I've encountered an install of 6" duct directly connected to the outside through the rim joist and covered by a simple mess screen on the end.  The other end has been placed next to the gas furnace as make-up air.  The cold air just pours in whether the furnace is active or not.  There is a clear need for make-up air as the CAZ is very small and the DHW and furnace are next within a couple of feet of each other.  Any advice on a simple mechanical option to better regulate this situation.  (The DHW is natural draft and the furnace is power-assist.  They are commonly vented.)   

Views: 265

Reply to This

Replies to This Discussion

Let it be known common venting of power draft and natural draft appliances is against code in many jurisdictions. Are the DWH and Furnace in conditioned or unconditioned space?

The units are in a conditioned laundry area, 16'X11'X7.5'  The DHW is 36K btu & furnace 45K btu. (Dryer is electric.)

Hi Jerry, your classic can of worms.

Each of us must decide where we will draw the line, that line where we can be blamed for making conditions worse and this is one.  Any improvements on air sealing could affect that CAZ, therefore, IMO, I advise the problems need to be fixed FIRST, before I will continue with an audit.  I usually catch these with a pre-audit visit, but that's my routine.

Here is some of what you face and I'm not a pro on HVAC as Maine prohibits auditors from doing anything beyond the specified CAZ test. 

1.  Venting as Bob mentioned needs to meet code.

2.  Replacement air also needs to meet code, such as vent area, high and low vents, and outside clearance.  A 6" duct is way too small.

3.  Any mechanical solutions need to be approved for that use, no shoe strings and cloths pins.

A common recommendation is a cold air trap with a bucket or a 180° turn in the incoming duct.  The short explanation is, they don't do as advertised and neither would bring the venting up to code.

Most burner units, and you have two, have combustion air kits that can draw air directly from the outside.  There are also powered dampers which open and close as needed and prevent the burner from coming on if they fail.  Whether either of these could be used to solve the problem and meet the codes would need to be determined.

Again, this is my opinion and perhaps others will chime in with some code requirements, beyond my scope.


Picture of uncontrolled make-up air duct next to furnace.



Glad they used insulated duct for the fresh air intake ;)

Have you considered sectioning off the water heater and furnace from the rest of the laundry area? Make the that unconditioned space. How about a vent between the laundry area and the rest of the house effectively increasing the size of the laundry room and eliminating the need for make up air?

Be sure to check local codes before making a final choice. As Bud said this may be a HVAC contractors issue, although I probably wouldn't call the one that stuck in the 6" flex for fresh air...


Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Robyn Smith replied to Chris Woods's discussion Looking to purchase Minneapolis Blower door system- Model 3 with DG-700 in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Hi Chris, I replied to you via email yesterday. I have what you are looking for!! Minneapolis…"
4 hours ago
Profile IconJohn Gillis, William C Gilbert, Brian Breckin and 4 more joined Home Energy Pros
6 hours ago
Paul Scheckel posted a blog post

Energy Ambassadors: Cuba and Vermont

Last winter i had the pleasure of hosting a Cuban colleague for a week in Vermont. We toured the…See More
8 hours ago
Linda Wigington commented on David Eggleton's group 1000 Home Challenge
"Two more projects have recently been approved as 1000 Home CHallenge candidates. We welcome Eric…"
Colin Genge posted events
Craig McManus replied to Timothy Renz's discussion Vinyl batt to metal frame ware house
"How can I air seal a metal building in the south to keep out humid air which is causing mold? Will…"
Tom White posted a video
Profile IconJeshurun and Chris Woods joined allen p tanner's group

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service