Need expert advice...Low Pressure Lockout

[skydiver2]

3000 sq ft house not including basement
Carrier 95,000 btu Infinity Series furnace
Carrier Model 24APA, Performance Series 5.0 Ton A/C unit
Carrier Infinity Control thermostat

We had the above 5.0 Ton A/C unit installed several months ago and it has never really worked properly. The unit will work for a day at the most and then show a 'System Malfunction.' Looking at the A/C unit itself it shows a 'Low Pressure Lockout' error.

At first, the pressure was indeed low and we suspected a leak in the lines. The suspected line was replaced and the unit was recharged. Again, it worked for a day or so and then showed 'System Malfunction' again with the same error. It keeps alternating between working fine for a day and then showing an error the next day! The technician has been out three times and can't find anything wrong. He says there may not be enough airflow...

We are using a 4" pleated box filter which was replaced and made sure there is good airflow by opening most of the vents on the main floor and all the vents on the second floor. We also tried to increase airflow by adjusting the A/C and CF airflow dip switches on the furnace. Different CF (continuous fan) settings were tried at auto/med/hi. However nothing seems to help. I did notice that the cooling coils in the furnace had some frost. I am quite sure nothing is blocking the return air or cold air intake.

Any ideas on what could be causing the Low Pressure Lockout time after time?? This A/C hasn't worked two days in a row! :confused:

[beenthere]

Open ALL registers and have them check the static pressure, to see how much air its moving, instead of them guessing.

[samtheman]

if i read your earlier thread correctly, you had 3.5 ton equipment replaced with 5 ton equipment. were duct upgrades made as well? I suspect you do have airflow problems. As beenthere said, the technician should be able to test the system and prove his suspicions instead of just guessing. What is the model number of the furnace? It may not be able to handle 5 tons of airflow. If you could post a few pics of the furnace installation and the model numbers it could tell us a lot.

[heaterman]

Did you use the same company for the second install? Sounds like there may be duct system problems coupled with an undersized blower. Assuming that you are using the same furnace and duct system that were installed with the original 3.5 ton system referenced in your previous thread.

[local553]

tech should take blower door off furnace run unit see if makes differance. also should have around 20 6 inch supply vents

[fishnfool]

tech should take blower door off furnace run unit see if makes differance. also should have around 20 6 inch supply vents

That would depend on if it is flex or metal, 20 6''Flex @.1 will leave you 360 cfm short for a 5 ton system!

[skydiver2]

Yes you are correct, originally they installed a 3.5 ton unit which was unable to cool the house. After I insisted on calculating the size of unit required, based on the size of the house, windows, etc they agreed it was indeed undersized and replaced it with a 5 ton unit. No changes were made to the ducting and the install was done by the same technician. The house has 16 vents on the two floors and 6 vents in the basement. Most of the basement vents are closed as it is cold there anyway. Initially I had many of the vents closed on the main floor in an attempt to cool the second floor. I have since opened most of the vents except for the basement.

The 5 ton keeps shutting down with a low pressure lockout. Last time the tech was out, he inserted a temp probe in the ducting just above the coils and noted the temp was 'Not as cold as he would like to see it.' and noticed some frost on the coils.

Yesterday I adjusted the dip switches on the furnace and set all thre to the default OFF position. The fan speed is noticeably slower and the A/C has not given an a low pressure error yet...

Here is a pic of the installation as requested. I have a Carrier Infinity 96 furnace. As you can see, the ducting had to be built out to accomodate the size of the coils. The sides were taped! Attached is a pic of the model no. etc. Can anyone confirm the settings on the dip switches for a 5 ton A/C unit on this furnace?? Now the tech is suggesting replacing a valve on the furnace?? :confused:

[force1]

I believe the infinity controller should recognize the installed units and adjust airflow, so you don’t need to set any dip switches.

[fishnfool]

If that return on the right is the only return going to that unit you are definitely undersized on duct work!

[wraujr]

If you went from a 3.5 to 5 ton unit there is a very good chance that your ducts are undersized.

How about another picture a few feet farther back showing the duct layout.
Then take some measurements of the ducts and report back.
What is your filter size??

[heaterman]

A 5 ton drive almost always requires #1 a large bottom return box coupled with a properly sized RA drop or #2 left and right side return to provide adaquate RA. Supply must also be sized properly. A second on site opinion may be in order here

[motoguy128]

A 5 ton drive almost always requires #1 a large bottom return box coupled with a properly sized RA drop or #2 left and right side return to provide adaquate RA. Supply must also be sized properly. A second on site opinion may be in order here


I agree. That seems to be a common notation in the installation manual of every furnace I've seen.

But hey... who reads those things anyway? Just us annoying over informed HO's. :D


I'm a little suprised the load calculation called for 5 tons. Garbage-in = garbage out. I'm not sure I would trust the numbers this company used, given that they overlooked the ductwork capacity. Just using the wrong design tempeatures could affect the result by 10%-20%. That's 0.5-1.0 tons.

Use a 70F indoor and 100F outdoor instead of a 72F indoor and 96F outdoor and you have more than a 20% difference in sensible cooling.