Carrier/Bryant Oversize or Undersize?

[mikemax]

I live in Cleveland, Ohio. My home is 13 years old and 2500 square feet, and I am in the process of replacing the original gas furnace/central ac with new equipment. I'm looking for both efficiency and comfort - both rank above cost for me. My research has led me to favor the Carrier/Bryant Infinity/Evolution line - the 95% efficient furnace and the 21 SEER (although lower than that for my configuration) 2-stage AC. I realize that I'll probably never get full payback on the 2-stage AC in this climate, but I like the idea of having better humidity control and being greener, and we plan on being in this house indefinitely, so I'm ok with that.

My preferred contractor just completed a load analysis of the home, indicating a heating load of 110,350 BTU, and a cooling load of 39,739 BTU. This makes the furnace sizing a non-issue - the 120k variable speed unit seems to be a good fit. But being on "the bubble" complicates my AC decision if I want to stay with Carrier/Bryant (they don't make their two-stage units in half-ton increments).

Options:
- 4 ton 2-stage 21 SEER - LOW=35000 BTU, HIGH=47500 BTU
- 3 ton 2-stage 21 SEER - LOW=26500 BTU, HIGH=37400 BTU
- 3.5 ton 1-stage 15 SEER - 42000 BTU

If I trust my contractor's load calculations, do either of the 2-stage options work for me? Despite the northern climate, I'd prefer to have the added efficiency and humidity control of the 2-stage unit. But it seems that the 4-ton unit's low rating is so close to my target that the unit would rarely hit the higher stage?

Is going with the 3-ton 2-stage unit, whose higher stage is 2300 BTU under my contractors load calculation, something I should consider? It seems as though this unit would come closest to providing the benefit I'm looking for - a much lower "low" stage for humidity control and lighter-weight cooling/temp stabilization, and a high stage for more significant cooling - but it would fall short of having the rated cooling capability based on the calculations.

I ask all of this because, from what I've read, having a slightly undersized unit might be preferable to having an oversized one - but I'm not sure how 2-stage AC's play into this.

Input would be much appreciated.

One additional note: the AC unit we're replacing is a 3.5 ton single stage unit, and has provided adequate cooling over the 3 years we've been in the home.

[mchild]

First of all it sounds like you are working with a good contrator. Not many are willing to do a load calc up front. Go over it with him and make sure he didn't make any incorrect assumptions such as the indoor temp you like and that he didn't adjust the outdoor design temps.

Next discuss with him what size of unit he would recommend. I bet it will be the 4 Ton unit as A/C is sized based on the sensible load not the total load. Assuming the sensible load is about 85% of your total, that would put it at about 34,000 BTU which is about the sensible capacity of the 4 ton unit.

There are other models within the Infinity line that will give about the same SEER and may meet your needs a bit better (possible better dehumification).

[dash]

What indoor cooling temperature do you like?

What did he use in the calc.?

If insulation can be addd to the attic,say you have R19 now and went to R30,he could easily run the calc again,to see if the 3 ton would be large enough.Could also consider any other improvements that would reduce the load.

If you can get down to a 3 ton,that save you operating cost forever.

The 4 will work just fine,if the duct system is large enough.

[tigerdunes]

mIKE

I would want to see the load calculations in writing.

What is your comfort level as far as thermostat setting for cooling in summer and heating in winter? Did you share this info with your dealer?

I am skeptical of such a high heating loss if your home has reasonable insulation qualities. What size is your existing furnace and AC? Has your ductwork been inspected by quoting dealer?

Going to a 3 ton AC condenser in a borderline situation is a mistake.

BTW, what is your electrical rate?

IMO

[mikemax]

I decided to purchase the residential version of HVAC-Calc and run the loads myself last night. My calculations were substantially lower than the numbers the contractor ran. I asked him for a copy of his, and it turns out that he'd oversimplified his calculations by doing whole-floor calcs and taking some other shortcuts. The end result was inaccurate square footage and a good deal of inaccurate detail.

I did the HVAC-Calc load room by room, and included all windows, doors, fireplace, people (I used 10 to accomodate visitors), etc. My numbers gave me a total heat gain of around 33000 BTU. That's with a design temp of 74 degrees (we don't plan to go any lower than that) and default Cleveland outside design conditions (88 summer, 5 winter, 94 grains of moisture).

At this point, I've made the decision to go with the 3-ton, 2-stage unit. Downsizing to a 2-ton is obviously too big of a stretch, and the 2-stage unit will have a 26000 btu lower stage to go with the 36800 upper when matched to the coil. The 3-ton has better SEER and EER ratings as well - 18+ and 13+ when paired with the furnace I've selected (the 95% 355BAV variable speed 120k btu unit).

