Ask Me Help Desk - Short Furnace Running Cycles

Short Furnace Running Cycles

My furnace only runs for 15-30 seconds at a time. It will kick off and then fire up 2 minutes later for about another 30-40 seconds. The desired temperature is maintained but the very short on and off cycles are very annoying. We were told it may be our thermostat so we installed a new one and we are still having the same problem. Is there anything that can be done the have the furnace run for longer, less frequent cycles?

Where is the thermostat located? This could be a factor if near a vent. The furnace might be oversized for the job. Some furnaces can be re-jetted to reduce the BTU level to allow for longer run times. Has the furnace always done this, or is this something new?

Eric D

Is this an oil furnace? Sometimes if the burn pan is out of level ( spot welds can break and the pan is canted) the furnace will automatically shut the burn cycle off.

I would check your fan/limit control. It has probably gone out of calibration. In many cases they are adjustable but they are not that expensive to replace.

For test purposes only you can jump the limit feature (safety device) out and run the burner to see if it runs a longer time. If so replace limit in not kep looking for the real problem.

Caution if limit needs replacing do not run jumped out for any great length of time. Your furnace could go to terminal melt down if the blower would fail.

Quote:

Originally Posted by JenScammy

My furnace only runs for 15-30 seconds at a time.

Is it a boiler or a furnace?

Quote:

Originally Posted by JenScammy

It will kick off and then fire up 2 minutes later for about another 30-40 seconds. The desired temperature is maintained but the very short on and off cycles are very annoying.

If it's a boiler, look at your pump and see if it's working. If not, the pump needs replacing. If you can't tell, feel the pipes. If they are smoking hot close to the system, but get colder the further you get from the boiler, it's definitely a pump related problem.

Quote:

Originally Posted by JenScammy

We were told it may be our thermostat so we installed a new one and we are still having the same problem.

Thermostats are only switches. If it turns the unit on, it's typically not the stat. Thermostats are replaced in error more than any other component on the system. And for ease of troubleshooting, always use Honeywell, or White-Rodgers as a second choice. Avoid Lux, MapleChase, and "toms house of thermostats" type models.

Quote:

Originally Posted by JenScammy

Is there anything that can be done the have the furnace run for longer, less frequent cycles?

Definitely. Locating the cause may be tricky though.

If it's a furnace... check your filter, make sure all vents are opened-return AND supply. Check the temperature of the air coming out of the vents when it IS running. Clean flame sensor (just in case) with clean steel wool. Unfortunately, the symptoms you are describing are vexing to troubleshoot without being onsite-that's why I suggested "boiler" (that and a lot of homeowners don't discern the difference).

If it's a furnace, it sounds like it may be "cycling on the limit" indicating an overheating of the unit. If the above suggestions don't remedy the situation, contact a professional, as safeties are NEVER to be ignored.

Do you have a model number and manufacturer? This helps greatly...

Quote:

Originally Posted by JenScammy

My furnace only runs for 15-30 seconds at a time. It will kick off and then fire up 2 minutes later for about another 30-40 seconds. The desired temperature is maintained but the very short on and off cycles are very annoying. We were told it may be our thermostat so we installed a new one and we are still having the same problem. Is there anything that can be done the have the furnace run for longer, less frequent cycles?

You should get a ohm meter and hook it to your wires in place of your thermostat and get the reading from the ohm meter this number is what your heat anticapater should be set on in your themostat so that your furnace will cycle properly

((To get proper heat anticipator settings requires an amp clamp and 10 wraps around the clamp in series with the gas valve. This is not a heat anticipator issue, as the unit is running for 30 seconds or so.))

You are correct but I find it much easier and more accurate to use a miliamp meter. No wraps and more accurate.

Quote:

Originally Posted by hvac1000

((To get proper heat anticipator settings requires an amp clamp and 10 wraps around the clamp in series with the gas valve. This is not a heat anticipator issue, as the unit is running for 30 seconds or so.))

You are correct but I find it much easier and more accurate to use a miliamp meter. No wraps and more accurate.

My bad... I don't personally use them, as I carry my amp clamp everywhere and have constructed a nice little "rig" that mounts fairly quickly... no more wrapwrapwrapwrap crap. I also wanted to straighten out the "ohm meter" post to keep people from chasing their tails. Gravesite was probably referring to a maMeter instead of an ohm meter.