return air and inspector
My wife and I recently purchased a foreclosed home as a starter home for our daughter. Its a small ranch home built on a slab with supply ducts in floor and a central return in hallway where filter is located. The home is only about 1200sq. ft. We have been gutting the home and we had our heating and air contractor add central air to the existing downflow furnace that is only a couple years old. When the township inspector came in to look at our electrical he looked at the furnace and told us that the return air would have to be changed. It has a cover or sheet of metal over the top of the return air opening and from that cover or cap there is a 14" round takeoff that goes about 7 feet over through the ceiling to the return air filter access. The inspector says that the top of the furnace or the return side cannot have a cover like that right on top of the furnace with the round takeoff but rather has to have a plenum that is the size of the factory furnace opening on the furnace and the takeoff off of that. Our contractor says that the blower speed is on a low setting and that the furnace temperature rise is within factory specifications and that the current setup is acceptable, so who is right? The contractor says that the inspector is a new inspector and trying to nitpick and not know exactly what he is talking about. All that I can find in the furnace installation manual on the return air opening on the furnace (Goodman 80% downflow) is that it says reducing return air opening size will reduce airflow. It does not say that it cannot be done. I do not want to argue with the inspector but I also want to know if this plenum really needs to be added. What difference would it make if there is a plenum and you still have the same size takeoff coming off of the plenum? And is the furnace temperature rise the correct way to know if the current return air is acceptable for the furnace?
Manufacturer recommended maximum temperature rise is established to insure that you have enough air flow to not over-heat the unit. That being said, I would make sure that you are not running at the max. leave a little room in there for when someone forgets to check the filter.
The duct connection that you describe sounds acceptable to me, the only reason that I can think of for not letting it go that way is insulation. Is the flat surface insulated with a high enough rated product?
How much AC was added? Is there adequate return air for cooling as well as heating? ICC requires 6 square inches of return duct area for each 1000 btu of cooling. With that as a requirement, 14" will only be adequate for 2 tons (24k btu).
"Customer Service" is not a department, it's an attitude!
Let me see if I read this correctly. The top of the furnace is the return, there is no plenum attached to it but simply a flat sheet metal plate, the circular return duct plugs directly onto the plate.
I can't site a code violation based on my jurisdictions adopted code but...that method is a horrible way to connect the return duct.
Imagine if you will, the return air is traveling thru the duct until it reaches the furnace. It will dead end into the bottom of the fan house and need to squeeze by a limited gap between the circular inlet and the housing. Entering the back of the furnace is prohibited at least by the mfg and probably by code for the very same reason.
Make the correction and the air will flow as it should and make the inspector smile.
Last edited by btuhack; 09-13-2011 at 11:04 PM.
no code violation with the return, however a proper install would consist of at least 3 ft of a plenum the size of the return opening on the A/H. This is to make sure the full coil see's airflow not just a 16'' round part of it.
but no nothing is wrong with that, unless uninsulated, and that can be disputed if it's within the building envelope.
It's hard to stop a Trane. but I have made one helluva living keeping them going.
try sending a picture and equipment size.
Why get in the middle of a pi**ing contest? Just ask the HVAC guy to come back and put a short plenum on the top of the furnace. Not a high dollar fix, and if the HVAC guy wants to fight with inspector, let him have a go. If the round inlet hole is directly above the top of the blower housing it (the top of the blower) will definitely lower the air flow regardless of whether the temp. rise is within the man. specs. Maybe your HVAC guy has never learned that you can't fight and win against City Hall, so it's always best to try and get along with them.
I don't think that is a code violation, but it is a poor way to duct return air. The airflow will not disperse and fill the entire furnace area but rather have a circle shaped area where the air is pulled through. May cause uneven heating on the heat exchanger, thus causing metal fatigue and premature failure... The contractor should install a return plenum - 24-36 inches would be ideal, but if space does not allow it put in as long of a plenum as possible.
As for the inspector, you might want to ask him what code number it is a violation of. I suspect the inspector is mistaken, but if he is right you can point to the specific code to your contractor and at that point the correction should be done at no cost to you. Make sure to be very polite and respectful with both parties. Offending either one serves you no good at this point.
It's not rocket-science...
It's electromechanical thermodynamic engineering
International Mechanical Code say's that for warm air furnaces the minimum unobstructed total area of the return and osa duct (if applicable) shall not be less than 2 square inches per 1000 btuh output rating of the unit and not less than specified by the manufacturer. Do the math and you will know if inspector is right.
And as cartercrew said if cooling is installed.
Politicians need to be changed like diapers, and for the same reason.
IMC 918.2 covers it but they have not posted the equipment size.I found it to be strange that the OP stated the contractor told them the fan was set to low position and within the temperature rise.
Have th3e contractor use
a short piece of "hard" round duct with a hard adjustable elbow then the flex.
Sounds like the 14 inch is a little small even for a 2 ton.
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
I don't like capping the return, would rather have plenum. Didn't know was code violation.
on a downflow furnace, the coil is under furnace, so airflow on coil not issue. but the blower can block up part of the duct opening.
questions asked, answers received, ignorance abated