low td no soloution

[cavalier2903]

ok i have a 13 degree temperature diffrence across my evaporator i cant find a reason i need help. here are all my readings and things.
13 degree temperature diffrence(this is the problem)
pressures 70/240 r-22 refigerant
15 degrees of superheat
25 degrees of subcool
electric heat strips are not energized
supply static .05
open return
piston size 065 size is correct verified by goodman mfg
system is a 10 year old goodman 2.5 ton
piston is clear no restriction
compressor pumps down
compression ration is 2.5
blower set to low speed and is turning in right direction
liquid line drier is not restricted
system is a straight cool condenser on an air handler with electric heat strips.
i am at a loss i can find anything please help me what could cause this my thinking is that there has to be something with the compressor because i had a similar situation with a 410 a tran only diffrence that one would not pump down. i replaced the compressor and it worked great

[Gary]

Just to make sure we are on the same page:

The evap TD is the difference between saturated suction temp (converted from pressure) and entering air temp.

Is this the TD you are referring to?

[Gary]

Your subcooling should be no more than 15F. This probably means the system is overcharged.

[cavalier2903]

im charging by superheat because im meterd by a piston and the td im refering to is my supply temperature minus my return temp im shooting for a good 19 or 20. and i tried it all removing gas and adding no effect on temperature diffrence best i had was 13 but its at an 11 now

[mad fridgie]

What are the air on conditions?

[Gary]

The air in/air out temp drop is called a delta-T or dT.

You need to remove refrigerant until the subcooling is 15F and then take a new set of readings.

[Gary]

We will need to know the air in and air out temps for both cond and evap, as well as pressures, subcooling and superheat.

[cavalier2903]

well not being on the call anymore i will do my best to add more info. my indoor temp was 70 supply temp of ac was 58. outsid was 85 not sure of the temp leaving the condenser

[Gary]

well not being on the call anymore i will do my best to add more info. my indoor temp was 70 supply temp of ac was 58. outsid was 85 not sure of the temp leaving the condenser

Okay... let's see what we can do with that,

240 psi gives us saturated condensing temp (SCT) of 114F.

114 - 85 gives us 29F cond TD. Normal is about 20-35F, so it is showing a fairly heavy load... however, overcharge can back liquid up into the condenser giving us a false reading.

I would guestimate that without the overcharge, we would be looking at about 25 TD, which would indicate a medium load.

(I am assuming good cond airflow since we don't know the leaving air temp)

Normal condenser load would indicate that the compressor is pumping okay and the orifice is in the ballpark.

Removing the overcharge would raise the superheat to a higher than normal level. Since the orifice appears to be in the ballpark, this brings us down to airflow.

I'm thinking the evap has too much airflow... OR... air bypassing around the coil.

(Here I am assuming that 58F supply temp was measured close to the coil, not way downstream at the supply vent)

Let me know if my assumptions are wrong.

Once the overcharge is removed, we may be able to go up one size on the orifice to better match the airflow... if this does not result in placing too heavy a load on the condensing unit (high TD).

[Gary]

system is a 10 year old goodman 2.5 ton


Has the customer had high humidity and/or mold problems over the years?

[cavalier2903]

the customer has been maintaining a constant temp but system always is running i am assuming. I checked when i was there to see if my air was bypassing the coil and it was not. My blower is set to low speed a thought i had that i did not check, u brought it up. To much air flow? I set the blower to low but did a previous tech possibly install the wrong size blower motor. Moving to much air across my evap coil even when set to low speed would definitely giv me low td. I keep coming back to a problem i found before with a 410 a compressor that was bad. pressures superheat and subcool all looked good on that one, diffrence was that one would not pump down. hmmmmm i hope i get a chance to get back there i really want to get to the bottom of this.

[Gary]

I keep coming back to a problem i found before with a 410 a compressor that was bad. pressures superheat and subcool all looked good on that one, diffrence was that one would not pump down. hmmmmm i hope i get a chance to get back there i really want to get to the bottom of this.

When a compressor is inefficient its capacity is insufficient to handle the load... but that would look the same as the load being too much for the compressor to handle. The low side symptoms are similar.

But its a different story on the high side. Excessive load would show up as high cond TD (high cond load). Inefficient compressor would show up as low cond TD (low cond load).

Unfortunately, we have an overcharge condition to confuse things.

[Gary]

With 12F evap dT, the room humidity would be about 70%RH... so if the machine has had this problem all along he probably would have noticed the high humidity.

[Gary]

the customer has been maintaining a constant temp but system always is running i am assuming. I checked when i was there to see if my air was bypassing the coil and it was not. My blower is set to low speed a thought i had that i did not check, u brought it up. To much air flow? I set the blower to low but did a previous tech possibly install the wrong size blower motor.

Another possibility would be something like a 3 ton AHU hooked up to a 2.5 ton condensing unit. You might want to check the model numbers to see if they match up.