Basement Cold Air Return Placement

[TheKingfish]

First time poster in need of advice. I am in the process of having a new furnace quoted for a northern Michigan part-time residence. The basement area (700 square feet, three rooms) has never warmed particularly well, and I have been told it would benefit from additional cold air return.

In this instance, installing a floor-level return is a physical challenge. Contractor #1 proposes to run a return in two rooms; in each case the return would be in the ceiling located about 7 feet from the ceiling heat vent. Contractor #2 proposes to run a single floor-level return in an area that would generally serve both of these rooms. This contractor has no faith in a ceiling cold air return.

I am wondering if I should insist upon floor-level placement. Best course of action? All thoughts are welcome.
Thank you,
TheKingfish

[tipsrfine]

Is the basement insulated? It should be cost effective to insulate it since you are in a cold climate.

[Shophound]

Is the basement finished? No return should be in a basement if it will have direct line of sight to gas burning equipment, such as a water heater, boiler, or furnace. If these items are in a mechanical closet that is air sealed from the basement but can get combustion air from outdoors, then you're good with a return in the basement (as long as it's not in the same closet with the equipment!).

[BadgerBoiler MN]

A return does not insure warmth. If you have an open door to the upstairs and the doors of the basement rooms are not sealed you likely have an effective warm air return. What you may lack is a separate zone for the lower level. Though most basements have lower heat loads than the upper floors, they usually do not have solar gain, shutting down the furnace for long periods of time and idling off the heat to the basement as well. Warm air rises while cold air naturally fall to the lowest level.

I would look into a basement zone. It works for our customers and in my own house as well.

[TheKingfish]

Thanks to all for your feedback; very helpful. The basement is not insulated and half finished. Safety wise, there would not be a direct sight line from the furnace to the proposed cold air vent. Furnace combustion air is drawn from outside.

A basement heat zone is probably more than I want, given my residential frequency. There is an open stairway to the upstairs. I am leaning toward the proposed single floor-level return vent.
Thanks again,
TheKingfish

[beenthere]

The type of supply register will determine if your going to have a warm basement or not. If the supply just throws the air across teh ceiling and not downward, your basement will be cold.

[vstech]

how much of the basement is below grade?
with an open staircase, you would need to have nearly all the return air drawn from the lowest part of the basement to get decent warm air down there.

in cold climates, I'd invest in radiant floor heating for a basement...

[RoBoTeq]

The return will do best if closer to the floor. With the return close to the floor, the statafied warm air in the basement will be able to move toward the floor rather than be recirculated back to the furnace without warming the air closer to the floor.

It is off course best to get return air in each room, if possible. If there are rooms that are going to be closed off with a door that cannot have return ducting to them, a one inch undercut on the door (1" above flooring. If carpeted, 1" above the carpeting) or a by pass chase in a wall between the room and the common area where the return ducting is will help.

If going with a bypass chase, have a grill installed low in the wall in the closed room and high in the same stud chase of that wall in the common area where return ducting is. Make sure the wall cavity is free of obstructions such as cross braciing.

[jtrammel]

Unless its an extremely small room then a one inch undercut is not sufficient. 1" x 36" = 36sq in. It's usuall more like a 6" undercut which is not feasible. Bypass chase or "jumper" duct is better if you want any privacy

[RoBoTeq]

Unless its an extremely small room then a one inch undercut is not sufficient. 1" x 36" = 36sq in. It's usuall more like a 6" undercut which is not feasible. Bypass chase or "jumper" duct is better if you want any privacy

This is not at all accurate. A one inch undercut of door is usually equivalent to a 6-7 inch round opening, which is more than enough free area for the air being pushed into a room to be allowed out of the room, especially when the area outside of the room is at a lower pressure due to their being a return grill there.

[jtrammel]

Jumper ducts have to be Sized correctly. Much larger than a regular return because you don't have the force of the blower to create pressure

[RoBoTeq]

Jumper ducts have to be Sized correctly. Much larger than a regular return because you don't have the force of the blower to create pressure

The force that pushes air through a jumper duct is based on the amount of air entering the room and the lowered pressure of the air outside of the room. You don't need an open door to have air transfer from a room.

