The AC guy tells me that you can NOT have a larger difference then 20°
between Outside Temp.and Room Temp.
100° Outside — 80° Inside not better!
Is this correct or is the AC not working right?
Hilmar
Edited 7/20/2008 10:52 am ET by h12721
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Edited 7/20/2008 10:52 am ET by h12721
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I don't know about if it is impossible to get more than that,BUT......
I would say that not only is it far too expensive to try and get a larger difference but the thermal shock to the body going in and out is not good fer ya.
That much of a drop will at least get rid of the humidity and make it comfortable.
I only try to get a 10-15 degree difference and am happy with that.
Clearly you don't live in Death Valley..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I've heard that "thermal drop" theory before and yet I've never heard anyone arguing for keeping your home no more than 20* above the outside temperature.
In a cooling climate why would any body in their right mind want to?
can have 73f inside while 100f outside here in GA.
With the windows open
Edited 7/20/2008 11:27 am ET by VAVince
Is this correct or is the AC not working right?
Now, I'm no hvac tech, try to not play one (much) on the internet, either. But, with the experience of having live most of my life in a majority cooling climate, it's my understanding that the equipment will only generate 20º of deltaT.
Now, that does not mean that if it's 110º out, you can only cool the inside to 90º.
Your house should be insulated enough to not rise to ambient temperature inside. Also, it should have enough thermal lag, that you are only air conditioning versus the temperature of hours ago, before it peaked at TDH.
So, a thermometer at the a/c return and the first outlet may/ought to show close to 20º difference. If the air is going in at 80º, it ought to be coming out at 60º--that's what the deltaT ought to be.
So, it sounds to me like your a/c tech is not working right; whether the equipment is or not, remains a question.
Ask him how you can have a refigerator or freezer, which work on the same basic refigeration cycle, and have 40-80* difference.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Some clarification ... if it is an evaporative cooling system (e.g. swampcooler), you are right ... they are 100% outside air ... and can only drop about 20 degF.
If you are DX cooling ... then you have a recirculated/return air system ... and the return air should be maybe 85 degF. Properly sized a DX system should give you plenty of capacity and supply temps of say 55 degF under any condition that it is designed for.
Ask him what he means by 20 deg delta T ... supply air vs. return air? Outside air vs supply air? Outside air vs. space temp? Get another opinion from a reputable HVAC guy ... especially if this guy can't answer your questions to your satisfaction.
"if it is an evaporative cooling system (e.g. swampcooler), you are right ... they are 100% outside air "Not true at all. It was 100F outside yesterday and our evap cooler kept the house at a comfortable 72F. We exhaust through Updux into the attic space, and that significantly lowers heat gain from the ceilings. If we crank it up to high, the cooler can reach a 35F differential on dry days (3% humidity).With refrigerated air, much depends on the unit specs, insulation, windows, etc. The typical "builder's special" unit installed in most new homes will struggle to maintain a 20F differential. When we lived in GA we had that situation and it was obvious on hot days when the a/c would hit a brick wall. It couldn't remove enough BTUs to overcome the heat gain.
Sounds like a nice system! We've lived with with evap cooling for about 15 years, and my experience would support your statement with one serious variable: the outside humidity level.I monitor a simple inside/outside temperature and humidity system here at my house. Our outside humidity runs from about 8% up to about 28% most of the time (spikes up sometimes, of course), but it's commonly around 12%-15%. It's easy to tell when the humidity goes up outside by the feel of the air coming out of the cooler. It seems that the cooler doesn't accomplish much if the outside humidity gets much over 30%.With the so-called monsoon afternoon t-storms that roll around here at this time of year, the effect is that at the hottest part of the afternoon it will rain for 10 minutes, the humidity will spike, and the cooler will start pumping warm moist air into the house. < G >Of course, with only three percent humidity you quoted you could store beef with one!
