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  1. #1
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    combustion air intake

    Hey Guys I have a Customers unit giving me some grief. Its a york TM9Y 2 stage. 96. When outside temp is cold the unit will cycle on with a what seems like a delayed ignition. but when outside temp is mild and unit comes on periodiccaly it cycles on fine inducer comes on burners like great. only noticed when really cold outside temp the burners will light with a slight boom almost as delayed ignition.

  2. #2
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    Quote Originally Posted by Mobiletec View Post
    Hey Guys I have a Customers unit giving me some grief. Its a york TM9Y 2 stage. 96. When outside temp is cold the unit will cycle on with a what seems like a delayed ignition. but when outside temp is mild and unit comes on periodiccaly it cycles on fine inducer comes on burners like great. only noticed when really cold outside temp the burners will light with a slight boom almost as delayed ignition.

    I was taught in my many combustion training classes, that anywhere in the country where the outdoor temperatures drop below 30 degrees, you shouldn't be using 100% outdoor air for combustion.

    Adding an open tee indoors on the intake pipe just after the vent enters the structure, will most likely solve this problem, and eliminate the problem of drifting snow over the intake vent.

    Combustion is based on the ability of molecules to mix with each other. Cold air doesn't mix well because it is too dense.

    Cold air can cause a furnace to become fuel rich because of the lack of mixing with cold air.

    Even an ASHRAE study in 1995 reported that cold air is not good for combustion, and can lead to poor combustion.

    Give it a try and report back with your findings.
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
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    Heating help dot com is having the same discussion. Here is NCI's instructor explaining it


    captainco Member Posts: 433
    December 8
    Is piping cold outside air directly to a burner a good idea? What happens to things when they get cold? They contract and bunch together. Their molecules move slower. Their surface area is reduced. You might compare this to putting sugar in cold tea versus hot tea. Doesn't seem to mix in the cold tea very well. If a burner is set up when the outside air temperature is 40 s degrees what will happen when the outside air is 10 degrees? The combustion process will totally change. Like it or not, combustion air needs to be heated to mix properly. Also during the off-cycle, cold outside air is still being pulled through the burner. This will make the oil in the drawer assembly cold.. Cold oil creates poor light-offs. If you ever wondered why a burner that was set up perfectly at the beginning of winter is all of a sudden sooted up, this might be the reason.

    Mechanical combustion air (Fan in a Can) is the best way for combustion air versus piping it directly to the burner. So little is understood about the best way to supply combustion air or how much is really needed. If air is piped directly to a burner it is going to have to be set up on the lean side to be safe!

  4. #4
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    Interesting, I wonder why I'm not seeing this issue with package units ?
    We have many days in the teens here in northern Ohio and I haven't seen any delayed ignition problems.
    Even the direct vent residentials seem to fire right off.

  5. #5
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    Quote Originally Posted by VTP99 View Post
    Interesting, I wonder why I'm not seeing this issue with package units ?
    We have many days in the teens here in northern Ohio and I haven't seen any delayed ignition problems.
    Even the direct vent residentials seem to fire right off.
    It was explained that using a combustion analyzer you can see the difference between 50° air and 10° air. More so with oil because you adjust the amount of air for combustion. Is there a air adjustment on gas RTU's?

  6. #6
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    Quote Originally Posted by pecmsg View Post
    It was explained that using a combustion analyzer you can see the difference between 50° air and 10° air. More so with oil because you adjust the amount of air for combustion. Is there a air adjustment on gas RTU's?
    Is the OP using oil ? I didn't look up his model number if so that would make more sense. With oil your relying on atomization of a liquid fuel.

  7. #7
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    Quote Originally Posted by VTP99 View Post
    Is the OP using oil ? I didn't look up his model number if so that would make more sense. With oil your relying on atomization of a liquid fuel.
    The York TM9Y model the OP mentioned is a natural gas or LP furnace.
    Instead of learning the tricks of the trade, learn the trade.

  8. #8
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    I have had this happen to a hanging propane furnace in a greenhouse. Furance is way oversized for the space, we had to bring in fresh air to avoid starving the flame, now I see why it has the occasional delayed ignition. Not sure how to fix, other than a smaller furnace sized for the space...which has been discussed...
    Philippians 4:13
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    Apostle Paul inspired by GOD.

  9. #9
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    It's always a tough sell telling people that youd like to install a tee for combustion air from inside .
    Then you go through the whole battle of, well ....doesn't that reduce the efficiency? Then you explain to them about denser air and molecules mixing and usually they don't get it and you end up having to tune the furnace to run higher oxygen to avoid possible problems when it gets really cold. So now they lost efficiency anyhow.
    They may not notice the problems , but a combustion analyzer would.

    On package units I haven't seen any delayed ignition but it could happen I'm sure and I can certainly see a change in combustion if I'm up on the roof at 0°f with a combustion analyzer as compared to 40 or 50 degrees.

    Sent from my SM-G950U using Tapatalk

  10. #10
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    For oil, throw this in the mix also. The effects of combustion on cold oil. Outside tanks should have nozzle line heaters. Technically all oil burners should use nozzle line heaters.
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  11. #11
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    Thread Starter
    Ok Guys i held off till i could get to the customers home. Iam in canada so the weather just turned brutally cold today. put a tee on the combustion intake 2 feet just inside the home Crossing fingers. customer will advise me if it worked will give him a few days. This unit is i Natural gas unit. 2 stage.

  12. #12
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    Quote Originally Posted by Mobiletec View Post
    ..... put a tee on the combustion intake 2 feet just inside the home Crossing fingers. customer will advise me if it worked will give him a few days.
    Thanks for the update, and we will be standing by for your report.
    Instead of learning the tricks of the trade, learn the trade.

  13. #13
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    If this is all in a small mechanical room, and you added the tee to the intake for the furnace, would there be a little concern about causing a back draft on the natural draft water heater sitting next to the furnace? In other words, should make-up air also be brought in just in case the termination drifts over with snow? Just playing devils advocate.
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