Furnace shuts down after a few cycles

[archive]

I have an older RED T LP furnace model DLAS075ADC. The air induct motor will kick on, then the heat element will light, then the gas will fire. Sometimes the gas will shut off after about 45 seconds and will try to relight. After 3 unsuccessfulls, I will have to turn thermostat all the way down and then back up to my set temp. Then it will run fine. Also, it sometimes will cycle for a shorter than normal period and the blower will shut off but the air inductor will remain running and it will cycle through again. Last night it went into safe mode around 1:30am. I reset it and it's running fine again. This can't be normal. a repairman said he can't find anything wrong, no codes, and it cycles fine for him everytime he's here. It never shuts down while he's present. Help! (When I say air induct motor, I think I mean booster motor. It makes a sort of whirring sound as well.) Also, flame appears to be yellow and irratic even before the blower kicks on.

pvplyr

[archive]

No it is not normal but the problem is intermittent. Very hard to diagnose when there at your home and almost impossible to do from here. Could be many things. It needs to be caught in the acting up cycle and checked at that time for component failure.

[archive]

I've been checking on return air. Could this affect it? Some of my vents are covered with rugs or furniture and can't get the full intake it would need.

pvplyr

[archive]

Yes that could definitely do it. All registers open and all return unblocked. The furnace has to have air to work.

hvac1000

[archive]

OK.Had the repairman come out. He cleaned the combustion area and the flame sensor. Of course everything ran fine while he was there. After about 4 hours, it started up again. Booster motor still running and cycling through after blower had shut off. Booster motor is still a little rattly but will go quiet if I hold on to the casing. Is the closet door to the furnace area supposed to pull shut when the blower turns on? After going into ignition failure, I reset the stat to about 74 degrees and it went in to regular cycle, but shut off after about 15 to 20 seconds after the burners ignited. Then it tried again and ran through the entire cycle flawlessly. Repairman was called and he thinks it might be the circuit board (brain) going bad. It is a honeywell that he he installed about 5 years ago. This is driving me crazy!!!!

pvplyr

[archive]

Having a heck of a time with this now. Flame sensor was cleaned on Thursday and by the same night it seems to be harder to keep lit. Goes through normal cycle and lights but sometimes the buner goes out after 15 seconds, sometimes 45 seconds and sometimes it actually runs completely through. Seems to run better when Tstat is set at 80 + degrees and the burner has tried numerous times to light. Repairman thinks possibly the ignition module again due to it's random behavior. I have to manually reset at least 4 times before it will cycle complete. Also, when in ignition fail should the inducer motor still be running?

pvplyr

[archive]

Did the service man test/check the hot surface igniter. They have to be in a impedance range for them to talk to the circuit board correctly. Also how did the service man clean the sensor? Light sanding OK but no metal scrub pad allowed. You paid for serviced and he did not correct the problem. He should come back with out charge and fix it correctly.

hvac1000

[archive]

The good news is I haven't paid him yet. my wife was there when he cleaned it, I was not so I don't know how it was done. It did, however, work good for about 4 hours after it was cleaned. I've also noticed that before the flame goes out the LED light on the board shuts completely off for a split second then comes back on and continues to pulse normally until the flame goes out about 2 seconds later. He said he also wants to cjeck out the duct work because a CA unit was installed after the furnace and the furnace blows hot air into the CA return vent as well as through the CA unit outside the house. It does not work anymore so I would like to completely disconnect all the ducts and tubes going to the CA and put the furnace back to original state it was before CA installation. The person that installed it years ago didn't install it properly I don't think.

pvplyr

[archive]

IF the entire A/C is self-contained outside with return air and supply ducts running to it, it should have had a close off damper placed under the furnace .

Should also have had one placed in supply duct where it connected to main trunkline.

You would also have a return air for a/c only in floor of home to be covered/closed off during winter.

ALL combustion air should come through sealed combustion flue/roof-jack or from underneath the home.

The air coming through closet door area would be return air, not combustion air.

The Tech sounds like he's guessing and grasping at straws at your expense. He needs to troubleshoot, locate fault and repair or say he doesn't know.

The remote sensor should be cleaned gently with steel wool and should not be touched with bare hands/fingers.

Take care and best wishes,
Robert

[archive]

Thanks guys for the responses. it will be in the 5os here all week so I plan to gget under the house and check all of the duct work. In all my years of being in this home, I have never been told of a damper under the furnace to the AC. It was my parents house before me. That would definitly explain the hot air going outside.

I was up on the roof the other day, I have no inlet vent up there. There is a hole cut into the floor in front of the furnace for air from under the house i presume. It seems the flame gets weak after about 30 seconds or so then burner goes out. It almost looks like there is not enough O2 to combust properly. The buner will continue to light and run for about 15 to 30 seconds until the chamber heats up enough to kick blower on. Then once it has heated up in the combustion chamber I can usually get it to cycle through completely. Does not make sense.

pvplyr

[archive]

Hi,

Whenever you install a self-contained A/C (entire unit outside), you install a close off damper under the furnace that keeps the cold air during a/c mode from blowing up through furnace (it is a waste and also will rust out heat exchanger).

You also install one where the supply duct from A/C connects to home main trunkline to prevent the warmed/heated air during Heat mode from going down through the A/C (wasted air).

You will cut a hole in floor and connect the A/C return air duct to it for return air during A/C mode.

You cover it during Heat mode.

Your combustion air will come through a double walled sealed combustion flue/roof-jack OR from underneath the home, depending on age of home/unit and set up.

Make sure burner and orifices are clear and not blocked by spider/spider web.

Thanks,
Robert

[archive]

the AC return air vent is in the living room and does have a piece of shhet metal that covers it in the winter. That's the only thing I was ever told to cover. There was some debris and such in the combustion area and around the burner orifice that was cleaned out during the flame sensor cleaning. Also, when the AC used to work, the furnace blower would run to blow the air. If I gather correctly from what you say, this should not happen either. Man the installer of these units really did not know what he was doing! I have alway had problems keeping the furnace running but they have never been this bad. It is getting pretty old though. About 12 years or less.

pvplyr

[archive]

Look at the outside A/C unit, does it have 2 large flex ducts coming to it ?

Does it have 2 refrigerant lines coming to it ?

Is there an A-Coil at bottom section of Furnace ?

IF it is a self-contained A/C, the indoor furnace blower should not run during a/c mode.

IF you have a split system with A-Coil inside at furnace, then the blower in furnace should run.

Thanks,
Robert

[archive]

There are 2 flex ducts and I'm not sure of the refrigerant lines. There is no Acoil on the furnace that I have found. It was an older ac unit and I think it was all inside its own housing.

pvplyr

[archive]

The blower motor for air circulation is inside the a/c unit. There is a t-stat with a lockout feature to keep the furnace and a/c unit from running at same time.

MOST HVAC Techs know little about MH units and these self-contained units are installed like yours often.

One of the flex ducts should be the supply duct and connected to the main trunkline.

The other should be the return duct and connected to floor return air grille in the home.

Thanks,
Robert