B-vent backdraft

11 years ago

As I've been replacing windows and generally sealing my mid-'80s house better, I've wondered whether or not I might have backdraft issues with the b-vent - mid-efficiency furnace, conventional hot water heater.

Replaced my kitchen microwave/exhaust fan (recirculating) with new one, that I vented outside.

I did a flame test next to the flue hat on the HW heater - with kitchen fan, and two bathroom fans running, air was being drawn back down the B-vent. I've had a CO detector go off in the past during high winds (the b-vent cap seems to be decently high off the roof - not sure what the rule is for it here in British Columbia.)

That also leaves the clothes dryer and central vac, which is vented outside, so I have a real potential problem. In the meantime I know we can't be running more than one fan or appliance at a time.

I am figuring on running a make-up air vent into the mechanical room - they're quite common here. A screened vent like the ones that bring fresh air to the cold air return, with a pipe, either rigid or flex, insulated, that runs to the floor with the basket/mesh end. I'm an alarm tech so I'm pretty adept at fishing wire and working in tight spaces, so I should have no problem feeding this new pipe from the outside in, where I have good access.

Best plan? I would love to eventually put in an HRV unit, but realise they're not intended to provide make-up air.

Eventually, the furnace will be replaced with an high-efficiency one, but I don't intend to replace the hw heater any time soon, it's new and I like the idea of hot water during power outages, as opposed to a power vent unit.

Any other suggestions?

Comments (5)

kalining
11 years ago
You have to have make up air vented into the home by code.
Your gas installer will know what size and how many. In
manitoba we have to have two 5 inch insulated pipes or flex pipes. One terminates at the furnaces dilution air at the flue and the other one terminates at the side of the
compustion chamber.
energy_rater_la
11 years ago
I've been waiting for a project to call for
combustion air, since finding a product called
Fan in a Can.
here is a link to an online site explaining usage
of this product.
I find that at my hvac supply house, that this
is an affordable product. not so much online.

http://www.fieldcontrols.com/cas.php

hope this helps.

best of luck.
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alan_s_thefirst
Original Author
11 years ago
I was trying to figure out the deal with the powered vent. I'm not sure if it is what I need - I have a small mechanical room, but the doors are louvred, and quite a few square feet.

The forced air would ensure there would be no backdraft, but I'm thinking there'd be a lot of heat loss.
jackfre
11 years ago
You need 50 cu ft/1000btu of input capacity. otherwise you need supplemental CA. The fan in a can is one unit. I prefer the Tjernlund CA Enforcer.

Currently your furnace and water heater are "common vented" into your b-vent stack. Be aware that when you replace your 80%...or so furnace with a hi eff 90+% you will no longer be venting the furnace with b vent. The venting tables in the nat'l Fuel Gas Code will detail this, but I think you will very likely need to re-line the existing flue for your water heater as you will be grossly oversized and the water heater will not operate properly without it.
alan_s_thefirst
Original Author
11 years ago
Because of the condensation? I know it needs to be lined a) because of corrosion issues due to the decreased flow and more condensation and b) it requires a smaller diameter because of it.

When my neighbours changed out their furnace, their HVAC guy ran flexible aluminium duct (I assume it was aluminium, could have been stainless.) I figured on doing the same.