 Re: It's said that induction fans (draft inducers) increase
Don Ocean wrote:
>
> Not an economically wise move..
I've already done it on a double-unit (installed 1981) in a small
commercial building (6000 sq ft). Round, flexible plastic and wire
dryer vent ducting (looks like an accordion) is about $10 for 20
feet. 4 lines (two for the primary combustion air and 2 for the draft
hoods) run directly to a duct that vents outside. Some sheetmetal
work, a few dozen rivets, and some metalized duct tape to seal the
cabinet. The unit is now closed combustion. The burners were turned
down to about 1/3 of full output, the primary air shutters were also
turned down to about 1/3 open to create slow flames with no red tips.
The furnace has been running like that since the start of the heating
season (roughly the past 2 months).
I probably spent $25 to make those mods.
> Just upgrade the furnace (...)
> Much more reliable.
Reliable? The current unit is as reliable as I'd want it to be.
> Keep in mind most gas water heaters are 45,0000 to 60,000 BTUH
> and also require combustion air.
See above.
> Induced air creates a hotter fire..
Because induced air is used for furnaces with inshot burners - which
must be force-fed with primary combustion air.
I doubt that they're much more efficient at combustion vs old, slotted
atmospheric burners:
--------------------
"Residential furnaces often do not provide combustion air adjustment
provisions because manufacturers have determined that the safety
gained by providing additional air to assure complete combustion
outweighs any potential savings that adjustment might provide."
http://www.testo.us/online/embedded...ustionGuide.pdf
-------------------
Basically, it's not a question of having 100% combustion (or "complete
combustion" as worded above).
For older furnaces, they've probably been setup such that they are
supplied with way more air than they need for 100% combustion. The
theory is that the more air that goes in, the better (faster) the
combustion draft will be and the less of a chance that combustion
products will enter the home. Hence the reason why stack temps are so
high, and primary air shutters are always turned full open. And again
we're back to the desire of preventing chimney condensation too. So
the flames are fast, flame residency time in the heat exchanger is
short, but 20+ years ago NG was cheap.
>
> Now we are talking all metal pipe
The supply ducting I'm talking about doesn't have to be metal.
> ..Which in itself is a radiator and will lose heat
We're talking about ducting outside air directly to the furnace
cabinet to supply the furnace with combustion air and draft-hood air.
The outside air will be cold, so naturally there will be heat to be
gained from it.
> Also the way the old furnace is built.. the heat exchanger
> will cool faster
I don't see how. Sure, the combustion air is all coming from the
outside, but it's not power-drafted, so the amount of air being pulled
into the burners is an absolute minimum that can be maintained by
natural draft convection.
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