Real Estate Forum

I haven't seen any posts in these groups in months, so I might just be
talking to myself here, but here goes.

Many articles on furnaces and furnace efficiency seem to link high(er)
efficiency with draft inducers.  In other words, a draft inducer
(draft induction fan) helps or adds to the efficiency of a furnace.

I don't see how a small blower motor that runs for a few minutes at
the start of a heating cycle is going to help increase the extraction
of thermal energy in the heat exchanger.

I can understand that the function of the draft inducer is to start
the evacuation flow of combustion gasses up the cold flue until
convective flow can be maintained on it's own.  I can't really see the
dire need for such a fan, but I guess someone thought it was a good
idea.

So what's the logic behind the draft inducer as it pertains to
increasing the efficiency of a natural gas furnace?

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Full-Quoter Zyp wrote:

> The answer lies in the flue gas temperature.
>
> On older natural draft furnaces, the flue gas temperature is
> about 450º or more.  To increase the effective efficency of
> the furance, you would have to reduce the flue gas temperature

I would expect that either a more efficient furnace means

1) a lower flue gas temperature (350º in your example)

or

2) the same (high) temperature, but less of it (you don't burn
as much NG, but the NG you do burn exits the flue at the same
temp (450º in your example)

In either case (350 or 450) that temperature is high enough to start
and maintain a natural draft - especially if the furnace has a
dedicated outside combustion air supply.

> The manufacturer's reduced internal spacing of the heat
> exchanger's to increase the transfer of flue gas heat to
> the conditioned air

Doesn't matter how they increased the heat exchanger's efficiency
(reduced spacing, thinner walls, etc) as long as they also didn't
increase the air resistance of the exchanger.

> The flue gas termperature of the induced draft furnace is
> now 350º or there abouts.  This heat exchanger space reduction
> doesn't allow for "natural draft" and hence the need for
> "induced draft."

You are equating a reduced flue temperature with reduced heat
exchanger "space".  And I'm not sure which side of the exchanger
you're saying is reduced - the combustion side or the household side.

If there is flow reduction within the exchanger then there will be a
heat build-up resulting in an INCREASE in flue gas temperature.

> There is a trade off for the cost of the inducer verus the
> increased heat transfer.

I think your explanation is full of shit.

Yes, I think heat exchangers toady (and probably up to 15 years ago)
have thinner walls and were closer-spaced vs heat exchangers 20+ years
ago.  But I bet the newer exchangers had more passages and maintained
the same cross-sectional flow area so they had the same air-flow
resistance as the older exchangers.

I'm talking about furnaces with only 1 heat exchanger (non-condensing)
and comparing those with and without a draft inducer.

So the furnaces with more efficient (but less durable or reduced
longevity) heat exchangers have lower flue gas temperatures, but the
question is -> has the flue gas temp. been reduced so much that it
can't start and maintain a natural draft?

> Also, the induction of the induced draft decreased
> the need for "dilution air" and less secondary air.

Or was the removal of the passive draft air intake a result of making
the furnace more efficient (overall) by not scavenging warm household
air and flushing it up the stack?

The presence of the passive draft air intake right below the flue is a
problem for a furnace with a draft inducer.  Where do you place the
draft fan?  Above, or below, the draft air intake?  If you put it
below the secondary intake, then how do you insure that the draft fan
won't blow combustion gas into the house through the draft intake?  If
you put the fan above the draft intake, then how do you insure that
the fan will pull combustion gas from the heat exchanger and NOT pull
household air from the open draft intake?

So if you've got a draft inducer, you've got to remove the passive
draft air intake.  I can't see how you can have both.

> For the older furnaces approximately 30 cu. foot of flue gas
> per one cubic foot of fuel burned.   The newer furnaces only
> create / need 13 - 15 cu. foot of flue gas for the
> same cubic foot of fuel burned.  No dilution air needed.

There is no reason why the older furnaces needed to be supplied with
twice as much air per CF of fuel.  If they did, how was it done?  Was
it forced into the burners under pressure?  Was it because someone
thought it was a good idea for them to have a passive draft air
intake?  What really happens when you close off the passive draft
intake on an old furnace?

> A new problem though also came with the reduction of the heat
> exchanger sizing.  The manufacturer had to increase the airflow.
> This was to decrease the heat exchanger temperture and prolong
> it's service life.

I'm not familiar with how heat exchangers have "shrunk" over the
years.  Presumably if you want an efficient heat exchanger, you have
to increase the surface area and make the walls thinner.  If you have
a fixed over-all size to work with, then the only way to increase
surface area is to have more chambers or pathways but put them closer
together.  If the walls are thinner, then you can place them closer
and not increase the air-flow resistance.

> So the older natural draft furnaces had a higher temperture
> rise than the newer induced draft furnaces.

Temperature rise is a function of how much NG a given furnace burns
per unit time vs another furnace.  You can mess with blower fan speed
and make one furnace appear to have a lower temp. rise vs another one.

