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Natural Gas Furnaces

Where pure fuel is definitely obtainable, the pressured air furnace simply dominates the area heating market; the colder the local weather, the higher the share of dominance.

Pure fuel is the bottom cost gasoline and the pressured air furnace is the lowest cost central system. This stays true even with the recent increases in the worth of pure gas, as gas oil and propane have seen greater will increase and more value volatility.

Great enhancements in efficiency have been made during the last 20 years. Any furnace lower than 80% environment friendly or more than 20 years old, is a candidate for alternative.

For any home that’s at the very least modestly insulated, replacing a 20 12 months old furnace is likely to be the largest single power management measure that can be taken to scale back heating costs.

Furnace Kind and Efficiency

Effectivity has no less than two ratings:

1. 1. a gradual-state or combustion efficiency
2. a AFUE, seasonal, standby, or cycle efficiency

The regular-state or combustion efficiency relies on how good the unit is at mixing air and gas in the burner, the place the combustion air comes from and how environment friendly the heat exchanger is at getting the heat from the combustion course of into the air being heated. The regular-state ranking will be up to 20% higher than the seasonal ranking.

The Annual Fuel Utilization Effectivity – AFUE, takes into consider the off-cycle losses. The regular state efficiency may be measured, but the AFUE can only be estimated primarily based on accepted factors utilized to the regular state efficiency. This ranking is influenced by how the furnace is vented, if it has a standing pilot or IID and its cycling efficiency.

All furnaces constructed and bought in the US at present have to be a minimum of 78% AFUE; to have an Energy STAR ranking, they must be at the very least 90% AFUE.

Estimate typical ANNUAL Furnace Effectivity (AFUE):

– old, gravity, conversion furnace with standing pilot – 50%
– outdated, compelled-air, standing pilot, 60%
– 20 – forty years previous, pure draft sixty five – 70%
– lower than 20 years outdated, pressured vent by way of steel flue pipe seventy five – 80%
– less than 20 years previous, condensing (plastic flue pipe) 85 – 90%
– less than 10 years old, condensing, multi-velocity fans, multi-stage burners, outdoors combustion air ninety – 97%

The above rankings clearly present the best way to cut back whole heating prices is to exchange an outdated heating system.

Outdated Gravity and Conversion Furnaces – they’re typically known as ‘octopus’ or ‘elephant’ furnaces. Any such furnace was designed to burn coal or wood and later transformed to pure gasoline. Most do not need followers, though some may even have been retrofitted with a small fan. They have a standing pilot that should be manually lit. It could even have been transformed to gasoline oil earlier than natural fuel. They’re centrally situated in a basement with a large return air duct in the midst of the house and several other massive heating ducts that radiate out to in the direction of the surface walls of the house. It is vitally giant, with a heavy hearth pot and ductwork.

Older Compelled-air Furnaces – that are forty years or older usually have cast-iron (high mass) heat exchangers and easy forged iron burners. They do have a fan, smaller ductwork, and a standing pilot.

Pure Draft Pressured Air Furnaces – the subsequent era of furnace has lighter weight heat exchangers, better burners and should have an computerized Intermittent Ignition System (IID). They require a chimney or steel flue for pure draft combustion and venting.

Pressured Draft, Non-condensing Furnace – have a fan that forces the merchandise of combustion out by a small metal flue and draw combustion air into the burner chamber. It has an IID. Utilizing exterior combustion air provides no less than 5% annual effectivity. It might use inside or exterior combustion air.

Any furnace with a metal flue pipe and open front that pulls combustion air from the inside, heated area, can’t be more than about 80% combustion effectivity, and is much lower seasonal efficiency on account of all of the heated air misplaced up the flue during off-cycles.

Condensing Furnace – has a plastic flue pipe; a second heat exchanger removes a lot heat from the combustion gases that the temperature drops beneath 212F. The condensation leads to further heat known as the ‘latent heat of condensation’ and numerous water that should be drained away. Condensing furnaces have IID and no standby vent losses.