I pay $10.15 per MCF for gas, and 5.6 cents per kwh for electric. I did consider heat pump "hybrid heat" options, but with the high efficiency units above, the northern climate and my reasonable rates, I couldn't justify the increment up.

Thanks everyone for your help - the questions certainly motivated me to double-check the numbers by running the calcs myself, and I think it was $50 well spent. Going with a 4-ton unit, even with 2 stages, would've been a mistake. The 3-ton will save me up front and will save me more on an ongoing basis while providing more comfort with better humidity control.

Still open to input, if anyone thinks my thinking above is wrong.

[dash]

Two issues.

One you need to take indoor design temperature into consideration to select the size of the equipment,along with sensible and latent loads.

Equipment is rated at 80° indoors in most all cases.The deduct for 74° is 835 sensible btus for each degree below 80° ,oer 1000cfms.

So 835X 6X1.2=6,012 btus,that's .5 of a ton.

You also need to look at the outdoor temp,standard is 95°,yours may be less ,so you gain capacity.


Second is 10 people/guests ,two per BR is enough,more might over size.

[tpa-fl]

Keep in mind that Manual J gives you the needs for the "worst case" conditions, which, in theory, you'll only reach about 2% of the time. Are you going to have 10 people over at your place on the absolute hottest day of the year during the hottest time of the day? Probably not.

Overall, it sounds like a 3 ton unit is the right choice, but this is assuming all ductwork is properly sized & sealed and air infiltration is as you told HVAC-Calc it was. You're actually closer to a ~2.75 ton need. Ever consider the "17 SEER" AC? I believe the 17 SEER system is closer to 1.5 ton / 3.0 ton for low & high respectively, which would probably be a better match. You're paying absolutely peanuts for electricity and the lower low stage capacity would mean longer runtimes and better humidity removal.

FWIW, with my own home, in Florida, the heat calc came out to 22,000 BTUs, installed the Carrier Inifinty 16 SEER heat pump and this system spends 90% of its time on low, which is only ~14000 BTUs. About the only time I see it switch over to high is if I've been out of town and bring the system off of Vacation mode and it has to drop the place by about 5F. Otherwise, it comes on low stage and pretty much stays there all day long. Nice low 40-45%RH, even with a tropical storm. I looked at the 19 SEER version of the system, but it's low stage is only ~75% of high, which would mean more cycling, less humidity removal, and even in Florida's extreme climate and high electric rates (I pay $0.10/kWh, $0.13/kWh after you add up the taxes & fees), the 19 SEER unit didn't pay back until about year 12-15.

[skippedover]

The easiest way to get an inaccurate load analysis is to guess at what the infiltration rate on the home is. You say he did a Manual 'J' calculation but was it J-7 or J-8? Each is acceptable but J-8 has a lot more variables to be accounted for. Other variables that can affect the rate are number of appliances that generate significant heat in the home or room (refrigerator, some high end wide screen T.V.s or home theaters, etc. as well as the number of people and/or fireplaces in the home. Oversizing an AC system is not a good idea but with a 2-stage you do actually have 2-units and we too sometimes spec out a unit that's oversized on 2nd stage (both furnaces and AC units) and undersized on 1st stage. Once you're satisfied all the variables of the load analysis are accounted for, I'd recommend going with the contractors recommendation.

[mikemax]

The contractor used blueprints only to do the load calc, and even then, only appeared to do the calc using rough numbers (house perimeter numbers, rough cut at windows). In other words, he obviously didn't REALLY do a manual J load, even if he's calling it that. He admitted that to me when I asked him to walk through his assumptions.

I made some minor adjustments to my own calculations, correcting for our actual ceiling insulation type, reducing the people count to 6, etc. My resulting number is 29,544, of which 25,435 is sensible.

I've replaced virtually every light bulb in the house (80 bulbs or so) with CFLs over the last few months - I guess renting "An Inconvenient Truth" turned out to be pretty expensive, at least in the short term :) So heat gain from lights should be greatly reduced. We're also minimizing indoor cooking during the summer months (microwave or grill whenever possible). We're turning off all appliances/devices not in use (no sleep or standby) as well. So heat gain should be reasonably low, but certainly not zero. We do have a rear-projection tv.

I did consider the 17 SEER unit. The pro was the 50/100 low/high split, the cons were that it was otherwise less efficient and that it didn't use the scroll compressor technology, which I've heard is a little more trouble free.

