HVAC - adding second unit

8 foot in bedrooms, 10-12 in living room

I have the same amount of conditioned living area and my HVAC is also 4 ton, and 14 years old. But I don’t have the same problem you are describing. I’d have a professional look at you your ducts - are they sized right for the warm part of the house, perhaps you have some duct leakage, perhaps the air ducts need to be balanced so that you are getting more air to the warm part of the house. Professionals can check all that and make corrections, if needed or warranted, cheaper than a second unit.

Honestly if your HVAC is properly sized it is not designed to keep up with 100 degree days, and the western exposure certainly can exacerbate the issue on that side of the house. For Baton Rouge, I believe the max heat design criteria for the HVAC is 92 F. When temps are higher than that for extended days, the system will run non stop during the day and may not keep up with the thermostat setting but that’s normal, and does not indicate a problem.

That said, you can can certainly consider a second unit, of course there are pros and cons to both. When my house was built I in 93, I asked about this with the builder and his HVAC sub contractor and the HVAC sub told me 2,500 sq ft conditioned space was right on margin where they typically installed a single system, much larger than 2,500 sq ft and they usually designed for a second system. Anyway, I was given the option of installing 2 units, at higher cost, but stayed with the single unit and it has been fine.

If you do go down the path of 2 units, make sure you have Manual J, Manual D, and Manual S conducted on your home. These are used to determine heat loss and gain, HVAC sizing, and duct sizing. You don’t want a HVAC contractor that uses “rules of thumb” for sizing.

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Considering he has 10-12' ceiling in the living area, 4 tons is too small.

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Impossible to know without a Manual J.

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Considering he has 10-12' ceiling in the living area, 4 tons is too small. OP where is your thermostat?

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Not saying you are wrong but that is the purpose of the manual J to spec out the size accurately. Depends on the building envelop - is his house tight, well insulated, leaky, etc My ceilings are comparable to the OP -9 to 14 ft high, and prob average about 10 to 11. I’ve conducted an online Manual J (loadcalc) and the Manual J Calc indicates a 3 1/2 ton unit for my house - I have 4 ton - 2,500 sq ft of conditioned space. I think I have a fairly tight house but I plan to have a blower door test and and room by room manual J conducted to identify problem leakage areas, size a new HVAC that I’ll likely replace in the next year or 2. Also a manual D to check my rigid duct work for leaks, etc.

By no means am I discouraging the OP in considering a 2 unit system as he plans for replacement of his old system, at times I wish I had 2 units, for those pesky times when a HVAC goes down, but I’ve been lucky and had few down times of my HVAC system over the years.

This thread is useless without calculations.

I also have ~2,500 sq ft, but have a 5 ton heat pump system that leaves the master suite (remodeled/reconfigured older home) poorly conditioned, cold in the winter and warm in the summer.
I am certain that we have duct issues, in quality, routing, size, etc.

A few questions
1. Are new heat pump units better, as in plenty good enough, at keeping the house warm when temps drop below 35 for an extended period of time? I’ve read they use a supplemental heater during those times, so I guess replacing the old outside unit means also replacing the inside unit. Is that right?
A related question is whether you think it would make sense to switch to a furnace and separate AC unit.

2. Since I really seem to only need additional heating and cooling in 2 rooms plus a closet, this seems like the ideal time for a mini split for that area. Do you agree? If so, what do you think of the pre-charged units they claim can be self-installed?

3. I’m thinking the best way to go about this is to first have a pro come and re-do my dilapidated ducting and then observe how the 5-ton single unit does with good ducts. Then if that doesn’t get it done, get a mini-split and replace the heat pump with a smaller heat pump when the old one dies. Make sense?

Thanks for reading this novella.

Sout Louisiana

I’ve heard good things about newer heat pumps.

A furnace for me would run off my propane tank which is really appealing, but when I got a quote to replace everything and go with furnace it was a nice chunk of change.

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? Is it 410A Freon

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Don’t know but I think it’s 25 years old. It’s living on a prayer.

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New air handlers can boost a condenser outside unit seer out put. In fact the system I bought for my guest house is 16 SEER, but with the air handler I bought, that boosted the SEER up to 17.5 seer. Yep, cost a couple hundred more, but should have a pay back in the first year in service

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I’m thinking the best way to go about this is to first have a pro come and re-do my dilapidated ducting and then observe how the 5-ton single unit does with good ducts. Then if that doesn’t get it done, get a mini-split and replace the heat pump with a smaller heat pump when the old one dies. Make sense?

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I’m far from being a HVAC expert, but I have tried to educate myself on the subject over the past year, from what gather experienced HVAC techs state that the most prevalent design flaw they encounter is oversized HVAC units and undersized ducts. My way of saying yes, have your ducts and air handler checked for leakage, air balance, etc. Last week we had a poster having a problem similar to what you described, and another person on this board board suggested he check for air leaks in his air handler and ducts, he found a major air leak in his air handler that he repaired himself and he stated he had immediate improvement in cooling.