Rule of Thumb: Combustion air flowrate estimate?

[HaVACman]

My first interaction with the forum: Hello out there.

I'm a 25 year career HVAC engineer too old but not too embarrased to ask if anyone knows a general rule of thumb:
What might the combustion air flowrates be for a 80% natural draft or 90+% forced draft boiler or furnace?

Combustion airflow is the issue. lets assume natural gas: which is what, around 1 scfm/1000 btuh?
Lets assume residential grade equipment 100,000 to 200,000 btuh input.

This should be easy - but I scrubbed the forums for FAQs and found nothing. I'm guessing my ASHRAE fundamentals doesn't have the answer either. Thanks so much.

 

[AbbyNormal]

First off natural draft then implies a draft hood, so you need primary/secondary/excess air for combustion and then you need dilution air to go up a draft hood.

The 90% induced draft residential type of furncace will probably pull about 50% excess air through the heat exchanger along with the products of combustion and then blow it out the vent pipe.

Here is an easy math version

Assume natural gas is methane, air is 20% O2 by volume, and the higher heating value of the gas is 1000 Btu/cubic foot.

Therefore for every 1000 Btu/hr you need a cubic foot

Perfect combustion, exothermic
CH4 + 2 O2 + 8 N2 = CO2 + 2 H20 + 8N2

So for each cubic foot of gas burned you need 2+8=10 cubic feet of air for perfect combustion. If a furnace was designed for 50% excess air, then each cubic foot of gas would need 15 cubic feet of air. The amount of excess air is going to vary with the appliance design.

Back to your Natural draft furnace, perhaps for 50% excess air then every cubic foot needs 15 cubic feet of air, then the draft hood could most likey need another 15 cubic feet of air for every cubic foot of gas burned, bumping the total up to about 30 cubic feet of air per cubic foot (1000 Btu) of gas burnt.

Harder math, natural gas is methane and perhaps small amounts of propane, ethane, N2 and CO2, air is 20.95% O2 by volume, perhaps 1% argon, CO2, trace gases and the remainder N2, and the heating value can be all over the place from about 900 Btu per cubic foot to 1200 Btu/cubic foot


Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

note the cubic feet i mentioned are just volumes, not a rate.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[GMcD]

Your local Gas Codes will require meeting their combustion and relief air openings as well. For example, the local Gas Codes where I am require the following for installations with less than 400,000 Btuh Input total:

" When an appliance is located in a confined space and the air supply is from the outdoors, the space shall be supplied with two permanent openings that communicate directly to the outdoors by means of openings or ducts. The openings or ducts shall conform to:

(i) Lower Opening - 1 sq. inch per 2000 btuh Total Input rating of the appliances in the space when the means of communicating air to the outdoors is by means of a horizontal duct, or

(ii) Lower Opening - 1 sq. in. per 4000 Btuh of the total input rating of the appliances in the space when communication to the outdoors is directly by an opening, or by means of a vertical duct, and

(iii) the upper opening shall be located as near the ceiling as is practical, but not lower than any relief opening of a Draft Hood or Draft Regulator, and shall have a free area of not less than the total area of all vents and chimneys from the appliances contained therein...."

Basically you have to have a low level combustion air inlet to the room as well as a high level relief vent. The Code requirements are somewhat based on the basic combustion formulas, but with saftey factors. Check your local Codes for what they require first.

 

[AbbyNormal]

Tjernlund sells a product that draws in combustion air, tempers it with room air and will pressurize the basement.

They show a chart showing combustion air in CFM for various gas/oil fuel inputs.

I looked at them about a year ago and for induced draft residential gas fired stuff, I think their chart was based on close to 70% excess air. They give numbers for natural, induced draft and retention head oil as well

http://www.tjernlund.com/combustionairintake.htm

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[RossABQ]

I have to believe the Tjernlund numbers are including the tempering air or something else, 70% excess air is very wasteful. I'll have to check their site. If they temper by direct mixing, that would appear to be using room air for combustion which triggers some Code issues.

For a rough rule of thumb, if you take gas input in CFH and divide by 5, it gives you combustion air requirement in CFM, including a little more than 10% excess air, which is an old commercial boiler value. Modern energy efficient boilers run very low excess air.

 

[AbbyNormal]

Industrial/commercial power burners could be low excess air 25%. The Tjerlund numbers were based on residential equipment.

70% is on the high side for residential equipment, perhaps they are making sure they exceed the requirement by a bit.

What combustion issues? Pretty much any furnaces outside of something that is direct vent, draws combustion air from the space.

Passive holes for combustion air and ventilation require an indoor negative pressure to draw in combustion air.

The Tjerlund product is supplying air to the space at a prescribed rate.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[RossABQ]

The "issues" are spelled out in IMC-M704.xx; they are not combustion issues but requirements where air comes from indoor/outdoor combined.

 

[AbbyNormal]

Is the IMC similar to working out what is a confined space or a non-confined space? Rules on the envelope being too tight and therefore requires a ducted supply from outside?

That Tjerlund brings in a deliberate amount of outside air for combustion.

Without sealed combustion, an applaince in a room with vents as GMcD pointed out, still burns room air for combustion. The vents allow air to come in to replace what goes up the flue, in a perfect world.

Can have problems with those passive openings sometimes though. Should debate some of the NCI guys sometimes.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[HaVACman]

Well I am certainly impressed on the quality of the responses. Thank you each. the 10:1 theoretical air-gas flowrate + the additional (50%) volume are memorable guidelines. It was good that the code venting reqrmnts were mentioned - had I not known about that. Very thorough all of you - and answers my need. Thank you. I'll look forward to returning the favor. -HaVACman

 

[AbbyNormal]

50% excess air is pretty good for residential equipment, just realize that the natural draft also has the dilution air

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

This link should hopefully still work. If you answer a survey at the bottom of the page, a combustion analyzer manufacturer will let you download an application guide.

In it you will find typical 'dry' flue gas compositions for various types of appliances and fuels.

http://testousa.com/fallpromo/

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChasBean1]

One good tidbit to add related to forced combustion air make-up:

International Mechanical Code 2006, Sec. 706.1 requires a minimum of 1 cfm for every 2,400 BTH/hr combustion input.