Delayed Ignition


Old 12-29-02, 08:04 AM
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Angry BOOM noise when furnace starts

I have a Ruud forced air HVAC unit (installed 1989 - I think). It has made a loud BOOM noise when it starts up (but ony intermittently ... then afterwards it works fine. I have had an HVAC person visit - all they did was clean the burner - but it still makes the noise.

It is obviously a delayed ignition. I can think of only three items that may be causing it: the fan starts up late before the gas flows so there is a build up of gas before the ignitor (hot surface) glows; the regulator needs adjusting; and/or the burner is "fatigued". The HVAC person thinks it is the burner.

Does anyone know if this BOOM is (1)dangerous and (2) what the source of the problem is?

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Old 12-29-02, 08:24 AM
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The gas valve won't open until air flow is first proved and then a delay period elapses to allow the ignitor to get up to temperature. Has the module been replaced or the wiring been monkeyed with? Usually this problem is caused by dirty burners, crossovers, orifices. Was the entire burner assembly removed and thoroughly cleaned? I'm not sure what is meant by fatigued burner but you may have a fatigued ignitor. It may not be getting hot enough every cycle. As for the regulator, he should have checked the gas pressure as part of the service call. Are you able to watch the startup sequence? Usually the BOOM comes when the burners farthest from the ignitor lite off late. Dangerous? I wouldn't want anyhting going BOOM in my house. With enough gas buildup, it could be.
Old 12-29-02, 08:46 AM
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Thanks BJ. He had previously just replaced the ignitor and no one has played with the wiring ( as far as we know). The burners - he stayed up there to watch the sequence but unfortunately the phenomenon did not occur despite putting the unit through a number of start-stop cycles. Also, he said he could really only effectively clean the burner main conduit, and did not clean the branches. "Fatigued" was my terminology. He just thinks that the burner is way old and worn, and may be the source of the problem. He did not check the gas pressure. I think that the gas line to the unit is 1/2 inch (I thought it is supposed to be 3/4").

They suggest changing the burner - which I will probably have to agree to since I am not the pro here.

Thanks anyway. I truly appreciate yout prompt reply.

Old 12-29-02, 10:44 AM
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Hello: Terrance

In my opinion, the service person was plain lazy, inexperienced or not knowledgeable on the problems of delayed ignition.

Intermittently or not, the delayed ignition problem, the reason and location can be determined whether it happens while on site or not, if a service tech knows the job. No excuses are allowed, if I were in charge, worked with me on the job and or that person worked for me. Period.

Is delayed ignition serious? You bet it is. If the condition continues there will be serious damage to the the heat exchanger. {firebox} Once tfirebox damage happens, it's too late. Furnace replacement is the only correctional means.

Additionally, there is no reason why all or any of the burners cannot be removed, inspected, cleaned and or replaced on that furnace or any other furnace, period.....> As in no excuses....> case closed.

Ambient temperatures can and often do cause expansion & contractions of the burners and crossover ports or slots. Same applies to the burners, especially ribbon style burners.

Cold ambient temps allow contractions of some parts and most likely to the burners, which can cause intermittently delayed ignitions. May be the reason your furnace has the problem intermittently.

If the cause of the delayed ignition problem cannot be visably witnessed while on site, all burners must be removed, inspected and cleaned. Any burner or part where any doubts may exist must be cleaned, serviced and or replaced to correct a delayed ignition or proper flame burning problem.....period.

That is what a customer can expect and pays for....when hiring a professional......period! I suggest you contact the company or person you hired and demand a return call free of charge for a second service call fee and demand the problem be corrected.

In doing so, be aware that you will be charged for additional labor and all parts required to correct the problem. Doing so will insure safe and proper furnace operations.

Other resident heating professionals replying within this forum, may have alternate solutions, suggestions, advice, opinions and or repair methods. Check back several times for additional replies.

If you need further assistance, use the REPLY button to add any additional information or ask additional questions.

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TCB4U2B2B Company Enterprises. Energy Conservation Consultant & Gas Appliance Diagnostics Technician.
Old 02-06-03, 08:32 PM
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Problem Solved

Dear bigjohn and Tom_Bartco,

Thanks for all your input. The heating company was incredibly lame ... saying that the burner was no longer avaiable. They lied becaused I simply logged onto Sears, identified the part, ordered it and installed it myself.

... and no more BOOM! when the heater starts up

Old 02-06-03, 09:42 PM
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Couild be???

I knew that Rheem/Rudd, had a major heat exchanger recalls in the early 90's..

They had heat exchanger chang out(and burner), and also new control board and HSI as well..

What's the model?
Old 02-09-03, 07:37 AM
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Ruud Model Number

Hi Jay,

The Model Number is UGUJ-07NA-GR

also the burner P/N I swithed out was 75-22476-14

If there is a problem with the unit as a whole, I would appreciate knowing about it.



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