Increasing the motor horsepower will not increase airflow any appreciable amount. Call the folks that installed the oversized condenser and have them properly troubleshoot.

 

Measure static pressure and compare it to the blower chart to see how much air it is moving. More often than not, the duct system is too small so putting in bigger blower wouldn't help anyway.

For a 3.5 ton condenser it needs a minimum of 1225 CFM to work properly.

 

So the capacity was increased 15% but not the ducts?
Who made that decision?

Same person deciding increasing Blower HP will solve the problem?

 

A 1/3 HP and 1/2 HP 1075 RPM motor will move the same amount of air when connected to the same blower wheel.

 

This would be true if the system was operating properly to start with. But if the constant-HP motor (rather than a genuine multi-speed motor) that he has was stalling at it HP limitations due to excess static - then adding HP would increase the stall speed by increasing the blower speed to the new, higher balance point. Moving more air in the process.

However; the OP stated that the motor was overheating and stopping as a result of the overheating. If the motor is not defective this would tend to indicate insufficient static pressure over-speeding the motor. In which case a higher HP motor would tend to increase rather than decrease the problem.

Right? <g>

PHM
----------

 

I thought more torque should help overcome the possible high static pressure and give more airflow until the blower wheel flys apart. A bigger indoor coil should have less static than the old coil. Let us know what the tech says was wrong.

 

The blower motor is not the sole factor in getting the desired cfm. It would also require a larger blower. The speed taps are actually horsepower taps and the highest one runs the blower at the motor's full rpm rating. Therefore, more available horsepower won't increase the rpm or cfm of the blower.

 

Your math was probably incorrect, and the 3 ton was fine for the house.

To move more air by changing out the blower motor. It would have to be a higher RPM and HP motor.

 

Which the blower wheel wasn’t designed for.

 

Thanks for the input. Sounds like increasing blower motor HP isn't a solution. I appreciate the feedback on that.

To address some of the questions comments received:
I'm 100% certain that the original 3 ton unit wasn't sufficient for my home. I have a 2500 sq. foot house in Georgia. I looked up many resources to determine correct tonnage and to be honest, I still believe it is too small. I should probably have a 4 ton unit. What I failed to recognize (and what the HVAC sales person and installation techs should have told me) is up-sizing just the condenser and coil isn't enough that I would experience restrictions from the other equipment (duct, blower, ect).

I don't know for a fact that the blower motor is overheating, that is just my suspicion. Everything can be running just fine, for hours and the unit blower will just shut off and then pop back on a few seconds later. Then it will go into a fit for a period of about 10 minutes where the blower motor just cycles on and off over and over while the outside unit is running and the thermostat is calling for cooling. This tends to happen in the later afternoon / early evening. It has happened in the morning as well, but mostly late afternoon. After about 10 minutes of this fit, the blower motor will kick on and continue to run like normal for awhile. I did have an HVAC tech out once to look at it but of course while he was here the unit was working properly and he couldn't find any issues.

Sounds like my next steps is to either go get a manometer or get the tech back out and do a static pressure test.

 

Thanks for the input. Sounds like increasing blower motor HP isn't a solution. I appreciate the feedback on that.

To address some of the questions comments received:
I looked up many resources to determine correct tonnage and to be honest, I still believe it is too small.

There is only ONE resource for determining the size needed.

ACCA Manual "J", "S", & "D"!

Lets start at the beginning

http://efficientcomfort.net/assets/documents/Sizing_Selecting_for_Proper_Humidity_Control.pdf
http://efficientcomfort.net/assets/documents/Another_View_on_Equivalent_Sizing.pdf
http://efficientcomfort.net/assets/....net/assets/documents/Bigger_is_Not_Better-Sizing_Air_Conditioners_Properly.pdf
http://efficientcomfort.net/assets/documents/Understanding_HVAC_System_Design_Issues.pdf

Not a fan of or an endorsement of Angies List but a must read:
https://www.angieslist.com/articles/difference-between-best-hvac-brands.htm

 

Then it isn’t a problem of the motor over heating. As it would take several minutes for it to cool down enough to run again.

