Thread: Furnace exhaust fan

Something I haven't seen before. Natural draft water heater and induced draft furnace utilizing a shared B vent.

The fan on the roof runs constantly. The controls in the utility room shows the amount of draw and the button on the right is utilized for the light only. My comments to the buyer is the appliances should not have to rely on a mechanical vent fan for correct venting and to have it checked out by an HVAC tech.

Speaking with an agent after the fact I found these were commonly installed as part of the MAC (Metropolitan Airports Commission) as part of their upgrades to homes in this area. I've inspected several hundred MAC upgraded homes and have never seen this.

Similar Threads:

• Condensate leak at furnace exhaust
• new construction - exhaust vent quest.
• White "powder" at furnace exhaust tube
• bath fans, exhaust to exterior
• That dread Exhaust Vent - Texas gang

There a after-market power venters out there. I have never seen one like this. If you captured the name of it's manufacturer, we could work from there.
Most of the ones we use here in the Southeast US are made to vent out horizontally. Check out their very well done website:
Power Venting | Oil and Gas Applications | Field Controls

I've always been taught that a power vent and gravity vent cannot share the same flue.
Did I miss something?

1) Don't accept anything an agent tells you.

2) They may very well be adding these vents to all the houses because

3) The power-vented furnaces are adversely affecting the natural venting of the water heaters.

4) What happens when the gizmo eventually seizes up?

That is an Exhausto fan. To be compliant, the fan must be interlocked to the gas controls of any attached appliances. If there is a natural draft water heater attached, it is unacceptable for a number of reasons. First, how would you interlock the gas control? Second, you would need a double acting barometric damper with a spill switch at each appliance so they cannot overdraft or backdraft. The fan would require a pressure proving switch, which Exhausto does make. The sequence is typically: on a call for heat, the exhaust fan is energized. Once the pressure switch senses the vent is working (based upon pressure in the vent termination), a switch closes allowing the gas control to begin its ignition sequence. If you lose power or lift the call for heat, the exhaust fan de-energizes which results in a drop in pressure so the proving switch opens causing the burner control to shutdown.

Now, how do you control the exhaust separately? Apparently you cannot since it runs constantly. So, in an area where the homes are being built like Tupperware for the Airport Sound Attenuation Zone, this fan is depressurizing the house 24/7 burning energy, pulling cold air in that will cycle the heater more often and could kill any other natural draft appliances in the home or pull CO in from the garage. Brilliant!

BTW, if these exhaust fans are used on solid fuel applications they require a battery powered alarm to notify occupants in the case of power loss.

You should check with the applianec mfrs to see what they say about this sort of venting and what min/ max draft is required. This might void the warranty and listing of either appliance.

Induced draft is still natural draft so it is allowed to be common vented. Power vented is positive vent pressure and it is never allowed to be common vented.

HTH

Originally Posted by Dana Bostick

I've always been taught that a power vent and gravity vent cannot share the same flue.
Did I miss something?

Both the induced draft furnace and the natural draft water heater are considered gravity or natural draft. They are allowed to share a vent.