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combustion air

don_185 Member Posts: 312
You are on the right track.I see this alot in my area with
gas waterheater.

The easiet would be the code book.Break it out and show him the math.

You could always break out the draft guage and combustion efficency tester that would also nail down the problem with
some math to back it up.

It can be a tuff job trying to convince a customer without
any math to back it up.


  • Mike Dunn
    Mike Dunn Member Posts: 189
    combustion air

    I was called in to trouble shoot a pilot going out alot on a Rheem nat gas 40 gal regular vent wh. After performing all the usual checks I have come to the conclusion that there is not enough combustion air in the room. There are about 220,000 btu's worth of appliances in this room and the only combustion air opening is a 4x12 floor register to the bar area above it.

    The owner does not believe me about the combustion air so I was wondering if there is some test I could perform while the appliances are running to show him the negative pressure in the room.

    Or, maybe I'm crazy and something else is causing the pilot to go out alot.
  • John@Reliable_14
    John@Reliable_14 Member Posts: 171
    This may help, you need about 110\"

    ASME Section VI Recommended Rules for the Care and Operations of Heating Boilers
    Reference 6.04 states:

    The boiler room must have adequate air supply to permit clean, safe combustion and to minimize soot formation. An unobstructed air opening should be provided. It may be sized on the basis of 1-square inch free area per 2000-btu/hr maximum fuel input of combined burners located in the boiler room, or as specified in the National Fire Protection Association standards for oil and gas burner installations for the particular job conditions. The boiler room air supply openings must be kept clear at all times.

    From experience the ASME rule of 1-square inch free area per 2000-btu/hr maximum fuel input of combined burners located in the boiler room works. I general rounded up to the next common size. Openings should be both top and bottom on wall/door.

    NFPA 54 - National Fuel Gas Code, 1992, section 5.3 Air for Combustion and Ventilation"It shall be assumed that wood louvers and grilles will have 20-25% free area. Metal louvers and grilles 60-75% respectively".
  • eluv8
    eluv8 Member Posts: 174
    pilot light going out

    How big is the house that the equipment is installed in? I had a similar problem on a rheem water heater due to the flue not being piped correctly. I was getting a backdraft condition blowing out the pilot. Had to extend the flue up and put on a pilot with a shield. Works great now, took me a little thinking to fiqure out what was happening on the roof as I had good draft most of the time.
  • Matt_67
    Matt_67 Member Posts: 284

    Some of the newer water heaters have a device to prevent the flame from rolling out - there is a glass spacer that holds a combustion air damper on the bottom of the water heater open - if it breaks the damper closes and there is no combustion air. Something you might want to check.
  • Mike are any

    other appliances in the area being affected?

    Did you check millivolts on the pilot/thermocouple? I will post a procedure seperately for you on this. I also recommend doing a combustion analysis on every piece of equipment in tht area.

    Is this a FVIR water heater? If so there have been case recently of very high flues temps on these causing pilot outage problems.

    By the way lack of air is not the same as a reverse chimney caused by mechanical exhausting. Mechanical exhausting requires make up air, this is seperate from air for combustion. Do an Appendix "H" test from NFPA 54 National Fuel Gas Code for the negative pressure problem.

    Is this unit flued into any vent with a power vented appliance? If so these have been a problem with the natural draft appliance (water heater) not being able to withstand the pressure created by the fan assisted appliance and the pilot goes out.

    Is there a dryer in the same area? If so the dryer has to have its own seperate make up air.

    If air is going to be provided eventually to this area I suggest doing mechanical means of bringing in air. Air openings no matter how well sized they are do not guarantee air being brought into the combustion zone. Mechanical are also interlocked with all the appliances insuring safe operation in the event of failure of the mechanical device bringing in the air.
  • Rich L.
    Rich L. Member Posts: 414

    Might seem to obvious but whats the condition of the thermocouple?
  • Mike Dunn
    Mike Dunn Member Posts: 189

    Been tested with a meter. And just for S&G's I put in a new one but to no avail. I totally believe it is a combustion air issue. Once spring gets here and the furnaces stop running I'll be willing to bet this problem goes away.
  • What readings did

    you have with the millivolt test on the thermocouple with the equipment running?

    I would advise you to also do a combustion test to make sure you do not have carbon monoxide issues. Lack of combustion air will definitely cause that to happen. DO NOT WAIT TILL SPRING!!!
  • here is a procedure to follow


    I am often asked about troubleshooting a thermocouple on gas systems. This will be a permanent reference that will give a step-by-step procedure.

    A thermocouple is a device used to satisfy pilot safety on many 24-volt gas systems. The thermocouple is a device made up of two dissimilar metals. They are joined together at the tip (Hot Junction). When heat is applied to that hot junction a small millivoltage is created. This develops because of temperature difference between the hot junction and what is called the cold junction. The flame has to envelop the upper 1/2" to 3/8" of the thermocouple and the tip should glow a "dull red". If the flame is adjusted to a sharp flame it will glow "cherry red" this will cause the tip to be welded and eventually the thermocouple will fail. The flame should be adjusted to a soft blue flame, not roaring or lifting. The normal millivolt output is 25 to 35 millivolts, on some you may even get up to 35.

