Air Handler Cold Air Return


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Old 03-28-05, 06:38 PM
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Air Handler Cold Air Return

I had a new HVAC system installed in my house when I added an addition to the house in 1999. I got a very nice Lennox system, at the time it was the largest residential electric furnace and heat pump made. The system is zoned with individual thermostats, has a variable speed air handler, and is 4.5 tons.

Since installing the system we have noticed that the house gets dusty very quickly. I think the problem is that the contractor installed the electronic air cleaner from my old 2.5 ton system onto the new 4.5 ton system. I dug out the paperwork on the old cleaner and it is rated at 1600 CFM flow, but the new air handler has a flow of 2150 CFM. So I think the new system is just overpowering the old cleaner. In addition the old cleaner has an air opening of 18" X 17" while the furnace opening is 20" X 23" so I think this restriction right before the furnace is probably also reducing the efficiency of the system. To solve the problem I bought a commercial Honeywell electronic air cleaner that is the exact same opening as the furnace and is rated for 2000 CFM. I will limit the air handler to 2000 CFM by changing the jumpers inside the air handler, it's very easy to do.

The last 8 ft of cold air return ductwork leading into the furnace is 14" X 20" or 280 square inches, while the furnace opening is 20" X 23" or 460 square inches. Should I enlarge this cold air return to match the size of the air cleaner and furnace opening? What is the minimum cross section that I should use to ensure the furnace is not being starved for air? Enlarging this small section of ductwork will be easy.

Thank You
Frank
 
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Old 03-29-05, 09:11 AM
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Ill go this way 60 sq" per ton is very tight and 80 sq" per ton is a very good job with the duct work. Now 4.5 ton could work at say about 360sq" and at 1800 Cfm. just fine
Since installing the system we have noticed that the house gets dusty very quickly. I think the problem is that the contractor installed the electronic air cleaner from my old 2.5 ton system onto the new 4.5 ton system.
Yes for sure the first thing here is take that EAC out and put in a good air bear like pleated air filter that will do a good job. So you dont have to be cleaning the AC coil all the time as with that EAC filter.What can I say we take out more electric filters than we sell so Id say dont put that new one in. People dont keep them clean.

ED
 
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Old 03-29-05, 05:45 PM
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Thank You for the reply.
Geez Ed I just spent $900+ on the new cleaner and a UVC addition. The small one I have now is 13 years old and has not given me many problems. I had to replace the air switch and power pack one time. I'm pretty good about cleaning them. Actually I have been using a regular furnace filter after the EAC, and I probably will do the same with the new unit.

After making this post I went out and did some more measuring. The AH is in the center of the cold air system, it is shaped like a "T", there is a cold air duct coming from each end of the house and the AH is in the middle. One of the air ducts is 128" the other is 70", where the 2 join the main trunk drops to the air handler and is 280".

Is there any use making the 280" any larger with only 198" supplying it? With the set-up I described what do you think the max CFM I should use?

Thanks Again
 
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Old 03-30-05, 09:01 AM
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Actually I have been using a regular furnace filter after the EAC, and I probably will do the same with the new unit.
This is why your unit works. In 2 or 3 days the wires are covered and dont do anything as a filter. Use a good pleated one after it for sure Good deal on the UV lights they are worth it.

Now duct work that 280 sq" is only 62sq" per ton that is very very tight thats at the low eend for sure now the other ducts 128sq" and 70sq" thats 198sq" tot. thats only 44 sq" per ton That duct would work for a 3 ton unit at 1200 cfm is all. The book I have calls for a 10X30 for 2000 cfm

ED
 
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Old 03-30-05, 10:16 AM
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Ed, one more question and I promise to leave you alone. You have been a big help. It is not practicasl for me to increase the duct size in the entire house. I have the 4.5 ton unit and would like to use it wisely. Other than increasing all the return duct sizes what can I do? Can I add a manual damper in the main trunk by the furnace to supply the furnace with the air it needs but can't get through the duct work?
 
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Old 03-30-05, 11:15 AM
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Yes ---AS long as it is on the intake side of the filter.Also so it can be closed tight and not draw air there from around the furnace when it is on heat " if this is gas or oil"

ED
 
 

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