What Is a Furnace Draft Inducer Blower? I'll tell you what it is ...

[.p.jm.]

You ever seen a residential system designed with NG at 11 " before ? :-)

[The Daring Dufas]

Yea, that was the required gas pressure input for Generac generators. :-)

TDD

[Vic Smith]

There's some kind of misunderstanding here about simple fluid flow and pressure, how gas burns, etc.. I can't get into technical details because I'm not an engineer. But I know....things. As I've mentioned, I cut down the flame on my old boiler by simply moving the gas supply ball c*ck lever until the flame was where I wanted it, and adjusted air flow to get a good flame. That was a vertical flame - the burner was directly under the boiler water jacket flue passages. So there's no question a gas supply line ball c*ck - intended as a stop valve - can regulate burner pressure effectively. Understand that the gas company supplies natural gas to residential customers in my area at 1/4 psi. By reducing the volume allowed through the valve I reduced the pressure at the burner orifices, and reduced the BTU per hour burn. In that case I lowered exhaust temps, increased the on-cycle, and came out ahead in terms of efficiency. I wouldn't try that with my current furnace, because it has a horizontal flames reaching into heat exchanger tubes. The nozzles have a good 3-4" of open air between them and the heat exchanger tubes. (BTW, that's a real good reason for an induced draft at startup.) I strongly suspect gas pressure/nozzle size are critical for safe and efficient operation on this one, and shouldn't be trifled with. They were easily manipulated on my old boiler.

So what's this all mean? If Home Guy plans on regulating gas burn on a new furnace, he should at least find one with a vertical flame. I don't know if they make them for forced air furnaces.

--Vic

[Home Guy]

I guess we can argue if a partially-closed supply valve is acting as a flow restrictor (reducing the quantity per unit time of gas that reaches the furnace) or if it is functioning to reduce the pressure of the gas reaching the furnace.

Obviously if the valve is partially closed, it must function primarily to reduce the quantity flow to the furnace. It could very well be that because of the size of the burner orifices that they are the real bottle-neck to quantity flow, and the pressure on either side of the partially-closed supply valve remains the same at all times.

I have been altering the size or intensity of two (2) furnaces over the past 5 years by turning the variable control dial on the pressure regulator inside these furnaces. This is the same control dial that must be turned (and depressed) to the "Pilot" setting in order to ignite the pilot light, and then turned about 90 degrees to another position to allow full flow to the burners. But instead of turning this dial the full 90 degrees, I turn it maybe 25 degrees to get a "lazy", less intense flame.

A third furnace (which dates to about 1981) is a "whisper heat" unit with electronic ignition (and a motorized combustion-air damper door and resettable over-temperature sensors on various parts of the cabinet). I had to re-solder the ignition transformer on the PC board control module of that furnace 5 years ago (basically when it came under my care and control) because I was observing intermittent ignition. That furnace does not have a pressure control dial like that found in the furnaces with a standing pilot. What I began doing this year is to regulate the flame intensity of that furnace by partially closing it's gas supply valve while observing the flames through the open damper door.

All 3 of these furnaces have long horizontally-placed burners that are positioned at the bottom of the heat-exchanger galley where the flames rise vertically directly from the burners.

[clare]

Propane at 11 inches is pretty normal, and NG can range from 4.5 to

14, with 7 apparently being roughly the "normal" for north american systems.

Still, my point is valid. It's a mighty small orifice, and the "shut-off valve" on a 1" iron pipe is going to make an awfully lousy regulator

[Home Guy]

I can only say that if you have access to an older-style furnace with easily visible, horizontal burners (not the "in-shot" type) then you can easily see for yourself how manipulating the main supply valve can reliably and consistently alter the size or intensity of the burner's flames.

[.p.jm.]

I agree. And dropping pressure upstream of the unit's gas valve in an attempt to regulate BTU input / output is dumb. As others have said, changing orifices is the correct way to do it.

And no, you don't just 'solder them shut and drill them out' with solder that melts at 400 F, like some idjit suggested.

[.p.jm.]

