 Re: It's said that induction fans (draft inducers) increase
Bob Pietrangelo wrote:
> If you created a sealed combustion unit, I do not believe you
> have enough air for combustion. I am new at this so this bare
> with me. I believe you require a certain amount of air per
> 1000 btu's of furnace power. do you have enough coming into the
> furnace?
Remember the following:
1) this is a two-stage (or double-unit) furnace. I will dig-up the
model number and post it later. This unit was sized and installed in
1981 to heat only the upper floor of a small commercial office
building. The upper floor is about 3000 sq. ft.
2) The second unit rarely, or never, comes on during the day, and I
believe it only comes on when the thermostat calls for heat in the
morning when the setpoint changes from 63f to 71f.
3) I have turned down the burners on the first unit (the second unit
has a manifold gas valve that is not adjustable).
4) there is no barrier inside the furnace (from side-to-side) in the
burner galley area, so the 2 intake vent lines (4" diam) can
essentially feed both sets of burners (when they're both in use) or
feed air to only the first set of burners (when only one side of the
furnace is running).
What this means is that you need to consider a situation where 4
burners running at reduced gas pressure are drawing primary and
secondary combustion air from two 4" flex lines (combined area = 25 sq
in.) equivalent to a square duct 5" x 5".
> With 8 burners you probably have 160-200,000 BTU's of heat.
Like I said, I will post the furnace model number and see if there's
any info on the internet about it, like BTU output.
> Since this is a twinned furnace set-up are both blowers
> coming on a call for heat?
(I'm adding "?" to your quotes because you don't seem to be putting
them in yourself)
The furnace has only 1 motor (I think it's 3/4 hp) and it drives both
blower fans.
And like I said, the second furnace rarely comes on when I'm there to
watch. I would bet that it only comes on during the initial heat
ramp-up in the morning.
> I am assuming you have a common return.
Yes.
> What is your manifold pressure at the outlet of the gas
> valve.
No idea. I don't have a meter to measure this pressure, and I don't
particularly care to do it anyways.
> Why would you have a 12" stack for 200,000 btu's that seems a
> little bit oversized. Are their any other appliances connected
> to it.
In the furnace room right beside this double furnace is a single
furnace (whisper heat, electronic ignition, motorized damper gating
the primary air intake). This is for the lower level of the
building. It's flue is being fed into the 12" stack mentioned above.
That furnace is not in use this heating season.
> What is the internal diameter of the chimney which drafts
> your equipment.
I don't know.
> Judging by the adjustments you made it does not sound
> like a properley calibrated combustion system. It seems
> as though you have decreased efficiency. Cold dilution
> air on a conventional natural draft furnace does not
> appear to be right.
Prior to my modification, the furnace room was very cold in the winter
because it had essentally an open vent directly to the outside. By
closing that vent, and ducting air from it directly to the furnace
cabinet, the room is no longer cold.
And in it's original state, the draft air would have been cold anyways
because as I said the room was cold. Not only that, but bottom of the
furnace cabinet was raised about 1" above the floor, allowing the fan
to pull some amount of cold air directly from the furnace room.
Internally, the bottom of the furnace was not closed or sealed.
> Mr Storm would probably be able to give you better answers
> to your questions, I am just trying to understand your logic.
I don't see Mr. Storm's posts because he posts from Road Runner. The
news server I use automatically cancels posts from Road Runner because
a lot of spam and flood posting have come from Road Runner over the
past few months. By all accounts, Road Runner is an irresponsible
operator when it comes to usenet.
I also don't see posts from:
.p.jm@see_my_sig_for_address.com
nicksans...@ece.villanova.edu
because their posts are also being deleted by the news server I use
(I'm not sure of the reason why in those cases).
> I don't think you have provided any scientific answers
> to any of the true questions asked of you.
The "scientific" answer is that I am seeking to limit the amount of
cold-air infiltration into the building, and I'm achieving that by
ducting primary combustion and draft air directly to the furnace
cabinet thereby creating a closed combustion system similar to (I
assume) the design technique used by high efficiency furnaces today.
And by reducing the heat output of the burners, I am essentially
modulating the furnace so that I am capturing a higher percentage of
heat and increasing the furnace run-time duty-cycle (again I believe
the same techniques are used in modern high efficiency furnaces).
> What area of the country are you doing these alterations in.
The climate that I experience is similar to Chicago, Detroit,
Cleveland, Buffalo or Toronto.
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