Power Vented System Question

[Terence McCann]

Hello all ( you too George [])

Is a furnace with a ventor motor & open burners considered a forced draft system? The burners jet into a tubular style of heat exchanger where you can see the flame (open to atmosphere so to speak). No draft diverter is noted.

Reason for the question: In this neck of the woods I see a lot of this style furnace tied in with a "el natural" draft hot water tank with a wye "B" vent style fitting going to a common flue. I've never noted a back-draft condition on the hot water tank.

Thoughts?

Tanks.

[dlee]

Terence, from your description the furnace sounds like a mid-efficiency unit, with draft that is probably best described as "induced." These need to be "fan-assisted" because they will not draft on their own steam, so to speak. Typically the flue gases are still under negative pressure with respect to the room air, though, so that the WH draft generally is not impeded when it meets the furnace draft. This is especially true if the installer has been smart enough to use a "Water Heater Wye" for the vent union. When T's are used the WH often will backdraft for a while before getting up to speed with the furnace draft. Even a wye union will result in WH backdrafting if the WH is running, and then the furnace fan comes on. Until the furnaces lights up, its start-up draft is cool ikn temp and will usually stop the WH draft dead in its tracks. I call that out if it lasts more than a minute. Usually doesn't.

This may be more than you asked for, or maybe even none of what you asked for. You just pushed my Answer Button.

-David Lee

[Terence McCann]

Thanks David:

One step further - code?

How would Brian put it: I'm a pushin'

[DLRambo]

Its totally common, proper and up to code to vent an "inducer draft" furnace together with a natural draft appliance into the same chimney, flue, etc. IT IS NOT CORRECT to vent a "forced draft" furnace with a "natural draft" appliance together.

Dan Bowers

[a46geo]

Originally posted by DLRambo

Its totally common, proper and up to code to vent an "inducer draft" furnace together with a natural draft appliance into the same chimney, flue, etc. IT IS NOT CORRECT to vent a "forced draft" furnace with a "natural draft" appliance together.

Dan Bowers

In fact, if you do NOT vent this way, the chimney would most probably be too large for just the hot water tank. In such a case, the chimney would need to be relined

George

[Terence McCann]

Thanks all.

[Chad Fabry]

Originally posted by DLRambo

Its totally common, proper and up to code to vent an "inducer draft" furnace together with a natural draft appliance into the same chimney, flue, etc. IT IS NOT CORRECT to vent a "forced draft" furnace with a "natural draft" appliance together.

Dan Bowers

I'm not sure why the above scenario wouldn't work, and I'd like a lesson.

In my mind, the air from the forced draft rushing past the wye connection at the water heater would create a venturi effect, drawing either air (if the water heater is off ) or exhaust gas from the heater. The way I'm picturing the situation, it'd actually enhance draft at the water heater. The only way this wouldn't work, is if there was so much resistance down stream of the water heater that it was easier for the furnace exhaust to go backwards through the wye and out the draft hood at the water heater.

[Brian G]

This isn't my best subject, but I'm gonna take a shot just for the heck of it. I think the problem might be a potential "air curtain" effect, where fast moving air effectively blocks much slower moving air.

Brian G.

Stepping Out on a Limb [:-sick]

[Jim Katen]

Originally posted by Chad Fabry

Originally posted by DLRambo

Its totally common, proper and up to code to vent an "inducer draft" furnace together with a natural draft appliance into the same chimney, flue, etc. IT IS NOT CORRECT to vent a "forced draft" furnace with a "natural draft" appliance together.

Dan Bowers

I'm not sure why the above scenario wouldn't work, and I'd like a lesson.

In my mind, the air from the forced draft rushing past the wye connection at the water heater would create a venturi effect, drawing either air (if the water heater is off ) or exhaust gas from the heater. The way I'm picturing the situation, it'd actually enhance draft at the water heater. The only way this wouldn't work, is if there was so much resistance down stream of the water heater that it was easier for the furnace exhaust to go backwards through the wye and out the draft hood at the water heater.

Forced draft equiment has positive pressure in the flue. Natural and "induced draft" equipment have negative pressure in the flue.

Dan's comments are correct. I'd only add that it's *best* to vent each appliance separately, even when code allows it to be done otherwise. They all work better that way.

- Jim Katen, Oregon

[kurt]

Would it be any more clear to describe the appliances as Category 1, Category 2, etc..?

Having an induced draft furnace & a water heater w/ an ell @ the B vent junction works fine; I look @ approx. 250 of these a year, & have NEVER noted any backdrafting @ the water heater, even in icy cold conditions where cold air curtain could have an effect.

The critical issue in this arrangement is adequate combustion & dilution air; if the mechanical room is an enclosed space, it could be a problem. If there is adequate air supply, it is "never" a problem, i.e., hot air goes up.