1. That chart is for when the indoor conditions are within human comfort conditions. It assumes RH is 55% or lower. Not that the indoor conditions are 85°F DB 76°F WB/66.6%RH.

If your system was working in a 100°F ambient, at those readings. You would probably find your SH and SC within 5° of each other. If the indoor was at human comfort conditions.

2. Originally Posted by 54regcab
The 31F superheat is high according to the R22 chart I have, should be 24F like you stated. (Chart says 22F but R410 may be different, close enough). According to the chart the 65 degree register temp is actually low, indicating poor airflow. The OP could be low on airflow and charge?

Hope the charts are helpful to the OP on diagnosing his system.
He probably is only moving 350 CFM per ton. Which is fine.

3. There's a lot of heat in that humidity and until that is removed, you aren't going to cool much.
In fact, that 20* split seems high for the conditions. I'd be looking at airflow as well and a "possible" charge problem.

4. I didn't realize dropping from 400 to 350cfm per ton would make that big of a difference, chart shows 72f discharge @ 400CFM/ton, he has 65f. What is the formula for the calculating BTU output based on delta T and humidity difference? I remember seeing it somewhere but forget the details. How close does airflow have to be in order to get the correct charge?

5. If the OP is getting 350cfm/ton x 2.5tons = 875CFM of 65 degree air into the 85degree 1100sqft house, how is it not cooling off? Is the system OK and the OP just has a ridiculous heat load coming from somewhere? If Delta T is good and airflow is good how can low charge be causing a problem?

6. If the system was undersized it can run all day and not cool off. My old house was undersized 95-100 degree day it wouldn't stay under 82-85 degrees inside. Ran all day 20 degree split on evap...

7. Originally Posted by Greend88
If the system was undersized it can run all day and not cool off. My old house was undersized 95-100 degree day it wouldn't stay under 82-85 degrees inside. Ran all day 20 degree split on evap...
2.5tons should be plenty for 1100sqft @ 100f unless the house has bad ductwork or unusual heat gain.

8. Actually those numbers sound pretty good with the exception of 20* split which sounds high. My SH chart shows 76 iwb/100 odb calls for 27* SH so you're not off by much. I would be looking for a problem in the duct system & not worry about the pressures yet. What doesn't make sense is the 20* split sounds more like low airflow but your SH should be lower than normal if that was the case. Could some of your readings be off? Is there much difference between the air coming out the registers than the air coming off the coil?

9. Originally Posted by 54regcab
If the OP is getting 350cfm/ton x 2.5tons = 875CFM of 65 degree air into the 85degree 1100sqft house, how is it not cooling off? Is the system OK and the OP just has a ridiculous heat load coming from somewhere? If Delta T is good and airflow is good how can low charge be causing a problem?
May have poor insulation and have a very large heat gain.

20°F delta at 875CFM is only 18,900 BTUs of sensible capacity.

10. Originally Posted by 54regcab
The 31F superheat is high according to the R22 chart I have, should be 24F like you stated. (Chart says 22F but R410 may be different, close enough). According to the chart the 65 degree register temp is actually low, indicating poor airflow. The OP could be low on airflow and charge?

Hope the charts are helpful to the OP on diagnosing his system.
Could you tell me where you got the superheat chart.
What manufacturer?
I'd like to put it up on my superheat page if I knew the manufacturer.
http://www.oceanhvac.com/superheat/

11. Originally Posted by 54regcab
I didn't realize dropping from 400 to 350cfm per ton would make that big of a difference, chart shows 72f discharge @ 400CFM/ton, he has 65f. What is the formula for the calculating BTU output based on delta T and humidity difference? I remember seeing it somewhere but forget the details. How close does airflow have to be in order to get the correct charge?
350 is find to get correct charge.

Delta times 1.08 times CFMfor sensible capacity.
H delta times .68 times CFM for latent capacity.
Enthalpy delta times 4.5 times CFM for total capacity.

12. Originally Posted by beenthere
May have poor insulation and have a very large heat gain.

20°F delta at 875CFM is only 18,900 BTUs of sensible capacity.
Using the CFM x Delta T x 1.08 he should be getting 14,040 BTU sensible with a 72f discharge temp and 1000CFM according to the chart. Latent heat really eats a LOT of capacity @ 66% RH!! If lower CFM is reduces sensible capacity then he's running well below 350CFM.

13. Originally Posted by 54regcab
2.5tons should be plenty for 1100sqft @ 100f unless the house has bad ductwork or unusual heat gain.
Should be and what it is are two different things, First Thing I would do if the system was running alright was do a load calc and find out the history of the building. If he has poor insulation it's no where near big enough, Or No ventilation in the attic etc... My old house was 1100 sqft and it calculated out to a 4.5 ton system and only had a 3.5 ton. Due to poor insulation and such. If it's a brand new house top of the line energy efficient design then maybe it is big enough but I doubt it. I've also seen if you put the same size 410a system in as R22 then your gonna have problems if it was "just" barely big enough to begin with.

Scenario we've had on a convenient store: Replaced a 5 ton R-22 system with a 5 ton R410a system, 410a system ran great but couldn't keep up like the R22 system use to be able to on 100 degree days. Ended up having to add another system to make up for the 410a system capacity loss at higher ambients. Also remember you loose so much capacity for every degree above 95 ambient. It's worse on 410a systems.

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