We have a Breezair unit with the cellulose pads and it does a good job of keeping them saturated so the air coming out is close to the wet bulb temperature. As long as the dew points are below 40F we get good cooling. And as I mentioned, we found that exhausting air into the attic with Updux vents instead of outside through windows greatly reduces the heat radiated from the ceiling and that makes a 5F to 10F difference in the indoor temperature.Yes, when the monsoon kicks in, the cooling efficiency falls off and there are a few days every Summer where even the best evap cooler is like a fish out of water. But 95% of the time it works fine and we get 2+ tons of cooling on about 500W of power.
Edited 7/22/2008 12:28 am ET by TJK
I stand corrected as husbandman pointed out ... it depends on the RH outside a lot and you can do more if it is low. BUT AS A GENERAL RULE OF THUMB, you should not expect much more than 20 deg delta T w/ evap cooling. Not true at all ... false ... true much of the time true.
The way I understand it (Admittedly not well)
You can not/should not have more then a 20 degree difference across the coil, which might be the Delta T measurement mentioned above.
Meaning the air entering from the return should not be less then or more then 18-20 degrees different then the air coming out the supply side.
My guess is either you are not understanding your HVAC guy, or you need to get a new one.
Hmmmmm.....was what 97 today, guess I better turn that thermostat up to 77 before the system explodes. Didja know that thermostats aren't called thermostats anymore? They're called "interfaces" now.
P.S. Did you get you "My coach can each your coach" shirts before they went illegal?
P.S.S. I hear Rockhurst University is going to sue K.U. for illegally using the color blue and Hawks together.
Never did get the shirt, didn't know they were "illegal" either.
Tell rockhurst we'll settle it over a game of football or basketball.
Well several years ago the high schoolers coulda taken yah, but these day, I dunno.....
Didn't read your OP carefully.
He's wrong unless he is talking Evap cooling which I assume he is not since you are in NY, right? Not between outside air and inside. wrong wrong wrong. period. He is a novice ... he better get straight before it's too late.
Coils are often sized for a 20 deg drop across them ... that is return air and supply air. Return is a bit higher than space temp and supply should be down to maybe 55 degF.
The AC guy tells me that you can NOT have a larger difference then 20°
between Outside Temp.and Room Temp.
Either he is confused or you misunderstood what he said.
A 20F difference between the inlet air temp and the outlet air temp is typical for an AC. However, the same air is recirculated and cooled further, so the outside air temp compared to the inside air temp can be much greater than that.
If you are not able to cool the building more than 20F below the outside temp, and the outside temp is above 90F, the AC unit is either under or oversized or not functioning correctly.
It depends on what dekta T the system was designed for. It has been since 1994 since I designed AC systems for multifamily but we typically used a delta T of 20* for design specs to plug into the computer model based on Manual J.
In Dallas for instance we would use 100* outside design with a 20* delta T for 80* inside. Meaning the system could keep up with a 100* outside and yeild a constant 80* inside, BUT the system by design would be running virtually non stop on those 100* days. In theory on a 105* day the system could only keep a space at 85* with these parameters. (people loads and air exchanges would have an effect as well)
Dry rooms use extreme delta Ts to remove all the moisture from the air and then reheat the super chilled air duct heaters to make it comfortable.
So, your tech might be right if that is how the system was designed (outside vs inside) for the hottest day of the year. But as a blanket statement he would be misinformed.
As stated before, he probably meant you cannot have more than 20 degree difference in air going into the AC and air coming out of the AC. Get an accurate thermometer and measure the temp of the air coming out of a vent. If it is 20 degree cooler than the air in the room, the AC is working fine. If the temp of the room does not come down, you have inadequate supply or the hot air is sneaking in somewhere.You get out of life what you put into it......minus taxes.
Marv
designing for 80 degF setpoint, in my opinion is much too high. And you can design to whatever temp you want ... if you want 60 deg ... you can have it w/ a DX type of system. Can't believe you only designed for 80 and 100 in Texas seems a bit wimpy for that climate.