Delta method good enough to fail entire A/C system?

Hello, experienced masses....
I am selling my home and have fixed everything that has been asked by the prospective buyer and the city so far but am having difficulty with the buyer's inspector in regard to his A/C performance spec.
The inspector told buyer that the temp drop from return vent to evaporator test point was 10 degrees. The buyer was told that my A/C system isn't performing as it should. I verified his finding of 10 degree drop after they left with MY thermometer. I'm not arguing his number or his method. Please read on to see what I AM arguing about (to myself).
The temperature outside at that time was only 75 F. and relative humidity was 70%. This is a pretty cool day for St. Louis in July. It was also overcast with virtually no sun. It was only 10 Oclock am and with no sun load, the house was quite cool. My probe-style thermometer read 68 degrees on my kitchen counter and in the return air duct. I think its accurate within 1-2 degrees. I believe it to be relatively accurate in terms of temperature differential, atleast. So as stated previously, I put the thermometer in the same test point as the inspector did with the system running and got 58 degrees. The magic delta figure to fail a system and scare a buyer seems to be below 15 - 17 degrees from what I've gathered in talking to realestate agents and dealing with several home inspections. But don't you have to take into consideration the temp and humidity and solar load to some extent before interpreting delta temp?
I just happened to have an A/C company out two weeks ago to give my system a performance test so I would be armed with a professional opinion on paper before negotiations started. This is because the previous owner had several big dogs, and apparently their big bladders found the warm condenser fins a common target. (I was told that dogs' urine is a common cause of condenser fin errosion). The missing fins in one lower corner area coupled with some rust on the thin cage that protects the condenser make the outside unit appear old and worn to be sure. It isn't a great selling point visually. But this house isn't the fricken' Taj Mahal either. I don't feel that someone can use the APPEARANCE of my A/C unit to demand $2000+ for a new one IF IT WORKS!
The A/C guy I hired wrote that the temp change was 18 degrees, pressures were normal and that 'system was operating normally at this time'. It was about 85 degrees out and sunny that day and he did the work around 2 pm. I tested the temp drop several days before he came out and actually got 21 degrees but I can't remember the temperature or sun condition then.
So how do you all feel about the accuracy of DELTA for the purposes of condemning an A/C system? Do I really have a problem? Or may the inspector have seen that the unit was around 15 - 20 years old with fin damage and just found an easy way to condemn it that isn't accurate? (much less than 5% fin damage)
I feel that for my 900 sq ft house with full basement (all cooled with ductwork) and poor insulation my electric bills are reasonable. I also feel that the A/C FEELS cold and the system has never been charged in my 8 years here. I mean, 58 degree temp in the furnace ductwork can't be that bad can it?
I thank you all for any responses yay or nay!!!
Tony
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Why are you so worried? You fixed everything the city required. That's different from the things the prospective buyers would LIKE you to fix for some unknown deal where they've yet to tell you what they would LIKE to pay.
A simple "My asking price already takes that (the appearance and performance of the unit) into consideration. You want a new unit? It'll raise the price. Lets deal."
Or walk away.
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You "FIXED" the system the AC guy listed his finding so the work has been done and should not be a problem move on!
If they want it replaced or something tell them no and that is it!
Wayne

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No shit!!!
Been there done that, and the "prospective" buyer just kept wanting more and more concessions.
Finally I hadda tell em to just go and find a different house.....ended up they bought mine instead, as is.......
--

SVL



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Thanks for your quick replies, guys. I know that I don't have to repair the system. I know that the buyer is going to request doing so for bargaining reasons, esp. since the house is in otherwise great condition.
I actually hoped I could get a concensus on if my originally posted Delta theory is wrong or right so I could be armed with some experienced information. If I find out I'm right then I will be less likely to budge on price 'just to get the deal done'. If I'm wrong it would be nice to know also cause I hate the taste of crow!!!
To summarize my standpoint as a homeowner, I feel that $100 to $135 a month for my electric bill is not out-of-line for the hottest three months in St Louis with poor house insulation, many single pane windows, and 1800 sq ft of space (half of it in the basement, ofcourse) to cool. The elec. bill is about $25 - $50 the rest of the season. Add to this that I feel the house gets comfortable and dry and I have approx. 65 degree duct temp upstairs on hot days...
So, is everyone else paying less a month for electric? Is everyone else getting 50 degree duct temps? Maybe I'm wrong. Maybe Delta temp should always be 15 - 20 degrees no matter what temp it is outside.
Please jump in if you have an opinion! Thanks, Tony.
"HA HA Budys Here" wrote:

