Thanks for viewing my post. I have had previous post on this site regaurding this issue. I used the manuel J short form at
and figured out that my ac unit is the right size.
The house will not cool to 75 on a 95 degree day. Today the ac contractor came out and "trouble shooted the problem." It was the tech supervisor. I asked to get a copy of the manuel J load calc so I could see it and he told me "it wasn't allowed to leave the office." Does this seem normal? Is that so secret? I believe they have something to hide. Based on the manuel J short form my 2.5 ton ac should be big enough for my 1700 sq foot house.
Next they went up to the attic and inspected the duct and took temps. "Oh you are getting 20 degree drop across the coil its working" said the guy. "you have no leaking ducts either." Next he took temp reading at the thermostat and sad it was reading normal. Then he took readings in the center of the room and said "they were 77-75 between the room's even though the termostat is reading 80". I beileve they are that temp because cool air is blowing in the room and from my measurements as soon as you turn the ac off those numbers jump up.
Next here is where he starts getting squirly. He took the temps at the vents (Well 4 of the 12 untill the one in the kitchen was too high to reach and he said he doesn't know what it is accomplishing anyway). Having previously mapped out the house with my $10 digital themometer I knew which vents where having a loss of cooling and I should him my map. The worst case is 71 degree at the vent when the pleneum has 60 degree's.
I told him that I believe that this is a big loss of efficiency and asked about the big drop. He said,"ah I don't know if that is normal but your room's are still 77 degree's) He then said he would take it to the designer's and let them look at it.
Now this is what pissed me off!! Joe blow ac guy was asked if he could measure how many CFM's where going into the return air and he said he could but he didn't have the equipment with him.
My AC will not get down to 75 on a 95 degree day. I believe they have designed the duct work wrong. The attic may not be venting properly.
Who should I get to confirm all my nightmare's. Obviously the contracter isn't going to admitt to anything?
I found a guy that is an energy star inspector and does Manuel J load calc's. Should I hire him or should I hire a regular residential home inspector?
Home inspectors can't help you. They don't know HVAC worth a darn. A well informed and equipped HVAC contractor would be your best bet. Good luck finding one.
But at this point you don't need to do that. Just get those schmucks to do what they said they would. Measure delivered airflow. Multiply that times the delivered temperature drop times 1.08 and that's your delivered sensible cooling. There's a simple method for calculating latent cooling as well but it escapes me at the moment (there's no humidity our here). If they installed all new ducts then you shouldn't have more than 6% losses. Since a typical sensible/latent ratio is 75/25 and accounting for 6% losses, you should have 21,150 BTUs per hour of delivered sensible cooling (give or take).
If that was a bit confusing just know that it's HVAC 101. Those guys should be able to measure that pretty easily. If they can't then find a contractor that can measure delivered sensible and latent cooling. You'll get the answers you need by measuring those aspects of the system.
try finding a honnywell certified home energy consultant that has the equipment to do honnywells home certifcation testing. with this equipment you can test for air flow,veolcity,cfm,duct loss, corect duct sizeing ect... with this you can get a computerized model of your home and pin point the problems. you should try there web site " i dont know there address".thay should point you to the most local company there are verry few.it sounds like a duct problem more than equipment.Its always good to get a 2nd opp. from an other company with a good reputation look for that at the beter bissness bord or good old word of mouth. but becarefull its a dog eat dog world and alot more hacks and fligh by night companys than honnest ones unfortionaly.
So tell them that you will be at their office in the morning and look at the calcs there...
Originally posted by smp7015
[B]Thanks for viewing my post. I have had previous post on this site regaurding this issue. I used the manuel J short form at
and figured out that my ac unit is the right size.
The house will not cool to 75 on a 95 degree day. Today the ac contractor came out and "trouble shooted the problem." It was the tech supervisor. I asked to get a copy of the manuel J load calc so I could see it and he told me "it wasn't allowed to leave the office." Does this seem normal? Is that so secret? I believe they have something to hide. Based on the manuel J short form my 2.5 ton ac should be big enough for my 1700 sq foot house./B]
You need to know what they designed the system to do, and what their target values were.
Then you need to find out if the system is working as designed.
You could have 2 possible problems,
1 The system wasn't propery designed for the load and your desired comfort.
2 The system isn't working as it was designed to do.
You need to stress to the installing contractor that it is in both of your best interests to find out what the problem is and fix it...
And they can choose to do it the easy way... or the hard way...(legal action)
11° rise in temperature from the coil to the supply grlle ,is a problem.
What type of ducts ,what insulation is on the ducts(R value),and what temperature is the attic??
Are the thermometers accurate?I've never seen that much rise,and I'm in Florida,ducts are in the attic here.
