Real Estate Forum

"HVAC Guy" <HVAC@Guy.com> wrote:
 

Roger wrote:
[color=#880000]
> You are right - in theory it shouldn't matter - BUT:  an efficient
> combustion is where you add exactly the amount of oxygen to the fuel
> (carbon and hydrogen, in whatever form it takes) to produce CO2 and
> H2O.

From what I know about most furnace burners, they take as much ambient
air as they need (there is no air-fuel mixing and injection such as
you would find in a car engine along with a mass air flow sensor) that
would insure that burning of NG in a furnace with such precision.

Prior to the 1980's, your average new NG furnace had an open burner
arrangement with simple circular air baffles, no vent motor (draft
inducer), and pulled combustion air directly from household air
surrounding the furnace cabinet.  It also had a secondary air intake
just below the flue to help draft the combustion products up and out
the chimney.

If you took such a furnace, and sealed the cabinet such that it draws
combustion air directly from a dedicated sealed duct directly from
outside the house (and you also ducted outside air into the secondary
air intake just below the flue) then you've probably just increased
the efficieny by 5%.  Since it was common to turn the burners full-up
on those furnaces, and to also fully open the primary combustion air
baffles, you could probably gain another 5% simply by closing the
primary burners to 25% opening and also turn down the burners to about
1/3 supply.  By reducing the primary baffles you are slowing the
flames (without making it too rich).  The heat exchanger captures more
heat (proportionately) if the flames are slow vs fast.  You are
slowing down the entire combustion process so there is more flame
residency time in the heat exchanger.  The furnace fan will run longer
(which it should, as most high efficiency furnaces today have longer
duty cycles as well).

The use of a draft inducer is really not necessary - I bet it's there
more as a safety mechanism (to insure that combustion products are
being evacuated at the start of a heating cycle) and in fact the air
being pulled in by the draft fan will tend to cool the heat exchanger
while the flames are trying to heat it.  What ever fractional gain in
flame temperature that might result from the draft fan would be lost
by the cooling effect of the cold air being force-drawn into the
burner and heat exchanger area.

I've modified an old Carrier furnace (installed around 1981) and have
sealed off the cabinet and have created a closed intake and flue
circuit with appropriate ducting.  I've also adjusted the primary air
baffles and have turned down the flames.

My next modification is to modify the fan control such that the blower
motor starts when the burners are turned on.  This will prevent some
waste heat from being lost out the flue while the heat exchanger
temperature is rising to it's nominal operating temperature.

> Furnaces without an inducer motor are built to provide enough air
> under a lot of conditions to burn all the fuel - (otherwise soot
> forms and makes the burner unsafe and/or unworkable),

I don't think you'd get any soot from a NG-powered furnace.  Have you
ever seen the flames?  Have you ever adjusted the primary air
baffles?  You really can't produce soot even if you completely close
the primary air baffles (you get lazy red flames but no visible soot
production).

> Choking off the air passages and adding a fan is just an easy,
> reliable and cheap way to control the combustion air flow.

Is the draft motor supposed to run all the time the burners are on, or
only until the heat exchanger temperature rises to the point that the
blower motor comes on?

> In many cases this arrangement also has the added benefit of reducing
> or eliminating outdoor air flow through the furnace when it is not
> heating.

Is it common (or an absolute requirement) that any furnace with a
draft inducer fan would also be ducted so that it is given a dedicated
outdoor combustion air supply?  If so, then you wouldn't need a fan
(presumably not rotating) to help prevent household air from being
drawn into the furnace and out the flue.

I think a draft fan is just an added resistance in the way of the
combustion products going through the flue.  Are the motors of these
draft fans also sitting in the flue (and being heated and corroded by
the combustion products) ???

> I know of no residential or commercial furnace that is more than
> 85% efficient that does not have an induced draft or forced draft
> combustion, so it is a pretty safe bet to say if there is no
> inducer or forced draft motor, it is not a high efficiency
> furnace (just because it is there does not mean it IS high
> efficiency)

I thought that once you went about 78 or 80%, you then had some
condensation going on, and the early high-efficiency furnaces (85%)
rusted out really fast and that's why you no longer see 85% furnaces
(you instead see 90+ furnaces because they have a second heat
exchanger and are specially designed to capture and deal with the
condensate).  So today there is a gap between 80% and 90% furnaces for
that reason.  Correct me if I'm wrong about that.

