Hi everyone, I've been reading this forum for the last few weeks as my home addition project has been ramping up and seems like there's a lot of knowledgeable folks here that can help. I'm having some difficulty deciding what to do about AC sizing for the new house size. (Warning, this is a pretty long post, but I'm trying to include as much information as I can).
The existing house is in Austin, TX, 1200sq ft, built in 1955 and renovated (somewhat cheaply) in 2007. It currently has a split system 2.5 ton Goodman AC condenser/compressor with a 3.5 ton blower/evaporator and gas furnace. (Don't know why the blower is oversized relative to the compressor). They did add some amount of wall insulation in 2007 - I can't say exactly what, probably minimal, it's the pink rolled-in stuff. The ceiling insulation is very poor because of the low-slope roof (and the fact that the reno in 2007 was cheap). I would say that the 2.5 ton system is good for most days, but on the hottest days (we've had 105-107 degrees recently), the AC stays on continuously and the house don't get below 79 or 80. Maybe that's an okay situation on such a hot day?
The addition we are doing is adding 675 sq ft of master bedroom, bathroom, extending the dining area, and adding a 4-season room. There are a lot of windows in the addition, and the added area is mostly west-facing. So total sq footage will be 1875.
The utility room has very limited space and we aren't building added space into it, so as much as I would want to, I don't have room to add a second evap/blower/furnace just for the addition. That means my options are mini-split(s) for the addition, or replacing the entire existing AC system. I don't really like mini-splits for a couple reasons: 1) I feel like they are a retrofit or a second choice, and I want to do this "right", and "right" to me means ducted HVAC. 2) I don't love the idea of not using gas for heat. I know it's Austin and so it doesn't get that cold that often, but if I have gas I'd prefer to use that. Basically, I want to do an HVAC replacement sized for the new addition.
The HVAC contractor has NOT done a Manual J. I know that's the first thing I'm supposed to get done, but honestly with the amount of fudging he's going to have to do on the existing square footage, I feel like he can make it say whatever he wants. Maybe it should bother me more.
Anyway, basically, he's saying that he would need 1.5 tons on the addition, and that the existing 2.5 tons could stand to be bumped up to 3 tons to handle the heat load on the hottest days (based on our input from saying that on the hottest days, we'd like it to be a little cooler). So that puts us at 4.5 tons, and there's no such thing as a 4.5 ton system, so it would be 5 tons. He's proposing an American Standard 14 SEER unit, single-stage compressor, new evap coil/blower in the untility room, new lineset, new tstat, add the ducts for the addition, for $9.5k.
But based on what I've been reading, a 5-ton system for 1875 sq feet to me seems pretty darn oversized. I'm worried about cost to run as well as short-cycling. It's not that humid here (not like Houston), but I'm still concerned. On the other hand, it does get really hot here and on the hottest days I could buy the argument that I really do need 3 tons for the existing sq footage. On the other hand, the addition will have good insulation.
So, what do the experts on here think in terms of sizing? 4 tons? Is 5 tons really needed? Remember, 1200 out of the 1875 sq feet will be pretty poorly insulated (and fixing that is not an option, attic access is impossible and I'm not ripping down the ceilings to do spray foam on the existing house). I've also read about the two-stage compressors, and honestly to me, the 5 ton with a two-stage compressor seems like it gets the best of both worlds: lower-capacity operation for most days, but has the tonnage to really cool the house on the super hot days and overcome the poor insulation.
Thanks in advance for the help.
tigerdunes
mike_home
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