New house undersized HVAC?

[Greg Bentz]

So I bought another house and it's my first split foyer home, not my favorite design but too hard to pass up the good deal. Issue is it was originally a 1100 sqft home with an unfinished basement. Has an old Gibson 2 ton 13 seer unit. Well the previous owner had the entire basement finished and all brand new duct work but didn't upgrade the unit. Now I am from Florida so I am used to heat but I am also used to being nice and cool inside my home. Well upstairs it's like a sauna and once you hit the front door mud area to head down to the basement it's about 10-15 degreers cooler and obviously in the winter the reverse. The home is about 1900-2100 sqft now if I had to guess by the same of the basement. So am I looking at a 3 ton or maybe 3.5 ton unit to be on the safe side?

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[Dom D'Agostino]

"Rules of thumb" or "guesstimates" for HVAC sometimes work, and sometimes don't work.

You need a professional to size the system, taking into account much more than just the square footage.
You didn't mention the home's age, the HVAC's location, or the type of distribution system, but older forced air heating systems had poorly designed ductwork, especially for upgraded or added central A/C.

Clearly, a finished basement adds to the heating/cooling load of the home, so invite a few qualified professionals to right-size the equipment for you.

[Jerry Peck]

Follow Dom's advice, and be prepared to spend a lot ... LOT ... lot more than you thought you would spend.

Redoing all that old ductwork, and the ceilings of the now-finished basement (which are likely hiding the old ductwork and will have to have parts of the ceiling removed to get to the old ductwork) is not going to be baby chickenish (NOT 'cheep, cheep, cheep').

[Greg Bentz]

The house was built in the mid 90's and even today I popped into the attic during the hottest part of the day and it was not that hot in there to my surprise. The handler is in the laudry room/workshop which is also insulated with R13. It's central air with a heat pump. The return is in the hallway of the second floor. The attic has spray cellulose and it looks to be in good shape as it's still fluffy and the level of it is above the wood beams. The ductwork is all new done in the past 5 years, I know because my the neighbor across the street was the one who installed it all. He's retired HVAC but the last time we spoke was during the winter so I had no idea how hot the house would be. I can survive the winter, I always dress warm inside and keep the heat at 66 usually lower until the wife complains. Also the walls of the basement have R13, found that out after pull doing some plumming repair to my outside spicket. All in all it's a typical 90's cookie cutter home


One note is my upstairs living room has vaulted ceilings. At peak they are roughly 18 ft high

[Ken Amelin]

"Rules of thumb" or "guesstimates" for HVAC sometimes work, and sometimes don't work.

You need a professional to size the system, taking into account much more than just the square footage.
You didn't mention the home's age, the HVAC's location, or the type of distribution system, but older forced air heating systems had poorly designed ductwork, especially for upgraded or added central A/C.

Clearly, a finished basement adds to the heating/cooling load of the home, so invite a few qualified professionals to right-size the equipment for you.

"Good advice from Dom" - Get a professional to review your home and size the unit and distribution properly. An energy audit would also help for improvements in insulation, if needed.

[Jim Luttrall]

So I bought another house and it's my first split foyer home, not my favorite design but too hard to pass up the good deal. Issue is it was originally a 1100 sqft home with an unfinished basement. Has an old Gibson 2 ton 13 seer unit. Well the previous owner had the entire basement finished and all brand new duct work but didn't upgrade the unit. Now I am from Florida so I am used to heat but I am also used to being nice and cool inside my home. Well upstairs it's like a sauna and once you hit the front door mud area to head down to the basement it's about 10-15 degreers cooler and obviously in the winter the reverse. The home is about 1900-2100 sqft now if I had to guess by the same of the basement. So am I looking at a 3 ton or maybe 3.5 ton unit to be on the safe side?

As others have said, get a professional to do a load calculation and put in the right size system. Almost all rules of thumb give you to big a system which then won't adequately control the humidity. A properly sized system will run longer, yielding better interior conditions than one that is over sized and starts and stops repeatedly. since you have a multi level home, consider a zoned system to overcome the temperature differences between floors. Or maybe a mini split for the basement.

[Scot Laudeman]

Sounds like a perfect application for adding a mini-split.

[Jerry Peck]

One issue with adding a mini-split is that, being ductless, they are designed for one open area or room.

Using a mini-split to try to adequately and evenly heat/cool separate areas and rooms, especially when there are doors, is a challenge for a ductless system.

[Greg Bentz]

One issue with adding a mini-split is that, being ductless, they are designed for one open area or room.

Using a mini-split to try to adequately and evenly heat/cool separate areas and rooms, especially when there are doors, is a challenge for a ductless system.

