How can I tell whether the cold air return for my furnace is properly sized?

Question

I have a new 80,000 BTU high-efficiency furnace. It's working well, but I'm wondering if my cold air returns are big enough. There are two - one on the main floor dining room and one on the main floor front living room. The rooms are next to each other, and we keep the doors between them open at all times. Both returns have 12" by 13" vents. One vent narrows to a 9" diameter duct leading into the plenum, and the other to an oval shaped duct that enters the plenum with dimensions of about 13" by 8". The narrower duct is about 8 feet long, running from the dining room to the furnace, which is in the basement, and the the wider duct is about 4 feet long. The house has two stories, but despite the lack of cold air returns upstairs, the furnace is heating the whole house fairly consistently. My questions - do you think the intakes are large enough for optimal efficiency, and is there an easy way for me to roughly assess proper size without referring to Manual D or hiring someone?

Answer 1

You are right in that the ACCA Manual D for Residential Duct Systems will probably give the answer you need. I don't happen to have a copy, nor does my local public library or the local university library. But it only costs around $50, which may be cheaper than hiring someone.

However, let's do a little bit of reasoning. But first, a word of caution. My background is in electronic engineering, not HVAC, so my thinking may be flawed.

At first glance, it would seem that the cold air return ducting should be as large in cross sectional area as the furnace outlet ducting. But in a heating system, the inlet air is colder and more dense than the heated outlet air. That means that the air flow going into the furnace will be lower than the air flow coming out of the furnace.

I will assume that the cross sectional dimensions of your furnace are 20" by 25", which is the size of my own 80,000 BTU furnace. That gives a cross sectional area for the outlet plenum of about 500 square inches. I will guess that the total cross sectional area for the hot air ducting is somewhat less than 500 square inches because you don't want too much standing air in the ducts to get cold when the furnace is off. Let's say that the outlet ducts total 300 square inches.

From this I would say that a minimum cross sectional area for the cold air return between 150 to 200 square inches is reasonable. You've got two return paths. The one with a 9" round duct is about 64 square inches. The other duct that is an oval measuring 8" by 13" is about 81 square inches. Thus you have about 145 square inches total, which seems like it is somewhat on the low side. Fortunately the path is short, so the blower should not have to work that much harder.

If your system includes air conditioning, the cold air return is probably undersized. For peace of mind, call your HVAC contractor and ask to see his calculations based on Manual D. If he is unwilling or unable to comply, you may have to buy the manual yourself, which is a small cost in comparison compared to your recent investment in a new heating system.

Answer 2

some furnaces circulate distinctive air and might substitute the rigidity in a room. greater significant the chilly air return is precisely that and in case you haven't any longer have been given one the chilly air will stay next to the floor and not be push (or pulled) to the furnace to be reheated. next do you have a duct to grant air to the furnace now which you have bumped off the chilly air return?

Answer 3

if it is heating the house its sized right, other wise it wouldent heat the house right, mesure both cold air returns, in sq ft,
mesure all heat duct outlets , add together, in sq ft, should be the same or heat ducts should be smaller in sq ft,
If it ain,t broke don,t fix it