Infinity AC not working properly after furnace PCB replaced

[rad]

Back in March, due to furnace blower issues and a service bulletin, the control board in my Carrier furnace was replaced & it worked great after that. Now I'm having cooling issues & I don't know if it's related.

So here's some background (sorry about the length):
58MVP100-F Nat gas furnace
CK5BXA048021 Coil
38TXA048331 Condenser (1 speed)
Infinity Tstat
Carrier zone control (2 Zones)

In Feb 06 we updated our Carrier system (installed April 04, Thermidistat thermostat) that included the Infinity 96 furnace, variable-speed blower, and 4 ton (I think) compressor (I think the comfort line as they’re NOT the 2 speed ones). When adding the new system 3+ years ago we made the older, existing system into an Infinity system (Infinitized them).

This “Infinitized” system has 2 zones (1 zone is the entire 3rd floor which almost NEVER calls for heat or cooling as we don’t use the 3rd floor, the other zone is 3 bedrooms & 2 baths on the 2nd floor, the stat’s in the master on the 2nd floor (ZONE1)) worked great last summer but now seems to have problems.

I noticed that the cooling seems to only come on at full blast until it reaches temp then shuts off. I also noticed that the dehumidify doesn't seem to work anymore. After noticing the strength of the system I went into setup thru the Infinity tstat and changed the Zone Airflow Limits to LOW on the 2nd floor zone (ZONE1, the one we use) & MAX on the 3rd floor zone (ZONE2, not really being used). This immediately slowed the airflow and seemed to work great.

Then a couple of nights later - I go to lower the temp & notice nothing is happening (I waited several minutes). So I go into the tstat settings and see the error (several times) "Airflow limited, Stagedown Occurred". I run it thru some tests & everything comes back OK. But the AC still won't come on. I then change the Airflow Limits to High on ZONE1, lower the temp again and on comes the AC but full blast (you can really hear it). If I change the Airflow Limit on ZONE1 to low WHILE the AC is ON, it will lower the fan speed and seems normal - but after it shuts off it won't come back on & I'll see the same Airflow error. The error only comes when the airflow is set to low on the Zone Limits. Also, as mentioned, the dehumidify seems not to work regardless of the airflow setting.

Any ideas? Could this be related to the replaced PCB in the furnace?

Thx.

[jrbenny]

LOW is your problem. Put it back where you found it and quit jacking with the darn thing.

When you choose LOW, you are limiting the amount of air the system will push into the zone. The system will only run so long at lower than optimum airflow before it wants to push some extra air to get back to a nominal airflow setting. Since, you put thing on LOW, it can't push that extra air.

There's a reason that section is the Install/Service menu and not the homeowner menu!

[rad]

OK, fair enough. But what about the other problems: the AC is way too strong, seems to ONLY be on the highest speed (which seems much higher AND louder then the heat), & the DEHUMIDIFY seems to not work anymore.

Thx.

[jrbenny]

Model plug or another setting is incorrect. The service company should be able to fix these issues relatively quickly.

[gasguy]

"(which seems much higher AND louder then the heat)"

The fan speed for cooling is almost always higher than it is for heating. Was it set up properly before the changes?

[qwerty hvac]

You almost always have higher fan speeds in cooling mode.

[rad]

This is significantly higher & pretty intrusive - both at the supply & return. I don't recall this last summer. Also, it's causing the tstat to reach temperature way too quickly to dehumidify. The other morning the humidity was at 58%. Last summer it was almost always at 50% (where it's set) except in the morning it was always LOWER (I put the room temp to 68 when I go to sleep, 75 during the day). Now the dehumidify has to work alot of the day to get it back to 50%. Here's what it shows consistently when I adjust the temp down:

1-2 minutes 980cfm, 615rpm, .20", then a few minutes @ 1400 cfm, 882rpm, .42", and finally 1600cfm, 995rpm, .50" for the remainder of the time, sometimes only a few minutes. The middle one is LOUD, the last one is really LOUD. The 1st one seems what it was like last summer & pretty quiet.

Nothing changed in the tstat settings as I wrote everything down b4 & after. Only the new PCB. I'd like to avoid another $200 service call. Thx.

[Alfa]

If the T-stat reaches the Settings too fast, it can not "dehumidify" too fast at the same time.

You mention the humidity last summer was 50%, but now reads 58% (that is a tad humid), than the Blower motor is running at (very) high speed.

The numbers you provided for the CFM indicate an air flow restiction or undersized ducts (less likely), hence the revving-up of the speed.

An ESP reading would help, but better call the install contractor.