Short cycle?

nmocci

Hello, new here and trying to figure out an issue that started since I had a tech come in and replace my primary control. 

Furnace- Miller Cmf80PO
old primary control- white-Rodgers 668-401
new primary control- Carlin 48245 

i had a guy come in and do a service and diagnose why I had no ignition/intermittent ignition/primary control tripping. He diagnosed it as a bad primary control. Replaced it and it started igniting. My problem is it will start a flame. Run for 45 seconds or so and then shut off. Then my air blower motor will come on and run by itself for a few seconds and then the motor will come back on and start making heat again. The furnace never did this until the new primary control was installed. It would always start a flame, run, and then the blower motor would come on and start moving air while the flame was still lit. I had to have the guy come back about a month later because I lost ignition again, this time due to a bad transformer. Replaced and running the same as before. I mentioned it was running differently to the tech. He said he should be doing it but thought it was because he has opened to door to the furnace while in the middle of a cycle. That was not the case because immediately after he left it did it again and continues to do it every time the furnace starts. I’m hesitant to call and have him come back and possibly waste more money. I have run the furnace through a cycle watching the primary control to see if any errors show up but it doesn’t seem to show anything. The lights just go out when the burner stops. And then come back on in a second. Any help is appreciated. If any more info is needed please let me know. 

Thank you,
    Nick 

nmocci

Another concern is that it is set for 45 second TFI. It has the option for 15 and 45. He left it set for 45. When the furnace stopped working and I had to call him to come check it out it was filled with fuel. I’m guessing because of the 45 second tfi coupled with me trying to bleed it two times. In doing research it seems that 45 second tfi is excessive and 15 is better but not best. Is this correct? Thanks 

EBEBRATT-Ed

15 sec trial is the way to go. Sounds like you need a professional and your tech doesn't sound too sharp.

If the control isn't going off on lockout (you don't have to reset it) then don't bleed it, that's not the problem.

If it does go on lockout stop resetting it the problem is in the burner or the oil piping.....oil supply..

If the burner is shutting off and restarting without being reset you problem could be the thermostat, thermostat wiring or the high limit control. Make sure the blower motor is up to speed, no duct work blocked and a clean filter

Is this furnace in a trailer?

nmocci

Thank you for the reply. Yes it is in a trailer. I am starting to realize that and that is why I’m not looking forward to asking him to come back. Since the primary control and the transformer it no longer faults out but continues to shut the burner off, fan on then a minute later the burner will come back on. I will start looking into how the wiring should be set up. I’ve been thinking about pulling the primary control and seeing how he wired it also. Maybe he messed something up I’m not really sure. I know it was a 3 wire control that was pulled out and I don’t have a solenoid on my pump. 

EBEBRATT-Ed

Furnaces are pretty simple. Hot wire comes from your breaker panel to the service switch at the furnace. You may have other safetys like a door switch, emergency switch etc.Then it goes to your combination fan/limit control and the power splits. goes through the fan side of the control and goes to the blower motor. also goes through the limit side of the control to hot on the burner primaryt control.

The off the primary to the burner motor, ignition trans and oil valve if you have one.

Of course the white neutral has to be connected to the blower motor and to the burner primary

EdTheHeaterMan

I have experienced this exact function with the Miller CMF80 on many occasions. There are 2 limit switches. One is part of the Fan Limit Control. This is an illustration from the manual

The second limit is on the blower deck. The function of the second limit is to stop the burner and start the fan if the Primary Operating Fan/Limit control does not bring on the fan. Once the fan cools down the plenum chamber the secondary limit allows the burner to operate... by that time, the primary fan/limit control has reached the necessary temperature to keep the fan operating along with the burner.

It appears that your system is functioning as designed. The fan is not coming on soon enough so the secondary limit is doing its job. Since this never happened before the parts were changed, my guess is that the repair person may have inadvertently adjusted the fan on setting on the primary fan/limit control while diagnosing or testing the system operation.

The remedy is to move the Fan On indicator adjustment about 5° to 10° lower on the primary fan/limit control. It's the pin in the middle of the 3 adjustment pins. To adjust, hold the dial steady with a firm thumb pressure on the metal dial and the plastic surrounding. with a fingernail push the pin adjustment one notch lower on the scale. Try not to move any other adjustments. If you are not comfortable doing that, then have the repairman back to do this and show him these instructions.

