Ducane furnace can't keep up

Discussion in 'HVAC Heating & Cooling' started by spfrancis, Nov 11, 2017.

  1. spfrancis

    spfrancis Member

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    So I have had this house for about 18 months. This is a Ducane CMPEO75u3. I had some similar problems last winter, but I thought I addressed it. I did find some stones in the exaust PVC, and I was able to clear those out. The main problem I'm seeing tat when the outside temps get down below 35degrees, it seems to have a problem keeping up. My inside temps will ranger from 60-62, when I have it much higher(68). The lights on the control unit seems to go between normal "pulsing green light" to a failure mode of 3 blinks and then 6 blinks. After a while it will cycle back into a working mode, but it will rarely stay in te normal mode. It as been frustrating. I have had 2 different HVAC guys out, but no one has been able to troubleshoot the problem. I would like to see if anyone on this forum has an idea about this. Or at least point a HVAC guy in the right direction. I tried changing out a pressure switch last year, since that was an easier change.
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Where is the in/out piping terminated? You mentioned 35-degrees, but is it when it is below freezing?

    I didn't find the troubleshooting guide with a quick check, but the pattern, speed, and which lights flashing should indicate somewhere to start.

    A closed combustion device has a few interlocks, and if those are not all activated, the system will shut down. These include, high/low gas pressure, a check that the intake air is clear and not blocked, sometimes, a differential pressure to verify that the exhaust is not blocked, high temperature, flame sensor, and there may be others. They all must be detecting normal conditions for the furnace to stay on. If the length of the intake/exhaust is too long (this could be because of the number of changes of direction as well as the actual length of the stuff - called effective length), the fan in the combustion/exhaust circuit may not be able to provide the needed air movement.

    When it can't keep up, is the burner lighting? Is the circulation fan operating? What is the outlet air temperature, assuming the fan does run?

    What else in the house uses gas? Sounds like you did not live there when this was installed. If the gas line pressure is too low, the thing won't be able to stay on, and would be running lean. Too low of gas pressure could be a meter issue, regulator issue, or the line is undersized for the size of the burner (which could be either too small, or too long, or too many things attached for the size of the pipe).

    So, need a bit more info to help rather than guessing.

    Basically, check out the operator's manual for the error indication you have and come back. It's probably on-line if you don't have it, but I only did a quick search and only found the installation manual.
     
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  4. spfrancis

    spfrancis Member

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    So it looks like three flashes means: pressure switch open, with inducer on. 6 flashes means softlockout. I assume that each flash pattern is its own error, and nothing new can be interpreted from the combo of the two codes. So I know that a HVAC guy did blow out the exhaust vent back in January. I thought that fixed the problem. So you bring up interesting points about length of exhaust, and the gas strength. I didn't mention before that this is propane, versus natural gas. I'm wondering if I could have the propane company come out and do some sort of test to make certain that the gas quality is good to the furnace. To answer one of your other questions, this is the only device that is being supplied by the propane tanks. To your point about number of turns of the exhaust vent. So it does probably take about 4-5 turns, as I will have to look when I'm out there(I'm not out there during the week, this is a home we head to for the weekend.) I don't know if I would be able to remove turns, but I could make the last couple of turns with a bigger diameter PVC(5") versus. That is something I can do on my own. Would that fix a constriction problem? I will have to check your questions about burner lighting. It looks to me like it falls behind when it is in this flashing mode(6 and 3). But then it must come out of that mode, and go into normal running mode to still provide some level of heat. Thanks for your detailed response, as this is driving me crazy, and we are supposed to host Xmas at the place...so I need some sort of fix. Oh so you also asked about what exact temp. So my comment about 35 is just a swag, but definitely when the temps are below freezing, it becomes very apparent. But in the higher numbers 35-40, it still may have a couple of degree variation from what I have set it too. So I am using the NEST thermostat. I assume that has no barring, but I thought I would mention. When it falls behind, the NEST is showing Orange, which means it is trying to run. But the furnace is usually in the flashing error mode.
     
    Last edited: Nov 13, 2017
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    I like my Nest thermostat...it's easy to get some good data from.

    The installation manual will give a figure for the maximum effective length of the intake/exhaust lines. It will vary based on the diameter of the pipes involved, but they normally only use one size. Effective length is the physical length plus a factor for each elbow and a (different factor) value for the termination. Don't remember the exact figures, but as a generic example, say the run was 35', but had 5 elbows, each elbow might add an effective 2', and the termination adding say 3', so your total effective length would be 48', not 35. You need to see the table in the installation manual and use their numbers and maximums.

