It's said that induction fans (draft inducers) increase efficiency - how?

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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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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. Adding too much oxygen means adding a lot of material - mostly nitrogen and the excess oxygen to the combustion process. This extra stuff still has to get heated up by the combustion process, but it adds no usable heat to the furnace output before it goes up the chimney and back to the great outdoors. 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), so under typical conditions, there is too much air added to the combustion. Choking off the air passages and adding a fan is just an easy, reliable and cheap way to control the combustion air flow. 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. This air flow cools the indoor space by way of heat exchange to the output side of the furnace, a waste of heat. 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) There are industrial process or power plant systems that use varous fuel controls, monitoring equipment and automatic or even continuously adjusted manual dampers to accomplish the same thing.

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Roger wrote:

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.

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).

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?

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 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.
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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
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Yes and no - A triple wall pipe, which preheats the air with the otherwise wasted heat from the flue gas is more efficient. If piped separately, it is more efficient when the furnace isn't firing AND you have a natural draft going up the flue - the typical scenario. 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, in that the combustion air has to come from somewhere - generally from someplace where that cold outdoor air flows past your feet while sitting in your favorite chair, so that you turn up the thermostat to stay comfortable.
wrote:

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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:

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

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).
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Maybe Paul's cancelling posts again.

But the combustion-air-heating energy wouldn't change, and the furnace would only pull its combustion air into the house. It wouldn't increase the total house air leakage more than that.

Hot thermal mass, esp in a boiler vs a furnace...

A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might have an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose roughly cfmdT = 259K Btu/h until it cools off, even with no wind pressure difference.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

You guys are funny! :) I love reading this group.
The answer to your question of "why induced draft" is simple. Hey Paul, can you enlighten?
--
Zyp



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On Dec 29, 8:57am, snipped-for-privacy@ece.villanova.edu wrote:

Atmospheric draft has dilution air going up the stack as well 24/7 in heating season.
Induced draft does not have this.
In a cold climate if you upgrade a furnace from atmospheric to induced draft, there is a good chance of suddenly having high humidity problems because of the loss of the 24/7 ventilation.
May not be intuitvely obvious to a guy living in a root cellar with 200 bunnies :)
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Full-quoter Abby Normal wrote:

And what if I modify it so that I have a dedicated (and sealed) duct supplying outside air directly to both the dilution and primary combustion intakes right off the furnace cabinet?
Will I still have air going through the stack 24/7?
And even if I do - will it matter?

Why not?
Does Induced Draft also mean Gated Draft? Is there a motorized barrier in induced-draft furnaces that prevents household air escaping through the flue when the furnace isin't running?

If so, then isin't that fixed by simply dialing down the furnace humidifier setting?
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Some people block off a draft hood, add spill switches and a double acting barometric when there are chronic furnace spilling problems, and these are highly trained people doing this. Otherwise no reason to mess with a draft hood appliance.
It is not so much a change in combustion efficiency, the dilution air going up the draft hood is heated air, and results in cold air infiltrating in.

Induced draft furnace does not have a draft hood.

Like a vent damper or a whsiperheat, no
The vent damper cuts down on the air leaving when the furnace is off. I would upgrade a furnace before adding a vent damper to an old furnace.

Maybe shut off the humidifier and bring in fresh air on a rise in RH
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Abby Normal wrote:

That's not the same thing.
I'm talking about taking a 25+ year old furnace and ducting outside air directly to the primary and draft intake ports on it's cabinet.

Not if you do what I said above. You won't have cold air infiltrating the home if you duct outside air directly to the cabinet of an old furnace.

So they don't pull household air into the flue then. So the draft motor does nothing directly to make a furnace more efficient. But because you have a draft motor, you no longer have a passive draft intake (draft hood) that allows household air to escape through the flue. So AFUE goes up a few points because your furnace is not the source of heated air leakage out of the house.

First you say "like a vent damper or whisperheat, no".
Then you say yes - "the vent damper cuts down on ..."
So is it yes, or no?
Is there a motorized damper or gate somewhere in the system if your furnace has a draft inducer?

If you duct outside air directly to the primary air intake and the draft hood intake of an old furnace, then you don't need to also add a gate or damper anywhere because you've just insured that no household air will escape through the furnace to the outside.
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you must be pulling my chain here.
google draft hood, dilution air,heat recovery ventilator
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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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Essentially you need to keep googling and figure out the drafthood and the dilution air part.
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Abby Normal wrote:

Bubba wrote:

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?
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Actually, from your last 10 bazillion posts, I and everyone else has figured you for quite the hard head. Why would I waste my time trying to teach someone like yourself that has proven time and time again that you wont accept anything? Again, because its a waste of time. Find it out on your own. I already know. Too bad for you. Bubba
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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.
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Why would anyone want to read and try to understand what you have to say? You're clueless and it shows each time you post.

Exactly why you should STFU. It plays a very important part in the system design. I suggest you read yourself until you figure it out. Hopefully it'll keep you busy for a long, long, long time so you'll stop flapping you gums like a ducks ass in here. Better yet, why dont you close off the draft diverter on all your appliances at home and test your theory. Report back to us. Bubba

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HVAC Guy wrote:

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



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