High static pressure due to undersized returns?

12 years ago

Hello,

I live in the Boston area and just had a 5-ton Carrier Greenspeed (25VNA060A003) installed at my house along with air handler FE4ANB006L00 and an Infinity Touch Control. As soon as it was installed, I started getting "replace filter" messages everyday, even though I would reset it. Now I believe a 5 ton system needs about 2,000 CFM (400 CFM per ton)... So I did an airflow verification test and the static pressure is off the charts. It is different every time I run it but it ranges from 1.1 to 1.7. It's come in at 1.25 several times. The infinity control allows me to run this test right from the thermostat. The reason I got the Greenspeed, was to replace oil for the most part, and heat my house on most nights unless it is below 5 degrees or so. I wanted the most efficient unit on the market to save money and energy, and I want it to last a long time. It seems to me that with a system with a static pressure this high, it's going to be VERY inefficient... Possibly as pricey as the oil I was trying to replace. And it's going to cut down on the lifespan of the unit with the blower working so hard. I'm wondering what the problem is, as the installer is having a tough time figuring it out. It's installed in 2,500 square feet, of a 3000 square foot house. (the other 500 sq ft. is still on oil to due ducting limitations.) The main return trunk is 20 inches. The returns coming from the rooms are as follows: Four 8" rounds and one 12" round return. All flex duct, going to a solid 20" return trunk. I found a rule that said that each 8" round, provides 170 CFM and 12" round provides 510 cfm. If my system is 5 ton, that means 2000 cfm. My returns, seem to add up to 1,190 CFM, correct? (170*4=680 + 510) = 1,190 CFM. Am I on the right track here, to say the returns are undersized? The duct guys that did it, assure me that if anything, the returns are oversized, but that doesn't match my calculation. Also, the tests vary in static pressure, even if the ductwork isn't touched. But do change significantly when we pull of the filter or the filter door.

I know the system itself might be a bit oversized for the square footage, but since it is a variable speed motor that can run at 40%, I didn't think it would be a big deal, because in the winter, it would have enough power to heat the house down to low temperatures, but in the summer, it could run on lower variable speeds and still dehumidify.

Thoughts?

Thanks!

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Comments (2)

bus_driver
12 years ago
HVAC is not my trade.I lurk here to learn.
Too-small returns would create starved intake. I know what problems that would create if this was a liquid handling system.
It is my belief, subject to correction, that a too-high static pressure is caused by insufficient area or restrictions for outflow from the air handler. Are the filters in the supply ducts?
mike_home
12 years ago
Ideally the static pressure should be at 0.5. Anything above 1.0 is unacceptable and needs to be fixed.

Was a load calculation done? Heat pumps need to be sized for cooling needs. The auxiliary heat will make up the difference between what the heat pump can supply on very cold days. The Greenspeed works well in very cold temperatures. Your heat strips should not come on very often.

A 20 in square return is very undersized. I have attached a document to help you figure out how big a 5 ton return needs to be.

This is what I recommend. Do a load calculation. I expect for your area the cooling load will be 4 tons or less. If it is, then have the plug in the Greenspeed changed so that it becomes a 4 ton system. The 4 and 5 ton condensers are the same, it is modified to work as either. You can ask the installer to refund the difference in price which is going to be about $800-$1000. In addition increase the return size. The bigger the better. The filter should be at least 20 X 25 for 4 tons, 25 X 25 would be better. When you redesign the return have a box installed for a 4 inch media filter. Hopefully you can get the static pressure down to 0.7 or less.

The controller is not going to keep the blower speed at 40% because the system is over sized. If you do any kind of set back in temperatures, it will go up to 100% capacity in order to get to the desired temperature. You are going to have the same problems in the summer. You are shortening the life of the very expensive variable speed motor if you don't address the static pressure problem.

Here is a link that might be useful: Rules and *Rules of Thumb for Duct Systems
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