Furnace cycles on and off during heating process

junior466

Hello all,

I have a Oil furnace with forced hot air that has been powering on and off during the heating cycle. Its a Patriot 80 with a Beckett burner.
What happens is lets say I call for heat at 70 degrees and the current temperature in the house is 60, then it will take approximately 1 hour for the temperature to be reached but during this process I can hear the burner turning off after about 5 minutes and the fan will remain on to move the left over hot air. In 2 to 3 minutes later the furnace will go back on again and the whole process will repeat itself a few more times before the temperature called for is reached.

I don't know if it helps but the furnace is 3 years old and has been cleaned (yearly maintenance) this past November. The filter is also brand new.

When I had the technician here for the yearly maintenance, he noticed that my basement has been recently air sealed and told me that the furnace was starved for air in which I believe he was talking about combustion air? He also suggested a Fan In A Can as a solution but said that I could also simply open a window.

I have only noticed this issue because I recently installed a Nest thermostat and it gave me a warning saying that my furnace wasn't running longer than 15 minutes at a time. So I did a test and sure enough the above keeps on happening.

Fred

That sounds normal to me. There's a high limit switch on the furnace that shuts the burner down when the heat exchanger gets to the upper limit. The fan continues as long as the thermostat continues to call for heat or the low limit is reached, in which case the burner comes back on and the cycle continues. Of course, if the furnace is over-sized there can be a lot of short cycles/bursts of heat but typically, in that case the blower runs until the low limit is reached and it shuts completely down (fan and burner) until the next call for heat by the thermostat. I suppose it's worth checking to see if the high limit switch is set to the recommended temp.

junior466

Fred, thanks for the prompt reply.
Pardon my ignorance but why is the "heat exchanger reaching the upper limit" so quickly? Is it the way is was designed to run? I was always under the impression that once heat is called for, the burner stays on until the temperature is reached.

Just to be sure I just finished running another test and sure enough when I set the thermostat from 72 to 80 the burner turns on, the fan follows shortly after and about 5 minutes the burner turns off, the fan continues running and 2 to 3 minutes later the burner comes back on. It continues until the temperature is reached.

Fred

Modern furnaces are designed to shut the burner down when the heat exchanger gets to a certain temp to avoid an unsafe condition. I have a couple forced air furnaces that I don't use often (they are backup to my steam system and I installed them as a result of adding central air to my home). I believe the upper limit on mine are set to 180 degrees, where the burner shuts down and stays off until the Heat exchanger gets down to around 140, then the burner kicks back on. If I set the thermostat to a constant temp, say 70 degrees and leave it there, the burner may not shut down because maintaining that temp can happen in a short timeframe. If I try to increase the house temp by several degrees, like you are doing in your testing, the furnace has to run for an extended period of time and the high limit switch is a safety device to ensure the heat exchanger doesn't get too hot where it can crack or worse yet cause a fire if left uncontrolled.

Eric_32

If the furnace doesn't have enough combustion air for the burner that is not a good thing... You could do a fan in the can but there is a better option... from the same company that makes the fan in the can.... Fields Controls

Fields CAS-1 and CAS-2 combustion air kit (depending on your burner). Bolts directly to the burner and pipes in fresh air from outside using 4" round metal pipe. An air hood gets installed on the outside of the house. Looks like a dryer vent termination except it doesn't have a flap, just a screen over the opening. that gets piped to the burner itself. Gives the burner all the fresh air it needs and doesn't make the basement cold like the fan in the can will. It is something an oil burner tech will have to install. It requires readjusting the air settings on the burner, which you will need special tools to do.

see this link from Fields:
http://www.fieldcontrols.com/cas-1-and-cas-2-oil?page_id=94

Brewbeer

The cycling is from the blower's inability to move enough air across the heat exchanger needed to carry away the heat generated by the burner. This can be a symptom of a burner that is too large, or ductwork that is too small.