But again I'm thinking that the draft motor is there as a saftey
thought - to insure the combustion products are ejected out of the
house, and they do little or nothing to actually increase the
*efficiency* of the furnace.  And if you had a truely closed system,
you wouldn't even need a draft motor.

[  Post Follow-up to this message ]

"HVAC Guy" <HVAC@Guy.com> wrote:

>... If you took such a furnace, and sealed the cabinet such that it draws
>combustion air directly from a dedicated sealed duct directly from
>outside the house (and you also ducted outside air into the secondary
>air intake just below the flue) then you've probably just increased
>the efficieny by 5%.

That might reduce the cold air leakage into the house by a few cfm,
since the furnace would not pull combustion air out of the house,
but how would it increase efficiency? Starting with outdoor air at
say, 30 F, we can either heat it to 70 F in the house, then heat it
up to the flue gas temp, or heat it from 30 to the flue gas temp in
one step, with the same energy, no?

Nick

[  Post Follow-up to this message ]

FYI - I didn't see Nick's post until Roger quoted it.  Nick's post
didn't show up on the NNTP server I use.

Nick Wrote:

> That might reduce the cold air leakage into the house by a few
> cfm, since the furnace would not pull combustion air out of
> the house, but how would it increase efficiency? Starting with
> outdoor air at say, 30 F, we can either heat it to 70 F in the
> house, then heat it up to the flue gas temp, or heat it from
> 30 to the flue gas temp in one step, with the same energy, no?

I'm thinking that AFUE ratings include or take into account the degree
to which a furnace prevents outside air leakage into a house, hence
any furnace that is designed for a closed intake and combustion path
would result in a higher AFUE rating.

The point is not that you are using cold outside air vs warm inside
air for combustion.

The point is that by having a closed intake and exhaust pathway you
eliminate the furnace as a point where outside air can enter the
house, or that household air can leave the house (via the flue).


Roger wrote:

> Yes and no - A triple wall pipe, which preheats the air with
> the otherwise wasted heat from the flue gas is more efficient.

Yes, and that is another modification that can be made to an original
low-efficiency furnace.

> If you use a mechanism to stop the air up the flue during the
> off cycle, it can still be more efficient in a back-handed
> sort of way,

I don't think that any such mechanism is necessary.

If you have a furnace where the combustion intake has been sealed such
that it can only draw air from the outside, and naturally the flue is
sealed so that it exhausts out the chimney, then you've got no reason
to close that pathway when the furnace is off.

You've got no reason to close it because the only reason to close it
is if you have a high, natural ambient airflow through the intake and
out the flue.  The distance from the exterior opening of the intake
and the top of the chimney is probably no more than 15 or 20 feet, and
they would be on the same side of the house.  Unless the geometry of
the house is very unusual, they would both experience the same ambient
air pressure, so you'd expect to get little or no net airflow from
intake to exhaust.

If in a given situation you actually would benefit from a damper
mechanism, then I'd be afraid that when it's open that you'd have way
too much air entering the intake for normal combustion during heating
(when the damper mechanism would be open).

[  Post Follow-up to this message ]

I can recommend several excellent trade schools where you can learn this
trade.. Or a good Engineering college that teaches thermodynamics.
This NG is not dedicated to educating others. It is a forum of peers to
exchange idea's and insults. Obviously your not of our ilk. ;-p


HVAC Guy wrote:
> Full-Quoter who is incapable of proper editing Abby Normal wrote:
> 
>
> Nope.  You just have to read and understand what I'm saying.
> 
>
> 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.

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Bubba wrote:

> Why would I waste my time trying to teach someone like yourself

How exactly have you tried to "teach" me anything?

You never respond directly to any concept or theory other than saying
"just get a new furnace" or "do a google search for the answer" or "go
get a degree in this or that".

> Find it out on your own.

There you go again.

[  Post Follow-up to this message ]

HVAC Guy wrote:
> Bubba wrote:
> 
>
> How exactly have you tried to "teach" me anything?
>
> You never respond directly to any concept or theory other than saying
> "just get a new furnace" or "do a google search for the answer" or "go
> get a degree in this or that".
> 
>
> There you go again.

Why? Are Mommy and Daddy no longer helping little 40 year old HVAVGuy?  ;-p

[  Post Follow-up to this message ]

Full-Quoter Zyp wrote:

> You [HVAC Gay Guy] are a clueless piece of s**t.

It must frustrate you that you can't come back with anything better
than that.

[  Post Follow-up to this message ]