Return size

[Chris Bernhardt]

New construction, 1738 sf, two story, in Forest Grove, Oregon.

When I went looking for the HVAC system filter, there was none and I could only find a 2 square foot floor return in the upstairs hallway.

The A/C has a 2.5 ton capacity.

Doesn't that return sound a little small?

Digging around on the internet what I found was 144sqin/ton is the bare minimum with closer to 180sqin recommended to prevent the evaporator from freezing up. Stick a filter in that return, maybe it's sure to freeze up.

Chris, Oregon

[Bain]

Chris,

You can't really determine whether the return is large enough without counting the supply runs and knowing what their diameters are. HVAC contractors use a cardboard, slide-rule looking thing called a Ductilator to make those calculations.

I actually own a Ductilator, but have no clue where it's currently hiding in my house. Mine was given to me, but Googling "Ductilator" should lead you to where you can buy one.

John

[Chris Bernhardt]

There are 10 supply registers whose agregate area adds up to more than the area of the return by at least 2 supply registers but of course I don't know the size of the ducts.

It just seemed weird. In this type of house I usually will see at least two returns: one downstairs and one upstairs both set up for filters.

I don't want to get a call back that there is not adequate cooling upstairs, the evaporator's freezing up and some dildo saying that I should have caught the obviously undersized return.

Chris, Oregon

[kurt]

It has to do w/blower size, CFM, and supply duct sizing. I'm not sure why AC tonnage would be a calculating factor.

This isn't even slightly scientific, but I've found that underized returns cause excessive "air" noise in the ducts. Undersized returns will also cause the returns to *oil can* inward when the blower is running.

This is equally unscientific, but simply checking air flow with your hand at each supply register will tell you a LOT. Yes, I realize this is highly subjective, but if you check a few thousand registers, you will easily get the idea of what is satisfactory air flow for a particular space.

[Richard Saunders]

100 square inches per ton is a common rule of thumb, so is 1 square inch per 2 cfm. Also, consider the size of the opening at the furnace, I would guess it wouldn't need to be any bigger than that.

[Jim Katen]

``

Originally posted by Chris Bernhardt

New construction, 1738 sf, two story, in Forest Grove, Oregon.

When I went looking for the HVAC system filter, there was none and I could only find a 2 square foot floor return in the upstairs hallway.

The A/C has a 2.5 ton capacity.

Doesn't that return sound a little small?

Digging around on the internet what I found was 144sqin/ton is the bare minimum with closer to 180sqin recommended to prevent the evaporator from freezing up. Stick a filter in that return, maybe it's sure to freeze up.

Chris, Oregon

Presumably, there's also a warm-air furnace. The code-required minimum area is only 2 square inches per 1,000 btu per M1602.1.1 and per G2442.2. A house that size in Forest Grove probably has an 80k - 100k furnace so the return is adequately sized for the furnace, if not for the AC.

I see two-story houses with only one return all the time. They never have even heating & cooling between floors. Even if they have two returns, the top floor is always warmer than the bottom floor.

- Jim Katen, Oregon

[Bain]

To avoid the calls from the dildoes, I always check the temperatures--via an infrared thermometer--on each level of the house and note them in my report. Like Jim said, with only one system and multiple living-space levels, there's always gonna be a discrepancy.

I looked at a quasi-modern, three-level house on Friday that only had one HVAC system. After the A/C had been cranking for a couple of hours, the lowest level was seventy degrees, but the uppermost level was seventy-six.

[Phillip]

Look at the size of the ductwork. The return ductwork should be at least the same size as the supply ductwork, if not larger.

If it is setting on the filter box in the closet look at the opening in the bottom of the unit. The filter should be larger than the opening.

Most unit have a built in filter slot. The return filter should be at least this side. The filter should be larger than the duct size for the proper airflow.

Like Jim and Bain said, when the system is serving more than one level. One of the levels is going to be cooler than the other.

[Chris Bernhardt]

Thanks, for the replies. It doesn't appear then to be obvious evidence of an issue.

I was talking with an HVAC contractor at a recent vendor fair and he was talking about the blower size, CFMs and duct size with respect to proper sizing of the A/C like Kurt said.

I am curious to know more about how to design a proper system and how to avoid the uneven heating between the 1st and 2nd story.

Do you think I can do that without buying manual J and manual this and that etc.?

Chris, Oregon

[Jim Katen]

Originally posted by Chris Bernhardt

. . . I am curious to know more about how to design a proper system and how to avoid the uneven heating between the 1st and 2nd story.

Do you think I can do that without buying manual J and manual this and that etc.?

Chris, Oregon

I think that it's a complex field of study.

The only way to get good control of heating & cooling of multi-level houses is to install a zoned system or to install separate systems for each level. Very few people want to spend the money to do this.

- Jim Katen, Oregon

[Thor]

Go to the ACCA ( Air Conditioning Contractors of America)web site http://www.acca.org/store/category.php?cid=1

and purchase Bob's house, read it and then read it again. This will not give you all the answers but will give you a great apreciation of what a good A/C contractor does and whats important.

[mridgeelk]

Monday I was inspecting a home for my clients with the owner present. I had turned the furnace on and was checking temperature and flow/volume, as I entered a bedroom with one client, the owner and the other of my clients left I noticed a low oscillating rumble in the room. At this point I thought I was hearing the "oil can" effect. As I leaned over to remove the register which was closed, I braced myself on the nearby bed, I was quite relieved to realize that it was a vibrating bed. The owner had turned it on to show my other client how it operated and left the room.

Originally posted by kurt

It has to do w/blower size, CFM, and supply duct sizing. I'm not sure why AC tonnage would be a calculating factor.

This isn't even slightly scientific, but I've found that underized returns cause excessive "air" noise in the ducts. Undersized returns will also cause the returns to *oil can* inward when the blower is running.

This is equally unscientific, but simply checking air flow with your hand at each supply register will tell you a LOT. Yes, I realize this is highly subjective, but if you check a few thousand registers, you will easily get the idea of what is satisfactory air flow for a particular space.