Duelling Calculators

[dancaselr]

I have two contractors who've bid systems. Both did a Manual J, but they came up with different results. Both contractors reached the same conclusion on my downstairs unit, but they reached somewhat different conclusions on my upstairs unit. I can accept that to a degree, since there's a little subjectivity (aka guesswork) involved in any such evaluation. However, I now find myself in a bit of a bind.

A little background on the upstairs unit: It's currently a 2.5 ton with a condensing unit replaced in the early to mid 1980's by the previous owner. The evaporator coil is original to the house (1975 ish). In the hottest part of summer, it runs constantly and can't keep up. The new furnace will be a Trane XV95 (2-stage, variable speed) which should help keep humidity down and make things more comfortable in the summer.


Contractor A says the upstairs unit came out to 2.2 tons. Given the history of not keeping up in the heat of summer, he recommends 2.5 tons and believes (and I agree) a new system that's properly matched will perform much better in that summer peak.

Contractor B says the upstairs unit should be 2 tons. Period. He's adamant about that value, and insists that 2.5 tons would be oversized and not control humidity as well. He'll put in 2.5 if I insist, but won't accept responsibility if it's oversized and doesn't produce the desired results. He will, however, "stand behind" his 2-ton calculation (though he stopped short of saying he'd change out the condensing unit/evaporator coil without charge).

When I discussed these two quotes with by beloved wife and came to the part about downsizing from a 2.5 to 2 ton unit, She shook her head and said, "I don't think so." (She prefers cool to warm and would be happy if I built her a sewing room inside a walk-in meatlocker).

To add fuel to this fire, I'm considering a 2-stage condenser, which comes in 2-ton or 3-ton sizes. Contractor A thinks I should use a 3-ton, Contractor B thinks a 2-stage in any size is overkill for the upstairs but of course thinks that if I do a 2-stage it should be a 2-ton.

And my beloved still doesn't like the sound of 2 tons in ANY configuration. She'll go with whatever I decide... but if it doesn't provide the right level of comfort, well, you know what they say: "If Mama ain't happy, ain't nobody happy!"

Based on your experience and professional opinion, which way should I go?


Thanks!

D.

[jtrammel]

First of all, assuming the duct is in attic and the ductwork is original 1975ish then it is not properly sealed or insulated if you seal it and reinsulate the duct, provided it is sized correctly, then 2 tons will probably work well, the existing 2.5 ton unit is probably not delivering 2.5 tons due to age and mismatch and you are probably losing 25+% due to duct loss. If you install a high eff unit on a leaky duct system you will not see the rated efficiency and often it intensifies hot/cold spots and humidity problems rather than solving them. The ductwork is critical for a long lasting efficient system.

[wahoo]

Cold air falls, so I'd "error" on the side of the larger unit. I'd also make sure of the condition of the ductwork as jt brought up. If you decide on the smaller output unit, and your wife is NOT happy.....uh.....ain't nobody going to be happy in your house!!

[motoguy128]

+1 As long as you don't use setbacks (turning it up when you leave and setting it down when you get home) and you seal or replace all of the ductwork and make sure they use a good equipment match, going from 2.5 to 2 tons will work fine if the existing unit is mismatched leaky and poorly insulated. Air leaks in a attic are HUGE!

If it makes you feel better, I took a leap of faith this summer and downsized from a 3.5 ton to a 2 ton upstairs that including replacing 1/2 the ductwork. THe smaller unit will remove humidity better and might run so long that if there aren't a lot of air leaks, she'll want the tmep higher because it almost feels too cold even at 75F. A lot of that depends on how well sealed your building envelope is. Mainly air leaks from the attic and at windows.

Match it with a 2.5 ton coil and tell your wife that it increases it's capacity above 2 tons. (its' true, it does... just not a lot) and the variable speed motor uses less energy so it dumps less heat into the space (also true, but not that significant).

Longer run times of the 2 ton will use maybe 10% less energy over a 2.5 ton, and on really hot days, you can just overcool the space 1-2F in the morning and close a few extra blinds. At my home at least, closing hte blinds probably equals almost 0.5 tons of capacity.

[BaldLoonie]

Cold air falls, so I'd "error" on the side of the larger unit.

