I just built a new home, 2518 sq feet in Waller County. The builder installed a 2.5 ton Lennox system that is really struggling to keep up. It never got below 77 in the house yesterday until after 10:00 pm. The builder sent me the manual J/S calculation, but I have no idea what any of this means. Any recommendations on who to have review the calculations to see if they were done properly?

It not cooling the house is all the proof you need that it was done incorrectly. Rule of thumb is 1 ton per 500 sq ft. so 2.5 tons on 2518 sq ft seems way off.

Can you post a screenshot of the calculations?

Can look it over and see if an error was made or if they carried a 0 in the wrong place.

Even the old "500 sq ft per ton" rule of thumb would have a larger unit. My first home, a 2800 sq ft, spray foamed house, built in 2011, had a 4 ton unit (2 stories) in Houston.

~egon

It's a 4 or 5 page document. I can email it to you.

Who is the builder?

a 2.5 ton system in a 2518ft house is NEVER going to keep up during the summer heat. Your builder is a moron. You're looking at a 3.5 ton system and if you have high/tall ceilings, probably a 4 ton unit.

Foamed insulated walls and attic ?

No foam, just standard r15 insation in the walls and blown in insulation in the attic. It does have tech shield radiant barrier on the roof decking

Holy cow. That is a 5 ton job all day long if not foamed if you are in Texas

Single story home ?



What height are the interior walls ?

One story home. 9 ft ceilings in most of the house with 10.5 ft ceilings in living room.

5 ton all day long

Why they put a 2.5 makes zero sense unless they were trying to cut cost but you see the end result

Problem is the Manual J calculations they did came up with a 2.5 ton unit. I don't know anything about these calcaulations so I don't know where they went wrong and how to prove it to the builder. They are telling me the system is functioning within the design specs but it runs constantly and can't get the house cool until late in the evening.

Loadcalc.Net

Send them to me at nowlivingforme@hotmail.com. I will also need a pdf floor plan and the specs for the insulation, windows, wall type, compass direction of the front door and whether it's gas or heat pump.

It will take me a few days to a week or so but I'll look it over and see what I think is up.

Email sent, I really appreciate it.

Another dumb question, are you sure you have a 2.5 ton?

Can you post a photo of the units sticker? The outside one?

~egon

Dumb question, but are you sure you don't have 2 units?

Positive I only have 1 unit. Here is the tag.

I'll hold off judgment until AgAcGuy12 gets a look at it (don't forget to tell him ceiling height in each room because you're heating and cooling cubic, not square feet). The ~8 year old 2.5 ton system at my place is handling about that much area, but about 2/3 of it has 8' ceilings and windows facing SW are tinted, however. The room with a cathedral ceiling has spray foam, house has full sun exposure all day, and the southwest most room has radiant barrier installed.

Aren't you on the east coast ?

JP76 said:

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Aren't you on the east coast ?

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A quick dive tells me he's on the coast in SC, and has a whole-home dehumidifier to help out his AC. Average July high is 88, August in 87 in Charleston. Probably not a good comparison with what we need in Texas.

Charleston, SC which is as far north as DFW. It feels worse here than it did in San Antonio from all the humidity. The dew point gets up close to 80 in summer and the HVAC definitely works to keep that in check. The oldest part of the house was built in 1959 and was relatively well sealed until all the construction work over here kicked off.

There are two passegeways tarped off like this, and one interior wall for the time being has become an uninsulated exterior wall. The HVAC is running at a disadvantage for now

Dr. Venkman said:

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It not cooling the house is all the proof you need that it was done incorrectly. Rule of thumb is 1 ton per 500 sq ft. so 2.5 tons on 2518 sq ft seems way off.

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I have to respectfully Disagree. There is no "rule of thumb" in 2022 when dealing with Hvac. We have science. The fact that the AC wouldn't cool past 77 when it was over 100 degrees yesterday is actually a pretty good sign that his unit is sized correctly or not, who knows without a lot more info. . We have design temps, and 100 degrees is well outside of it. What is the Op design temp, should all be on the Man J.

If they arrived at 2.5 tons from a man j on a modern standard build house of 2500 sq ft, then we need to know what assumptions were made and where you are located. It won't fly in texas….anywhere.

