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Discussion Starter · #1 · (Edited)
New Thermal Zone 2-stage furnace (apparently Rheem's budget label) that keeps cycling even when the T6 Pro thermostat is set at the same temp as the room (so theoretically no heat call). Cycle: 30-second inductor fan purge, 30-second low heat firing, then heat stops and the main fan runs for 90 seconds while giving a low pressure sensor error code; then the cycle starts all over again. Does this for 4 or 5 cycles then quits and status code goes back to "0". If the room temp drops, then the furnace responds by going to high stage until the call is met, but only after cycling 3-5 times as previously described. The installer has ordered a new control board but I'm curious if this is some sort of intelligent or comfort mode that we're not familiar with. Any ideas? Thx
 

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New Thermal Zone 2-stage furnace (apparently Rheem's budget label) that keeps cycling even when the T6 Pro thermostat is set at the same temp as the room (so theoretically no heat call). Cycle: 30-second inductor fan purge, 30-second low heat firing, then heat stops and the main fan runs for 90 seconds while giving a low pressure sensor error code; then the cycle starts all over again. Does this for 4 or 5 cycles then quits and status code goes back to "0". If the room temp drops, then the furnace responds by going to high stage until the call is met, but only after cycling 3-5 times as previously described. The installer has ordered a new control board but I'm curious if this is some sort of intelligent mode that we're not familiar with. Any ideas? Thx

I would’ve tested the thermostat prior to ordering a board. It very well could be the board though


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Discussion Starter · #3 ·
I changed the tstat to show 1 stage furnace and flipped the 10-minute AutoStage function DIP switch on the board to force it into a different mode, but it still does the cycling thing. The installer originally put in a T6 tstat with only 1 heat mode and I think it was giving that same low pressure error code as well, and I think that's why he upgraded it to a 2-stage tstat.
 

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If you’ve checked the pressure switch and the thermostat then it’s probably the board


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The problem is the low pressure code.

It is not running long enough to heat your house because the switch is opening.

Could be a restriction in the venting or it is too long or not sloped back to the furnace properly and is waterlogged.

Point is he needs to solve why the pressure switch is opening.

Rheem used to have a entry line called Weather King and maybe they changed the name. Actually a pretty good quality unit and not a el cheapo unit.
 

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Discussion Starter · #8 ·
Installer even disconnected inductor exhaust vent pipe from the unit and let it run without restrictions at all. No change. The manual says the low pressure error code will prevent the low heat burner from igniting, but it ignites fine during the cycling.
 

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Discussion Starter · #9 ·
So far I'm pleased with the setup; just want to see it operating correctly. I didn't want to put a lot of money into a top-of-the-line model for this house because property values here are starting to decline. The system replaces a 450,000 BTU gas fired boiler that finally quit working. And now I have cooling for the summer.
 

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The pressure across the affected switch should be checked with a manometer.
 
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Discussion Starter · #11 ·
Thanks for the comments. I still don't understand why the furnace is cycling at all when the temp setting on the thermostat is the same as the room temp. Think we should put a voltmeter across the W1 wire to check for a 24V heat call? Is a humidistat calling for these cycles? When the 2nd stage heat comes on (finally) I get lots of water draining out.
 

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The reported temperature is rounded and stats work in fractions of a degree internally.

Honeywells use an algorithm to determine cycle length, on/off times, not a fixed differential.

Good stats allow you to adjust how aggressively to cycle - for honeywells, it's a cycles per hour setting. They come from the factory set to 5 when 3 is more appropriate for most systems.

If the stat indicates a call for heat, it's fine. If not, check for voltage between W1 and C with door switch depressed.
 

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Discussion Starter · #13 ·
If the new board doesn't correct the problem, I'm sure the installer will next check the low pressure switch itself. May just be a stuck lp relay on the board. When I read the serial number to register the unit, I noticed that the furnace was mfg'd June, 2017! Basically an NOS furnace that's been sitting in a warehouse somewhere for 3 years. Installer says he was not aware of this.? When he first unpacked it, the condensor overflow device broke in his hand, probably from being brittle. Not happy about this, but the coil and outdoor condensor are fresh. (Overflow device has since been replaced)
 

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Lets start in the beginning as your description is a bit confusing.

The furnace should not try to start unless there is a W1 signal.

If there is none and it tries to start then you have a electrical problem. Is that what is happening?

Some Honeywell thermostats/tstats ship with a wire cheater device in case you do not have enough wires to hookup C at the tstat. Did he use that?

If he did it can confuse and screw up circuit boards and they should all be thrown away . My company will not let use use then because of all the problems and we run extra wires.

If it is acting up while there is a call for heat then we have another problem. If the intake is partly plugged it may not produce enough draft to close the low fire pressure switch but on high it may.

Let us know what the problem is.
 

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Discussion Starter · #16 ·
It's a new install in a 1920's home. New thermostat line; 8 leads. T6 Pro 6320U stat for 2 stage furnace and 1 stage condenser. Dedicated 110V line in. Intake for inductor is near the furnace and exhaust is routed through an opening in a header to outdoors with minimum of bends. I have not measured for voltage at W1 when the furnace cycles but about half the time it idles and returns a "0" code on the readout without ever igniting the High burner and I've just assumed from this that there was never a heat call from the stat. When it goes through the cycle and the low burner fires for 30 seconds before cutting off, the main blower circulates a little warm air but blows at room temp by the time it ends the 90-second delay. If I set the thermostat 3 degrees above room temp to force a W1 call, the furnace still goes through 2 or 3 cycles before the High burner stays on and I get good heat.

I'm not sure why the low burner would be able to ignite and burn for 30 seconds when it cycles if there is a low pressure sensor fault. Wouldn't it fail to ignite at all?
 

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Discussion Starter · #17 ·
The comments on the tstat function by "user_12345a" was helpful because I've suspected this cycling is part of a comfort feature. But yesterday I re-programmed the tstat to show only 1 heat stage and flipped a DIP switch on the control board to put the furnace in "AutoStage" mode. The intent was to override the algorithms in the T6 tstat to test for basic heat function. The AutoStage setting ignites the low heat burner for 10 minutes whenever there is a W1 call, then ignites the High burner if the heat call persists after the 10 minutes. After running the tstat 3 degrees above room temp to force a W1 call, the furnace still went through the cycling (30-second purge, 30-second low stage fire, 90-second main blower after low stage burner extinguishes with a low pressure error code). The tstat is re-programmed back to its original 2-stage settings now. I eventually get heat, but it seems that the furnace is going about it with a lot of extra steps.
 

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Low pressure switch opening fault is internal to the furnace and has nothing to do with stat settings.

Need to check and see if pressure switch is actually opening during the cycle. If it is, can check pressure across it to see if the switch is failing or there's a problem reducing the draft.

Plugged condensation lines, blocked exhaust/ combustion air venting or venting done wrong, plugged pressure switch hoses, inducer problem can cause pressure switch to open.
 
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Is the furnace properly grounded?

Some high end units will give you a error code if it is not grounded properly.

Grounding is critical in newer units and can cause weird problems.

Check from the furnace to a copper water line if nearby and use a cheater wire from the water line to your meter if necessary. Should have almost 0 ohms resistance.
 

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Discussion Starter · #20 ·
Thanks. I'll try that. I ran a voltmeter across the low pressure switch and it reads OK. Voltage dropped when the inductor fan started up.
 
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