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Variable AC Setting

Posted by bparker (My Page) on
Wed, Jun 22, 11 at 13:14

I have a 2 1/2 year old home with a split AC system. A 2 ton for upstairs and a 5 ton for downstairs Both are Carrier variable speed systems. The air handlers are in the attic and I live in SW Florida so it does get hot up there. I had my first routine maintenance check, about a year after installation, and the tech told me the system was wired incorrectly and the air handler was only operating at low speed. We debated about rewiring it to the intended setting but I declined as the AC always worked great and it was very quiet. I also monitor the humidity and it's always 40% or below. I've been happy with it ever since. Today I had a routine maintenance check up on the system and the new tech was quite adament about rewiring the system so it would operate at the higher speed. He kept saying air flow is your friend and operating at the lower speed could cause sweating of the ducts and subsequent damage although after 2 1/2 years there is no visible damage. So, I let him rewired the fan so it operates on high but I really don't like the noise of the high setting.

I'd like a few other opinions about potential problems by operating the system on low as I have since I moved into the home. I'd really like to go back to the low setting but don't want to damage the home and end up with costly repairs.

Thanks, Bruce


Follow-Up Postings:

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RE: Variable AC Setting

verify its still pulling humidity out of the air. When my non variable speed airhandler was running on the highest of 4 settings in A/C mode it didn't dehumidify. I lowered to 3rd highest of 4 settings. It then worked great. Its true, it shouldn't be too low, but the highest setting isn't always good either. Lower air speed creates a colder coil and dehumidifies better. However it might not pump as much cold air into the house and might not be as good for the unit. You want the correct setting basically.


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RE: Variable AC Setting

What is the size of your fist and second floors? It seems unusual you have a 5 ton unit for the first floor, and a 2 ton unit for the second.

I suspect the 5 ton unit is oversized and that is why it works so well in the low stage. I don't see why this would cause a problem.

Can you post the model numbers of your condenser and air handler? Perhaps you can program the condenser to stay in the low stage with your thermostat.


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RE: Variable AC Setting

Thanks for the input. Here are the model numbers and I think they are builders grade and I'm not sure which is the condenser or air handler. Here goes - 25HBB324A300, FV4BNF002, 25HBB360A300 and FV4BNB006. The upstairs is approx 1000 sq ft and the down stairs about 2000 sq ft. By the way, although the rewiring was just done this morning I see the humidity is increasing. The thermostat is pretty basic with just an updown temp setting.


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RE: Variable AC Setting

The 25HBB is a Carrier Base model heat pump. The FV4BN is a Carrier Performance variable speed air handler.

I looked up the product data for the air handler. For a single speed 5 ton unit, the air flow can be set to either 1750CFM for nominal AC cooling, or 1400CFM for better dehumidification. We don't know the original speed of the air handler. The higher speed is not allowing the AC to run long cycles and dehumidify your home.

I suggest you describe what is going on and ask them to lower the fan speed. I suppose if the fan is going to slowly the coil could freeze up. But it sounds like you had no issues until the tech decided he needed to "fix" it.


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RE: Variable AC Setting

Thanks. I'm going to have them back to "unfix" it.


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RE: Variable AC Setting

Hi Mike. I thought some on your message and have a question. Do I have a variable system or a two speed system? I always thought a variable system would run at a low speed under normal conditions and ramp up to a higher speed when required.


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RE: Variable AC Setting

I am not an HVAC expert, so I will tell you what I know from reading the Carrier literature.

It looks like you have a single stage heat pump. This is a builder's grade type of equipment. But your handler is capable of variable speed, which is better than what a builder would install.

The air handler blower would change speeds if it were matched with a two-stage condenser. But you don't have this, so you will be set to one speed for AC cooling. The speed may change when you are in heat mode.


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RE: Variable AC Setting

When they switched the airflow of the fan coils, did they check the refrigerant charge as well, since it is quite likely it may need to be adjusted with such a change in airflow?

I would not want 1400 cfm (280 cfm/ton) on a 5 ton system. This is more appropriate for a 3.5/4 ton system. Carrier's fan coils come from the factory with a nominal setting of 350 cfm/ton. 400 cfm/ton is standard. 1750 cfm is where your 5 ton unit should be set at and 700 cfm is where your 2 ton unit should be set at. 350 cfm is a very good setting for better dehumidification. Any lower, you sacrifice efficiency and capacity of the system.

Noise issues you may be encountering are due to improperly sized/designed ductwork.

While you may have liked it the way it was before due to lesser noise, 350 cfm/ton is where it should really be at. On the control board in the fan coil this corresponds to a 5 ton outdoor unit.

I'd like to know what the installers adjusted the airflow to. From the factory these units are set to high speed (3 ton - 1050 cfm - for the 002 and 5 ton - 1750 cfm - for the 006).

Hope this helps.


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