Is cold air return in basement needed?
myriad
| Posted in Energy, Heating & Insulation on
Finally getting around to finishing my roughly 1000 SF basement. HVAC guys put 4 hot air vents in ceiling. Have moved them into the stud walls ready for drywalling. Question is, should I include a cold air return? I can place one in one of the walls close to where the furnace is with out too much trouble. What do you think?
Replies
This is a question of basic science, two items can't occupy the same place at the same time. The items being the air that's in the room and the incoming air. New air will only enter the room as fast as the old air exits.
They may be counting on the new air pushing the old air out which will eventually find a return intake. This counts on open doors, door gaps, etc. - of just a wide open floor plan.
A closed room will have more predictable results if the air is returned via duct.
Jeff, is correct but a little more thought needs to apply here.. In that heat rises and thus the floor of a basement tends to be a very cool area. If there is a cold air return on the floor, the warm air that would normally tend to collect near the ceiling will be sucked down into the furnace..
To make this work the heat vents need to be on the opposite side of the room. if they are near the cold air return that's all that will happen is you cycle warm air into the cold air return..
You can't put a return two close to the funace; 10 feet in the rule of thumb, but it depends on the individual house. Otherwise, you could cause back drafting.
Basements are usually negative pressure zones so you usually don't need to worry about losing air supply through over-pressurization, and putting a supply register can actuyally help system performance, esp the draft in the flue.
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Sojourners: Christians for Justice and Peace
Whats back drafting?
When ever cold air returns are installed in a basement 'back drafting ' can occur.
The cold air return draws air into the furnace and can draw carbon monoxide into the basement from the chimeny.
George
So there seems to be 2 schools of thought here then. Some say yes some say no. I have radiant heat in the slab so I will probably only use the ducts for AC or recirculating the air anyway.
Back drafting is where the flue gases get pulled back into the house.As someone noted they can contain carbon monoxide which is a potentially deadly poison.Note that having a return too close to the furnace won't always cause backdrafting: less than 1/2 the time in tests I've done has backdrafting occurred. (In my tests I take the blower compartment cover off during operation while I measure the draft in the flue.)To me, that makes the risk of backdrafting too high for having a return near the furnace.
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Sojourners: Christians for Justice and Peace
so myriad needs to ask is his HVAC if the design was for backdraft prevention - would the returns be too close to avoid a problem
>>so myriad needs to ask is his HVAC if the design was for backdraft prevention - would the returns be too close to avoid a problemSince they put the registers in the ceiling, it sounds like the only design criteria were cost and ease of installation.
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Sojourners: Christians for Justice and Peace
?if the supply & return in the basement are balanced and given good flue draw then seems like you shouldn't have backdrafting?
Its also location of the registers. A return close toi the furnace might not be balanced by a supply register on the other side.
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Sojourners: Christians for Justice and Peace
Is it possible to have backdrafting in a 94% efficient furnace? Mine has "dedicated" intake and exhaust lines (via PVC pipe to the outside of the house--not through the chimney).Just curious.
I'm not sure, but I don't think so
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Sojourners: Christians for Justice and Peace
Backdrafting occurs, as Bob described pereviously, in a gravity vented flue system, and is caused when the pressure difference that causes gravity venting (due to the stack height and density differences between flue gasses and outside air) is overcome by a greater pressure differential. Worst case would be when a relative negative pressure exists in the area when a draft hood is open to an occupied space and flue products are literally sucked into the house as opposed to being sucked out.
Is it possible with a sealed combustion system like you describe? No. I'll add the caveat that this applies in the absence of any serious issues that would prevent the system from operating properly or gross installation errors. Since gravity and inside air are not used in this case, rarely if ever is backdrafting possible. If, a situation exists where the draft inducer cannot develop enough pressure/flow to operate properly, the unit will not allow the gas valve to open and will not fire.
Backdrafting of what? If the furnace happens to be a high efficiency one with outdoor intake, how would it backdraft? Are you referring to a commonly nearby gas water heater with a standard, open flue? He was writing about return registers; he already has the supply registers, which would certainly not contribute to a backdraft. I'm confused by your response.
Thanks to everyone for their informative contributions! Just for the record, I have a high-efficiency furnace and no HW heater (Takaki tankless in the mud room closet on the 1st floor). I would have to locate aforementioned return in a wall dividing a hallway with the furnace room. I am getting the drift that backdrafting is not going to be a problem. I have attached a 'pdf' file showing a possible location.
>>Backdrafting of what? If the furnace happens to be a high efficiency one with outdoor intake, how would it backdraft? Are you referring to a commonly nearby gas water heater with a standard, open flue? He was writing about return registers; he already has the supply registers, which would certainly not contribute to a backdraft. I'm confused by your response.He didn't specify what sort of furnace, natural draft, 80+ or 90+.The first 2 can backdraft if the air pressure around the furnace gets low (and the amount of positive draft in a natural draft and 80+ furnace furnace isn't much - typically .01 - .02 water column inches. About what it takes to blow out a birthday candle from a few inches away.)A return register is not allowed too close to a furnace - that's why they have disconnects now on the blower compartment door.Yes, having supply registers in the space might prevent depressurization from a return, but might really isn't good enough, don't you think?
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Sojourners: Christians for Justice and Peace
Bob, put I put a whole new heating system in my house, including new ductwork, this fall/winter and I had questions about this issue also.I asked the tinners on the jobsite and looked at new homes in the local Parade of Homes to see what they did.The sheet metal guys said that they usually just cut in a dampered grill right into the return boot.Some of the Parade homes were done like that and a couple of them had no basement return air at all.
I have a 94% efficient two pipe system so I don't see backdrafting of flue gases as a problem.I also don't see the return boot/return air grill as ideal either because it seems to me the furnace would draw more air from it,as a least resistive path,than from the return ductwork serving the upstairs.I sized a duct for the amount of supply air coming into the basement,and ran it from the main return trunk down to the floor near the middle of the basement.Just like upstairs,supplies are near the outside walls,and returns to the inside.
I had to learn a bunch from the HVAC-talk site to do my project but never saw this topic addressed.What a petty bunch they are over there though.
Barry
Yeah, with a closed system it isn't an issue
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Sojourners: Christians for Justice and Peace
I used to work at a HVAC supply house. When I built my office one of the guys there told me to install a cold air return near the floor. I have had no heating or cooling problems.
If you are closing off rooms I would put returns in the rooms.
I finished our basement with a cold air return located near the floor. It's much better to place it low to suck out the cold stratified air. It is close to the furnace and actually on an adjacent wall. I also included a fresh air return with motorized damper to bring in outside air. See buildingscience.com. This setup will pressurize the house (slightly) and prevent backdrafting. Heating appliances should be sealed, powered vent, or have dedicated combustion air as well. Good luck. Joe
For years I design HVAC system and provided some consulting/troubleshooting on systems that didn't work well.
I only put returns where I want to have consistent and controlled air movement.
I have returns in my basement, some high, some low. If the system is balanced there will be no backdrafting concerns. IF you have gravity vented fuel burning appliances you have to be sure of what you're doing so as to not create a negatively pressured space.