I shudder to ask, but .... what is your excuse for not putting in orifices of the capacity you want instead ? They are cheap, and easy to change.

[Home Guy]

The orifice on the long horizontal burners is nowhere near where the flames are combusting. These orifices are mounted right to the fuel rail-pipe coming off the gas valve, where the burners are loosely inserted over them. It's cool to the touch.

[Home Guy]

The orifice on the long horizontal burners is nowhere near where the flames are combusting. These orifices are mounted right to the fuel rail-pipe coming off the gas valve, where the burners are loosely inserted over them. It's cool to the touch.

[.p.jm.]

So is your brain.

[clare]

He's an "idjit", but surprisingly enough, it actually CAN work - since the flame does NOT occur at the jet. It would be very unlikely, in many furnace designs, for the jet to approach anything close to 300 degrees F.

[clare]

Hey. I hate to agree with Home-guy - but in this case he IS RIGHT.

[clare]

Sure, you can "control" the flame, but accuracy and repeatability are not really in your lexicon when you do this. Soldering the "jet" and re-drilling it is a much more "scientific" , repeatable, and accurate way of doing it. (and getting a jet of the same size/format as a permanent replacement is even better) It's not just the size of the jet either - the "format" of the jet - the taper on the inlet side(if any) and or the outlet side, can significantly affect the volume of gas flow through a jet of a given "bore"

[.p.jm.]

True - and it's STILL a dumb-ass thing to do in a combustion chamber. Can you say 'roll out' ?

[.p.jm.]

No, he's not. He's talking about 'some asshole DIY'r could get away with it', not 'the correct and safe way to do it'. Which said 'correct way' , as it happens is also very inexpensive and easy to do.

[.p.jm.]

And it 's total bullshit.

Thank you. Why not skip the bullshit DIY'r crap and just give the correct answer ?

And the velocity and pattern of the outlet gas, and the quality of the air / fuel mixture, thus the quality and efficiency of the flame. So why not just do it right ?

[Home Guy]

Accuracy is not called for here.

Anyone dialing-down the gas supply to the burners will not know ahead of time what the exact precise flow rate should be that they are intending to achieve. Much the same way that you won't initially know what setting your barbeque's controls or gas stove need to be in order to get a certain temperature on the grill.

In this excercise, you dial the gas down, you let several hours (or days) pass, you note how the duty cycle of your furnace has changed, and you either leave it alone or you adjust it up or down as needed.

And also more effort for no real, effective gain in terms of the desired end result.

If the furnace has a variable dial on it's gas valve/regulator, then use it. If it doesn't, try the main shut-off valve.

Yes, you know that changing the jets by getting smaller ones (or making them smaller with solder) will give you a smaller flame and less BTU's. But you really don't know by how much until after they're in place. And if the new jets are too small or not small enough, then you've got to partially dis-assemble your burners and change the jets.

Makes about as much sense as saying that replacing the potentiometer in my stereo's volume control with fixed resistors is a better way to control the intensity of sound coming from my speakers. Does it matter if the fixed resistor is "more accurate" ?

[Home Guy]

Yes, I am.

No.

I'm saying that the orifice jets do not experience high temperatures because of where they are mounted, so your claim that their size can't be changed by filling them in with solder is wrong.

I never said that modifying them with solder is good or bad or risky or safe.

But if you want such an opinion from me, then I would say that filling them in with solder and re-drilling them to a smaller size is perfectly acceptible. Solder is dimensionally stable in this setting, and the distance between the orifice and the combustion site is such that there is no fine detail in orifice shape or construction that can have any meaningful effect on downstream combustion other than the size of the opening.

You see, this is what happens when you have no real experience with 25+ year-old furnaces, other than to tell the customer that "yup, yours needs to come out and you need to fork over $4k to me for a new-fangled one".

After all, that's what you tell people when their pilot light blows out

- isin't it?

[Steve]

Your stereo doesn't need a specific fuel/air mix to operate the way it was designed to. FWIW, I have *NEVER* seen a furnace gas valve with a "variable dial" on it.

Oh, and while I am at it, HomeBoy.... you sure that your name isn't really Homer Simpson??