Need help deciding between 2 or 3 ton unit after ManualJ

[njh123]

I am replacing an existing 2 ton system for my second (top) floor, which is one zone of a two zone house. I had a ManualJ done to verify the existing system tonnage was appropriate. The Total Cooling Load came out to 28,876.

The existing system is an 18 year old Trane and has been struggling a bit the last couple of years. We've been in the house for 5 years, but it's hard to remember exactly what it was like when the system was working at peak performance. It certainly had trouble keeping up on the hotter summer days, but in seemed adequate most of the time (in the rooms we frequented). We don't keep it super cold, so that probably helped.

Anyway, the companies I have talked to say that if the ManualJ is at 29K, you would be better off with a slightly bigger system, 2.5 ideally. Problem is that the system I want doesn't come in a 2.5 ton size, only 2 and 3. Now, both of these systems are 2 stage cooling, so I (and my HVAC pro) figure that with a good t-stat I would probably do well with 3 ton since it could likely run in the 1st stage the majority of the time and would have the reserve capacity for those really hot days.

So my question is, do you (the experts out there : ) agree with this theory? The 2-ton system would be great price wise, since I wouldn't have to upgrade other system components to support the 3 ton. With the 3 ton though, it seems like it would offer the best comfort potential (especially with an expanding family). Or do I just scrap this line and get something that is 2.5 ton (seems the problem here is that I end up spending more money with or without the tax credit qualifying systems).

Love this site guys, I don't think I could count the many hours I have spent reading up on all of this stuff at this point. I like to make informed decisions on things.

[HVAC9900]

If it were me, would go with the three ton, especially since it's 2 stage and upstairs.

[dash]

Assuming your Man. J is accurate,your HVAC Pro is correct.

[amd]

Is your contractor absolutely sure that the existing ductwork can even handle 1200 CFM? (cubic feet per minute of airflow)

There's no point of having a high end system if it can't run at rated capacity and efficiency; low airflow kills performance.

[njh123]

Is your contractor absolutely sure that the existing ductwork can even handle 1200 CFM? (cubic feet per minute of airflow)

There's no point of having a high end system if it can't run at rated capacity and efficiency; low airflow kills performance.


The contractor says that my existing duct work can handle it. I have 12" ductwork (vinyl flex??, it is a round, black plastic covered, flexible material) that all the contractors have said is in good shape. I have two of these as input coming from two separate 12" x 24" returns (which he has indicated I will need to expand the size (or at least flow) of one of them to make sure there is enough "input flow" available.

Does this sound adequate?

[Irascible]

I'd go three ton as well. Based on anecdotal evidence and my subjective experience, heat load calcs do a poor job of accounting for how much of the downstairs heat load the upstairs ends up taking. If the home has large open areas that transmit air from down to up, then my guess is that actual load is probably more than calculated.

Upgrading the returns is likely a must. The supplies may need upgrading too.

[njh123]

Thanks for everyone's feedback.

To upgrade the returns, he has suggested high-velocity returns, which don't require more physical space, but have less surface area and as a result, more airflow capacity. Has anyone else used such a return and been successful? Any cons to think of?

[dash]

Is your contractor absolutely sure that the existing ductwork can even handle 1200 CFM? (cubic feet per minute of airflow)

There's no point of having a high end system if it can't run at rated capacity and efficiency; low airflow kills performance.

Good point,since there's a good chance they weren't large enough for the 2 ton system.

[dash]

Thanks for everyone's feedback.

To upgrade the returns, he has suggested high-velocity returns, which don't require more physical space, but have less surface area and as a result, more airflow capacity. Has anyone else used such a return and been successful? Any cons to think of?

High velocity requires special equipment/fans.

He likely means something else. Can you describe what they plan to do, a little better.

[njh123]

High velocity requires special equipment/fans.

He likely means something else. Can you describe what they plan to do, a little better.

Sure. Basically they plan to leave everything the same in the system except for upgrading the return grille to this "special" grille (with less grille and more open space). This is supposed to get the extra airflow I need through the return. Also they plan to upgrade the line set from what was going to be 7/8, to over an inch (1 1/8 I think) based on the expected 30 ft run.

System is an ASX18 with AMV80.

[BobbyBJr]

That may or may not work depending on the size duct running to the return grille. If the duct itself will not carry the required airflow, you can make the grille as large as you want and not gain much. I'd be very surprised that you can increase the size of your unit a full ton and not have to do some duct renovations to the supply and return. It is possible, but unheard of in this area because most duct systems are undersized as they are. Alot depends on the air handler's fan capability and the filters you are using, but the ductwork really determines the final efficiency of that brand new unit.

[udarrell]

That may or may not work depending on the size duct running to the return grille.

If the duct itself will not carry the required airflow, you can make the grille as large as you want and not gain much.

I'd be very surprised that you can increase the size of your unit a full ton and not have to do some duct renovations to the supply and return.

It is possible, but unheard of in this area because most duct systems are undersized as they are.

Alot depends on the air handler's fan capability and the filters you are using, but the ductwork really determines the final efficiency of that brand new unit.
BobbyBJR, is speaking to the realities that exist out there.
Their ductwork change is merely cosmetic, it will do very little to improve airflow.

Have them do a manual D & get everything right so your high SEER system has a chance to perform for you! Click udarrell for more information on duct system sizing. They have to get'ter Right, or it won't work right!