Sq footage or Manual J ?

[sdeemer]

I have found many different ways that residential A/C sizing is determined. WHich is more accurate? Our house is not cooling well. New home; extremely energy efficient; double pane windows, etc. etc.

2845 sqft home. The builder put a 4 ton unit zoned (upstairs/downstairs). But we have been unable to keep the house cool and uthe unit runs all the time. The builder says this is sized appropriate. But I son't know if they determined sizing by sq footage or did a load calculation. If they did one they aren't able to provide it to me.

[pstu]

Your question is really a forensic one. Do you really care more about the builders thought process, or about getting your system cooling well? They are two different projects.

I submit you will increase your odds if you call someone trained by the National Comfort Institute:
http://www.nationalcomfortinstitute.com/

Their process will use several instruments which a hack will not even own, and is supposed to identify how many BTUs your system is really putting out, and how many of them are getting to your rooms. Give it a try.

Hope this helps -- Pstu

[dan sw fl]

Originally posted by sdeemer
Which is more accurate?
New home; extremely energy efficient; double pane windows, etc. etc.

2,845 sq.ft. home. The builder put a 4 ton unit zoned (upstairs/downstairs). But we have been unable to keep the house cool and uthe unit runs all the time. The builder says this is sized appropriate. But I don't know if they determined sizing by sq footage or did a load calculation. If they did one they aren't able to provide it to me.



Manual J performed by an experienced person with ALL the building envelope parameters along with your inside air specs ( 75'F & 48% R.H.) defined accurately is the most appropriate method for determining cooling load
prior to equipment selection and duct design.

How many zoned systems has the mechanical contractor installed in the last 2 years?

With your current situation, you need to hire an air balancer to obtain an unbiased evaluation of the equipment/system performance as previously suggested.

[beenthere]

That could be oversized, undersized, or the correct size, for all we can tell from the little info you gave us.

Has the hvac contractor been back out to check the operation of the unit.

It could be under charged, or over charged and cause the unit not to cool right.

[cem-bsee]

good air flow from all supply registers?
what is ambient? temp & RH%
2 story w/bsmt? slab? crawl?

do you buy a car with engine sized for the max number of passengers -- simular to sizing HVAC based upon sf!

[-80guru]

Manual J.

[NormChris]

This short article will answer your question

http://hvac-talk.com/vbb/showthread.php?threadid=33866

[mjk_na]

Manual J is better in your case.

The AC has to be installed properly as well (proper charge, no leak, and correct duct sizing)

[sdeemer]

thank you all for the info. i have a 2-story with slap, no basement, but several attacks (above 2nd floor). i assume the HVAC contractor would be the one the builder is contracted with with has been out 3 times to rebalance the system. they said everything was working fine - but thus the problem is not fixed. temps now currently 65-70 at night and 92-98 during day. with high humidity (in houston it's horrible)

there are 2 thermostats. the one down is set at 76 and up at 77. this is what they suggested setting the upstairs 1 degree warmer and leaving the fan on ON not auto. down is on auto. they are both the same digital thermostats. upstairs seems to be 77 or 78 most of the time. the air blows much harder from the registers upstairs than down.

it's downstairs that has the problems. study, living area and dining area and kitchen. now they are already 79 and it's only 10am! (therm always stays at 76)

i still do not think they put a big anough A/C unit in here. but yes, this is the builder's problem as everything is warrantied for 1 yr. and i am going to keep writing them letters and calling and keep copies of it all incase i have to sue them.

but i also think maybe we should spend $100 or whatever it cost to have someone (unbiased) come evaluate our system. that was a good idea - whose ever it was. that might give up more leveredge. it just seems strange that all the other homes around me (all brand new) have the same size units (i asked them) and most are 300-400 sq ft less, half are 1 stories.

[coolwhip]

48% RH ?

[hvac hero]

lol I dont need a manual J to tell me that a 4 ton system on a 2845 sq ft house in houston is wayyyy undersized. If I were you, I would purchase the load calc program & run it for your house. You can purchase a 1 time use for it. It'll give you something to go by & compare to theirs if they ever show it to you. I'm thinking their copy will conveniently have been misplaced.

[Panama]

Originally posted by hvac hero
lol I dont need a manual J to tell me that a 4 ton system on a 2845 sq ft house in houston is wayyyy undersized. If I were you, I would purchase the load calc program & run it for your house. You can purchase a 1 time use for it. It'll give you something to go by & compare to theirs if they ever show it to you. I'm thinking their copy will conveniently have been misplaced.

All I can say is that they must build really lousy houses in Houston if a modern 2845 sq ft house can't be cooled and dehumidified adequately with properly designed and installed 4-ton system.

[dan sw fl]

Originally posted by sdeemer
2-story with slab,
, but several attics (above 2nd floor).

has been out 3 times to rebalance the system.

they said everything was working fine - but thus the problem is not fixed. temps now currently 65-70 at night and 92-98 during day. with high humidity (in houston it's horrible)

one down is set at 76
and up at 77.

this is what they suggested setting the upstairs 1 degree warmer and leaving the fan on ON not auto. down is on auto. they are both the same digital thermostats.

upstairs seems to be 77 or 78 most of the time.
the air blows much harder from the registers upstairs than down.

it's downstairs that has the problems. study, living area and dining area and kitchen. now they are already 79 and it's only 10am! (therm always stays at 76)

i also think maybe we should spend $100 or whatever it cost to have someone (unbiased) come evaluate our system.


