Furnace short cycling and condensate backup


Old 03-04-07, 06:09 PM
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Furnace short cycling and condensate backup

Hi all. I wanted to ask for advice here before enlisting the services of a professional.

First, a little background: My furnace is a Rheem Classic 90 high-efficiency installed in an upflow configuration. The unit was installed new when the house was built by the previous homeowners. We moved in last summer.

During the recent cold spell in February, we started turning the thermostat down at night to 65F from the daytime setpoint of 69F. I noticed that the furnace would begin short cycling about 20-30 minutes into the long runs required to recover the temperature to the daytime setpoint in the morning. Once the short-cycling began, the unit would cycle every 5-10 minutes until finally reaching the setpoint.

I began to investigate and first checked the airflow via temperature rise measurement. The temperature rise was found to be within specification (~63 versus spec of 50-80F). I did notice a puddle of condensate below the draft inducer and discovered that the when the inducer shut off, a few drips would leak from the connection of the draft inducer to the flue pipe (as shown in the picture here: http://home.insightbb.com/~pletch_spam/Inducer-leakage.jpg ). When the inducer shut off, I could also hear a rush of water through the drain lines as the secondary heat exchanger drained of condensate. The negative pressure from the blower motor must be causing a few inches WC to be retained in the heat exchanger.

The downstream pressure switch was also found to be waterlogged and I assume that the switch tripping is the cause of the short-cycling. Listening to the inducer when it runs, I think there may be some water collecting in the housing as well.

Between normal full cycles to maintain steady state temperature, everything drains and the furnace will run continuously. It is only on long cycles that the short cycle is observed. However, I do always see a few drops of water leak from the previously mentioned spot any time the inducer shuts off after a cycle and I always find evidence of water in the pressure switch tubing.

I have cleaned the condensate trap, flushed all of the condensate drain lines, and ensured the lines were kink free. The furnace is installed level. At this point, I am confused why condensate would back up into the secondary heat exchanger since the drain route appears open. Any suggestions of other potential root causes or possible remedies would be greatly appreciated.

Here are a few more linked pics:

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Old 03-04-07, 06:54 PM
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I don't know where the trap is that you spoke of but it isn't in any of the pictures. I suspect that there SHOULD be a trap in the condensate line where it exits the side of the furnace. There would also need to be a cap on the short vertical piece of pipe. I recommend the EZ-trap. The clear U-bend is helpful to see what is hapening in there. Here is a link. http://www.eztrap.com/documents/brochures/S2B1-EZT-107-Brochure.pdf
Without a trap, the air entering the drain keeps the water from exiting until the furnace stops.

Old 03-04-07, 08:10 PM
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Condensate Trap

Thanks Kfield.

The condensate trap is located in the main blower compartment. First time around, I didn't bother to take the door off to snap a pic.


I am quite sure the trap is liquid full. I ensured I filled it when I flushed the drain lines.

You do propose an interesting thought though...

I wonder if the inlet nozzles have subsurface dip tubes on them and the one on the drain line from the secondary heat exchanger is broken off or something. Then, you wouldn't maintain a liquid seal on that side of the condensate trap. Unfortunately, there is no way to disassemble the trap and inspect the interior from what I can see. It is opaque molded plastic.

I'll look into this further.
Old 03-06-07, 03:16 PM
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Trap OK

The dip tubes in the condensate trap are ok and the trap is full of water.

Maybe the gasket for the inducer fan is embrittled/degraded allowing air into the system which disrupts the flow of condensate. This would possibly explain why condensate drips from the inducer after it stops at the end of a cycle.

I have someone coming to take a look at the unit Thursday morning. I do not have the motivation to yank it off and deal with resealing it to the collector box.

I will relay the technician's findings to this post for posterity.
Old 11-07-08, 03:39 PM
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Similar symptoms on 80% Rheem Classic

I did a remodel and now have a brand new Rheem RRPL-B 4-ton 80,000 BTU outdoor gas-pack. I got a RiteTemp 6020 thermostat at Home Depot and hooked that up, including the C wire so I don't have to use batteries.

I'm seeing similar symptoms to DrPressure, although he's saying after 1/2 hour, whereas mine happens right after turn-on, for the first half hour or so. I haven't sat still and observed it all the way to the set point.

We have a set point of 69 F, and turn it on in the morning with the house at 65 F and the unit outdoors at 35 F.

The unit short-cycles for the first half hour or so, about 7 minutes on, 1 minute off. A funny thing is that I can hear the relays clicking in the T-stat near the start and end of cycles. It's nowhere near the set point, and the mfr. says this T-stat has no heat anticipator feature, since it's digital and uses a low-mass thermistor.

Once things are warmed up, it seems to function fine. With the house at 70 F, I can set the T-stat to 78, and the unit will kick on and run for at least 20 minutes.

I've had our dealer tech look at it, but unfortunately it was after warm-up and it functioned fine. He adjusted the gas pressure anyway, and took it down from about 78,000 to about 67,000 BTUH. No help there.

Any ideas? I'll try bypassing the T-stat tomorrow morning.

- Ned

Last edited by nedflanders; 11-07-08 at 04:07 PM.

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