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Old 03-11-2008, 03:01 PM   #1
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


Hi to you all!
I have a dilema with my RUUD Pack Unit furnace. Every day the first light off of the furnace causes a "KABOOM" just after the thermostat calls for heat! This doesn't happen everytime the furnace lights, only if it has been off for maybe 6 hrs or so! We have a wood stove that we use most of the time when it is real cold, maybe below35-(VA) and the gas (LP) furnace if the fire goes out over night or it is borderline too warm for fire but too cool inside. The unit is about 12 or so years old and we have had no problems with it until this winter. We recently had the unit serviced to see if any problems were apparent, but none were evident! The service tech checked manifold pressure on the gas, which was right on the specs and cleaned/blew out the unit w/ compressed nitrogen to remove any dust/dirt/ debris. The unit lit perfectly for him with all blue flames on the three burner tubes with no "kaboom" (of course the unit had already kaboomed for the initial start up of the day) I am a Master Electrician and a fairly knowledgeable person with some common sense in many different fields and am an avid DIYer. I generally take care of whatever I own and fix what needs fixing but this has got me stumped. I clean the unit and inspect the combustion chamber and oil motor bearings about every 2 yrs and don't see anything wrong! The other day I removed the 2 burner panels to try to watch the unit start for the day and nearly had my face roasted by the flames flying out of the unit--twice! Anyone know what could be causing this problem? I have heard the neighbors pack units do this for years but they had the whole units replaced! I am not ready to go to that length until repairs can no longer be made to my unit or it is unsafe and cannot be fixed!
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Old 03-12-2008, 08:33 AM   #2
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


some how the pilot track(if you have it there)that lites the burner sections is delayed and your getting the BOOM with the chamber building up gas...all 3 chambers should lite up close to the same time check the track with a wire brush or view how the flame gets to the burners to lite them off.check the orifices off the burner header(removable with a box wrench) should be clean no soot.with the long OFF time as you describe is it possible the heat exchanger gets cold overnight and causes a lack of combustable air(too cold)with a lower temp dropping down into the exchanger.when you say it is OFF your basically doing a lower night temperature setting there.if it doesn't do it when the unit is maintaining the higher occupied temp and is staying warmed up within the exchanger...it has to be the lack of temp on the pilot track..check it out might see something.
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Old 03-12-2008, 09:10 AM   #3
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


Thanks for the info. This unit doesn't have a "pilot light" per say it is electronic ignition. There is what looks to be a "spark plug" with a large orange wire like a spark plug wire in a car. I will remove the burner tubes and orifices and do a through clean w/ wire brush as you suggest. I will post my findings later. Thanks again.
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Old 03-12-2008, 03:42 PM   #4
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


OK,so it is a spark then pilot lights proofs out,and the main comes on....take a look at the PATTERN of how the flame gets to the burners to the left and right if the pilot assembly is situated in the middle.once that spark stops and the pilot is lit the main gas will open...see if the burners all lite up and witch lags... that's the boomer.
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Old 03-16-2008, 06:02 PM   #5
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


Biggles, I took the furnace burners out to examine them more closely and there is nothing visably amiss! The burners are stainless and other than obvious heat discoloration, are perfect. The cross burner tube that ignites all burners is stainless and perfect also, with all slots open and clean-(no soot). I wire brushed them anyway! The igniter is over by the far right burner tube and the end of the crossover tube is right by the hooded pilot assembly. The ignition process happens so fast that it is hard to see any delay on any tube! The igniter has been cleaned and I sanded off the ground path for the spark above the igniter to shiny stainless in case it wasnt getting a clean spark right off the bat. I also removed the igniter and the small gas nozzle that ignites the crossover tube, it had a little rust inside and I couldnt get much flow blowing by mouth, but it must have a small orifice to restrict the flow to a minimum. I cleaned out the rust and put it all back together and it works fine with a small "soft" blue flame. When I returned the unit to service it still "booms". I don't know what else could be causing the problem. Any ideas? The ambient temperature was about 65-70 so I don't see how "cold combustion " air is an issue. Ithought I may have a small internal gas leak that builds up over night and fires off on the first light up, but I can neither smell gas nor find any leak with soapy solution. Could the gas valve be defective and sending gas too fast to the burners before the pilot has a chance to start things correctly? Also I removed the burner orifices and cleaned each of any soot or obstructions that might send gas outside the tube. That is the extent of all I have done to this point.
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Old 03-16-2008, 07:45 PM   #6
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


