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Posted by Zyp on January 1, 2008, 7:43 pm
HVAC Guy wrote:
> 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
> ---------------------
PITA;
HVAC Gay Guy, read the book again. And by the way, Whittman and Johnson is
a good book, but not the ONLY book. [I happend to own all 5 editions.] It
is the opinon of author's. And, attend some HVAC classes either from your
local supplier, or your local community college.
And why are you trying to re-engineer somthing the manufacturer's already
have. Do you think for one minute the reason they went to the inshot
burner [not really new - it was introduced in the 1960's once before in a
furance called the "stubby"] was because of positioning? The inshot came
out of a need to lean out the mix. More efficient. And provided less
technician / home owner tuning. [Keep YOUR fingers out of the burner.] It
does create a side benifit for multipoise furances though and the wholesaler
can now stock one furance instead of four different styles. But the fact
remains, the inshot burner is more efficient, creates less nitrousoxides,
and less maintenance. It's cheaper to prouduce as well. It came on-board
in the late 1970's to early 1980's a the burner of choice. After the
furance recalls, all manufactuere's went to it.
The newest mid-efficiency furances now have a stack temperature of 270º -
300º [much lower than what I explained earlier.] And yes you moron, the dew
point of condensation is considerably less, and yes you MUST use a metal
double wall flu to remove the condensation. [It deters the formation of
water.]
The whole point here, the induced draft is requied. The inducer is meant to
[just pull the fumes through the firebox.] The heat exchanger is too
restricted to create a natural draft. It's [the inducer] not to create a
positive pressure on the flue vent. Otherwise you couldn't join more than
one fuel burning appliance to one chimney flue. Read your GAMA guides.
The newer furnaces are more efficient in fossil fuel useage, more efficient
in their electrical usage, have less maintenace items, and provide the
buying public with a safer product. Have a better warranty and a longer
warranty. Much safer than the furnace of yesturyear.
So get off your trip that the old heaters are better. And while you're at
it, go pound sand.
--
Zyp
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