losing a return vent to increase total return size?
I have a return vent that whistles loudly when furnace blower is on high speed. I have been told by the tech that checks the furnace that my returns are too small (seems to be a common condition from what I have been reading here). I had a contractor come and look at my return ducting to see how it could be increased. Some info first,
- single story house is 1400 sq ft
- 2 stage furnace only (3 ton I think), no AC
- supplies all under house (10 total)
- return ducts in attic going to ceiling returns
- return ducts are 1x12" diameter going to 8x20 grill, 1x8" diameter going to 8x14 grill
The issue is that the box above the furnace for the return ducting (return plenum?) is at the edge of the sloped roof and also has the flue pipe beside it so there are really only two sides accessible. The contractor suggested removing the 8" duct and taking a new 12" duct off that side and running that with flex duct to a new grill in the hallway. He would use a 14x20 grill for that and also expand the box and grill of the other 12" duct to a 14x20 grill.
So this plan would give lots more area in terms of grill size, but not really that much in terms of duct area. I am reluctant to lose a return vent especially since it is in the kitchen/dining room and this area can be separated from the rest of the house by closing a door.
The furnace tech said a rule of thumb for return size is 1 sq ft per ton of heating. The question is do you calculate by duct size or the size of the grill? It doesn't seem right to use the grill instead of the duct but I see lots of examples of that?
I suggested branching multiple ducts off of a single hole in the return plenum but the contractor said you will get uneven air flow that way. I have attached a couple of pictures of the return plenum to illustrate how awkward it is.
are the returns to small or is the furnace to big .is the air flow set correctly
Well I'm not sure about the furnace size, it was installed by the previous homeowners. It is only about 10 years old and still working so I don't want to replace it just yet. So having this furnace I guess the returns are too small. As for the air flow I'm not sure what that is. I do know that this furnace has a variable speed blower. The whistling noise from the return grill only happens when the blower is running at the maximum speed.
I had another HVAC contractor come look at the setup today and he had similar recommendations. He did suggest removing the existing ducts and going with two new insulated flexible ducts and using much larger boxes for the return grills. Also suggested running the flexible ducts in a loop which he said would reduce noise coming from the blower. Makes sense I guess.
The new, suggested 20"x14" grille is too small (560 cfm),i.e; 2 cfm per sq. inch (280 sq. in.) = 560 cfm.
The 12" pipe is good for 700 cfm which is okay, times 2 gives you what you need (1400 cfm).
Find a grille that will work with the 12" pipe and go with that.
The pipes, boxes and attic need insulation.
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