I agree with Dash, you shouldnt see that much rise from the coil to the grille. You can argue with the guy your originally hired and play that game as long as you want or you can simply call someone else. Understand the next guy has no obligation to do anything gratis. He will have to charge for the work he does.
Before you do that, you may consider documenting the temperatures at the return grilles and return plenum, supply plenum and supply registers. Along with the model and serial numbers from all the equipment. Do your little map thing and set up your argument and send a letter to the owner of the company. Explain that you feel the temperature rise in the ducts is to blame (because if those numbers are true, then the ducts are to blame). 2.5 tons is probably not undresized as you may have found out. I wouldnt bet there isnt any leakage in the attic either. Many installers are now using a "bubble wrap" type of insulation. If this is used, it is not meant to be wrapped tight to the duct, it should have an air gap. Otherwise it is virtually uselss.
If your attic is hot (hotter than you figured using your load calculation) then consider a method to ventilate it better. Gable ends and a power venter may be a viable option.
"The worst case is 71 degree at the vent when the pleneum has 60 degree's..."
Really sounds like you're sucking attic air/outside air into your ducts.
Have you actualy seen your attic and wall insulation or was it all sprayed in 20 yrs. ago?
As far as this contractor is concerned, hold him accountable! Those of us that do what we say and say what we do, want those that don't held accountable. They just give us all a black eye! Believe it or not, a lot of people will make the assumption that this guy represents all of us in the industry. You've gotten some good advice here from a technical standpoint, but unless you, the customer, demand satisfaction, then you probably won't receive it (at least from this particular individual.) Those of us that consider ourselves a cut above the others welcome the opportunity to respond to questions or concerns on the customer's behalf. I wholeheartedly agree also, STAY AWAY FROM THE 'HOME INSPECTOR,' the proverbial "jack of all trades, and master of none."
Everyone has a purpose in life..........even if it's to be a bad example.
Seek first to understand, before seeking to be understood.
There's a simple method for calculating latent cooling as well but it escapes me at the moment (there's no humidity our here).
Irascible it's Total BTU=4.45 x cfm x (delta h enthalpy)
Sensible BTU=1.08 x cfm x (delta T temp)
Latent BTU=.68 x cfm x (delta W specific humidity found on physc chart)
I've never run a manual J, but I just took a minute to review the sheet on the website. It appears at first glance it might oversize the unit slighty since it considers infiltration at a high rate-I've never seen a house that is "poor" and has 1 A/C per hour. Most residential houses are nuetral or slighty negative if an bathroom exhaust fan is running. 2 people per bedroom seems excesive to me. Most homes have 3 bedrooms but only 2 people live there.
Nor does manual J take into consideration motor heat from the fan motor. Granted they are fractional horse motors, but you get a large AHU with a 1 hp motor and you'll get some heat gain.
As I said before I come from a commercial background and use Trane 700 to run loads.
If you can e-mail me a sketch with dimensions I'll be happy to run a load on your house and send you the info. seancantrellXXXXXXXXX
And as everyone else has stated here, "stay away from home inspectors". My best friend is one and he missed a refrigerant leak in the house we bought a couple of years ago. I had to point it out to him when we were looking at the house.
[Edited by lusker on 07-05-2005 at 11:08 PM]
Will the temp go below 75° at night and do you run the AC all the time?
The manual J doesn't leave the office because they never did one.
The tech was told not to give away any info to you.
They are trying to figure a cheap way to fix your problem.
With a jump that big from 60 to 71 your attic is definitely WAY to hot. Air will not get "sucked in" from the attic AFTER the blower unless you have a venturi contraption built in your attic which is very unlikely.
The heat gain is causing that. 71 blowing into a 77 room will probably not compensate for the heat load coming in, or at least balances out at 77.
Look into better/more insulation for attic or better/more passive ventilation. If you do more insulation, concentrate the buildup around the ducts, if possible. If they are hanging, you are out of luck.
I had the same problem with a home- was sized right and temp drops were "as they should be" but the house would not cool below 80 in August.
Look into lowering your heat load while you are at it. Solar Screens over your windows are worth every penny in comfort and savings.
How old is this house and what city are you in?
something is obviosly wrong. i bet a new apprentice went to town in there and did alot of little hard to see but critical errors. i get to train these new guys in and it makes me feel like a babysitter rather a tinner. most of the things they do, no one would know. a homeowner would look at it and say "wow that looks nice". me being a perfectionist looks at it from a tinning standpoint and says "looks like a 5 yr old kindergartner installed it, then i get to fix it and it makes me wanna strangle these newbies! a good hvac man is hard to find, 85% of the workers dont care or arent gonna last. its either in your blood or its not.