AC Register Discharge Temperature

[MLD]

I have a Central AC system that's been around since '73. The air temperature coming out of the last register in line has consistently been

54-56F year to year even with an OAT of 90F.. My daughter has a new system and initially the register air temp was 70F. Service Tech says he found a leak. Now, after the repair, the air temp doesn't get lower than 67F and the Tech says that's normal and everything is working OK. OAT is in the high 80's low 90's--Is she getting a line of bullshit?? MLD

[<kjpro]

You would normally get a minimum of 12-15 degree drop. It&#39;s all going to depend on a lot of data.

So what&#39;s her temp differential?

[MLD]

Don&#39;t have the exact numbers but for a ballpark evaluation--Return Air Temp (room temp)=73F with a register discharge=67F. That low a differential (6 deg ) would seem to indicate that the unit still is in trouble and needs help. One possible explanation and it&#39;s a BIG reach-- is the fact that the AC air has to travel from basement to a second level and is being heated as it makes its way up the line--It&#39;s not that long a run, however, and the ducts that are visible are well insulated --no negative comment from the Service Tech as to this being a possible explanation. Using my own system as previously noted--return air temp=77F with a register air temp=56F. giving a differential of 21F which seems consistent with a good working system. .She is dealing with the people that installed the system and based on this information do you think that she is being given a snow job when they tell her everything is working normally? MLD

[<kjpro]

Yes, she&#39;s getting a snow job. Is it the air conditioners fault? Don&#39;t know, it could be a ducting issue (heat gain). Either way, it&#39;s not operating correctly!

In order to get to the root of the problem you need to take measurements at the furnace or air handler. Measure the inlet and discharge air as close to the evaporator coils as possible and see if your differential is still 6 degrees. If so, this points you to the air conditioning system. If it&#39;s

12-15 degrees, then this points you to the poorly designed ducting system. Another possibility is that the air handler is moving an extreme amount of air by the evaporator. Do you have the model numbers of the equipment. Lets see if it&#39;s a matched system or something that&#39;s been cobbled together.

It&#39;s best to get a second diagnosis from a service company that can actually see and test this system. They should be able to diagnose the system rather quickly after they gather all the required data.

[MLD]

Your comments are extremely helpful and appreciated. Being on the East coast with the AC unit in the mid-west it&#39;s beyond my following through on your suggestions. However, based on your remarks it&#39;s time for the AC company to be brought back in to do a more professional diagnosis and troubleshooting job. Again--thanks for your input. MLD

[udarrell]

A 6 degree split is unacceptable! There could be hot air from a non conditioned area being sucked into the indoor blower downstream of your temp check point. It is important to determine whether there is a light or heavy heatload on the evaporator & condenser, therefore go outdoors, if it is a round condenser, take a reading of the discharge air temperature split as compared to the surrounding outdoor ambient temp. Find out if it is a single speed condenser fan motor or two speed then ask the tech, contractor or go to the condenser company&#39;s website, then if possible, look up the Expanded Data on that condenser.

- udarrell

[<kjpro]

consistently

differential

Where in the mid-west?

[<kjpro]

He stated: Return Air Temp (room temp)=73F

He stated: OAT is in the high 80&#39;s low 90&#39;s

His posted temps pretty much tell you the load conditions. And just what do you think the expanded data is going to tell this homeowner?

[MLD]

measurements

Cincinnati

[<kjpro]

IIRC, there&#39;s a poster here that lives there... maybe they&#39;re close and will reply or e-mail you?

[Bubba]

WHO? :-)

Ok MLD. Spill the beans. What company? I could use a good laugh. Bubba

[udarrell]

That depends on where he is taking the return air temp, if the furnace sits over a return air chamber he may not get the mix of the hot air entering the blower. That is a common occurrence when the chamber under the airhandler is not properly sealed off from the attic where it comes down the interior of the walls, etc.

I assure you the condenser split will provide some information that may be helpful. He did not check the condenser split, I only view the unread posts & don&#39;t recall that he posted the head pressure(?) The Outdoor Ambient Temp (OAT) does not indicate the actual amount of load that the condenser is discharging, the condenser delta-T (Split) provides an indication of it. If he did the saturation temp on the gage would give a good indication of the split. Goodman Expanded Data tells me everything I need to know concerning whether the unit is delivering the mfg?ers specs.

- udarrell

[<kjpro]

This guy is NOT a tech, therefore he doesn&#39;t know the "head pressure".

Likewise, he wouldn&#39;t know what to do with the Goodman Expanded Data (that&#39;s

*if* it&#39;s actually a Goodman).

[udarrell]

Well, as usual I agree with you kjpro, an HO wouldn&#39;t know how to use the Expanded Data & wouldn&#39;t know the head pressure & corresponding temperature. Though they should be able to take the temp split of the condenser. If it were drawing real hot air from a non conditioned source, an incompetent tech would probably way overcharge the system trying to get the suction line beer can cold & sweating. That would further reduce the E-Coil split; a six (6) degree indoor split is pathetic. They need to find & use a competent tech!

- udarrell

[MLD]

Thanks to all the comments, I&#39;ve got the answer that I was seeking, which was to confirm that the Service Tech was passing Bullshit along with a Snow Job. What is needed is a more competent Tech and the owner of the company to know how his guy has performed in the field. As an aside, from your perspective, it is a safe assumption that the average HO knows very little about the mechanics of field troubleshooting, the nomencalture and data that is required to resolve a proiblem. Fortunately (or unfortunately ) I went through a year of hvac study, however, it was aimed at system design but did not include any meaningful "hands on" work at the component level. I can appreciate the training and experience needed to be proficient in diagnosing, troubleshooting and solving a problem. I did that for years on a product that cost about $2 million; one of its control components ran around $50,000 each---shotgunning was not an option. MLD