[jtrammel]

The force that pushes air through a jumper duct is based on the amount of air entering the room and the lowered pressure of the air outside of the room. You don't need an open door to have air transfer from a room.

I understand that but the force of pos pressure on a room caused by having supply air with no return is not near as great as the force of having a return that is ducted to the equipment which has the force of the fan to pull the air, therefore if you have a 6" supply forcing air in then you need a larger return than one that would pull ~100 cfm if connected to the return side of equipment via ductwork.

[RoBoTeq]

I understand that but the force of pos pressure on a room caused by having supply air with no return is not near as great as the force of having a return that is ducted to the equipment which has the force of the fan to pull the air, therefore if you have a 6" supply forcing air in then you need a larger return than one that would pull ~100 cfm if connected to the return side of equipment via ductwork.

Not true. For residential, and most commercial applications, it is the pressure differential that forces air from one point to another. HVAC system air is only pushed by the blower on the supply side and not pulled from the return side. That is why we refer to the negative side of the system as "return" air and not "suction" air.

As long as there is an opening between the area being pressurized by the HVAC system and the area being depressurized by that same system, air from the positive side is going to travel through that opening to the negative side. Just close a door to a room with no return air ducting, turn on the HVAC blower and feel around any openings in the door to feel just how much air will push through those openings and at what rate.

[jtrammel]

A 1" gap on a standard 30" wide door will allow approx 47cfm to pass through.

[RoBoTeq]

A 1" gap on a standard 30" wide door will allow approx 47cfm to pass through.

At what SP? What is the SP of 47 cfm of air going through a 1" door?

You have been led to some false beliefs.

[jtrammel]

If there is not an easy enough pathway for the return air to get back to the filter grille it will cause that room to bottle up the excess supply air causing pos pressure in that room and will push the air out through any hole in the thermal envelope because there is not adaquet space for the air to flow under and around the door. It also causes the other side of door to be neg and pull in air from any hole in the thermal envelope

[RoBoTeq]

If there is not an easy enough pathway for the return air to get back to the filter grille it will cause that room to bottle up the excess supply air causing pos pressure in that room and will push the air out through any hole in the thermal envelope because there is not adaquet space for the air to flow under and around the door. It also causes the other side of door to be neg and pull in air from any hole in the thermal envelope

Not necessarily so. If there is no way for supply air to exit a room, the supply air will simply stop flowing into that room. Only if there are severe leaks in the room going to the outside ambient air wil the conditions you describe occur. Obviously, if there are major leaks to the outside of the house and not enough controlled leakage from the area to the return path, some things need to be addressed.

[jtrammel]

This explains how these scientists and engineers tested it and what they found. I had this problem at my own house that i rented at one time. The doors were undercut but there was a noticable pressure difference when door was closed. I talked my landlord into paying me to onstall a ducted return to the master bedroom and solved the issue. Since then my company has invested in blower door, duct blaster, flow hood, ir camera etc and have found many many master bedrooms like that. If its not feasible to run a ducted return we will jumper duct with flex to deaden sound transmission and if you dont oversize the duct then you still have a pressure difference from the rooms. I by all means dont know everything about these types of problems but i do attempt to solve them all the time.http://www.bestofbuildingscience.com...3-4_p42-45.pdf

[RoBoTeq]

This explains how these scientists and engineers tested it and what they found. I had this problem at my own house that i rented at one time. The doors were undercut but there was a noticable pressure difference when door was closed. I talked my landlord into paying me to onstall a ducted return to the master bedroom and solved the issue. Since then my company has invested in blower door, duct blaster, flow hood, ir camera etc and have found many many master bedrooms like that. If its not feasible to run a ducted return we will jumper duct with flex to deaden sound transmission and if you dont oversize the duct then you still have a pressure difference from the rooms. I by all means dont know everything about these types of problems but i do attempt to solve them all the time.http://www.bestofbuildingscience.com...3-4_p42-45.pdf

Well, you are certainly over thinking this one and making a proverbial mountain out of a mole hill. Our industry has been doing very well with undercutting doors and installing jumper chambers without having to spend a lot of time and money for such testing unless situations call for it. Personally, I have not run into a residential system that cannot be resolved without having to bust the bank.