I was told that my central air conditioner needs a larger return duct
strobo
| Posted in Mechanicals on
I was told that my central AC needs a larger return duct. The symptom was that the low pressure line at the compressor was icing up on the outside. Does this make sense? I had thought that if the system was low on freon that this was a symptom.
I am handy but need some input into the size requirements of a return duct versus the output ducts. I would assume them to be about the same size.
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Replies
John,
I'm not an AC tech, but a common cause of a freezing low-pressure line is low airflow over the evaporator coil (the indoor coil in your air handler). In other cases (I have read) the cause may be a refrigerant leak.
If your ducts are undersized, it is certainly possible that the undersized ducts are contributing to low airflow over the evaporator coil.
Airflow over the coil should equal 400 cfm per ton of air conditioning capacity. If your contractor is telling you that low airflow over the coil is causing the problem, then ask whether the airflow was measured. How much airflow does your unit now have? How many tons is your AC unit rated at?
The duct design should be reviewed for ACCA Manual-D compliance- they could ALL be out of whack for the CFM needed by the coil to stay above freezing. It's pretty common in older houses to find ducts designed for only half or 2/3 of the flow requirements, but so grotesquely leaky that "get by", until somebody seals them, increasing the net system efficiency, but introducing an ice-up problem.
It's also common to find systems with lower-than-spec refrigerant charges (or stuck refrigerant volume valving) having frost issues too.
Re-commissioning the whole shebang may be called for, including measuring & adjusting the refrigerant charge. But if the compressor is oversized for a tight version of your ducts, it may be time to think about replacement (either the ducts, air-handler & coil, compressor, or all of the above.) Most central AC is oversized for the loads, and going with a smaller unit and oversized ducts (within reason) increases both comfort & efficiency.
THANKS The info was most helpful. The return duct was completely colapsed for about 10 feet of its 30 foot length. I replaced it today. I think that a worker in the attic last year started the crush and it cascaded as the suction got stronger. I hope that the new duct will not destruct from delamination as the old one did. the unit is a trane 16 xi or something like that. It has a multi speed air handler. I hope that the 18" return duct is big enough. I will give it a try. If it doesn't work I guess I will add another parallel return. john