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Furnace cycling every 6-10 minutes, normal?

48K views 23 replies 8 participants last post by  pacnw  
#1 ·
This is my first post and I'm a brand new homeowner, so I'm hoping your expertise can help me!

I moved into my home in June and today was the first time using the heat. We used the AC all summer and had no issues;worked great!

However, it seems that the furnace cycles every 6-10 minutes. As the system heats up to the desired temperature set at the thermostat, I will hear the furnace ignite, followed by the blower a few seconds later, the whole system will run 6-10mins, at which point the furnace turns off, followed by the blower about a minute later, and pretty much exactly 60 secs later, the whole cycle begins again.

I spent about an hour timing the whole process with a stopwatch: the shortest cycle I timed it running was 6:32 and the longest was 9:10. It was off a consistent 52-57 seconds between turning off and the whole cycle starting again.

It is definitely heating, reaches the desired temperature, and shuts off completely when achieving the set temperature, as it should.

Sorry in advance as I don’t know many technical details of the unit but it’s a central hvac system with the AC condenser outdoors and the blower and GAS furnace in the tight attic crawl space. My home inspection documents noted that it was a 100k BTU furnace. It is a 1100 sq/ft single story home. The thermostat is a Honeywell TH6220D1028. I have verified that the thermostat seems to be setup as a 2 heat/1 cool conventional system and the Furnace Cycles Per Hour is set to 5. As it cycles, I do hear the typical "click" from the thermostat turning things on and off.

Is this normal or is it short cycling? Some things I’m reading suggest that it can be normal for a furnace to cycle up to eight times an hour, but my previous residence didn’t do this, though it was an older hvac system and a two story home. I've also read that if this is truly an 100k BTU furnace, that it is way more powerful than recommended and this could be causing this cycling issue. In the thermostat settings, I also noted that the Compressor Protection Function is set to 0 minutes instead of the default and recommended 5 minutes. Is this a concern?

Any advice and input is greatly appreciated!
 
#2 ·
Sounds like horribly oversized equipment.
Where in the world is this house that requires 100k BTUs for such a house?
 
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#3 ·
Possible one reason the thermostat cycles back on after only one minute is behind the thermostat where the wires hook up to the thermostat subbase the hole is large and it has cold air in that chase, cooling off the thermostat.
 
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#4 ·
The furnace is oversized for your house. It cycles off because an inner safety is getting overheated from inadequate airflow. Either get a smaller furnace or enlarge the duct system to move the amount of air needed by the furnace.
 
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#6 ·
I've reset both Stage 1 and Stage 2 CPH to 3. It hasn't been very cold today, so I haven't had much change to test. However, I did run it under the previous 5 CPH and it did indeed seem to be doing just that, running about 8-12 minutes with 1 minute breaks in between as it reached the target temperature (which it did in under an hour, and then stopped completely as it should).

I'm in the San Francisco Bay Area. 100k BTU is only based on what one inspector noted. I have no idea if it is, in fact, that big, as it's in a tight attic crawl space and I can't easily access the unit myself.

I'm going to keep an eye on it, but it seems that there is not much I can do. It may be a combo of the CPH and overpowered unit and, from what I have researched throughout this forum, it sounds like I should just enjoy my home as there is no easy fix and I may just have a lower overall lifespan of the unit, but is not dangerous. Correct?

Also, since Compressor Protection is currently set to 0, should I increase it at all or just leave it as is?
 
#13 ·
The recommendation to change from 2 stage to 1 was received in a posting I made to the HVAC Reddit page, which I had posted to before I found this awesome and more reputable forum. Judging from the reaction, I will not be changing that setting!

I haven't been able to get to the unit itself yet as it is in a tight attic crawl space without much clearance/ease of access. And again, the 100k BTU is based solely on one report on one home inspection; I have no idea if that was an estimate or really is the size of the unit.

Interestingly, this morning, I timed again. As it was reaching temperature, the shortest cycle was about 7 minutes, with almost exactly a minute between cycles, and the longest was the last cycle which about 15 minutes at which point the desired temperature was achieved and the system stopped cycling as it should. It almost seems that as the system is "learning" how long it takes to achieve temperature, the cycles are getting longer.
 
#14 ·
Further update: This morning, with the system correctly following it's set schedule, the timings were as follows:

On for 8 minutes 15 seconds, off for 52 seconds
On for 7 minutes 7 seconds, off for 52 seconds
On for 7 minutes 4 seconds, off for 55 seconds
On for 14 minutes 35 seconds, off completely as at this point the set temperature had been reached.

Is the system just in the process of "learning" how long it takes to reach the desired temperature as part of the thermostat's Adaptive Intelligent Recovery feature?

It seems to be keeping within the set 3 CPH (if I understand CPH correctly) and it seems that the last cycle before it reaches temperature is always the longest, not to mention that the other cycles also seem to be getting longer as well.
 
#15 ·
No.
Thermostats don’t “learn” how long to run. They run until the temperature is reached or until the cycle is interrupted.
That is not running 3 cycles per hour either.
You need to have a good technician diagnose your system and check temperature rise as well as the size of the duct system.
 
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#16 ·
A simplified example of 3 CPH. Would be on for 10 minutes, off for 10 minutes, on for 10 minutes, off for 10 minutes, on for 10 minutes, off for 10 minutes. With the set temp having been reached each time it shut off.

Sounds like your riding the high limit. Most likely due to not enough air flow.
 
#17 ·
Thank you so much for the CPH explanation as I was very confused and research only made me more confused. I understand it now.

Besides the wear and tear, are there any issues with running the heat periodically or should I be ok until I can get a tech out? I have a CO detector, of course. I keep it at 65 F. I'm in Northern California, so when it's on, it's typically only for a little while in the morning or the evening when I'm home. It seems to reach the set point temp pretty quickly, within about 30-45mins.
 
#19 ·
On another forum, one user responded that he had the exact same issue I have and it was solved by reducing the gas pressure and installing a new limit switch.

How plausible does this sound?

Just trying to gather the most accurate troubleshooting info possible that I can share when I have a tech out.
 
#20 ·
Your furnace and installation are not the same as anyone else’s. YOU don’t need accurate troubleshooting information unless you’re doing the troubleshooting. You need to know how to select a good troubleshooter.
 
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#24 ·
Do you have any paperwork that shows the model number?

Do NOT do any of those other dangerous and unprofessional suggestions!!!!!

Without doing other things, changing the stat to 1 stage is not correct. Additional steps may be taken to use the equipment as a single stage set up, but that means a qualified tech needs to be involved.
 
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