low airflow

[swa2796]

We are working on a local duct sealing program and we are finding many, many residential air conditioning systems with severely low system airflow. Program rules disallow sealing the ducts if the existing airflow is less than 300 cfm/ton. Normally, we like to see 400 cfm/ton, but this is very uncommon in existing systems. Oddly, although we are routinely measuring airflow in the 200-300 range, we are rarely seeing ice-ups or other problems that we'd expect. Any thoughts on why these systems are running without problems with such low airflow? Any thoughts on whether 300 is a reasonable cutoff? We are using a flow-plate (which simulates a pitot-traverse) to measure airflow, which I've double checked against the duct-blaster to verify accuracy. I'm pretty confident that our measurements are accurate within about +/- 10-15% overall. We're averaging about 30% of all the systems we see having airflow less than 300 cfm/ton. Climate is New England.

[udarrell]

I have seen numerous tests run on a lot of residential systems where the airflow was far too low.

The most cost effective way to address the problem might be invest in every way possible to reduce the heat-gain heat-loss and the reduce the size of the condenser.

If you could drop one tonnage the ductwork & blower might deliver enough to make it work properly. Figure out how much airflow CFM it will deliver & size down A/C to the level!

The over capacity equipment to airflow, simply does not deliver anymore than the airflow CFM will allow! That is a total inefficient waste of energy!

If they have a TXV it will tend to throttle down the refrigerant flow which will protect against slugging the compressor & may help reduce freeze-ups.
http://www.udarrell.com/air-conditioning-seer.html
Click Field Tests at the top of the page for several Airflow Field Evaluation Test Projects. - udarrell

[4l530]

Would you happen to know the running pressure of the cooling system in this low airflow situation?
I would guess just enough heat in the (although reduced)airstream to prevent freezing or something happening to raise the pressures above what you might expect (overcharged for example). Just a shot in the dark.

[udarrell]

Originally posted by 4l530
Would you happen to know the running pressure of the cooling system in this low airflow situation?
I would guess just enough heat in the (although reduced)airstream to prevent freezing or something happening to raise the pressures above what you might expect (overcharged for example). Just a shot in the dark.
Well, how many techs check CFM airflow, I would say a low percentage of them.

How many techs are actually using Superheat or Subcooling or Approach methods to verify proper charging? How many actually follow the mfg'er's charging instructions to the letter?

If the pressures are real low, how many will add refrigerant to bring them up? The overcharging might help to keep it from freezing but that would again be a very inefficient & costly situation.

After you learn what tonnage the ductwork & system will handle, you then decide which will be the most cost-effective way to go. Mid to Long-term it is usually finding ways to reduce the tonnage required to utilize the airflow being delivered, if that is at a reasonable SP.

The blower curve would need to be checked to see tonnage it could handle, at the required static pressure specs.

There is a possibility that could be done by using a proper sized TXV & changing out the condenser to a reconditioned 10-SEER unit, ha. (Where costs are the critical factor.)

Energy Conservation & saving the customer money with an effective payback period should be the paramount priorities. - udarrell