Do you think that the 4th time the mechanical contractor visited he would be any luckier in enhancing the performance?

Air balancer fee for half-day service
might be in the $250 range.
However, do NOT expect your builder to actually
address your concern until REALLY FORCED
with presentation of independent report and
setting of date for a court appearance.

Let me guess ...
Air handler is in the attic.
------------------ Upstairs ___ Downstairs
Supply Air Temp ...._______ ....________
Return Air Temp ...._______ ....________

Supply air duct chase is __"x___" and located ___
Return air duct chase is __"x___" and locaetd ___

The downstairs ducts need to handle
> 800 CFM (> 1.1 square feet of metal duct or minimum 14" flex duct with 16" diameter actually preferred).

Send plans, window sizes/specs and
house orientation for full evaluation and
comparison of calculated air flow rates per room
to the air balance test report.

Electric usage per day = ?
___ 80 kw-Hr
_X_ 70 kW-Hr
___ 60 kW-Hr
___ 50 kw-Hr

2,845 Sq. Ft house in Houston with a 4-ton might be sufficient if you have < 11 tinted windows
AND no sliding glass doors
or 15 windows if None of the windows face East or West.
(hvac hero says it is 'way undersized' ...
but I need to deal with actual facts prior
to providing a more realistic evaluation)

How do I know the approx. A/C unit size required?
I have performed probably
> 400 Manual J calcs in the last few years.

[beenthere]

Originally posted by Panama

Originally posted by hvac hero
lol I dont need a manual J to tell me that a 4 ton system on a 2845 sq ft house in houston is wayyyy undersized. If I were you, I would purchase the load calc program & run it for your house. You can purchase a 1 time use for it. It'll give you something to go by & compare to theirs if they ever show it to you. I'm thinking their copy will conveniently have been misplaced.

All I can say is that they must build really lousy houses in Houston if a modern 2845 sq ft house can't be cooled and dehumidified adequately with properly designed and installed 4-ton system.


Not really.

At 98 OD, the temp is 3° above unit design rating, and 76° indoor is 4° cooler then the units design rating.
Or approximately 9500 btu's lost sensible capacity on a 4 ton unit.

ARI is 95OD, and 80ID.

[mark beiser]

Originally posted by dan sw fl
How many zoned systems has the mechanical contractor installed in the last 2 years?

That isn't really an accurate gauge of how good a company is at zoning.

There are a number of companies in my area that have installed thousands of zoned systems that are absolute garbage.

[mark beiser]

Originally posted by hvac hero
lol I dont need a manual J to tell me that a 4 ton system on a 2845 sq ft house in houston is wayyyy undersized.

Hard to make a statement like that without knowing anything about the construction...

With some types of construction, his system could be significantly over sized.

[pstu]

FWIW I have done a load calc on my 3300 sqft Houston home, with design temp 99F HVAC-CALC tells me total heat gain 55,600 BTUH, which would be 4.5 tons. I could go down to 4 tons if I had good windows, which I don't. If I am not mistaken the equipment selected should be about a ton larger due to de-rating, am I right here? This business of de-rating is something I did not understand until recently.

What I actually have appears to be 6.5 tons in 2 systems (one old system has been hard to identify). I've been measuring runtimes all last summer and the larger system, a 3.5 ton Trane, could clearly be downsized 0.5-1.0 tons and still do the job with this house, this duct system, etc. During most of the heat last summer it never ran more than 75% of any hour (the strongest day observed, August 27 2005, ran 88% of the peak hour).

I suppose that pretty much means that Manual J is confirmed to be a good model here. Any house with 2 systems would need somewhat more than half the total load, as the two halves of the house will have peak needs at different hours.

For the original poster, he needs to be thinking this is more than a hundred dollar diagnosis. I would gladly pay 3 times that for such a service well done. A load calc would indicate clearly what the AC size *should* be, if the house is still not cool enough then there should be a problem robbing the AC system of its ability to cool. But of course any hack can say the word "undersized" and hope the result will be a fat juicy sale. Hate to say it but selling a hack answer like that appears to be an easier sale than doing it the right (ACCA) way, to most homeowners.

Leaking ducts is one common thing which can rob a system of its cooling power, and waste energy dollars besides. Somebody trained by National Comfort Institute should be able to evaluate this need, as well as his overall system efficiency:
http://www.nationalcomfortinstitute.com/


If the homeowner is contemplating adding any equipment (not necessarily the right answer), I would suggest a whole house dehumidifier. That would surely improve the comfort level of the house, and he may find himself re-thinking what temperature is needed to be comfortable. He won't have to worry about overwhelming his duct system, which upsizing would likely do. And finally he has a working answer for those spells of wet mild weather when humidity is hardest to control.

Hope this helps -- Pstu

[Edited by pstu on 06-18-2006 at 02:29 PM]

[praha99]

Without any info rule of thumb in my parts is 600sq/ton

[sdeemer]

thanks for the info. i did actually purchase the 1-time use program yesterday. i have been playing around with it trying to figure it out. as i said i'm just a girl no A/C exp, and my husb travels alot. but, the program says it should "key word" be simple enough for the novice person.

also i checked and i have 18 total windows none tinted; all double-paned. 11 of these face east/west directly. i do have a front porch (small) but no trees to speak of as this is a brand new neighborhood. although i have planted some for the furure.