That's kinda my feeling about an upstairs unit. The Manual J doesn't look at the heat that comes up from downstairs. So while it may say 2.2 tons, there's likely more than that just because some of the heat that comes in downstairs will come upstairs. The downstairs unit won't remove all of it. And if it is a tad big, that excess cold air will fall and reduce operation of the downstairs unit. BUT, gotta make sure your duct system will handle the CFM needed by a 2.5 ton. Usually we find that the ducts can't handle what's there. That and leakage or heat penetrating the ducts are why upstairs units don't cool.

[beenthere]

That XV might move a lot more air then your old furnace did. And low air flow may be why it didn't keep up.

How much hotter then temp set point did/does the upstairs get, and what temp are you trying to maintain upstairs.

[dancaselr]

That XV might move a lot more air then your old furnace did. And low air flow may be why it didn't keep up.

How much hotter then temp set point did/does the upstairs get, and what temp are you trying to maintain upstairs.

The upstairs unit we're replacing moves a good deal of air. I don't think that will be a problem. That said, I do intend to seal and re-insulate the accessible ductwork (in the attic, as is the unit).

We have another problem that we'll be rectifying with this install, and that's the position of the upstairs t'stat. It's in a spot that would probably be OK if the bedroom next to it had the door open all the time, but instead it's mostly closed (it's my server room, among other things). We've already got a better spot picked out for it, and I th8ink that in that spot it would "keep up" better. However, in the hottest part of the summer I've seen it in the upper 80's upstairs (110 outside, of course)

How cold do we want it? Cold enough to make Mama happy. Fortunately, she's on the far side of menopause now. A few years ago, even the walk-in cooler would have been too warm for her some days.

D.

[jtrammel]

Did they account for the "server room" a lot of computers and other electronics can add a lot of internal load. It would also be a good idea to have a return air duct installed in any room where the door is closed most of the time

[Rodney28334]

Make sure you relate to the contractors that your wife likes it very cool. You said with the 2 ton it was in the upper 80's when over 100 outside. I dont know your location but most load programs size for example 77 in door temp for 95 out door temp. If your wife likes it 70 or 72 even in the hottest days then you may need to go with the bigger size. You will pay for it the rest of the year with lower run times and higher energy use (both much lessened with 2 stage) Just make sure they know about server room and your wifes needs.

[motoguy128]

I dont know your location but most load programs size for example 77 in door temp for 95 out door temp. .

You can;t make that geenralization. Indoor temps are normally 75F in all climates, except maybe hot dry regions. Outdoor temps vary. 95F would be too hot for at least 60% of the homes in the US. Most of the East coast is 90F or lower. Northern climates are 88-90F. Some parts of Texas are 97-100F design. Then some area will "throw you for a loop". Northern FLorida and many parts of Georgia are only 92-93F... the same or lower than many parts of the Midwest.

Then there's wet bulb temps... which impact latent capacity requirements. So air leakage, expecially from reverse stack effect will have a large impact as well as mositure transfer from vapor drive from a hot humid attic into hte cooler dryer home.

That gets me thinking... why don't you have a vapor on the attic floor in vented attics? A vapor barrier is installed in the walls... why ignore the attic?

.

[martyinlincoln]

Get the 2.5 ton and keep the Mrs happy. A quarter or half ton over sized isn't going to effect humidity on a house large enough for two systems.

[hkempf]

Everyone else has pretty much said everything I would say, I would also look into installing make up air returns in every room to keep the air circulating good, most people's misconception on a two and two and a half is that a 2.5 ton from the 80s is probably losing 3/4 of a ton by now and the new technology along with sealing and insulating ductwork will take you much farther than that extra 1/2 ton.
Another analogy people don't think of is the newer a/cs are designed to run longer and should not be oversized to cool or heat faster, think of it as a car, driving in town is stop and go, like an ac starting up and cooling to quickly, it wears on the unit, where do you get your best mileage, on the freeway, less wear and tear from not stopping and going and better fuel mileage, just like heating and cooling, let the furnace and ac do their job like they were designed, and that is to run for longer periods of time, saving you energy and life on your unit, and removing more humidity the longer it runs making you feel more comfortable despite the temperature reading

[hkempf]

Also look for a company that stands behind their work, my company if you let our comfort consultant come in and do a load calc free of charge and you are not happy we give you a year test drive, if you are not happy for any reason we will refund 100 percent of your money and you can have someone else do the work not losing a penny, now that's confidence in our work