Multi-Stage Condensing Furnaces – in addition to the above, have multi-stage burners and variable speed distribution air followers to more closely match the furnace output with the house load. There can be a small effectivity gain over fastened rate items, however the biggest benefit is improved area comfort from extra even temperatures.


Typical Furnace

1. Thermostat calls for heat
2. Security Pilot confirms pilot is lit and opens essential fuel valve
3. Fuel burner ignites and begins to heat the heat exchanger
4. The heat exchanger thermostat (fan restrict management) reaches its upper set temperature and turns on the distribution (ductwork) fan
5. The burner continues to fire and the fan continues to run till the thermostat is happy
6. The thermostat reaches its set level and sends a signal to shut the primary fuel valve, turning off the burner
7. The fan continues to run until the heat exchanger thermostat (fan limit management) reaches its decrease set point and turns off the fan

Pressured Draft and Condensing Furnace

Same steps as above with the addition of:

– draft control strain switch proves the combustion fan is running earlier than permitting the principle gasoline value to open

Word: The Fan Restrict Management is also a safety; if the heat exchanger gets too sizzling, the Control shuts off the gas burner. This might occur with a failed distribution fan or plugged ductwork.

Retrofit Choices

There have been two retrofit options that were fashionable in the 1980’s and 1990’s that improved the efficiency of present furnaces. An automated Vent Damper and Intermittent Ignition System (IID).

An automatic Vent Damper closes the flue throughout furnace off instances. This reduces the off-cycle flue losses of heated air up the chimney. This can improve the effective annual efficiency by 5 – 10%. For more information see Vent Dampers.

An Intermittent Ignition System could be added to most furnaces which have a standing pilot. It is because the gasoline they saved was very small in comparison with the primary cost of the equipment with installation, and that they had a high failure fee. Nonetheless, they proved to be mostly uneconomical and a maintenance problem. IID models installed as factory original tools are highly dependable and are required by Federal law.

Although vent dampers and IIDs are nonetheless accessible, a furnace previous enough to learn from one ought to be thought-about for alternative. Additionally, a vent damper may interfere with correct compelled draft operation so it should not be put in on many newer furnaces. New furnaces, even the non-condensing units, have smaller flues, forced draft, and no open ‘draft hoods’, which means they unfastened less up the flue.

Vent dampers are extra common on boilers, which are inclined to final longer and have much larger substitute prices.

Furnace Sizing

It is important for comfort and efficiency that a furnace be accurately sized for the space. Traditional practice has been to over-size a furnace and use a couple of 100,000 BTU furnace for any ‘average’ house, and maybe 200,000 BTUs for a big home.

Furnaces are the least efficient when they’re warming up, and cooling down. This cycling additionally produces more uneven temperatures in the space, lowering consolation. Subsequently, a furnace that’s over-sized goes through extra cycles and has a decrease over-all efficiency.

One of the best approach to size a furnace is with an correct Heat Loss calculation of the home.


Discover: According to one directory, there are over 185 brand names of residential furnaces; this itemizing on no account is intended to be all inclusive or endorsing. It’s only offered as a comfort to seek out a number of major manufacturers that can be found in a lot of the US and Canada.

Go to the Amana internet site at

Go to the American Standard internet site at

Go to the Bryant net site at

Go to the Provider net site at

Go to the Coleman internet site at

Go to the Heil net site at

Go to the Lennox net site at

Go to the Trane internet site

Go to the Rheem internet site at

Go to the Ruud internet site at

Organizations and Directories

The Gas Appliance Manufacturers Affiliation (GAMA) is a national trade association whose members manufacture over ninety p.c of all the residential, business and industrial gasoline appliances made within the United States. GAMA also represents manufacturers of sure products which use oil or electricity as their vitality source.

The GAMA web site features a membership directory with hyperlinks to most US Manufacturer’s web pages.

Go to the GAMA web site

Source: Textual content Bob Fegan 12/2008; pictures Bob Fegan; logos of firms listed and linked above;

© 2008 Vitality Solutions Center400 N. Capitol Street NWWashington, DC 20001 All rights reserved.

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