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-   -   Delta T, do I have a concern? (http://www.diychatroom.com/f17/delta-t-do-i-have-concern-144038/)

viper 05-16-2012 10:58 PM

Delta T, do I have a concern?
 
I monitor our HVAC pretty close and it usually performs at about 18-20* delta T measured directly across the A coil. This is a 2.5T system in the midwest in a leaky home. Current outdoor temp is 77*, RH is 40%, Return side is 76*, outlet is 65*. It is moving a fair amount of condensate and I am going to go measure that to see if that is the issue. It has not run all day and got to near 80F in the house today.

I know my filter is brand new and no major leaks anywhere. NOt sure how much condensate might be typical.

Condensate volume was measured at 60ml/min.

Condensate temp is 46*F

plummen 05-16-2012 11:51 PM

Id prefer to see it closer to 20 degrees myself,but it needs a chance to get some humidity out of there before youll really notice a temp drop. :)

viper 05-17-2012 12:22 AM

When I build the thing, I installed a vent right in the main plenum above the A coil. I took a look in there and the A certainly has crap on top of it. I also made a cover for the front that makes it a little more accessible. I am not sure if I could be fighting an A coil clog by chance? Condensate temp in the mid 40s but air temp is not coming down?

Seems to be moving plenty of air but I have not tested it. I do know that the filter clogged up enough over the winter that my wife saw dirt in the vents. I found a very fouled filter so apparently they need changed very regular around here.

I do know it is not performing as it did so something is likely up. The condensate volume has reduced to 30ml/min but no improvement in delta T, at least not with the cheap temp meter I have right now. My son screwed up my good one so I will have to fix that tomorrow as well...:whistling2:

I figure I might try to blow out the A coil and throw some gauges on it tomorrow. Regarding the temp differential on the A coil lines, the liquid line is maybe "slightly" warmer than ambient, and the suction is cool but not cold. There certainly is no ice on the A coil that I can see.

plummen 05-17-2012 12:29 AM

Definately change those filters monthly.
If it doesnt start cooling better as the humidity drops Id throw some gauges on it and check your superheat to see where youre sitting. :)

viper 05-17-2012 12:35 AM

Do you think you would even waste the time to clean the A coil or is there always a certain level of dirt on them? From the top, there is nothing really on the sides, just the center flat part.

I know there was a very slight leak on a service port back in the day where we used an O ring cap to seal it. It did not leak back when completed but that may have changed.

REP 05-17-2012 12:40 AM

If the filters were dirty,its certainly possible that the underside of the "A" coil is dirty too.I have seen coils so dirty that they impede air flow but all that has to happen is to coat the fins with dirt to change the rate of heat excahnge off the coil .
I see that cleaning the coil top and bottom is the first step.Then if you don't get a 18 to 24 degree differance through the coil you have other problems.

plummen 05-17-2012 12:51 AM

Quote:

Originally Posted by viper (Post 923153)
Do you think you would even waste the time to clean the A coil or is there always a certain level of dirt on them? From the top, there is nothing really on the sides, just the center flat part.

I know there was a very slight leak on a service port back in the day where we used an O ring cap to seal it. It did not leak back when completed but that may have changed.

If you can access it easy enough it wouldnt hurt to look inside the coil,but if its got any decent amount of blockage its usually noticable and if its blocked really good it should be icing up I would think.
Center/top part of coil is commonly covered by tin depending on coil :)

viper 05-17-2012 01:02 AM

This one is a 20yo tempstar, decommissioned for 5yrs. Been running for 3yrs now. I think I should probably look at the A coil. Everything was fully cleaned and inspected 3 years ago but I guess a lot can go wrong in 3 yrs. My gut usually helps me and I have a suspicion it is dirty. Maybe not all the issue here but.... There is also some concern of the stupid temp meter I am using. A digital instant read probe type. I usually use a bare K thermocouple that I trust.

I will say that for some reason (maybe normal), the vented air that is coming out of the condensate tube is actually VERY cold. Low 40s. The system is getting ready to shut off. Been running solid for 2hrs now to drop temp from 80* to 72*. It is still generating about 35ml/min of condensate but may be normal due to the leaks of the home.

When we first installed it (home never had central AC), a friend and commercial HVAC guy had to come see it because he did not believe that it was literally dumping 5gal bucks of condensate. Still maintained a 10* dt though. It pulled right down to 20* dt after I got all that water out. Incredible that a 1000sf home could have about 20-30gal of water in it!

plummen 05-17-2012 01:04 AM

If its got a carrier coil Id start there looking for a freon leak! :laughing:

viper 05-17-2012 01:12 AM

I thought tempstar did their own? Being 20yo, I am hoping this was before they started cheaping out wall thicknesses.

beenthere 05-17-2012 05:40 AM

If the vapor line is not very cold its probably not a problem of the coil being dirty. but if you do a mild chemical cleaning of it every year. it will never have a problem of being too dirty.

Sounds like it is slightly low on charge.

if you had a good thermistor temp probe that you could lay on the coil at different areas. You would probably ee that the coil temp in some areas is colder then 40, and as you got near the refrigerant circuits end, it is close to 60.

viper 05-17-2012 10:54 AM

I got my good meter fixed and ran some numbers. Temp testing over the A coil is all over depending on where I test in the plenum. As high as 23* dt, and as low as 12* dt.

Liquid line to A coils is 81* before entering the A coil, 70* coming out. I will get the A coil cover open and get a pic.

viper 05-17-2012 02:31 PM

Have not got a chance to open the A coil yet but I did put gauges on. 205/55 is where I am at. Ambient is 86F,RH 27%. dT is showing about 14*. Liquid line is 93*F, vapor is 70*F. I need to start running some numbers to determine where I am at. Any help would be appreciated..

HVACDave 05-17-2012 03:23 PM

205/55 @ 86 F outdoor ambient would indicate you are a little low on charge, as BT had suspected. You would likely see closer to 225/65 at that ambient temp. You currently are pushing 40 degrees of superheat (which is higher than your average for AC which is about 20) and 10 degrees of subcooling (could also be a little low)

viper 05-17-2012 03:43 PM

I too came up with 10* SC in which I thought was in the good zone of 8-12?

Also, I am curious how you got the SH values without a wetbulb value from in the home? It was my understanding that you need that value? I do not have a way to get RH or wetbulb so I guessed based on the outdoor air which is at 30% and added a bit so guessing 50% RH. That put me at 30F pressure temp, and 10-12 SH for a total of 40-42. You are right on and my suction is 70 so that is an issue but I am curious how you got your values. I was also looking to make sure my numbers were right.

Will it be OK to charge based on pressures and suction temp or is weighing in the charge mandatory? I am not yet sure how to calculate the weight based on above calcs. Or do you need to have the factory charge data for that?


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