Hi everyone,
I'm looking for a second opinion. My furnace is a Lennox model EL296UH110XV60C-05 which is a 2 stage furnace. I also have 3 zone heating controlled by a Harmony III board and 2 stage AC. All about 5 years old and heat runs primarily on the main floor zone. I don't know if it's always been happening but my problem is in the start up sequence when we turn up the thermostat by 4C in the morning.....inducer and ignitor start, flame goes to H1, fan starts at 450cfm, inducer speeds up, flame goes to H2, fan starts ramping up but at around 750 the limit switch shuts everything but the fan down. After about a minute with the fan at 1090 the furnace re-ignites and everything seems to work but the E250 code keeps flashing. Filter is clean, just the cheap gauze type replaced regularly and AC coil is spotless. This happens even if I have the coil access panel (17"x36") off the back of the plenum so I cant understand how it can be getting too hot. I'm assuming the limit switch re-sets itself somehow as the E250 stops after a few cycles. Any idea what might cause it to shut down during startup?
Is it possible one of the dampers is stuck closed? Did you try each thermostat to make sure you're getting airflow from all zones out of the vents?
It could also just be a bad limit control although that's not very common. If you measure the temperature where the limit is inserted or just above the furnace what kind of temperature are you reading?
Like you said this could've been an issue since day one. Without measuring the temperature rise it's hard to know what's going on. That's basically the difference between the temperature in the supply and return ducts.
That is a big furnace so maybe there is not enough return air. You can get away with a single side filter but it needs to be a minimum 20x25x1".
There would also be dip switches so you can increase your airflow and see if that helps. Look at the attached picture and on the control board in the furnace you can adjust those three dip switches. First try increasing by 6% and see if that solves the E250 error. If not try 12, 18 or 24% increases.
Thank you for your reply. The dampers, zoning and vents are all working properly and the filter is 20"x25x 1". I've even tried leaving a second zone open 50% but that doesn't help.
When it's running on H1 the temp rise is 18C - 54C. But when it's first starting up and running on H2 just before it shuts down the rise is 18C - 68C. I'm taking the temps at the filter and about 12" above the top of the coil which is more or less the same level as the furnace sensor.
As for the dip switches, the Harmony III controls the furnace so they're on the damper board. If I did increase the speed does it affect the initial start up speed of 450? As I said, it's just as the fan starts ramping up (650-900) that the limit shuts everything down.
Well the low fire temp rise for your furnace is 35-65F and you have 36F and the high fire is 45-75 and you have 50F, so I'm not seeing a problem. It's not very common to have a limit fail that way on that model but I guess anything is possible. That limit should be good for 120-150F and the part # you need is 73W18. If you registered for the 10 year parts warranty you can likely get it for free and just pay the labor.
Sorry, I forgot you mentioned the Harmony. Does this Harmony have a sensor? I wonder if maybe this is what is causing the E250. I believe you can adjust the PIAB jumper to increase the fan in 10% increments and can adjust each zone separately. BTW do you get the E250 regardless of what zone is used? I attached the manual below in case you didn't have it. See page 27,
QuoteNOTE: Integrated control DIP switches 14 through 19 are not disabled, and can be used in conjunction with Harmony III zoning system to increase or decrease airflow volume during heating operation
If you look at Page 67 it shows much higher CFM than what you are using. Something is likely not setup right but your temp readings are what are confusing me. You may need to hire a Lennox tech with Harmony experience to review the setup.
If the back of the plenum is open, the closed dampers are likely not an issue. A high limit fault is likely, or the sensor on the harmony. If I recall correctly, the sensor on the harmony is a resistor, so they can get glitchy sometimes
Thanks for your responses.
Admin.....I gave you temps in C and I noticed you're using F. In Fahrenheit they're more like 66-130 in low fire and 66-155 in high fire.
And yes, the Harmony has a sensor. I didn't follow the wires far enough and thought they went to the furnace but they doubled back and go to the Harmony. I also increased the fan by 20% but that doesn't change the 450cfm start-up speed so it does the same thing...just as the fan starts ramping up a bit in speed the switch shuts down the flame.
I also did another experiment. I thought maybe I was getting a pressure build up within the coil for some reason so I removed the coil access panel and then also removed the triangular cover of the coil itself so I could actually see the limit switch. I then started the furnace and it still trips the limit with the back of the plenum wide open.
I already have the Harmony manual but thanks anyways.
Sorry, you clearly said C! Those are high temp rise numbers and if increasing the fan speed didn't work we must have a ductwork issue.
I would check the manifold gas pressure just to make sure both low fire and high fire are set properly though.
If you remove the bottom blower door panel will the limit still trap when the furnace runs? You'll have to hold in the door safety switch manually.
Whereabouts do you live and how old is your home and roughly how many square feet?
I removed the door panel and I think it worked. I say think because it worked to the point that the fan was running full speed so it didn't shut down just as it started ramping up. I had to stop after a few minutes because my finger got sore :) Tomorrow morning I'll try again by taping the door switch to see if it works the entire cycle.
The house is 5 years old located in Edmonton. 3100 sq. ft. 2 story....1700 on the main and 1400 on second floor. Basement is not finished so heat almost never runs in that zone.
Re: manifold gas pressure, I'll have to get someone out to do that so I'll wait a bit just to see if it is a ductwork issue or not. Thanks again.
If the basement is unfinished it may be easy to add a return air and that may solve the problem.
Make sure the filter is in the right direction, I've seen that happen due to that too lol
I tested it with the blower door panel off and the blower now running at 1280. Everything works properly. So, I cut an 8"x14" hole in the RA and replaced the door panel. It's better but still not right. Now it goes through the startup sequence as it did before and runs properly in high fire for about 2 minutes with blower at 1280 before the limit trips and shuts down the flame. The fan keeps running and after 5 minutes the flame relights on high fire for about a minute, drops to low fire and keeps working properly. For some reason now I no longer get the E250 that I was originally getting.
Since airflow seems to be the problem do you think I should try a second opening in the RA?
samihabib....thanks but the filter is definitely in the right way.
Have you inserted a temp probe into the limit hole to take a measurement? To see what the true temperature is at the limit? There is a possibility the limit is tripping prematurely.. the limits temperature setting should be printed near its part number
Have that manifold pressure confirmed in both high and low fired. Followed by clocking input at the meter in high and low fire
I just wanted to say thanks for all the advice. It seems my problems go beyond air flow so I have a technician coming tomorrow.