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Forced air furnace burner short cycling

We have a 2 year old Olsen BCL145S furnace. installed in the basement of our 90 y/o, 1300 sq. ft. house. During a heat cycle, the burner cycles on/off 2 or 3 times; the blower sems to cycles properly. Limit switch settings are: off-100, on-130, high limit-230. We use pleated filters, replaced regularly. When it was installed, we replaced the "octopus" of heating and cold air return ductwork with rectangular central ducts, with branches for the individual registers. Lost a lot of intake and outflow sq. in. when we did that. Tried adjusting the limit switch to off-90, on-115. Didn't help. Could the problem be inadequate ductwork? Would it help if I vented some heat into the basement? I am worried about premature wear on the furnace. Should I be? Thanks for any help.

Re: Forced air furnace burner short cycling

Short cycling can be caused by several things.

First, check out the t-stat---dirty contacts can cause this symptom---remove the t-stat cover & take a white envelope & run it thru the contacts of the t-stat to clean them & see if there is any change.

If that doesn't work, if you can see the 2 wires in the t-stat that control the heat, mark their location in the hold-down scews, turn off power to the furnace, remove them & short them together, restore power to the furnace.

The furnace should come on---allow it to run for 10-15 minutes to see if the short cycling has cleared up---if not the problem is somewhere else in the control system.

Re: Forced air furnace burner short cycling

Olsen BCL145S is a 145000 BTU furnace when using the proper nozzle, smaller nozzle less BTUs. You can put a grill in your duct work in your cellar to get rid of some of the heat but without doing a Load Calc. you won't know what size to put in and could rob some heat from the other rooms. You may also have to get more air back which means one or more returns added. I'm not a big fan of pleated filters even though they may last a little longer, the blue or green throw away's are less restrictive. Your furnace sounds like it's oversized but it's hard to tell from here.
Just for comparison I have an HTL-100 that puts out 100,000 BTUs (depending on the nozzle) in my house of just over 1000 sq. ft. My furnace is also oversized and now uses a .5 nozzle which puts out 55,000 BTUs. All the modifications I did were per Beckett Oil Burner Tech Support. I would put the limit switch back to the factory settings since it didn't help anyway. Good Luck!!! :)

Re: Forced air furnace burner short cycling

If the blower is running normally but the flame is going on and off it is an indication that not enough air is moving through and the high limit is being hit. Many furnaces have 4 speed motors, check th wiring diagram and see if the heat is on the lowest speed. If it is move the jumper to the next speed and see if that helps.

Re: Forced air furnace burner short cycling

Take the cover off the limit switch and watch it as the furnace runs, if it hits high limit and the burner shuts off , then turns back on it could be air flow. If it doesn't hit high limit and goes off then back on at random temps then it maybe a control problem. Good Luck!!! :)

Re: Forced air furnace burner short cycling

Wow! Thanks to all who've responded so far.

I'll try everything suggested so far, one by one, and hopefully something will work. My serviceman keeps saying it's the limit switch settings, but I think that's just fixing the symptom, not the problem (if it fixes anything at all and so far it hasn't.)

I've been leaning to airflow. First, I'll put the original course foam filter back in. Then, I'll check the wiring specs for blower motor speed adjustments.

Would prefer not to cut heat runs into the cellar (a heated cellar would be nice, but I worry about pressurizing the cellar and really screwing things up). Cutting more return runs would be a major pain. But I'll do what I gotta do.

Never thought of the T-stat or downsizing the nozzle (currently 1.20, per factory spec), so they're on the list.

Just to let you all know, we ended up with a probably oversize furnace, first, because our old one broke in January and we live in far northern New York. So we pretty much had to replace it with whatever we could get that day in a real small town. Second, the replacement is the same size as the old one, which ran fine (until the heat exchanger cracked, now I kind of wonder about that). But like I said, we also changed both the hot and cold ducting and lost a whole lot of square inches of duct size.

I'll run all these by my serviceman and see what he says.

Again, thanks!

Re: Forced air furnace burner short cycling

Hi Sten,
Posted my reply before your last post came in. What a great idea! Have to leave right now, but will try that soon as I get back. Thanks! Gary

Re: Forced air furnace burner short cycling

And right now, I need al the luck I can get. :)

Re: Forced air furnace burner short cycling

OK, here are today’s changes.
1. I have an old mercury t-stat, and I don't see any contacts. So, I'm guessing that tip won’t fly for me. I'd prefer not to take it down to run the furnace without it, although I sure will if that's what it comes to.
2. I replaced the pleated filter with the course foam filter that came with the furnace.
3. I changed the blower limits to: off-90, on-115. Hoped to vent a bit more heat and hopefully stop the short burner cycle.
None of the above fixed the problem.
4. I looked at changing the blower motor speed, but the wiring diagram doesn‘t l look like the wiring on the blower at all. So, I’ll wait until I can find someone more comfortable with electricity to try that.
5. I think maybe a nozzle change will also help, but want to talk with either Olsen or Becket about that to find out what to downsize to.
6. I’m still not sold on putting a heat vent in my cellar. Seems I’ve read something about over-pressurizing a cellar and messing up how a furnace runs (especially the chimney draft). Any thoughts?
7. I’m looking like crazy for a place for another return air vent. I think that will help (I lost a pile of return air when I reducted), but it’s not easy to find a place where the ductwork can be conveniently run into the return air plenum.
8. Most interestingly---I watched the limit switch cycle. Blower tabs work fine. But, the dial rotated to about 200 and stayed there for at least a minute, probably longer (hi limit is set at 230). Then, all of a sudden, the limit switch unit made a loud click, the dial jerked right up to 230, and the blower shut off. The dial rotated down to about 185, the burner turned back on, and the dial rose to 200 again. It stayed there for many seconds, then it clicked and shot up to 230 again. Again the burner turned off. The dial rotated down and the burner turned on again briefly before turning off, apparently because the t-stat was happy.
Again, thanks to all for all the help! I really, really do appreciate it!

Re: Forced air furnace burner short cycling

Your furnace is the same size as the old one and your ductwork is smaller because of the change you made. Your not moving as much air as with your old furnace which is why JLM suggested changing your blower spead to move more air. I don't think you have a multi spead blower motor but could be wrong.
The only thing that's changed that could cause whats happening is air flow, both supply and return. I don't know if your going to be able to downgrade your nozzle small enough to make any difference. Lowering your limit will only start your furnace earlier and make it run longer, your still not moving the amount of air that it's been designed to move at any one time. With a furnace that size you may have a 6 or 7 ton blower motor which equals 2400-2800 sq. ft. maybe bigger ( 1 ton = 400 sq. ft.)

Another thing you could do if you have a cheap multi meter is check your Temp rise. Some where on the inside it should say what it is, or you may have to call Olsen. Usually it is 35-65 but that could vary. Drill a 1/8 hole in the supply and return plenum and using the multi meter check both temps and subtract. If it's to high it's more than likely Air Flow. Let us know how you make out. Good Luck!! :)

Re: Forced air furnace burner short cycling

The manual shows a 4 speed blower motor and talks about changing the speed. I just can't see how to do it since the wiring diagram doesn't look like the actual blower wiring. Also not sure what blower model it is, which doesn't help. The manual doesn't say and there's nothing to ID it on the furnace. I am getting more and more sure I've got to figure out a way to more air in/out. Don't have a multi-meter, will ask my serviceperson to check that. Really starting to lean towards electric heat with solar panels :) Thanks!


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