So your saying the previous owners lived in a hot house instead of a cool house with the 3 ton A/C.

Are you trying to get your house down to 66 degrees.

 

I can't speak to the previous owners, what temp they liked the house at or what other measures (if any) were used to keep them comfortable during the hot months. What I can say is the previous owners were elderly and in my experience the elderly tend to like it a little on the warm side.

No, I'm not trying to get my house to 66 degrees. I'm trying to maintain 72 degrees. The system will run the better part of all day but by 4pm or so the thermostat reads 76-78 degrees while it has been calling for 72 all day. Part of this is due to the cycling of the blower motor, but even previous to the blower motor cycling issue the system has struggled to keep the house at around 72.

Another note, the 3-ton unit I changed out was a GE brand. It looked old as dirt, I even imagine how inefficient it was.

 

What resources did you look up for equipment sizing?

 

So I can't post links on the forum yet. But I found various online resources with different calculators and methods. Trane's website has some good info on selecting size. As well as hvacdirect .com.

My sales and service tech also agreed with the up-sizing. I originally wanted 4-ton but they convinced me 3.5-ton would be sufficient. Although I'm now not entirely convinced. The first 2 summers I was more conservative on my thermostat and kept it at 75-76 degrees and even then the system seemed to run all day in order to keep that temp and if I had guests over the temp would encroach on 80 degrees inside even with the thermostat set for 75 degrees. This summer I bumped the thermostat down to 72 and about a month ago is when I started experiencing the blower motor issue.

 

When I first got out of trade school I spent 16 months with a contractor that did residential and light commercial. When he added a coil to an existing furnace, he always up sized the motor. I never questioned whether it was correct or not, just did as I was told.

 

You need to look at the "Entire System" your concentrating on just the equipment. Nothings been discussed about the ducts!

AGAIN
There is only ONE resource for determining the size needed.

ACCA Manual "J", "S", & "D"!

 

I'm not debating you and I don't believe anyone is. I fully recognize I'm not an expert in AC system sizing. I was able to get a rough idea of what the tonnage of the what the condenser unit should be. I wasn't aware that other components needed to be considered at that time as well. I would have thought that my sales guy and the service techs would have made that point. The issue I'm posting about here isn't regarding the sizing of the system.

I understand now that the entirety of the system is likely not sized correctly. There is little I can do about that now without spending (tens of?) thousands of dollars to have all of the duct ripped out and replaced. The most important thing at this moment for me is to determine the cause of the blower motor cycling like it is and determining if a larger blower motor would have a positive or negative impact. From the responses, it seems mostly it would have a neutral or negative impact. Now I need to determine if replacing the blower motor, even with a direct replacement in size would be of any benefit or if there is another reason why the blower motor is cycling like it is.

 

I sure hope you don’t actually buy into that sizing square foot calculator on the hvac direct site. That’s about as wrong as you can get.

 

I didn't. That had me putting in a 5-ton unit which really made no sense to me. I was just gathering some information to get an idea of what should be in place. Like I said, the sales and service techs agreed I needed a bigger unit, but then again they also didn't say anything about needing to re-size the duct and other aspects of the system. So I don't know.

At this point, I'm looking to get the blower motor cycling issue resolved. The overall system sizing or re-size that may be needed will be a different discussion down the road when I have enough funds to take care of an overhaul such as that.

 

You’ll have to get your tech to look into the blower problem. To see what’s wrong.
As we cannot give any DIY information here on things you should check.

 

Yep, I understand. My original question was regarding up-sizing the blower motor and if that would help such an issue I was having. That has been answered with a probably not. I said in a previous post my next step is to get a tech to come do a static pressure test or get a manometer and do it myself. Hopefully that will shine some more light on the situation.