the leakng gas valve in the off position would burn off if the pilot lit on a call for heat the BOOM has to be the gas exiting the orifice and mixing to ignite...do you have adjustable air mixing covers where the orifice fits into they should all be the same setting on the openings.might try throttling down the main gas valve at the furnace(on the supply pipe into the unit) to lower the gas pressure to TEST if the BOOM is eliminated?changing the main GAS valve and with no positive results seems like the last thing to try.try the throttle down just slightly to lower the burner flame and cycyle the unit after a cool down(if you have AC on that furnace put the fan in the on position to accelerate the furnace coolng down.i am thinking... blocking ONE ORIFICE at a time and testing might narrow down which chamber is BOOMING...piece of duct tape over the orifice wrapped around the header will starve the chambe(just for testing OK on that one)QUESTION have you been in front of the furnace when it BOOMS want to know usually you will get a flame moving off the burner track if it is happening...just a thought here any chance your ducts are booming from the fan starting.that woul be aced out if you hear the boom before you feel the heated air.if it might be the fan starting, check it out with the fan from the stat...then it could be heated air BOOMING in the cool duct on start
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Old 03-16-2008, 10:03 PM   #7
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


the burners are fixed air...no adjustment. it is definitely not fan booming! I have watched light up and nearly had face burned! it is like gas floods into burners faster than pilot can ignite properly. its like throwing a match into chamber of gas...a big WOOSH of flame. I will try to eliminate one burner at a time, but might take a couple of days since need several hours of cool down(as I mentioned in outset only does this after unit been off for hours or overnight) I will try to lower gas pressure after that. (btw which way to turn allen screw to lower pressure?)
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Old 03-17-2008, 07:29 PM   #8
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


just adjust the main shut off outside the furnane on the line coming into it,just a last suggestion how about flipping the orifices around to get the chamber that booms closer to the pilot burn on start.your gas pressure might be to high but you sad this never happened before this running this fall/winter a tech needs to read the WC(how gas is read with the valve open)and the burners running.touching the adjustment screw under the cap changes the burner temp from its design...but FYI SCREWING IT IN (CW)OPENS THE GAS MORE FLOW (CCW) takes the pressure off the spring,and throttles the gas down thru the valve....count your turns either way,and do a 1/4 turn at a time
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Old 03-17-2008, 08:11 PM   #9
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


i had a gas tech come a few weeks ago to check things out for this problem. he checked the gas pressure in the unit with manometer and it is exactly where the specs say about 10" wc. are you suggesting i adjust the main regulator from the gas tank or the one inside the unit with the "on/off" knob? when you say "flipping" the orifices around do you mean change their location respective to one another? Didn't you want me to block them off one at a time with tape? i'm getting confused with what you are wanting me to try.
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Old 03-17-2008, 08:43 PM   #10
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


ACE that idea of the tapping off of the orifices your still going to deal with hi gas pressue trying to ignite.standing in front of your furnace you should have a unit shut off on the line into the furnace throttle that down a little and restart the cycle.on the 'flipping" the orifices....if you pick up the booming chamber move that orifice closer to the pilot ....check the tag on the furnace 10" WC sounds awfally hi should be stamped on min/max info.
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Old 03-19-2008, 09:43 AM   #11
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


Inlet gas pressure should probably be in the 7-10" WC range, but your manifold pressure to the burners should normally be in the 3.5" range. Make sure this is the case. If your gas valve is set up improperly, then you will not only cause dangerous conditions ( ie. BOOMING) , but you will also have incomplete combustion and could cause damage to other parts. You should probably also have your heat exchanger checked.... everything else you wrote about seems to be working as per design. Hope this helps.
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Old 03-19-2008, 01:54 PM   #12
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


