2nd stage cooling isn't running. New system.

7 years ago

I had a new system installed last week. Lennox XP16, multistage unit.
XP16-036-230-09
HEAT PUMP

CBX27UH-O42-230-6-04
VARIABLE SPEED AIR HANDLER.

I'm beginning to wonder if 2nd stage isn't turning on.

I live in Fort Worth Texas.
My house was built 6 years ago, 2100 sqft, with foam insulated walls, encapsulated attic with radiant barrier. On a 100 degree day, my attic is 75-80 degrees. My home is extremely well insulated.

My old unit (carrier) brand, 3 ton single stage unit leaked and compressor burned up, so I had to replace it. This unit before it crapped out on me ran extremely well. If it was 100 outside, I had no prob getting the temp down to 68. Even lower at night.

We normally set our t-stat to 71. But for testing purposes for the new system, I've been setting it lower, just to see how it will preform compared to my old system.

With this new unit, it's just not performing how I think it should. It struggles to get down to 73 during the day. At night, I'll set the t-stat to 65 (only to see how far I can push the unit). The temp will only get down to 71. At night the temp outside comes down to 80. So you would think the system shouldnt have any probs getting down to 65.

So today after work, I tested my theory. I used a clamp meter to measure the amps at the compressor. The RLA on the tag reads 15.3. The meter read 7.5. I tried forcing the unit into 2nd stage to see if the amps would rise, nothing happened. The unit sounded the same and reads 7.5 amps. I then tried forcing it into stage 1 only. I had the same results.

I've measured the difference at the supply and return and I'm ranging between 18-20. So those numbers are great.

I had the installer come out 3 times since install. He's adjusted pressures and added freon. He insists the unit it's running fine. But I know my home. I built this house, I know what it's capable of.

Any suggestions?

I'm at a loss.

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Comments (5)

mike_home
7 years ago
What thermostat do you have?

You should hear a difference in air flow between the first and second stages. It sounds like an installation problem.
sktn77a
7 years ago
Well, you need to have the installer show you that it's operating on stage 1 and then stage 2. As mike says, they should be a very niticeable difference in airflow on second stage. Does your thermostat show a call for second stage? Is the thermostat set up for 2 stage operation? Check the thermostat wiring also.
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i have a tri-level with basement ... that a quad level ... if there is such a term ... cold air falls ... if its sent upstairs ... it will fall... until it hits the lowest level .. in my case.. the basement ends up 65 ... and the top floor... whatever i set the temp to ... i spend a lot of time with the FAN ONLY running.. i do not have two zones ... in the alternative.. a door on the higher level.. will trap and maintain the conditioned air upstairs ... [now this will be a decorating conundrum] my point being... who says its not working ... perhaps its just that the cold air isnt staying up there... and the hot air from the lower level is constantly going up there.. thats why the lower unit doesnt run as much ... try temporarily hanging a bed sheet across the stairway .. or whatever.. to trap the cool air up there.. and see if that makes a difference ... [to keep the basement... from going total arctic... i use an adjustable twist shower curtain rod with a heavy floor length drape as a air dam ...] if it does.. and the system then runs to specs.. the the problem is not the A/C ... its gravity ... good luck with that.. lol .. ken ps: in winter.. its just as bad .. the upstairs is 90.. with all the rising heat ... and the basement.. again.. is 65 ... crikey ... more running of the fan to even temps thru the spaces ... because warm heated air.. wont stay DOWN in the basement
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PRO
Air and Energy Soutions
7 years ago
Oh yeah, one more thing. The formula for sensible capacity is :

temperature drop across the evaporator coil (deg) X Air flow (CFM) /1.08

For example, if your temperature drop was 20 degrees and the airflow was 1200 CFM, the sensible capacity would be 24,000 BTU or around 2 tons of sensible heat.

This will give you the amount of sensible heat removed. Most, if not all, residential systems are designed at .75 SHR. This is sensible heat ratio and means that the sensible heat removed will be 75% of the total capacity. To get a good ROUGH calculation of the total capacity your system is removing, divide the sensible heat by .75. Again, this will be a rough estimate, but will give you an idea if your system is producing close to what it is supposed to be. You will notice that even at a 20 degree drop and 1200 CFM, your system will not produce a full 36,000 BTU. This is pretty normal. Systems are designated by "nominal" tonnage and many times do not produce the entire rated capacity, but should be close. Hope that helps.
stoveguyy
7 years ago
I would feel bad if my 6 yr old unit died. What exactly leaked? Was it at a joint which installer placed? Or was it due to a component actually failing and leaking?
PRO
Austin Air Companie
7 years ago
last modified: 7 years ago
There is a procedure that Lennox has to determine proper modulation of the compressor to determine if it is modulating properly from 1st speed to 2nd speed. If it fails this test the compressor is noted as bad.

This is a complicated test and should be performed by the dealer. Many clauses with in manufacturing equipment warranty guidelines prevents you and others from attempting to repair it. Get your HVAC installer to perform the test and watch him while he does it.

I post the procedure only to give you an idea of what your installing contractor must do to recognize the failure of proper compressor modulation from 1st to 2nd speed.

PROCEDURE
-- Do not ATTEMPT THIS YOURSELF... You will void your Manufacture Warranty.

NOTE −
Block outdoor coil to maintain a minimum of 375

psig during
testing).

1. Turn main
power OFF to outdoor unit.

2. Adjust
room thermostat set point 5ºF above the room

temperature.

3. Remove
control access panel. Install refrigeration

gauges on
unit. Attach the amp meter to the common

(black wire)
wire of the compressor harness. Attach

thermometer
to discharge line as close as possible to

the
compressor.

4. Turn
toggle switch OFF and install switch in series with

Y2 wire from
room thermostat.

5. Cycle
main power ON.

6. Allow
pressures and temperatures to stabilize before

taking measurements
(may take up to 10 minutes).

7. Record
all of the readings for the Y1 demand.

8. Close
switch to energize Y2 demand. Verify power is

going to
compressor solenoid.

9. Allow
pressures and temperatures to stabilize before

taking
measurements (may take up to 10 minutes).

10. Record
all of the readings with the Y1 and Y2 demand.

11. If
temperatures and pressures change in the direction

noted in Two−Stage
Modulation Compressor Field

Operational
Checklist on Page 36, the compressor is

properly
modulating from low to high capacity. (If no

amperage,
pressures or temperature readings

change when
this test is performed, the compressor

is not
modulating between low and high capacity and

replacement
is necessary).

12. After
testing is complete, return unit to original set up.

2nd stage cooling isn't running. New system.

Comments (5)

mike_home

sktn77a

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