Based on my 29,544 rating, I'm inclined to go with the 3-ton dual-stage, with roughly a 2-ton low and 3-ton high. If my numbers are accurate, I'll still be a little oversized, but the 2-stage will spend most of it's time in the 2-ton+ mode, and if I've understood what I've read on this forum, that plus the variable speed fan should be able to effectively and reasonably efficiently control humidity while supporting long(er) and more efficient run times. The system will have the 3-ton capability to fall back on the small percentage of time that my system requires it. All much better than the 13-year-old 1-stage 3.5 ton unit I had, or the 2-stage 4-ton unit the contractors inaccurate calculations initially suggested.

Hopefully I'm on the right track. I'm disappointed that my contractor took shortcuts and didn't do full calculations, and I'm tempted to seek another, but none of the 3 other contractors I had out to the house seemed any more thorough.

I wish it were easier to find a professional who I could trust with work like this, without feeling like I need to get up to speed enough myself to be able to assess whether they're giving me good advice. But, unfortunately, if I had done that in this case, it looks like I would've gotten some bad (and expensive) advice, and I would've been an unhappy customer paying too much for cooling. As it stands, I may or may not make the best/perfect decision, but I think I'll make a much better one than I might have before.

I keep hearing "find a contractor you trust, then let him ...". But that, my friends, is the rub. If I could find a contractor I knew I could trust, of COURSE I'd let him make all these decisions. But finding that person (and being reasonably sure) isn't easy to do. If anything, this experience has further validated that for me. And it sounds like I'm far from the only one who's experienced this.

Thank heaven for this board - not only the direct feedback I've received to my post, but all the information contained in all of the other threads as well. Not sure where I would've started without it.

[dash]

Did you read about derating the capacity for 74 degree indoor temperature,serious issue often overlooked??

Losing half a ton from standard ratings ,check it out.

[mikemax]

Dash,

Thanks, Dash. It seems that this adjustment means that a 3-ton unit (vs. a 2.5) makes even more sense - the extra rated cooling capability might be required to overcome the indoor operating conditions and provide sufficient sensible cooling capacity? And variances in outdoor temps will affect that as well.

I used 74 for calculation purposes - our actual goal is to strive for 75 or 76 with better humidity control.

[teddy bear]

I'm looking for both efficiency and comfort - both rank above cost for me. My research has led me to favor the Carrier/Bryant Infinity/Evolution line - the 95% efficient furnace and the 21 SEER (although lower than that for my configuration) 2-stage AC. I realize that I'll probably never get full payback on the 2-stage AC in this climate, but I like the idea of having better humidity control and being greener, and we plan on being in this house indefinitely, so I'm ok with that.
But being on "the bubble" complicates my AC decision if I want to stay with Carrier/Bryant (they don't make their two-stage units in half-ton increments).

Options:
- 4 ton 2-stage 21 SEER - LOW=35000 BTU, HIGH=47500 BTU
- 3 ton 2-stage 21 SEER - LOW=26500 BTU, HIGH=37400 BTU
- 3.5 ton 1-stage 15 SEER - 42000 BTU

If I trust my contractor's load calculations, do either of the 2-stage options work for me? Despite the northern climate, I'd prefer to have the added efficiency and humidity control of the 2-stage unit. But it seems that the 4-ton unit's low rating is so close to my target that the unit would rarely hit the higher stage?

Is going with the 3-ton 2-stage unit, whose higher stage is 2300 BTU under my contractors load calculation, something I should consider? It seems as though this unit would come closest to providing the benefit I'm looking for - a much lower "low" stage for humidity control and lighter-weight cooling/temp stabilization, and a high stage for more significant cooling - but it would fall short of having the rated cooling capability based on the calculations.

One additional note: the AC unit we're replacing is a 3.5 ton single stage unit, and has provided adequate cooling over the 3 years we've been in the home.
If you want humidity control for the damp cool weather you get in green grass climates, consider adding a whole house dehumidifier. A whole house dehu will maintain <50%RH thourghout the home including your basement regardless what your cooling load is. The better ones like Ultra-Aire remove moisture without any overcooling at 2-3X less electricity. 5-6 lbs of moisture removed per KW verse the a/c overcooling 2 lbs. per kw. The investment in a high seer 3.5 ton a/c and the dehu wood be less than the 2 speed a/c. The dehu will also keep your a/c ducts/a/c coil drier during the shoulder season. Most a/c contractors are not aware of the benefits dehu. The dehus maintain <50%RH when its raining and 65^F-70^F for a couple days. Dehu TB

[tigerdunes]

Mike,

with your inexpensive electric rates, you would be doing yourself a disservice by not getting the Infinity heat pump model to make your system dual fuel. And what does your load calc say about BTU heating loss for the size of the Infinity gas furnace?

IMO

[mikemax]

I did give the heat pumps a long look, but I just didn't think the payback was there. But I'm open to exploring it a bit more.

I posted a separate thread on dual fuel vs. straight gas heat/ac in my situation.