EDIT: The commentary above is subjective. The design temperature for Harris county is 95* a 1%. So, the AC should be designed around comfort at 95* or lower with the expectation that 1% of hours in the year, it will be above that limit.

https://www.energyvanguard.com/blog/we-are-the-99-design-temperatures-oversized-hvac-systems/

I cannot say exactly where the OP lives, the design temperature there, or the building envelope quality.

Jason_InfinityRoofer said:

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Dr. Venkman said:

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It not cooling the house is all the proof you need that it was done incorrectly. Rule of thumb is 1 ton per 500 sq ft. so 2.5 tons on 2518 sq ft seems way off.

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I have to respectfully Disagree. There is no "rule of thumb" in 2022 when dealing with Hvac. We have science. The fact that the AC wouldn't cool past 77 when it was over 100 degrees yesterday is actually a pretty good sign that his unit is sized correctly. We have design temps, and 100 degrees is well outside of it. What is the Op design temp, should all be on the Man J.

If they arrived at 2.5 tons from a man j on a modern standard build house of 2500 sq ft, then we need to know what assumptions were made and where you are located. It won't fly in texas….anywhere.

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I don't know, Jason - let me preface this with how I normally really value and agree with your opinions, but "100 degrees is well outside of design temperature"? Where are you living in Texas? I'm in the northern tip and still saw 106 multiple times this week. And you mean to tell me there's no margin in HVAC design? And while a rule of thumb is not proper design, it definitely is outside a typical ratio of square footage to tonnage. My $.02, it smells like the undersized both from a square footage standpoint, and from a performance standpoint. Maybe the calcs are using the ridiculous DoE requirements (78* when home, 85* when away, 82* when sleeping).

Prince_Ahmed said:

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JP76 said:

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Aren't you on the east coast ?

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A quick dive tells me he's on the coast in SC, and has a whole-home dehumidifier to help out his AC. Average July high is 88, August in 87 in Charleston. Probably not a good comparison with what we need in Texas.

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It's 95 right now with a heat index of 109, while at my parents place in Hays County it's 97 with a heat index of 100. Yep, no comparison whatsoever.

The Fife said:

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Prince_Ahmed said:

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JP76 said:

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Aren't you on the east coast ?

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A quick dive tells me he's on the coast in SC, and has a whole-home dehumidifier to help out his AC. Average July high is 88, August in 87 in Charleston. Probably not a good comparison with what we need in Texas.

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It's 95 right now with a heat index of 109, while at my parents place in Hays County it's 97 with a heat index of 100. Yep, no comparison whatsoever.

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Oh, there you go, a single day just nullified my argument. Setting aside the fact that you also have a whole-home dehumidifier (which the OP hasn't mentioned having), along with any mention of home specs... in the last 130 years, the highest temp recorded in Charleston was 105. On average, you have 64 days above 90 each year, and 127 above 80. Houston has, on average 101 days above 90, 206 above 80... and it typically has over 30 days above 100.

In 14 of the last 20 years, Charleston hasn't even broken 100.... but sure. They are totally apt comparisons

Jason_InfinityRoofer said:

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Dr. Venkman said:

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It not cooling the house is all the proof you need that it was done incorrectly. Rule of thumb is 1 ton per 500 sq ft. so 2.5 tons on 2518 sq ft seems way off.

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I have to respectfully Disagree. There is no "rule of thumb" in 2022 when dealing with Hvac. We have science. The fact that the AC wouldn't cool past 77 when it was over 100 degrees yesterday is actually a pretty good sign that his unit is sized correctly or not, who knows without a lot more info. . We have design temps, and 100 degrees is well outside of it. What is the Op design temp, should all be on the Man J.

If they arrived at 2.5 tons from a man j on a modern standard build house of 2500 sq ft, then we need to know what assumptions were made and where you are located. It won't fly in texas….anywhere.

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The fact that it doesn't cool past 77 is all the info needed. Unless there is a refrigerant leak, the unit is undersized. Bad assumptions, incorrect calculations, who cares? Science could have said 2.5 tons, but the A/C is still struggling.

