The truth about 20 degrees of seperation

[mosestr]

Fellas, I was listening to a local Homebuilder here in Houston and he stated that no matter what the temp. is outside, you should be able to get your house cooled basically to whatever temp you want it to be. The day I called it was about 100 w/humidity. My house was about 76. I was telling him that during the times of approx. 12p-7p I could only get down to 75, but anytime after that my house was freezing (low 70's) I thought I was all good until I talked to him and he said that things could be better. For record. 1 unit (5 ton) 2700 sqft. 2 story. aver. elec bill 300.00

[Mr Bill]

Originally posted by mosestr
The day I called it was about 100 w/humidity. My house was about 76. I was telling him that during the times of approx. 12p-7p I could only get down to 75, but anytime after that my house was freezing (low 70's) I thought I was all good until I talked to him and he said that things could be better. For record. 1 unit (5 ton) 2700 sqft. 2 story. aver. elec bill 300.00

I live in Houston and my opinion if your a/c is working that good I would say "You Are Blessed"

[mosestr]

Thanks MrB for that confidence builder. I have put ALOT of long hot hours in my attic cleaning coils, changing duct, adding insulation, and radiant barrier. ALL of you on this site have been a great help with your advice and know-how. Thanks a million. You made me into a shade-tree a/c guy.

[leapfrog]

Originally posted by mosestr
Fellas, I was listening to a local Homebuilder here in Houston and he stated that no matter what the temp. is outside, you should be able to get your house cooled basically to whatever temp you want it to be.

True. Given any reasonable indoor design temperature and a specified outdoor design temperature (and humidity), all it takes is a properly designed and installed air conditioning system. Unfortunately, not all contractors take the time to measure a home's heat load and then to engineer, specify, and install the system accordingly.

[TLTrex]

My supplier told me that, by law, a system is to be considered properly sized if it can obtain a 17 degree Delta T. He is a reliable source of info so I take his work on it.

[leapfrog]

Originally posted by theestimator
My supplier told me that, by law, a system is to be considered properly sized if it can obtain a 17 degree Delta T.

"By law"? Did he mean the law of physics, the law of the state, some Federal law, or his mother-in-law?

[Edited by leapfrog on 08-03-2006 at 09:33 AM]

[Mr Bill]

Originally posted by theestimator
My supplier told me that, by law, a system is to be considered properly sized if it can obtain a 17 degree Delta T. He is a reliable source of info so I take his work on it.

So if it's 104° outside with a 100% humidity and the humidity level in your home is 70% you going to tell me your supposed to get a 17° TD? yea maybe if you have a 5-ton a/c on a 1000sq.ft. home, under the conditions I just described it would not be unusual to maybe have only a 10° to 12 ° TD I have seen this many times in my 30 years of travels and the a/c is working perfect.

[sskzekeman]

I don't see anything wrong with the proposition of 17 degrees between the outside and inside temps For 104 outside, 87 inside should be easily attainable.

[TLTrex]

Originally posted by leapfrog

Originally posted by theestimator
My supplier told me that, by law, a system is to be considered properly sized if it can obtain a 17 degree Delta T.

"By law"? Did he mean the law of physics, the law of the state, some Federal law, or his mother-in-law?

[Edited by leapfrog on 08-03-2006 at 09:33 AM]

As in if you get sued and take it to court with a seventeen degree split or better justice will smile in your favor. I will try to find a source.

[leapfrog]

Originally posted by theestimator
As in if you get sued and take it to court with a seventeen degree split or better justice will smile in your favor.

17 degrees is easy enough to get....just slow the blower down. Even an extremely undersized system could get a 17 degree split under those conditions.

I think your supplier is going to have a tough time coming up with a citation for his statement.

The only defense against improper sizing is an industry-accepted design tool such as ACCA's Manual J, combined with the customer's specifications for IDT and ODT (if they vary from the default 'standards').

[Panama]

Originally posted by leapfrog

Originally posted by theestimator
My supplier told me that, by law, a system is to be considered properly sized if it can obtain a 17 degree Delta T.

"By law"? Did he mean the law of physics, the law of the state, some Federal law, or his mother-in-law?

[Edited by leapfrog on 08-03-2006 at 09:33 AM]

The law of thumb for a very dry climate.

[chillbilly]

If a system is designed and installed properly, there will still be a few days (2.5% according to Man J) where the design temperature will not be achieved.
This is normal.
A temperature split of 17 degrees is certainly acceptable, with 20 degrees being exceptional.

[matt_from_bh]

Interesting discussion.
So what is the verdict on the 20 degree "rule"? Is it a reasonable goal?

And is it possible in any house to break 20 degrees: Say maintain 75 when outside is 100?

I live in central NJ and have been houses (not mine) that seem to be able to go past the 20 degrees.

And it has always amazed me how they manage it but commercial buildings and spaces definitely extend past the 20 barrier. It seems that no matter how hot it is outside inside these stores or offices its always 72 (or lower).

[compresser100]

My a/c is 2 years old and it is a 2.5 ton unit. also it is a york r-22. it keeps my house at 70 degrees all day even here in the hot oklahoma wheather. it cycles on and off just fine. no problems

[BobbyBJr]

Oh yeah, I have been in several commercial buildings this week that were not breaking 20 degrees. Many factors come into play, but the one I am seeing most often is all this new computer equipment and people being moved in without any thought to the air conditioning until we get a heat wave. Also, the areas that adopted the International Mechanical code are requiring lots of outside air and this, if not properly accounted for, can wreck havok on the conditions. So, yeah, it is reasonable to expect it to be 20 degrees cooler inside than outside, but you better have a proper load calc done and make sure it accounts for everything if that is your expectations.

later,
Bobby

[sooner]

I have no trouble maintaining 70deg inside when its 105deg outside. And humidity is usually in the 40's inside with an occasional 50 to 52 on mild rainy days.

[beshvac]

There seems to be a conflict over delta T's.

1. Indoor to outdoor delta T's are not limited by anything but design and equipment size vs. load.
2. Return air to supply air delta T's are normally 15-20. the 15-20 has nothing to do with outdoor Temp to indoor temp.

[icyflame]

[QUOTE]Originally posted by mrbillpro
[B]

Originally posted by theestimator

So if it's 104° outside with a 100% humidity and the humidity level in your home is 70% you going to tell me your supposed to get a 17° TD? yea maybe if you have a 5-ton a/c on a 1000sq.ft. home, under the conditions I just described it would not be unusual to maybe have only a 10° to 12 ° TD I have seen this many times in my 30 years of travels and the a/c is working perfect.

That could be so if your system is trying to recover from from a sudden load gain. As your system recovers the delta T should increase.

[master mechanic]

Delta T depends on indoor humidity. The higher the humidity the lower the delta T . If you are exceeding 20f you have a airflow problem.

[watercop]

Originally posted by beshvac
There seems to be a conflict over delta T's.

1. Indoor to outdoor delta T's are not limited by anything but design and equipment size vs. load.
2. Return air to supply air delta T's are normally 15-20. the 15-20 has nothing to do with outdoor Temp to indoor temp.

I was wondering when someone would get 'round to mentioning this little detail...