Like just about anything, the conventional NG furnace is setup for a generic set of variables by the manufacturer. Like a vehicle with a carburetor, a skilled tuner can get hold of it and make it run better. Most service manuals outline adjustments for furnaces but most service personnel never bother with a tuneup and leave it factory set. :-)
TDD
OK, it's best to compare apples with apples, but the35 year old 75% efficient basic furnace I removed, right sized, would still be as or more efficient than a 95+% furnace of twice the required capacity. My old furnace WAS pretty well optimized, although it was slightly oversized for my house.
20X30 ft 2 storey mid--seventies brick and siding house in south-central ontario - heated exclusively with natural gas - including domestic hot water, for $700 per year. - and virtually no difference when the "old" furnace was replaced with the new high-tech non-condensing furnace escept a siseable difference in the hydro bill.
Yes - EVERYONE just KNEW you could not run a gasoline reciprocating engine lean of peak without burning valves. It was generally accepted "proven fact" until "Lindy" showed them otherwise.
Hey, I never said ir did anything for efficiency - I said BECAUSE the furnace is so efficient, a natural draft may not be created immediately on startup - so the "draft inducer" gets the air movement up the stack started to make sure you get ptoper ventilation. It IS a safety item. Any other advantages would be bonus side-effects.
It was OK for him to fly until he started shooting down Japanese Zeros.
TDD
"hr(bob) snipped-for-privacy@att.net" unnecessarily full-quoted:
If your furnace has a variable-control gas regulator valve, you could have more easily just turned that to dial-down the amount of gas reaching the burners.
Even if it doesn't have a dial control, I bet there's a valve on the gas line running to the furnace, maybe about 3 to 6 feet away from the furnace, and you could have also partially closed it to reduce the gas flow as well.
snipped-for-privacy@snyder.on.ca unnecessarily full-quoted:
If you've extracted so much heat from the combustion process that the exhaust gases are cool, then you have no choice but to force the combustion gases out of the furnace - and out of the house - by using a draft fan because you won't have any natural convection happening in that case.
The draft motor is one of those extra electricity-consuming devices on modern furnaces that are overlooked when comparing the total electricity usage of old vs new furnaces. The proper operating of the draft motor, and the proper operation of the sensors that monitor it, electrical inter-connects, relays, etc, are additional points of failure for the modern furnace that older furnaces simply don't have.
(rest of the unnecessary full double-quote deleted)
There's no reason why it can't function as one.
By partially closing off the gascock valve, you're not necessarily reducing the inlet pressure (the pressure beyond the valve going to the regulator).
When the burners are not going, the pressure on both sides of the gascock will be the same. When the burners light up and get going, there might be a slight reduction in the pressure between the gascock and the regulator valve in the furnace.
Two problems with this source. First, it's the wrong sort of source to support your claim. It is a book about the topic of energy conservation. It is NOT a specialized HVAC manual. It's intended to provide a broad and fairly basic overview of energy conservation, and as such includes chapters on building design, lighting, HVAC, etc. Second, it's woefully out of date, as it was published seventeen years ago. You had to have noticed the page where electronic ignition is described as a "new development in gas heating systems".
It is imprudent, if not highly dangerous, to use as a technical guide an old, outdated book that only touches on gas furnaces in a brief, general, and decidedly non-technical manner.
Not if he adjusts the blower speed to give the proper temperature rise across the heat exchanger. Run the fan too fast, and yes, you are correct.
Which is an extremely mickey mouse way of doing things. Changing the orifice (jet) sise is the correct way if there is not an adjustable pressure regulator in the system. The ball valve (or whatever) on the
1 inch iron pipe feeding the furnace is a pretty gross adjustment.
Any cred you may have built over the last 2 days just went down the crapper, HomeGuy.
With propane at 11 inches water column pressure, a .078" diameter orifice (jet) produces just over 50,000 BTU per hour. For natural gas at the same presuure you are looking at something like a 0.100" orifice.
You are going to effectively regulate that flow with a valve on a 1" iron pipe???
NOT!!!
You ever seen a residential system designed with NG at 11 " before ? :-)
Yea, that was the required gas pressure input for Generac generators. :-)
TDD
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
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.
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
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.
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.
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.
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