    Failing to close the pressure switch could be caused by various reasons, some of which are listed:
    - debris, a bird's nest, leaves, etc. in the intake
    - snow or ice (I had this on mine when they didn't install the termination properly...the condensation from the exhaust ran down and plugged the inlet on a cold day), or something else blocking the exhaust
    - a cracked pipe
    - a failed induction fan
    - a bad pressure switch
    - depending on where the pressure switch is, it might have a hose, and the hose could be loose or the clamp incorrectly installed, or the tubing split at the connector

    An open pressure switch will lock out the burner, so without the burner, yes, it will fall behind!
     
  6. Stuff

    Stuff Well-Known Member

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    With the inducer running feel how much pressure there is at the outside exhaust. Compare between when it is working and when it is not.

    Do you have an intake pipe from the outside or does it suck air from the room? That being blocked could also trigger the pressure switch.
     
  7. spfrancis

    spfrancis Member

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    The input seems to be just in the closet thtat houses the Ducane. I only see the one pipe that goes to the outside for the exhaust. To your question about the pressure on the exhaust. When it is running normally, I feel good pressure coming out of the exhaust, and it is a strong flow of warm air. When it is in error mode, I feel a light pressure of cool air on the exhaust. The one comment above about the "termination" So the end of our exhaust is just a pipe that is pointing horizontally out to the driveway. There is no cap or anything. I don't have anything special ending the pipe. I'm not sure why it was not installed with a cap or something. The worrisome thing is that the house before me was a rental, and that is why we found stones in the pipe when we moved in last January. The one hvac guy came out and blow it out from the inside, based on that error code. We saw some of the stones come out. After that, my buddy and I went and rented out a wetVAC, and spent a bit of time pulling out the stones we could see, so the pipe section we can see looks to be clear of blockage. So I don't really know what happens after the turn. So what would be the best way to identify a problem in the flow. I could just crawl down there and cut the pipe shorter..and then have to rebuild. I'm not sure if there is any snake-camera technology that would help me view the pipe all the way through(that isn't cost prohibitive). On anther note, I have had a tough time googling around to find the install manual for the Ducane CMP furnace to determine what my effective length is. When I go to the ducane site it shows me different series that are upflow or downflow that start with 95 or 80. I'm guessing this is the latest models, and not something that is probably 7-8 years old. I have switched out the pressure switch last winter, but it was at the end of the winter season. I check out the hose that goes from the pressure switch, and it looks like visually like it is okay. I took it off and blew into it, and it seemed to stop air flow when I blocked the other end of the hose. So the idea with the fan is that it is coming on,versus not coming on (bad induction fan)? I see another thread in another site that people say that these smart-valve setup are pretty well known for issues.
     
  8. Stuff

    Stuff Well-Known Member

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  9. spfrancis

    spfrancis Member

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    So I will check the pressure again when I get back out there. Probably this weekend. It is a 3hr drive to get out there, so I will have to see. So if the inducer motor is the problem, that sounds like it could be a DIY job? Or do you think something that really needs to be done by a professional HVAC guy? If it is a DIY job, do I really need part numbers from that the motor, or can a parts supplier know the part from the serial number of the Ducane unit, and the model will be enough to order the right motor?
     
  10. Stuff

    Stuff Well-Known Member

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    It is a DIY job. Model and s/n should be enough for the parts counter but sometimes store gives you the wrong part. All they do is shrug afterwards saying that's what the book/computer said.
    It should be less hassle if a pro handles it. 3 hours from home and missing a tool or part and your weekend is shot.




    On the other hand this inducer (80M52) seems to be almost universal for the CMPE series and the discussion specifically mentions your CMPE075U3: https://smile.amazon.com/dp/B003JV1ODE
     
    Last edited: Nov 15, 2017
  11. spfrancis

    spfrancis Member

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    That seems like a great price on the inducer. I think I need to sit in front of the furnace as really watch as the things go from normal mode to error mode. It sounds like I will probably really hear what is going on. I will post what I see, and determine if it looks like the inducer is really having a problem. Thanks "Stuff" for the help. I also have to see if the temps will be low enough to really cause the problem on the weekend.
     