EdTheHeaterMan

Worst case scenario is the Fan Limit Control is failing and that will need to be replaced also. Sounds like your furnace is getting old. When he's finished let him know that this repair should be covered under the warranty for the previous two parts replacement. If they can't stand by their workmanship, then you should think about finding another repair company.

Maybe when it's time for the new furnace the new guy will get the job. I loved doing those furnaces, In and out in 1/2 a day when the existing roof jack was in good condition.

JUGHNE

Do you have an "old school" type of thermostat?
If so it may have a "heat anticipator" adjustment under the cover.
Either a slide pointer or dial.
I have seen those accidently get set to "shorter" run time.
Then fire would come on, run short time, tstat shut fire off, then fan switch hot enough to make fan run a bit, then tstat starts fire again for short run........and on and on.
Show us a picture of tstat with cover off if you are not sure.

nmocci

Thank you all. I was able to make a small adjustment to the fan limit control and now it is working properly. I have a newer style thermostat with a battery. It has pins on the back that I played with but it didn’t help any so I just reset it and put the pins back to where they were. It seems to be functioning like it should now. I may look into replacing both the limit switches before next season just to be safe. 

    Thanks again, Nick

EBEBRATT-Ed

@EdTheHeaterMan

I only worked on a few Millers. Never like the short stacks, not much draft.

I replaced one in a trailer with a brand new Miller. combustion test and everything was fine. The women complained about a slight oil smell on and off for a couple of weeks. Couldn't find anything wrong.

Finally pulled it out into the driveway and stripped the jacket off.

Found a pin hole in the HX at a bad weld joint. The supplier wanted us to change the HX but I demanded and got a new furnace.

Left a bad taste in my mouth for Millers

EdTheHeaterMan

Can't blame you. @EBEBRATT-Ed... I know Lennox makes excellent stuff but the engineering department there threw me under the bus on a problem job, Installer Error was the diagnosis when in fact it was an Architect Error for specifying a Pulse furnace that was too wide for the closet he provided in the drawings. I would never use a Lennox product after that, and I had the jokers remove the 15+ pieces of equipment I had left from a "floor plan" with them,

Told the credit manager that is was not going to pay for the stuff and that he can come and get it if they want it.

So one bad experience can spoil you on a product. Did very well with American Standard for 35 years after that.

BDR529

Most Millers blow air out the bottom-down flow.

Have blower asy, or motor replaced?

Reads like it is bouncing off high limit. Wrong direction on blower.

nmocci

I got a new fan limit control and out of the box it doesn’t turn my fan on until 30 degrees over where I set it to come on. Set 110 actually comes on at 140 on the dial. So I guess I’ll be going back tomorrow to try to get another one. Is this common? 

EdTheHeaterMan

Where are you measuring the temperature and what are you measuring it with? I don't believe you need a new control by your last comment

EdTheHeaterMan

If the small adjustment you made solved the problem then why did you need a new fan/limit control? I would disregard the numbers on the control, I would go by the function. Does the control make the furnace operate properly? The spring that operates the switch on the cam that turns with the dial is holding tension on the turning action of the bi-metal spring that causes the rotation based on temperature. This is not rocket science. you just need to make the adjustments according to the conditions in your equipment.

That part was not specifically for the Miller furnace in your home. It fits other miller furnaces in other homes, and it also fits many other brands of furnaces in many other homes all across North America and abroad. The control must be adjusted based on the conditions of the equipment it is installed in.

Respectfully submitted
Mr.Ed

nmocci

I was able to get it working better but it isn’t consistent. I think the dial is sticky. Sometimes the fan comes on before the limit shuts the motor off and sometimes it doesn’t. And it will give me my original problem. That is why I bought a new fan limit control. The issue I had was I set the on for the fan on the dial at 110. It wouldn’t come on until 140 on the dial. At which point the limit switch had already cut the motor and the furnace has stopped running. If I lowered it anymore on the dial the fan would never shut off because it forced me to move the off lever so far down. Hopefully that makes sense. I understand how the limit control works and that it doesn’t need to be 100% accurate. I don’t have a lot of temp range to work with where my limit control is 135 degrees. I’m guessing it’s lower then your average furnace because it’s in a confined space etc.