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I think you are over concerned , your unit works. Either they buy the house or they dont. Everybody wants a better price or deal or new AC , you dont have to do anything it is your house.
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Delta may be inaccurate if the compressor is kicking on/off automatically in mild weather (not on 100% of time). Cooling capacity also depends upon airflow and duct size. So the question is, does it have a history of keeping you cool enough during hottest days of the year? The home is what it is, so if they want you to replace something now instead of when it becomes no longer functional, they should be willing to pay for it.
BTW my last electric bill (6/22) in northern IL was under $60.00 (572 kWh) including a couple of PC's that run 24/7. But I just run a ancient wall shaker downstairs or portable A/C upstairs when I need it, and box fan exhausting from an upstairs window on mild nights. Most of my lights are fluorescent, and blinds (and my neighbor's tree) limit solar gain.
--
David Efflandt - All spam ignored http://www.de-srv.com /

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Thank you very much for your inputs. Especially about your electric bill experience (that could point to my system being inefficient) and about delta value.
I wonder how cold an evaporator can actually get in a properly sized, properly running home system? I thought 58 deg. was pretty good. I am an auto mechanic; car systems are different enough that my experience seems useless here -- Car evaps get down to just above freezing and are positioned about 4 feet from the driver, though.
But if it turns out I'm paying way too much in electric that's all I need to know. Then the system is running too long or simply inefficient. I'll check with the neighbors and see what they're paying....
Thanks, Tony
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ComboverFish writes:

Don't confuse termperature with heat.
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This is Turtle.
Tring to get a good ideal of electricity cost from a idenical system next door to your is almost useless. You can change just one thing in the building of the house and make your electric bill go way up or way down and insulation and building materials go. Also the temperature setting of the thermostat and life style of being there and not being there is a big thing. Everytime you open a door to go out your adding to the cost operate the system. Just too many variables to get a good ideal.
Now let's get picky here.
The 58F discharge air seems reasonible but anything colder seems to me to be a restrictive air flow of the system. At 58F discharge air from the system puts the evaperator coil at about 40F depending on the type system you have. So if you had a 50F discharge of air would put your evaperator coil at or below freezing. So here is my take on discharge air temperature in general on hvac system.
57F to 65F is the normal range. 55F to 65F is normal on trailor houses only. 56F or below on houses and not trailor houses --- You have a problem somewhere or you have a walk-in cooler and not a hvac system. Now if your asking for 60 to 65F in the house you can get below the 56F discharge temperature a little and still be normal.
Now to your auto discharge temp.s. 40F to 45F discharge air on 134-A is normal and runs your evaperator coil at about 30F to 35F and also depends on the type system you have.
Now using what i've said here to test a system --- Also sucks to , to compare it working properly. There is really no sure fired way to check a system as to working properly other than use super heat , run manual D , and have a real hvac service man look at all the readings and give a report and TD is just a item to confirm the real reading. TD is just for people to talk about when they don't have a ideal as to what the system is doing.
TURTLE
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ComboverFish writes:

That old, and performing poorly? It is simply worth next to nothing. Deal accordingly.
Cf Proverbs 20:14
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The "delta" method is valid during a limited time property inspection. A properly working system should have a 15-18 delta. No higher than 20 or so. High is no good, the evap will freeze up when the temp hits the dew point.
Dana, General contractor, HVAC, Electrical, Plumbing, Certified CREIA home Inspector

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This is Turtle.
1) Checking the T.D. / Delta-T is not a very good test of the proformance of a system for on high humitity day it will make the TD number look very poor and on a dry day make the numbers look very good and it being the same system but at different times. With the TD you can make a system look anyway you want it by picking the time and days you want to check it. It sucks to check a system this way.
2) Call all of them over to see the results of the TD but let the house get up to about 95F or more in the house and then turn it on and you will have a 30F to 35F [ TD ] . You can make the number what you want when choosing the time to check it.
3) If you have a 58f discharge temperature and about 70f to 75f indoor temperature. a TD of 10F or 15F can be just fine.
4) one for a thought. Take a brand new tune to perfection 14 seer hvac system flower rater type and not expanion valve type evaperator coil and turn it on in outdoor temperature of 50F and you will be luck as hell to get a 7 to 10F TD. You need a low ambiant controller to get it to work in 50F or less outdoor temperatures.
5) If your agent is going to use the TD to see if the system is working properly you don't need a hvac professional at all and just get the yard boy to check and turn up your system.
6) Using TD for a standard good test of a system is really just a joke talk about.
TURTLE
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Great post, Turtle! This reinforces my theory that temp difference can vary greatly. If the house is really hot when you turn the system on, the evap is going to cool down relatively quick compared to the house's ambiant temp. So to check delta a couple minutes after firing up the A/C on a really hot day would probably yield some gawdy number... Thanks for the time and help, everybody!! (except alt.hvac specifically -- try ignoring a post if you don't want to answer it -- it works better for everyone! That's why I wrote 'crosspost' in the subject)

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