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jmking1982

AC sizing for addition+replacement

jmking1982
10 years ago

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.

Comments (14)

  • tigerdunes
    10 years ago
    last modified: 9 years ago

    Not more than 4 tons. You could go with a two stage AC condenser. Matching evap coil. Also recommend a two stg 80% eff var speed furnace, the smallest available with a 4 ton rated blower.Plus correct two stg thermostat. Ductwork should be evaluated as to size, design, insulation qualities no less than R8, adequate returns, etc.i probably would consider ripping out old and replacing over reworking old.

    I would not take any shortcuts on insulation for your renovation project.

    IMO

    This post was edited by tigerdunes on Tue, Jul 16, 13 at 6:41

  • mike_home
    10 years ago
    last modified: 9 years ago

    You should consider making the addition a separate zone. This will help keep a constant consistent temperature between the two area. I would also recommend spending the extra money on a 2-stage AC.

    You need to find a HVAC contractor who will spend an hour doing a Manual J calculation. There is no excuse not to do this especially when new construction is involved. The guy you have is already over sizing your system.

  • udarrell
    10 years ago
    last modified: 9 years ago

    You can try this Free Online Whole House Load-Calc.

    READ all the Instructions!

    Use the calculator to figure Infiltration CFM:

    You can easily reduce infiltration rates yourself, therefore, using the calculator, I’d use 0.4 ACH (Air Changes per Hour) be sure to add the (Air Changes per Hour) CFM into the ‘Fresh Air Recommended ‘line-slot, or it won’t figure the Infiltration & fresh air Btuh.

    This calc seems to show considerably higher loads than a spreadsheet calc so don't hype it in any way; go the other way if anything!

    It will feel okay on those exceptional hot days if indoor temps goes to 80°F as long as it is below 50% Relative Humidity. So you could use 80°F indoor design & 50% RH.

    Here is a link that might be useful: Free Online Whole House Load-Calc

  • tigerdunes
    10 years ago
    last modified: 9 years ago

    I hate to question udarrell but why would OP want to use 80 deg fah as inside design temp?

    I wouldn't...

    IMO

  • udarrell
    10 years ago
    last modified: 9 years ago

    Because that load-Calc seems to show a lot more heat-gain than a correctly performed spreadsheet calc.

    Plus, he doesn't care if it goes to 80°F on those unusual heat wave hours; you can be comfortable at 80°F with air movement & below 50% RH..

    That might get it closer to the actual heat-gain.

    I've done them both ways & knew what the actual heat-gain was on a design day running window Air Conditioners.

  • tigerdunes
    10 years ago
    last modified: 9 years ago

    Well I understand what you are saying.

    However this brings into question the reliability of this software for sizing purposes.

    I think if i was using this software, I would run it at both 80 and 75 to see how this affected the heat gain.

    IMO

  • ionized_gw
    10 years ago
    last modified: 9 years ago

    OP, have you considered sealing the attic and spray foaming the bottom of the roof deck? That is what is recommended in my climate (Gulf coast), especially if there are ducts in the attic.

  • jmking1982
    Original Author
    10 years ago
    last modified: 9 years ago

    Thanks for the responses so far.

    @ionized, the addition will be insulated as you described - spray foam on the underside of the roof decking. Ducts for the addition will then be in insulated space. Unfortunately I can't do that on the existing house because the roof is low-slope and there is limited to no attic access. What's up there right now is some limited amount of blow-in insulation.

    I am getting a quote on a 4-ton two-stage system, we'll see what he comes back with.

  • tigerdunes
    10 years ago
    last modified: 9 years ago

    jmking

    You realize furnace will need to e replaced as well?

    Or if existing conventional furnace has a 4 ton rated blower, you will be limited to a 13 SEER condenser.

    IMO

  • jmking1982
    Original Author
    10 years ago
    last modified: 9 years ago

    Yes, the furnace/blower is being replaced too. Existing furnace/blower is 3.5 ton so I can't use that with a 4 ton condenser unit. I don't think keeping that and going to only a 3.5 ton condenser would be quite enough.

  • jackfre
    10 years ago
    last modified: 9 years ago

    I think if you went with a mini-split in the addition in a couple years you would be looking to replace your exisitng ducted system with them too. They are substantially more efficient and quieter than duct work too.

    I would suggest you leave the existing system as is. As the minisplits are variable speed you can oversize a bit for all the window load and still be good as a 1 1/2 ton ms will turn down to about 6k or so btu.

    Before you touch your existing system or choose to extend it I strongly recommend a duct leakage test to see how your current system is performing. If you go ducted, I wouldn't pay for it until I had a duct leakage test done. Google" DOE duct leakage" to see what I'm talking about.

    I'm remodelling too. New addition for big kitchen, up and down baths, laundry and small porch. I have all ready torn the 6 yr old ducted system out completely and gone mshp. I'm adding a 15 kbtu ms for the kitchen addition. I guess we look at things differently ;) good luck with the project

  • jmking1982
    Original Author
    10 years ago
    last modified: 9 years ago

    Got three quotes for 4-ton systems. All quotes include addition ductwork, new lineset, 10 year comprehensive and 10 year parts warranty.

    Comfortmaker 16 SEER N Series R410A
    Outdoor Model: NXA648GKA*
    Indoor Model: EN(A,D,W)4X48*21**+TDR
    SEER rating 14.5
    $8950

    Comfortmaker 16 SEER N Series R410A
    Outdoor Model: NXA648GKA*
    Indoor Model: EN(A,D)4X61*24**
    Furnace Model: *8MX*0902116**
    SEER rating 16.0
    $9485, plus this one is eligible for either $300 or $700 worth of rebates.

    Ameristar
    Outdoor Model: M4AC4048A1000AA
    Indoor Model: M4CXC049AC1CAAA+TXV
    SEER rating 14.0
    $9165

    To me, the middle system looks like the way to go. It's the same condenser as the first one, but with a larger indoor unit. Is that how they get the higher SEER rating - larger evap coil with slower blower speed?

    What do people think of these quotes?

    Thanks in advance.

  • tigerdunes
    10 years ago
    last modified: 9 years ago

    I certainly would like to see more quotes.

    Very frisky pricing from low end to mediocre HVAC.

    Does this include new ductwork which you should insist on?

    IMO

  • jmking1982
    Original Author
    10 years ago
    last modified: 9 years ago

    Not in the old part of the house, only the addition. But that ductwork was installed as part of the renovation in 2007 so it's not horribly old.

    The GC allowed $2K for addition ductwork, so if I take him at his word (and his 33% profit), these quotes are like $7K for the replacement system. Are these quotes really that high?