Mobile home furnace (C) Daniel FriedmanCombustion Air FAQs
Q&A on Heating Appliance Combustion Air Problems

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Combustion air defects & hazard FAQs:

Questions & answers about the neccessary amount of combustion air for heating appliances to assure safe and effective operation.

This article series explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters.

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Combustion Air Requirement FAQs

Backpressure Burn at an oil burner (C) Daniel FriedmanQ&A on how to recognize, diagnose & fix inadequate combustion air, sooting, or burn marks at oil-fired or gas-fired heating systems.

Watch out: inadequate combustion air not only causes improper and wasteful operation of heating equipment it can also produce fatal carbon monoxide gas hazards indoors.

These questions & answers were posted originally at COMBUSTION AIR REQUIREMENTS

On 2017-09-18 by Sam - diagnose improper gas burner flame

This is a problem that occurred about a month ago. I had the local gas company came out first and checked it and said that there is no gas leak from their end and it has to do with the unit itself and they cannot touch the boiler.

Wife called local boiler company and had a tech came out, took our money and left by saying that he does not know anything about this type of boiler.

Wife made second call to the local BUDERUS tech expert; tech came out and said that he himself cannot pinpoint the problem, took our money and said that the unit needs servicing (complete cleaning) and it will cost toooooo much for us to have him fix it

Honestly, I think that he himself didn't know what the problem is. He told my wife that "your husband can do that himself (after passing through my garage and seeing all my tools and equipment) because I see he is well verse in repairing different things".

When I came back home, I decided to take on the task myself. Lo and behold, after going through a series of troubleshooting, I determined that it was some of the burners that were not lighting and therefore causing the Raw Gas scent.

I have checked everything so far and cannot come up as to why it is doing that. I know for sure that all of the orifices are getting gas. I checked that by running the unit and wetting my finger to block off the orifices one at a time and feeling the gas on my fingers.

Fact: when I blocked off the orifices that the burners were off, the flames shot up much higher on the others that were lighting. When I blocked the ones that were lighting, ta-da, obviously the flames went out. Note: the flames are of a nice crisp blue color with NO yellow in them. Also, sometimes ALL the burners would light up when turned on.

When you say diagnostic, what do you mean? Do you mean it has to do with the settings, circuitry or control board etc.? I don't think so, because it is cycling through properly. I can put it on and off from the Logamatic R2107 control. This boiler produces domestic hot water also.

Finally, I was asked to check the inlet gas pressure and the pressure at the manifold, but I don't have a manometer to do that. All I can say is there is NO blockage, and I do see an increase in pressure when I block off one of the orifice. And, sometimes ALL burners light properly, so if there was a problem with the pressure, it should be off all the time (the burners that are not lighting). Would that be a reasonable assumption?

I still believe that there is insufficient oxygen coming in the chamber for four burners to achieve its stoichiometric point.
What are your thoughts on that?
Thank you.

On 2017-09-18 01:25:45.830096 by (mod) -

Is this a new problem or has this always been a problem - that might be diagnostic. Also has anyone checked the gas pressure?

On 2017-09-17 21:05:58.321128 by Sam

Last comments are from me.

On 2017-09-17 21:05:03.376534 by Anonymous

Thank you for your response. I have taken the system apart for service and did not find anything abnormal. Firstly, the system is a seal system and nothing (very doubtful anything) like spiders can get into that area that you mentioned.

Secondly, inside the combustion chamber was ABSOLUTELY CLEAN when I took it apart so much that I can literally eat food from inside the area if fallen inside. I mean it is that clean.

Thirdly, I checked all the orifices and burners properly and NONE of them were clogged or have absolutely NO signs of being clogged. I cleaned them out anyways. The burners are made of STAINLESS STEEL just to point that out. Even when the noted burner(s) are not lighting, the gas still flows through the orifice(s).

Only one or two burners may be out at a time and it's ONLY the same one(s).