    The other part of this safety pilot system is the electromagnet (power unit). It is if you will the LOAD and we can say the thermocouple is the SOURCE. The electromagnet is made up of a coil of wire and "U" shaped iron core. When the thermocouple is heated and the millivolts generated the coil will be energized and create a magnetic field. The magnetic field will cause the "U" shaped iron core to be magnetized; it in turn will hold open a seat allowing gas to pass through.

    When this system malfunctions it typically causes the pilot to go out and the gas will not flow. The first thing that should be done when arriving at a pilot outage situation is to do some visual checks.


    Once those things are addressed it is a good idea to take some millivolt readings. It should also be mentioned that many times it is the policy of some to replace the thermocouple on a call and clean the pilot. It is not a bad thing to do, however it is statistically about 85% of the time it is the thermocouple giving the problem. It is the other 15% of the time that taking readings can solve other problems.

    You need a multimeter with a DC volt scale, as the millivolts generated are DC volts. There are four readings we are going to take they are

    OPEN CIRCUIT - this is taken with the thermocouple disconnected and the meter leads attached to the outside of the thermocouple and the other meter lead attached to the tip of thermocouple. The pilot-on-off knob will have to be held manually to take this reading. This measures the output of T'couple the readings must be above 17 to 18 millivolts.

    * CLOSED CIRCUIT - This measures the millivolts used by the coil in the electromagnet. A rule-of-thumb is this reading should be roughly half of the open circuit. It is taken using an adapter screwed into the magnet and the thermocouple screwed into the adapter.

    CLOSED CIRCUIT LOAD - This reading is taken the same as the previous reading except the burner is now on. With a proper flame this reading should be about the same as the previous reading. With a lifting main burner flame or excessive drafts or chimney pull, this reading may reduce from previous reading (flame being pulled away from the thermocouple). With the cold junction being heated this reading may increase. If the "cold junction" is heated excessively it will break down.

    DROP OUT - This is the final reading. It requires the pilot to be blown out. It measures the ability of the magnet to hold under reduced MV input. A good unit should drop out below 6 MV's - normal is 1 to 2 MV's. The allowable "drop out" time is 180 seconds yes three minutes. It is more likely to be a minute and half to two minutes. There will be an audible "click" when the magnet shuts down.


    A normal set of readings

    OC- 30 millivolts
    CC- 15 millivolts
    CC(load) -15 millivolts
    DO- 1 millivolt

    The best way to be able to diagnose these readings is to use MILLIVOLT CHARTS these can not be displayed here but I can provide them if you e-mail me.

    Thermocouples from different manufacturers vary as to their dependability. The only thermocouples I recommend are made by Johnson Controls. The K15 and K16 series are the best. If you are having durability problems then use the K16RA, which is a nickel-plated high ambient or corrosive environment thermocouple. The Husky (K16) will fit most applications and for those that it does not the Slim Jim (K15) will fit.

    To repeat the adjustment of the pilot flame to envelope the upper 1/2 to 3/8 of the thermocouple is important, the flame should be a soft blue flame not roaring which will cause the tip to glow a "dull red" versus "cherry red".

    The combustion condition (excessive temperatures) in the chamber is also an issue and this will require a combustion test and draft measurement to insure that excessive temperatures are not being applied to the pilot. In some cases on water heaters it may be necessary to alter the pilot adaptation to get better quality performance. This however should not be done unless you have had proper training.

    The possibility of the equipment operating in a depressurization environment will certainly lead to thermocouple failure. In addition if the equipment is flued together with a "fan assisted" furnace or boiler this can lead to problems. There are solutions to this also but training is required.

    The thing that I find is often a problem is the environment in which the equipment is operating. Many times corrosive chemicals and airborne contaminants are being drawn into the air gas mix and a chemical reaction takes place. This again requires attendance at a training session by a professional combustion person to help you to see the various affects this will have.

    Last of all the failure to put all the doors and covers back in place on equipment. The failure to do this will cause an alteration in combustion air and the flame stability is affected.

    The design of some equipment is also a problem. When there is high demand for heat (very cold weather) the temperatures that are created in the chamber have an adverse affect on the pilot and thermocouple system. The addition of the K16RA thermocouple can offer some assistance toward extending the life of the thermocouple in this situation.

    Insufficient air for combustion and dryers operating in close proximity to equipment also lead to problems.

    Last of all and this is not directed at any one in particular but just plain lack of service personnel and installers knowing what they are doing.

    My book "Circuitry and Troubleshooting" Volume II addresses many of the things in question here.

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