My theory is that older furnaces had their gas valves set on full max
and had their primary baffles set wide open so they'd be able to throw
out as much heat as was being lost by homes 20, 30+ years ago.  When
you had mechanical thermostats, you want a furnace that can blast the
heat out because of the hysteresis effect of the old thermostats.
Today, you have furnaces that aren't blasting the heat out because
houses are better insulated, and electronic thermostats are better at
controlling the temp within a narrower band vs the older stats.

> I hope this helps you understand why the induced draft
> blower on the newer 80% furnaces is needed.

Seems that induced draft is needed more because of the use of inshot
(mono-port) burners and not really needed for multi-port (slotted)
burners with adjustable primary air shutters.  That's probably the
basic reason why there is a draft motor on all newer mid-efficiency
furnaces.

> One other note is the understanding of the inshot burners and
> how that increased the effective efficiency of today's mid-
> efficiency furances.

It's not clear why inshot burners are more efficient.  They have a
smaller flame size and flame area when compared to a long, ribbon or
slotted burner.  The long slotted burners would seem to be able to
more efficiently (or evenly) heat the interior of the exchanger galley
compared to the inshot burners.

Seems that a basic reason for the use of inshot burners is because of
the desire to place the burners at the top of modern mid-efficiency
furnaces instead of the bottom.  You probably can't place a slotted
burner at the top of the heat exchanger.  As to why you want the
burners at the top, seems it's to help drain the condensate by gravity
(although I don't understand exactly how):

--------------------
"The combustion gases soon reach dew point, and liquid condensate
forms and drains by gravity to a condensate sump or drain trap.  The
burners are located on top to allow the draining of condensate by
gravity".

Page 694

Refrigeration & Air Conditioning Technology By William C. Whitman,
William M. Johnson, John Tomczyk
Published 2005
ISBN 1401837654
---------------------

[  Post Follow-up to this message ]

Full-Quoter who is incapable of proper editing Abby Normal wrote:
 
>
> you must be pulling my chain here.

Nope.  You just have to read and understand what I'm saying.

> google draft hood, dilution air,heat recovery ventilator

And what would that do?

A draft hood is essentially a hole in bottom of the flue.  The hole
allows warm, household air to be pulled into the flue and out the
chimney.  I question the necessity of the draft hood.  If there's an
air imbalance in the house (or some return air vents are blocked) then
combustion products can flow back or be sucked from the flue and into
the house through that hole.  When the draft hood is working, it's
pulling warm household air into the flue.  Now why you want to COOL
the flue gasses as they make their way out the chimney is just
stupid.  You really don't want the flue gas to cool because you want
the flue gasses to exit the chimney before it condenses.  By pulling
household air into the flue from the draft hood, you are cooling the
flue gasses (because even warm household air is a lot colder than the
combustion gas).

Dilution air is just another term for the warm household air that gets
pulled into the draft hood.  It could also mean outside air that is
added to indoor air, but the proper term for that would be "makeup
air"  - but that's not part of this discussion.

Heat recovery ventilator is completely unrelated to what I'm talking
about.

The HRV is a useless piece of shit.  It's fans consume energy, it's
filters and drains require maintainence, and it will no doubt require
repair from time to time.  It's simpler and more cost-effective to
simply have the furnace pull some small amount of return air directly
from the outside if you need some air turn-over in the house.  In a
normal house, you've got a close dryer and a kitchen fan and maybe one
or two bathroom fans that are pushing household air out of the house,
so there will be some natural air leakage into the house to make up
for that anyways.

----------------
The really stupid thing about the HRV is that in the spring and fall,
when the outdoor air is cool (particularly at night) but the house is
warm, what you really want to do is to connect your furnace air return
directly to an outside intake, and basically fill your house with
outdoor air (instead of running the A/C).  In that situation the HRV
works against you - you don't want to pre-heat the outside intake air
with exhausted indoor air.  How many modern HVAC systems are ducted
and gated to allow the homeowner to cool their house by filling it
directly from an outside air intake (and dumping the return air
directly to an outside exhaust vent) ?
----------------

Again, what is wrong with taking a 25+ year old furnace (a furnace
with a draft hood, open burners (not inshot) and running outside air
directly to the cabinet (to the primary air intake grill and also to
the draft hood intake) so that primary air and draft air come directly
from outside the house?

In that situation, combustion gas has NO way to enter the household
air, no matter how tightly the house is sealed, and warm household air
can't leave the house via the furnace.  And it requires no motors,
gates or dampers or control electronics.  Just some flexible ducting.

[  Post Follow-up to this message ]

Abby Normal wrote:

> Essentially you need to keep googling and figure out the
> drafthood and the dilution air part.

Bubba wrote:

> I suggest you read yourself until you figure it out.

An indicator of those that are unsure of their own knowledge is that
they simply respond by saying "I know the answer but I won't tell you
so instead you should keep looking and reading".

As in the above 2 examples.

Who else wants to highlight their own uncertainty by saying the same
thing?

[  Post Follow-up to this message ]

Full-quoter Zyp wrote:

> He [Don] and everyone else has a right to tell you [HVAC Gay Guy]
> to STFU.

And I have the right to tell you that STFU is not the correct answer.

> Now go pound sand.

I'll be doing that at the beach soon enough while you guys are trying
to figure out how to screw the home owner of your next service call.

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