I have gas 80% furnace and is controlled by Honeywell thermostat (rthl221b1008). It didn't use to do this before, but it is acting weird for the last day or two now. It would run for about 10 minutes and turn off, then within 30 seconds it will turn on again. I thought maybe the batteries were almost out of power. I replaced them with new batteries (AAA), but it still acts the same way. The temprature outside is round 40. I looked in the manual about reducing the number of cycle. There is no mention of it. However, I searched online for this and I come across conflicting information. One website stated my thermostat should give you control to adjust cycle, but I went through all the setting for my thermostat and didn't find anything like that. So, why is my thermostat acting the way it is and how do I control cycle rate with this thermostat if it is possible?

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    What make and model is your furnace? – ThreePhaseEel Jan 1 '17 at 4:45
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    It is unlikely a basic 7-day thermostat like the Honeywell rthl221b1008 would have a cycle control like you describe. It might be an issue with the furnace high limit tripping and resetting. Check for plugged filter or restricted airflow. – user39367 Jan 1 '17 at 5:04
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    It's likely not a thermostat issue, but an issue with the furnace itself. There are many systems and and sensors designed to keep you safe from fumes and overheating. It sounds like your level of understanding is basic , as such I would recommend that you call a furnace company for service. – Tyson Jan 1 '17 at 5:27
  • @ThreePhaseEel It is Honeywell rthl221b1008 – ThN Jan 1 '17 at 6:35
  • @chris It is odd that you mention restricted airflow. Air is not blowing out with same speed as before. – ThN Jan 1 '17 at 6:43

Q: Why cycling on and off? Air is not blowing out with the same speed as before.

A: Insufficient airflow at the furnace is causing the high temperature limit to trip.

One potential cause for this is an obstruction, such as a clogged filter or partly closed seasonal damper. Another potential cause is that the blower motor is pumping less airflow than it should, possibly due to a failed run capacitor or worn bearings.

On most furnaces the high limit will not self-reset in a short period of time like the OP describes. Instead, the furnace will lock out with an error code or trip a switch that requires a manual reset. The high limit control needs to be carefully inspected to verify it is being tripped and to ensure that it has not been compromised in some way. If in doubt, higher a professional.

  • I ended up hiring a professional and his diagnose was not enough airflow. That still doesn't make sense, because I replaced my filter about 3 weeks ago with 3 month filter. He did say that it was my pressure switch that was tripping the furnace off. So, he removed the filter, my furnace started to work as it should. – ThN Jan 6 '17 at 17:04
  • Many different types of filters out there. Some are too restrictive for residential systems despite nothing saying that in the sales material. You must have a pretty modern residential furnace for it be equipped with a cut-off switch that is triggered by excess pressure drop across the filter. Check the owner's manual to see what it says about the trip threshold and how to select an appropriate filter. – user39367 Jan 6 '17 at 18:57

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