. From there, you can base your decisions on whether to replace ductwork with actual evidence.

If you'd like to take this a step further, you can have a professional in your area perform a Manual J on your property along with a blower door test. This will tell you what sized unit is ideal for your space based on actual loads and leakage of your house. You should also perform a Manual D which will give you the optimal duct sizes based on your system. If you've done these, you may actually find the duct isn't the problem but rather the unit is oversized, undersized, or some combination of unit and ductwork.

Yes, according to the AC vendor who went out to inspect the unit after it stopped working. Ok if I understand the ductwork is too small but this isn't necessarily way the unit stopped working. It could have been any number of things.

However, it is important the current 12 in duct work be replaced with a large size. Is this correct?

I've spoken with a couple of other AC techs who said:

The SEER of the unit means nothing as far as correct size, this is just an energy rating and a motor most certainly could have gone bad. It is a mechanical part and prone to failure at any time, usually the most inconvenient time. The invoice does not provide me with enough information to determine what else was checked to determine the motor was defective, what is written means little in condemning the motor. Incorrect duct work COULD cause a situation that may shut the unit down. Dirty coils COULD create a situation that causes the unit to shut down.

Another said:

Don’t take this wrong. He sees you as an uninformed homeowner. Someone who doesn’t have a clue how the system works or whats wrong with it. He thinks he can give you a snow job, convince you new ducts are the best solution, and charge you a lot for a simple job. My suggestion ... get his proposal. Then call another a/c vendor to the house, don’t tell them about the burned out motor, just ask for a proposal to change the ducts.

[R J]

How clean the indoor evaporator coils isn't going to impact the outdoor condenser motor, unless you're running controls for low ambient operation that directly control the motors On/ Off cycles.

If the outdoor condenser coil is dirty, the excess heat will be a contributing factor to reducing longevity of the motor. If the coils are unable to reject heat efficiently, the motor will be subjected increased operating temperatures. A failing run capacitor, will also cause it it to overheat or trip on thermal overloads.

Much like the indoor evaporator, a dirty condenser will cause the compressor to operate at increased compression ratios, increased discharge temps, as well as increased energy consumption.

You may not have to replace all of the ductwork. If access is limited, I would replace the most problematic areas. I assume the 12" round duct is directly off the supply and return of your unit? If so, I would focus my attention there as that portion is a choke point for the entire system.

If you're running a 1" pleated air filter (Merv 11+), consider upgrading to a 4" style filter box. One of the biggest mistakes homeowners make is to install the most restrictive filter into their unit from Home Depot and choke the system. The more surface area, the less of a pressure drop across the filter. If you run fiberglass filters, none of this will really impact you very much.

[john3232]

Originally Posted by R J

How clean the indoor evaporator coils isn't going to impact the outdoor condenser motor, unless you're running controls for low ambient operation that directly control the motors On/ Off cycles.

If the outdoor condenser coil is dirty, the excess heat will be a contributing factor to reducing longevity of the motor. If the coils are unable to reject heat efficiently, the motor will be subjected increased operating temperatures. A failing run capacitor, will also cause it it to overheat or trip on thermal overloads.

Much like the indoor evaporator, a dirty condenser will cause the compressor to operate at increased compression ratios, increased discharge temps, as well as increased energy consumption.

You may not have to replace all of the ductwork. If access is limited, I would replace the most problematic areas. I assume the 12" round duct is directly off the supply and return of your unit? If so, I would focus my attention there as that portion is a choke point for the entire system.

If you're running a 1" pleated air filter (Merv 11+), consider upgrading to a 4" style filter box. One of the biggest mistakes homeowners make is to install the most restrictive filter into their unit from Home Depot and choke the system. The more surface area, the less of a pressure drop across the filter. If you run fiberglass filters, none of this will really impact you very much.

Thanks will do.

Yes, the unit is using air filters from Home Depot which aren't restrictive.

[Voebe]

My move would be to get a third and fourth opinion.

Use this as an opportunity to shop around for an a/c guy you can trust.

[jnojr]

Originally Posted by john3232

The house is 1440 sq ft. and a 3 ton package unit was what the builders installed.

In 2017 the AC was replaced

My AC has a 10 year warranty. I'd call the company that installed this and lean on them to make this right.

[Pink Jazz]

Originally Posted by john3232

Thanks will do.

Yes, the unit is using air filters from Home Depot which aren't restrictive.

Many newer homes are designed and tested to use MERV 6-8 filters, so the advice of not using 1" pleated filters is no longer applicable to most newer homes that are Energy Star qualified since their systems were designed to use them from the start. The advice of not using pleated filters often comes from techs are are not properly trained to work on Energy Star systems installed in newer homes. We use the Rheem green label pleated filters from Home Depot.

[john3232]

Originally Posted by jnojr

My AC has a 10 year warranty. I'd call the company that installed this and lean on them to make this right.

So does mine but the fan motor went out it was the middle of summer and the property is a rental. The guy who installed the unit was swamped and couldn't get over for 3-4 days. I showed the guy who did come over by the warranty and he said yes... he could pick up the part. However, I would be charged for this and the cost would be roughly what it would be if he installed a new blower fan from his truck. It was bull**** but by 1 pm it was 90 degrees inside the home and I needed the AC fixed that day.

From what I've been reading the ductwork needs to be bigger and perhaps explains why this property has had AC issues since I've owned it.

In any event I am addressing this now and well before the weather heats up.