Jamie Hall

Assuming that the filter really is clean, and the correct filter, the cause is most likely what @Brewbeer said -- inadequate ductwork. Also, it is likely that the installation has been doing this all along and you just didn't notice it. However, there is one other possibility: has something -- such as furniture -- been moved or change so that one or more of the vents, particularly the return air vents, has been blocked at least partially? And are all the vent dampers fully open? Wouldn't be the first time...

junior466

Fred said:

If I set the thermostat to a constant temp, say 70 degrees and leave it there, the burner may not shut down because maintaining that temp can happen in a short timeframe. If I try to increase the house temp by several degrees, like you are doing in your testing, the furnace has to run for an extended period of time and the high limit switch is a safety device to ensure the heat exchanger doesn't get too hot where it can crack or worse yet cause a fire if left uncontrolled.

That's probably the reason why I never noticed it. I have always kept a constant temperature throughout winter but this year I installed a wifi thermostat and started turning the heat way down remotely when out for the day.

Thank you very much for your help.

BobC

Large setbacks are really not worth it unless it's for days at a time, it just takes too much energy to bring the structure and belongings back up to temperature for short periods of time.

Bob

junior466

Brewbeer said:

The cycling is from the blower's inability to move enough air across the heat exchanger needed to carry away the heat generated by the burner. This can be a symptom of a burner that is too large, or ductwork that is too small.

I see. My house is about 1200sq. How oversized can my burner be? Wouldn't that be a huge waste of money for whoever bought the system? (I purchased the house 2 years ago).

If the duct work is too small, would opening the registers in the unfinished basement help or it's not that simple?

Apologies if my questions are silly.

Fred

Having those registers open will help, probably a lot, especially if the furnace was sized to include that space. The more registers you close, the less air you can move away from the furnace.

junior466

Jamie Hall said:

Assuming that the filter really is clean, and the correct filter, the cause is most likely what @Brewbeer said -- inadequate ductwork. Also, it is likely that the installation has been doing this all along and you just didn't notice it. However, there is one other possibility: has something -- such as furniture -- been moved or change so that one or more of the vents, particularly the return air vents, has been blocked at least partially? And are all the vent dampers fully open? Wouldn't be the first time...

The filter is clean and it's not a restrictive type. I do have a couple pieces of furniture partially blocking two vents that I am about to move this very minute. All dampers are fully open and return vents are not blocked. They were dirty though but I ran a test after cleaning then and it still ran the same way.

Since we are talking about ductwork, there's been an addition added to this house (about 13x15 room) and I believe it's the reason why the furnace is new. They needed a little more power. But at the same time I've noticed that no new return vents been installed. Only 3 exists in the entire house. Before cleaning the other returns, I could hear a really loud suction noise from the one closest to the furnace. Now it's a little quieter but could this be it?

junior466

Just wanted to drop a thanks to everyone in this thread that has helped me. I appreciate it.

mark schofield

on a Williamson forced air furnace in my daughter's house, the blower motor can actually be wired for 3 different speeds. The Carlin burner, as per Williamson set up instructions, can use 3 different nozzle sizes from .65 to .85. I would imagine these two interventions or changes would influence the temperature in the heat exchanger.

pecmsg

A little reading on sizing and why
http://efficientcomfort.net/documents/Understanding_HVAC_System_Design_Issues.pdf

billtwocase

You have an air flow problem thru that furnace. You can make sure you have a non-pleat air filter in it for a short term remedy. Use a basic air filter that is not as restrictive. You should have all the supply vents open, and all returns unobstructed. You probably don't have enough return air, and do not allow one put in the basement. You will open up a whole new can of worms creating a negative pressure with the furnace exhaust

776v63

Will this effect draft readings?

Side note; how the hell do you quote another comment? I can never get it to work.

Fred

776v63 said:

Will this effect draft readings?



Side note; how the hell do you quote another comment? I can never get it to work.

You should be able to click on the "Quote botton across the bottom of the Poster you want to quote and it will drop that post into your Comment box.