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AC Register Discharge Temperature MLD 06-28-2007
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Posted by MLD on June 28, 2007, 12:09 pm
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



Posted by on June 28, 2007, 12:38 pm

> 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


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

So what's her temp differential?



Posted by MLD on June 28, 2007, 1:36 pm

>
> > 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
>
>
> You would normally get a minimum of 12-15 degree drop. It's all going to
> depend on a lot of data.
>
> So what's her temp differential?
>

Don'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'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'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





Posted by on June 28, 2007, 2:00 pm

>
> >
> > > 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
> >
> >
> > You would normally get a minimum of 12-15 degree drop. It's all going to
> > depend on a lot of data.
> >
> > So what's her temp differential?
> >
>
> Don'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'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'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


Yes, she's getting a snow job.
Is it the air conditioners fault? Don't know, it could be a ducting issue
(heat gain). Either way, it'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'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's a matched system or something that's been cobbled together.

It'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.



Posted by on June 28, 2007, 2:16 pm

>
>>
>> >
>> > > 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
>> >
>> >
>> > You would normally get a minimum of 12-15 degree drop. It's all going to
>> > depend on a lot of data.

        12 - 15 SUCKS, that is TERRIBLE performance, and very often,
assuming the system is not grossly oversized, will not be enough to
cool the house.

>> >
>> > So what's her temp differential?
>> >
>>
>> Don'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'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'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
>
>
>Yes, she's getting a snow job.

        Then it should be nice and cold in thre. What's the problem ?


>Is it the air conditioners fault? Don't know, it could be a ducting issue
>(heat gain). Either way, it'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'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's a matched system or something that's been cobbled together.
>
>It'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.
>

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