"statman" is correct in that manifold pressure should be 3.5" wc, inlet supply pressure should be (according to label on unit) 7" wc max & 5" wc min. The "TEST" pressure is 12" max. The gas tech checked on the inside regulator and found 10"wc. I don't remember weither he removed the allen screw on the down stream side of the valve to reveal an opening to hook his manometer to or if he hooked to something else but I remember that it showed 10". This unit is 14 yrs old from date of manufacture (on label)and I purchased new. It was originally set up for natural gas but was converted with factory provided jets and such. Could the supply regulator or internal regulator have simply gone bad and be allowing too much gas pressure through beginning this year? Logically, in my mind, if such was the case, I would think the booming would happen each and every time it lit...but it doesn't. It happens ONLY after been off overnight or off 4+ hours. After that lights smooth as silk each time! The gas tech said that he has noticed complaints now and again of this happening after it gets extremely cold outside...he said that occasionally moisture is present in the regulator(main one) and it freezes in the regulator or makes it stick! Their solution to this problem is to wrap a cotton rag around the regulator to prevent cold temp from freezing moisture. He said that had worked on most cases in his experience. The flames on the burners are not raging or flaming wildly as if gas pressure was way out of specs...they burn evenly and nice and blue, only the slighest hint of any yellow! Guess I may have to get a RUUD service person to come out next time and pay AGAIN for service! Thanks for all the input guys! I will post more info as I can on my success/ failure with this problem.
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Old 03-19-2008, 09:11 PM   #13
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


First I wanna say I haven't a clue as to anything about furnaces.
But wanted to throw in with my little bit of both technical(ford tech) and life experiences.
From reading everything I was leaning towards a volume/pressure issue,not from any real knowledge just because the symptoms point to a rich mixture which could only be caused by 1 of 4 thing. F.A.S.T
Fuel-To much?
Air-Not enough?
Spark-Weak?
Timing-Late?
Common sense as a technician makes me follow that path pretty much. Everything seems okay it looks like though from your inspections.

I have personal experience from doing a swap from natural gass to propane though on an electric stove. It was also done by a licensed person with factory parts,but never(even to this day as we still have it) has it since cooked anything right.
we have to cook everything in the oven at 250*,because there is no more adjustment persay. Anything less and it won't cook,anything more and it will burn. We don't cook at that house(Lake house) so we never cared,we've got a grill.

What I'm thinking is that maybe density difference's between natural gas and propane gas are causing flow problems while not necessarily having pressure issues.
Sorta like if you change from regular gas to race gas,you get more burn/bang with less consumed volume.

Just an idea..
And the specs your looking at for the 5" HG and stuff,are they written for the propane or natural gas? I remember someone telling me the difference between the two gases wasn't much but I would think it would be a considerable variable since you seem to be having abnormal combustion.

Last edited by DefEddie; 03-19-2008 at 09:13 PM.
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Old 04-03-2008, 11:11 PM   #14
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


I have a Lennox Pulse furnace that does exactly as you describe. Only happens on the first startup after sitting awhile. Gas is propane. New ignitor, spark plug, filters, inlet air filter, etc. I haven't been able to figure it out either. I did read in another post while hunting on the net, that it could be low gas pressure. At first i didn't think that sounded right. But it could create a boom with a minimum amount of gas trying to light. I have not changed my gas valve but thought it could be that also. If you find out the problem, I'm interested, David
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Old 04-04-2008, 04:12 AM   #15
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Ruud Pack unit "KABOOMS" upon First Light Up of the Day!


This post is a couple of weeks old so I hope you have resolved this but if not first of all if this unit has a window in the bottom door then that door is an integral part of the combustion chamber and ignition should not be initiated with it off if you notice the little draft fan/ventor motor on the horizontal shelf is pushing combustion air down and forcing the flames up the chimney with door off no draft and singed chin whiskers.
Delayed combustion is usually a gas pressure problem or the igniter is not perfectly set into the gas stream because your problem only happens on cold start that makes it more interesting the only thing I could think to do would be instead of blowing out the cells with nitrogen would be to actually use a heat exchanger brush and clean each cell to see if improper combustion in the past may have sooted up the chambers. I would think if you actually had a crack in your heat exchanger 1. you would only have combustion issues if the indoor fan was on during ignition and 2. it would happen all the time
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