Return air in basement?

Hope to get all the conditions clear on this.

Furnace in unfinished basement about 12 x 30 ft 8 ft height

Finished side of the basement is about the same size.

2 return air ducts in the finished side that is separated by a solid wood door.

No vents to exchange air between sides.

Supply vents in the unfinished side are sealed.

The return air ducts are less than 6 feet log. One vent is about 10 feet away and the other is about 15 feet away. both around the corner.

There is NO passthrough at the floor joists. It has been blocked.

The furnace is 80 percent efficient with induced draft.

My thoughts are that the combustion air source has been reduced by the refinishing and the blower may create a negative pressure that would cause combustion air around the burners to be drawn into the return air ducts.

I suggested a HVAC expert evaluate the situation.

There is no evidence of spillage on the 4 year old furnace.

any suggestions?

You do home energy tune-ups and may have a micromanometer or magnahelics gauge that could test the internal house pressures to determine if there is very high (or low, depending how you look at it) negative pressure in the furnace room.

With induced draft venting, the unit fan is strong enough to overcome any regular/normal house depressurization values. Juss make sure there is a CO detector in the area of the unit in case of fan failure…and in case the unit does not shut down upon a venting fan failure.

Some info:

You answered your own question 10 feet is the magic number


Why is 10 feet considered a “magic number” when negative pressure is the active ingredient in these situations? As older, loose houses are retrofitted for energy effiiciency, I feel this guideline or code requirement will have to change to reflect house pressures.

My thoughts are these guidelines or codes are set up so that in the event of equipment failure the conditions are such that it reduces the chance of a health issue in the structure. We all known things fail.

I would think that this current set up may be okay while everything is working but a failure in any number of places may put the occupants at an unnecessary risk. I did recommend mulitiple carbon monoxide detectors. I just hate to keep referring out to experts but in this case I provided the information to the client as to my thinking and that he may want to consult an expert in the field.

Thanks for the great info.

You are probably right codes change periodically as to different needs but until they do all we can do as professionals is just make recommendations.

If the furnace is in the unfinished side and the returns and supplys are in the finished side with a relatively tight separation between the two, the only concern would conbustion air. Whats the btu of the furnace and is there any vents to the exterior at the unfinished side of the basement.

An interesting thing happens when we install higher efficiency heating appliances…they need need much less air to operate efficiently and safely.

(1) since their is better burner design leading to better mixing of fuel and oxygen in the air, less excess air is required for complete combustion

(2) since a fan driven induced draft unit has a forced draft, there is no need for draft hoods/barometric dampers to keep pressures in the fire chamber at/near disgn values…therefore, this secondary air supply is not needed.

Thus usually there is enough leakage from the exterior or from other areas of the house. The house will bleed air into the family room through all the gaps created by wiring, plumbing and heating penetrations. Check the pressures in the room near the heating applaince to ensure they do not get severely negative.

I have seen passive supply air ducts into the mechanical room actually suck air out of the room (tested with smoke) when the exterior hood is in the lee of the wind…most of this side of house goes negative in strong wind gusts!!! The theory is to have supply air vents high and low to suply air but when they are both in negative pressures from wind conditions, they are both sucking air from the room.

As mentioned before, the WETT Assoc in Canada has a homeowner video of a fire in a zero clearance , high efficiency, airtight wood fireplace during which in strong wind gusts, smoke emitted from the combustion air wall supply hood in the lee of the wind while the fire burnt downwards through the firewood and was being supplied with combustion air from the chimney…a fire burning downwards…Chris Angel must be around!!! The single wall, uninsulated metal combustion air supply pipe was installed horizontally on wood drywall ceiling strapping/furring under the floor joists of the floor supporting the fireplace.

I find some of the standards committees setting the guidelines/codes do not have enough building science and field experience or field contact to properly move ahead to make rules that are up-to-date with research and new equipment. Up here it’s a bit of a honour/feather-in-the-cap to sit on these commiittees but they don’t necessarily bring enough to the table!!