To address your last suggestion of having a blocked shutter, this unit does not carry a shutter on it. Instead, it has an inducer fan which is located in the fan blower box. The fan comes on to rid the exhaust gas through the flue; however, I don't know if it also creates a vacuum or blow air for the intake air to get to the burner assembly. If it supposed to blow air, then I think that may be the problem at hand. Air does not blow through the d

Note: The motor works fine. This mo

I am stumped as to why the said burners would light sometimes and sometimes not. I would think that if something is really wrong with them, they would not light up at all.
Any other thoughts / suggestions on this are greatly appreciated.

On 2017-09-17 by (mod) -


When some of the gas burner tubes ignite and others do not, I suspect that the burner gas metering orifice is plugged (spiders can do that as can debris) or that the actual flame orifices are clogged with rust and debris, or the air shutter is blocked.

This is a dangerous condition, risking ignition of a burst of unburned gas.

Your heating service tech would typically start with a visual inspection for obvious debris and dirt, clean the system, then test its operation again. IF the problem persists then she will look more deeply into parts like the gas orifice, air shutter, burner tubes.

Your separate suggestion of being air starved is an interesting one. I would think that you'd see improper combustion flame colour - more yellow, less blue, when the burners are ignited.

The system is in my opinion unsafe to use in this condition.

On 2017-09-17 by Sam

I have a Buderus Logano GA244 sealed gas fired boiler and not all of the burners are lighting up at all times. I was getting a strong scent of Raw Gas coming out from the unit. I determined that when some of the burners are off, that's when the smell appears.

The unit goes through the cycle properly without any hiccups. Gas is flowing through ALL orifices (4) when it is on but sometimes one or two burners would be off even though gas is flowing through them. Sometimes ALL four burners would light up. There is no problem with the ignitor nor the pilot; those are functioning properly

I think that when the unit is turned on and the burner closest to the pilot goes on, the flame is not JUMPING to the two outer burners sometimes. Important information to know: unit seems to work fairly good when the burner tray cover is off.

On the contrary, if cover is in place, sometimes they may not even want to light or if the unit is on while cover is off, and attempt to put on the cover, some of the burners would go off.

Overall, I think the unit is STARVING OF AIR.
What are your thoughts / feedback on this problem? Thanks.

On 2016-06-22 by (mod) -

Keith please see our detailed answer at

On 2016-06-22 by keith preddie

can anyone go through the control sequence between louvered openings and the gas fired equipment?

Question: soot problems after oil burner puffback

(Jan 6, 2015) mahgy said:
we had a furnace blow back a while ago, still noticing soot/black build up coming out of the heating vents on the walls.. this is a rental property. is this dangerous to our health? landlord does not seem concerned



No one can assess exposure levels of soot nor health risks accurately from a simple e-text, but in general, breathing soot is not healthy. If the heating system is no longer producing soot (which is itself unsafe) then the duct system may need cleaning.

Question: visible clues of adequate combustion air

(June 25, 2015) Anonymous said:
What if there is no visible combustion AIR


Anon you'd need to be more specific. Most building codes and standards discussing combustion air supply include a square-foot area size specification. For example if a heater of input BTUh amount X is in an open space of Z square feet then the area is deemed capable of providing adequate combustion air. Those guidelines presume a standard minimum rate of building air leakage or air exchange rate.

In the article above you can read details at the paragraph titled

Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50

Question: article correction

(Jan 14, 2016) Joe said:
With your cubic ft volume for adequate combustion air equation, you use 140 MBH for the total input BTU of a 180 MBH boiler and 40 MBH DHW. Should you be using 220 MBH total for that?


hanks Joe, we've corrected the math in the article above. I appreciate your careful reading. We can use all the editing help we can get.

Question: combustion air source for water heater in a trailer

(Mar 19, 2016) Anonymous said:
does combustion air for mobile home water heaters enter from under the trailer



In some installations it might;

Watch Out: if your water heater is gas or oil fired and lacks adequate combustion air the unit is unsafe and could kill the home's occupants.


21 June 2016 keith preddie said:
can anyone go through the control sequence between louvered openings and the gas fired equipment?


Keith please see our detailed answer at



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