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Old 06-12-2009, 10:46 PM   #1
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HVAC Freezing Up - Help


Last year I had a new AC system installed. The old unit was a 3 ton and the installer recommeded a 4 ton. Well, from day one the system would freeze up from time to time. I could not figure out any consistancy at the time. Well, this is the first spring I have had it and have determined that the unit freezes in the evening when the outside temp goes down.

I called someone else out for a second opinion and he basically told me that I was sold a unit that is too big for the set-up in my home. He said my R/A deck is too small (See pic below) and my fur down duct system was designed for 1200CFM. He recommended I have the other guy come out and test with a tru-flow template to determine CFM range.

The options he presented were:

1. Downsize the condenser to a 3.5 ton. I dont feel this is an option due to the fact that the one I have is less than a year old.
or
2. Raise the R/A deck and create a plenum in the attic and supply air with new ducting. The current R/A deck is 10" tall, 36" deep and 60" wide with two 24" X 8" intakes on each side of the closet.

I'm wondering what cost effective options I may have here. I'm going to call the other guy back but I have a feeling he will not take care of it.



Any help will be appreciated.

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Old 06-12-2009, 11:10 PM   #2
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HVAC Freezing Up - Help


Icing up is caused by basically two reasons 1. lack of air flow 2. Low on charge.

The real problem is if you had a 3 tn and installed a 4 tn not only the return needs to be modified but the suppy air may need to be altered.

Just because your return is able to take in 1600cfm/4 tn 1200cfm/3 tn your supply needs to be able to deliever 1600/1200 cfm.

Looks like the installer didn't size properly. What is the sq ft area of your home that is being cooled.

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Old 06-13-2009, 03:30 AM   #3
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HVAC Freezing Up - Help


Plumber101 is correct call the contractor back out to
increase the supply and return duct work.

I would look at some other issues as well.

1. Is the water heater sitting on wood. Should be on non combustable material.

2. Does the vent to the right of the water heater go to any living areas? If so you could be risking death. Hopefully it is only combustion air for the water heater and furnace and is only drawing in outside air.

3. The platform the units are sitting on is also the return air duct. Then thru the small vent in the door. You can try to enlarge the vent on the door as large as possible.
Then seal the door to the return air platform/duct with foam tape. You do not want to be removing from the air from the equipment room and circulating it into your living areas. CO poisining and shorting the burners of combustion air or back flowing of the exhaust fumes.
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Old 06-13-2009, 05:23 AM   #4
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HVAC Freezing Up - Help


Thanks Plumber101.

The airflow issue is exactly what was provided to me as the cause. He mentioned that the installer overcharged the system to try and prevent the freezing.

The house is 2000sf.
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Old 06-13-2009, 05:38 AM   #5
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HVAC Freezing Up - Help


Frazid,

Answers to your questions:

1. As designed the water heater is in fact sitting on wood. Would putting one of the plastic or metal pans under it be acceptable?

2. The vent to the right of the water heated does not go into a living space. It also does not vent outside. I think it vents to the wall space then possibly up to the attic.

3. The vents in the doors are in fact too small. I actually leave the one door open in an effort to increase airflow. This has only been a consistent problem over the past month or so.

Thanks.
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