Thanks for the all the advice. I did speak with my neighbor yesterday and he said everyone in this neighborhood has the same issue. I also measured my ducts and obviously my 20x20 return would not be enough for any larger unit and even adding a second return would not work since the duct work is too small. I am leaning towards a 1 ton 12,000 BTU fujitsu for the living room/kitchen(all one big room). The bedrooms are fine since the door are shut in there so the cold air does not sink down to the basement. I'm hoping that combined with the central air coming up to the living room and the mini split in there as well I should be cool enough in the summer. The air coming from the vent is 22-23 degrees below the outdoor temp which is good. Another idea would be to replace the old 2.5 ton gibson unit with a more current gen 2.5 ton with a better seer rating but I may just go with the mini split first and see how it does.


My gibson is the original one from the 90's seer 13*

[Bob Kenney]

I live in a split foyer in MD. One of the big problems with the design is simply that it is like a wind tunnel. The upper and lower floors have no separation in terms of heating and cooling. Cold air is heavier and winds up in the lower floor (basement).

Two things help. Registers in the upper floor are closed during the winter and all heated air is delivered to the basement. The opposite is done in the summer and the cooled air is delivered to the second floor. The second thing is a properly suspended ceiling fan in the foyer can assist air movement throughout the year.

[Joe Stoddard]

A 2-ton unit ?should? be adequate for your newer 2000SF house - and the worst thing you can do is over-size AC. However that assumes that the duct system can deliver the air necessary (both supply and return) to the various parts of the house, that the fan speed is correct , that the evap coil and Lineset are correctly sized, that the unit is adequately charged, that there is no unusual solar gain (wall of windows pointing the ?wrong ?way) and on and on. The size of the compressor is only part of the story, and often a smaller, less impactful part than people first think. Get someone who knows what they?re doing and does not have anything HVAC-wise to sell you to examine everything, do a proper cooling (and while they?re at it - heating) load calculation and reverse-engineer of the existing ductwork. It?ll cost you $3-400 but could save you 10x that amount or more buying stuff you don?t need. My guess is there are some air distribution issues that can be greatly improved for a fraction of the cost of new equipment and etc. In any case the wrong thing to do is to start throwing money at it before you know what you have.

[Jim Luttrall]

The air coming from the vent is 22-23 degrees below the outdoor temp which is good.
Another idea would be to replace the old 2.5 ton gibson unit with a more current gen 2.5 ton with a better seer rating but I may just go with the mini split first and see how it does.


My gibson is the original one from the 90's seer 13*

22-23 degrees cooler than the outdoor temperature is not a good measure of how the unit is performing. The difference between the supply register and the return register temperatures is a better measure. Around here, 22-23 degrees cooler than outdoors would be very bad. 105 - less 22 = 83 degrees which is about 10 degrees short of acceptable.

The higher efficiency unit will NOT cool any better than the old unit. It should cost less to run but don't confuse energy efficiency rating with cooling capacity. Buying a new truck that gets better mpg won't help haul any more load than the old one.

Bottom line, like everyone has said, get a good HVAC guy and pay him to check the system and run the numbers.

[Jerry Peck]

Buying a new truck that gets better mpg won't help haul any more load than the old one.

That's a great analogy to use as it should be easy to grasp by anyone, regardless of their DIY level.

[Greg Bentz]

I live in a split foyer in MD. One of the big problems with the design is simply that it is like a wind tunnel. The upper and lower floors have no separation in terms of heating and cooling. Cold air is heavier and winds up in the lower floor (basement).

Two things help. Registers in the upper floor are closed during the winter and all heated air is delivered to the basement. The opposite is done in the summer and the cooled air is delivered to the second floor. The second thing is a properly suspended ceiling fan in the foyer can assist air movement throughout the year.

I do have a fan suspended from my vaulted ceilings in the living room(main level) It hangs 8 feet above the floor ceiling. I closed the registers in the basement but it didn't seem to make a difference. All week the living room hasn't dropped below 78 even when set to 72. My AC guy has been swamped so for now I put a large portal AC unit up here which has helped a lot with the fan combination. You're totally right about the cold sinking down to the basement, I guess I've just never dealt with this type of home design. I did end up tenting my sky light and my green house window as well. I took all the measurements for my guy so he can go over them and from doing the calculations myself I def can not go up in size.

The main trunk is 8x12
4 round steel ducts feeding off that which are 6.4 inches in diameter


So we may end up looking at a 1.5 ton in the attic to cool upstairs

[Jack Feldmann]

Why not get a HVAC professional to come and look at it. You may consider making it a zoned system for both levels. It might make a huge difference.

[Rod Butler]

Sounds like a perfect application for adding a mini-split.

I agree. A small (2-ton) mini split will run around $10 to $12 K and provides very nice zoning. You can also get one that has multiple evaporators (fan section) with a single outdoor condensing unit. Great suggestion.