    Last edited: Nov 15, 2017
  12. Stuff

    Stuff Well-Known Member

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    With stones in the pipe who knows what could have happened. Still worth disconnecting exhaust and shaking to verify no more in there.
     
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    IF the thing sits in a closet, keep in mind that there are some fairly specific requirements for airflow into the room in order for it to function properly. If you can, try leaving the door open and see if it works more reliably.

    The door needs to have a fairly large amount of louvers or openings in it to support combustion properly. The manual should say, but a fairly common number is 1sqin/K BTU of the burner...louvers or a screen will restrict some, so the actual opening would need to be larger. Then, the actual room that is connected around it via that opening needs a minimum volume. Keep in mind that all of that combustion air that gets exhausted means you are pulling in air from cracks and leaks of the house to support that combustion...pulling in unconditioned air so the burner can work. This is one reason why more and more units are coming with closed combustion (where the makeup air comes from outside verses inside of the building).
     
  14. spfrancis

    spfrancis Member

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    It does sit in a closet. I did open the door on Saturday when I started seeing the problem. My feeling is that the door opening on Saturday didn't seem to make a huge difference to the inside temps adjusting in a timely fashion. I will really pay closer attention to that. The door definitely does not have louvers. My feeling is that the previous owner(since they rented, and it was more of a pure investment property) just didn't go their in the winter. I know that when they sold it, they had the whole house winterized, so the furnace probably never came on. During the walk-thru, we had to get them to address the problem of the furnace not heating the house. The HVAC guy did some work, and it started to work. I'm not 100% sure what they did back then. Later that winter we discovered the issue with the stones in the exhaust. it seemed to be better, but we didn't have a super cold winter in early 2017. I will monitor when the temps start to dip. It looks like the next 10 days down there will not drop much below 35 for the lows.
     
  15. spfrancis

    spfrancis Member

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    so I did a test the last 24 hours. I put the temp(remotely) to 68, and over night the outsides temps were in the 38-40 range. It seems that it tried running all night, and it still can never get up to 68. It hoovers around 64-65range. So I would have to assume that it is still doing the waffle between normal mode and error mode. This is with the door to the closet open. I will check the other things discussed above for additional testing. Thanks.
     
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    IF there are not two pipes coming out of the top of the furnace (intake and exhaust), it is relying on drawing combustion air from the room. Having it in a closet without at least the minimum air opening is a real problem whether it is the only problem, it's hard to say.

    When you looked, did you see any error indication on the furnace? FWIW, if the WH turned on during that time, that additional burner may have still exceeded the available air needed, and caused the furnace to lock out.

    The following is from a State WH installation instructions manual, but is pretty common for all burners that do not have closed combustion (ie.,getting their combustion air from directly outside).
    The confined space shall be provided with two permanent openings communicating directly with an additional room(s) of sufficient volume so that the combined volume of all spaces meets the criteria for an unconfined space. The total input of all gas utilization equipment installed in the combined space shall be considered in making this determination. Each opening shall have a minimum free area of one square inch per 1,000 Btu per hour of the total input rating of all gas utilization equipment in the confined space, but not less than 100 square inches. One opening shall commence within 12 inches of the top and one commencing within 12 inches of the bottom of the enclosure.
    Note, if you use louvers...they would need to be much larger than that minimum of 100sqin because the louvers block a lot of that area...you need free air passage. That passageway to unconfined space still h asvolume requirements, and if that is too small, you might need to duct it to outside. That requirement is at least 50CUFT/1K BTU...so, if your WH has a 40K BTU burner, you'd need those openings into an area that had at least 2000CUFT of volume - with an 8' ceiling, that's a room a little bigger than 12'x12'. If your furnace is 120K and the WH is 40K, you would then have 160K worth of burner, and need 160sqinx2 of opening in the door, and it must connect to a free space of 50*160=8,000CUFT, or at an 8' ceiling, 100'x100'! I'm certain your house isn't that large! This means, drawing it from outside would be the only option.

    FWIW, part of getting an inspection is to have followed both the local and national codes, but those dictate the appliance must be installed per the manufacturer's instructions.

    As we tighten up our houses, open combustion devices make it harder and harder, which is why things are moving to closed combustion burners. Throw in a big gas stove, maybe a gas insert fireplace, then turn on your range hood and bathroom fans...you can be sucking all sorts of outside air in through any cracks...and, as the house gets tighter, that gets harder and harder. Older houses weren't that much of a problem, because they often leaked like a sieve, at least in air. This is why new homes built in more enlightened places (Canada comes to mind), require fresh air ventilation that would help ensure both a means to provide fresh air to vent out excess moisture but also to bring in enough air so your combustion devices can not create a lower pressure in the house.
     