Prince_Ahmed said:

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The Fife said:

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Prince_Ahmed said:

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JP76 said:

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Aren't you on the east coast ?

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A quick dive tells me he's on the coast in SC, and has a whole-home dehumidifier to help out his AC. Average July high is 88, August in 87 in Charleston. Probably not a good comparison with what we need in Texas.

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It's 95 right now with a heat index of 109, while at my parents place in Hays County it's 97 with a heat index of 100. Yep, no comparison whatsoever.

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Oh, there you go, a single day just nullified my argument. Setting aside the fact that you also have a whole-home dehumidifier (which the OP hasn't mentioned having), along with any mention of home specs... in the last 130 years, the highest temp recorded in Charleston was 105. On average, you have 64 days above 90 each year, and 127 above 80. Houston has, on average 101 days above 90, 206 above 80... and it typically has over 30 days above 100.

In 14 of the last 20 years, Charleston hasn't even broken 100.... but sure. They are totally apt comparisons

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You made me dig deeper and from a load calculation perspective Charleston and Houston are equals. I went to loadcalc.net and made a simple building with 200 sq ft walls and 16 sq ft windows on the N/S/E/W, 400 sq ft area with 8' ceiling, 1 person, ductwork in the attic insulated at R6. Less than 1% difference in cooling requirements. So yeah, Houston is a pretty valid comparison in terms of what it takes to cool a house because of the energy it takes to deal with the extra moisture in the outside air. Feel free to run the numbers and see for yourself.

FWIW The dehumidifier isn't a factor here, it was tied in to a 3 ton system that died this winter. The second HVAC system is 2.5 tons and lost the den+foyer (also being rebuilt) and will go on a mini-split, so the decision was made to move the area serviced by the dead system to the one that works because the manual J for its coverage area was 32K BTU cooling. That's a bit less than 2.5 tons but the calculator on loadcalc.net doesn't take into account radiant barrier. Surprise, surprise... since the HVAC is no longer oversized humidity isn't a problem. The dehumidifier will go to the garage after the construction project is done.

This means nothing at all for OP so until we hear back with a second opinion on the load calc or it turns out their equipment was installed wrong or is defective we don't really know anything.

If the ductwork is as tight as what I've run into everywhere else I've lived that's what my money is on.

Normal design outside temperature for B/CS is 97 degrees and the load is usually designed for a 25 degree split. Which means that if it is 102 outside, the house won't get much below 77 (IF everything is done correctly). That being said 2.5 tons does sound light without running or seeing the load calculation.

I was in 2 different houses in BCS On Sunday and i think we got to 103 or 105. One is a 20 year old unit on a 20 year old house. R13 in walls, 30 blown in attic, 4 ton unit on 1900 sq ft, double pane windows from 2002 and it had the house cooling at 73 degrees. Other is a 53 year old house, unit is 4 years old, 4 ton on 1980 sq ft. R11 in walls, r19 in attic, single pane alenco windows from 1969. This house was set on 74 and holding at 74 during the high of the day. Previously this house had a 3.5 ton unit on it and whenever it got to 100 it could not cool the house under about 77/78 and would never cycle off and run nonstop.

Prince_Ahmed said:

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The Fife said:

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Prince_Ahmed said:

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JP76 said:

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Aren't you on the east coast ?

---

A quick dive tells me he's on the coast in SC, and has a whole-home dehumidifier to help out his AC. Average July high is 88, August in 87 in Charleston. Probably not a good comparison with what we need in Texas.

---

It's 95 right now with a heat index of 109, while at my parents place in Hays County it's 97 with a heat index of 100. Yep, no comparison whatsoever.

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Oh, there you go, a single day just nullified my argument. Setting aside the fact that you also have a whole-home dehumidifier (which the OP hasn't mentioned having), along with any mention of home specs... in the last 130 years, the highest temp recorded in Charleston was 105. On average, you have 64 days above 90 each year, and 127 above 80. Houston has, on average 101 days above 90, 206 above 80... and it typically has over 30 days above 100.

In 14 of the last 20 years, Charleston hasn't even broken 100.... but sure. They are totally apt comparisons

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FYI Houston averages 7 days of triple digits, and we currently haven't broken 100 in 654 days.