Oil Furnace Short Cycle

Jeff Lawrence_24

Is there an air filter in the ssytem anywhere?

That happened to me a while back with a gas furnace and the evap coil for the A/C was plugged up with dirt.

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Scott B

Oil Furnace Short Cycling

11 year old rheem imperial oil furnace short cycles. Temp. rise is ok at 70degs(60-90spec), heat exchanger was just cleaned, cfm is adequate, just replaced hi-limit switch but keeps shutting down on high limit. All combustion numbers are good except must have O2 at 7% for good CO(50ppm stable). Stack temp at 450F. Had F6 head in it with 1.10GPH nozzle(CO was off the chart), swapped to F3 head and down-fired to .65 x 70 solid. Fan speed is high. System was heavily sooted on arrival. Don't know why it is still short-cycling. Anyone have any ideas? I would appreciate any thoughts.

Scott B

No Evap Coil

There is a good quality air filter on this system but it has no A/C coil. Return/supply ducts are adequate to create proper cfm for the unit. Air flow at the registers is pretty uniform. I don't think there are any blockages in any ducts. It is a 2 zone system and it short-cycles with both zone dampers open and even with the blower door removed.

lchmb

Has to be something

blocking the air's ability to leave the system. If the blower is clean I would start checking the duct work. Did somebody close the damper's? Is the coil plugged? Something is keeping the heat in there...good luck..:)

Ken D.

Short cycle

Any chance the unit is over fired? Or the blower is going backwards? Are all the registers open? I have seen many times where duct lining has fallen down and partially blocked duct. If you suspect such, then you may have to cut open duct to see. Belt tight? Is a higher fan speed possible? How is the draft? Is the limit switch positioned according to specs? I have also seen extra filters installed in ducts where no one knows about it until it blocks.

Mitch_4

Off on limit

You said heat rise is good,...and you said it is cycling on limit. Cannot be. If it cycles n the limit then the heat rise is excessive. Check for plugged "A" coil, or filter.

Most likely the a coil as it will limit the amout of heat escaping, but allow build up in the cabinet.

Pull the coil and look. I bet they did drywalling recently and its plugged tight.

Mike

Dale

Temp rise

Since the temp rise is ok I think there is a local air flow problem near the limit. I would remove the limit and fit a thermocouple sensor where the limit came out of. See what the temp actually is there. There could be a crack near the limit or some piece of sheet metal affecting air flow. I would also pull the combustion chamber and vent and blow it down from both sides.

Scott B

Air flow at limit

I tested for temp at a point approximately 12" or so directly above the limit in the plenum. It was only reading 147degF when it shut down. Hi-limit is a 180degF spec shut-down. Do you think something is creating some kind of excessive temp build up at the limit without affecting any points above the limit?

Also, are you familiar with the chamber lining in this particular unit? You remove the burner, then you remove the plate and the lining is attached to the plate. The chamber is about the size of a 5-gallon bucket with the holes on one side. The holes on this one are facing down. Do you know if this is the proper orientation?

clammy

things to check

To understand you correctly the burner is short cycling not the fan correct if so i would run the unit with the blower compartment door off and see if the unit cycles on limit if not your filter is either to restrictive or dirty and or your return ducts are blocked or undersized for the fan .Has anything been done to your system recently and has any one pull the fan to ensure the blades are fulled with dust and dirt.Is your high eff filter system new to the picture peace clammy

Scott B

blower door off

Even with the blower door removed while the system runs it still shuts down. The things that are certain are that it definitely is the hi-limit that is causing the shut-down, it must be a build up of heat at the limit. It's not return air restriction. The fan has enough power to pull the blower door shut from a pretty good gap. The hi-limit is new which makes the system run a little longer than with the old one. The hi-limit is in the factory position with the button facing the heat exchanger.

Mitch_4

wont work

Radiant heat from the heat exchanger screws up the read. Limits are positioned and selected to compensate for this. Which is why your limit can be set ot 250° and have the max rating plate temp say 180°. the radiant heat affects the limit setting.

As you do not have an A/C I think you have a problem in the heat exchanger

Mitch_4

also check

the baffles in the heat exchanger area and the cabinet insulation. If airflow is being directed away from the high limit probe, it will trip. The glue on hte insulation may have let go. If it has put it in place and use a strip of tin and some zip screws to act as an insulation retainer and get it back where it belongs.

Dale

Heat exchanger

I guess that if you haven't already done so get the parts drawing for this unit to make sure the orientation of all the assembly is correct. I would pull out the fiber 5 gallon bucket heat exchanger area and make sure there are no restrictions from the outlet to the vent up to and out of the furnace, good time to look for heat exchanger holes too. I think you have an air diversion problem past the limit or a crack/hole in the heat exchanger near the limit. You may also want to take off the upper duct work to prove to yourself that you don't have an airflow problem. If you can use a magnahelic to do an external static presssure check for air flow, although given your temp rise this doesn't seem to be the issue.

Scott B

New Info

I have been told that the problem is due to an excessive horizontal run of flue pipe (20'). Maximum for this furnace per the manufacturer is 10'. Plus the combustion liner holes are supposed to be facing up not down. The liner and burner mounting plate are "one piece" from the factory so it must be a factory defect. They'll cover the part under warranty but not the labor to replace it. I was also told that an "in-line" draft inducer such as the Field DI-2 will correct the long horizontal flue run issue. I've searched Rheem's web site for detailed info on this furnace with no success. Does anyone know a good site to get detailed specs and drawings for specific furnaces?

Student_2

rob

FYI....

some older Rheem Imperials were A series that had problems with chamber and burner mount. There should be a B series upgrade package that you may need. The A series would soot up when they failed. The A series also had a line voltage fan control and limit, these also could be worn out or even inadvertantly reversed. The B upgrade includes a new fan control (24V) and thermal limit. Good Source of info is Ted @ P&N in NJ 856-786-1703.

Dave Flood_3

Limit Switch

Scott,
Is it possible the new limit switch is defective? Reading your posts, the temperatures you've given seem to be in range.


Dave Flood
www.trainingontheroad.com

steve_29

If you have a t87 make sure your heat anticipator is set properly.

Scott B

Could be defective...

Dave, it's quite possible the new limit is defective, but I want to correct the other major issues with this furnace before I start second guessing parts reliability. Unfortunately, the customer has not gotten back to me yet regarding the more important problems like the apparently upside-down combustion lining and the 10' over-the-limit horizontal flue run. Also I want to put it back to the F6 head with the 1.20 GPH nozzle per manufacturer specs and retest. It's like pulling teeth trying to find schematics and exploded views of this furnace. Any ideas where to find such info?