  17. spfrancis

    spfrancis Member

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    Jim, Thanks fot the information/input. I wasn't able to head up there last weekend, but I do plan on making a day trip up there this Sunday. I understand that having the furnace in a closet was not a good design. I'm trying to figure out if that is contributing to the problem that I am seeing. I would think that leaving the door open would have fixed that problem, and my system would have been able to hold am temp consistently. Since it was not, I am thinking that I will have to look elsewhere, but probably need to address the door in the near term. My goal is to gut this house in 5-7 years, and have all of these areas addressed. In the meanwhile, I hope I can find the root cause of this problem, to keep the system working okay. So you asked one question. Did I see an error. The flashing is 6 blinks and then 3 blinks. Here is what I can tell that means: So it looks like three flashes means: pressure switch open, with inducer on. 6 flashes means softlockout.
    I'm going to do a better check of the exhaust piping. I'm going to check the hose on the pressure switch(may just replace it), and watch the system closely, as it moves from running mode, to failed mode. Maybe there is an indicator that shows more of what is going on. I have already changed the pressure switch, so no problem there. I will also verify that intake air is coming from the room. Hopefully this is a good plan.
     
  18. spfrancis

    spfrancis Member

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    So I did a bit more of research yesterday . I did see a couple of things of concern. The inducer motor definitely seemed to get pretty hot(to the touch), which may be normal, but something I noticed. There is a bit of water pooling up on the left side of the picture. There is 3 hoses coming into that area. The one hose(which is a little larger) is going to the inducer housing, and where the PVC for the outside is. The second hose is going to the pressure switch. The 3rd larger hose is going back to the housing(so not sure that that hose really does). It is hard to tell which of the 3 hoses is leaking, but it almost looks like it may be the 3rd hose that goes to the back housing. As you can see in the picture, there is a bit of rust where this may have been happening for a while. On a correction from before, there is an input hose that follows the same path as the exhaust hose. It definitely never seems to have the same pressure as the exhaust hose, when is it in normal mode. When it is in error mode, they both have very light pressure. It does look like the fan/inducer stops when it move from normal mode to error mode. I did verify that my pressure switch(which is at 1.71 WC PF that I replaced, was the same as the one I pulled out. So when I was testing yesterday, the outside temps were 50, and it is still pretty easy to have it fail, but it is less noticeable, as the temps kinda keeps up. WHen it gets down to 30 or so, is when it goes into a tail spin, and probably just doesn't stay on enough. I'm almost still thinking possibly a failing inducer motor, or whatever is causing this pool of water on that 3 hose area. I have taken off of the middle small hose(pressure switch), and made sure it was air tight. Any thought would be appreciated.
     

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  19. cacher_chick

    cacher_chick Test, Don't Guess!

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    Pull the hoses off and make sure they are all clear. If the hoses are blocked, the pressure switch will not function and the unit will go into lockout.

    Check that connection where all the hoses are connected together too, there may be a trap and/or blockage there.

    This is not a problem of the furnace "not keeping up". The furnace is locking out because it sees a fault.
     
    Last edited: Nov 25, 2017
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Carefully check all of the hoses...sometimes, they develop a split at the connector, or lose their resiliency and don't keep a good seal (some may use a clamp, but I don't remember seeing any that will help with the sealing). Once in awhile, they may develop a leak if they rub on something, so you want to check their entire length.

    The exhaust will tend to have a higher volume than the intake because you're adding all of the combustion gasses to that inlet air in the combustion chamber. IN a condensing furnace, unless it's running at max output, generally, there will be some condensate, and that tends to be slightly acidic. There's probably a trap somewhere, too, and over time, the lines and trap can get partially, or even fully clogged, letting that condensate back up and potentially leak, and even, worse case, block off sensors so they can't work properly.
     
  21. spfrancis

    spfrancis Member

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    so a question about how to properly blow out. I do have a air compressor. I'm not sure if that is too strong. I read a few things about blow out procedures. I was able to find a bit about cleaning drain lines, but there is a lot of different thoughts online. Any links that folks would recommend for blow-out procedures?
     
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