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  1. #1
    Join Date
    Nov 2005
    Posts
    3
    I realize you don't do DIY advice here; I'm just looking for some insight.

    My house has an an older high-efficiency furnace, "first generation 90%" according to an HVAC pro I know who dropped by casually but did not diagnose. It's a Kenmore model 867.769010, sold by Sears in (I think) the early '80s. We do not have central air. The furnace operates the way it always has over the 5 years we've owned the house, but a couple of friends recently pointed out that it's not acting as it should.

    First, the blower kicks on instantly when the thermostat turns on the furnace. I've been told that the blower should not come on until after the burners have started to warm the heat exchanger, but this is not what happens; blower start-up is instantaneous.

    While the furnace is operating, it always seems as if the airspeed in the ducts is faster/louder than it should be. The wiring is set up so that the blower should be running on low speed, but it sure is moving a lot of air. It's a 50k btu furnace in a small house (600 square foot first floor, no ducts to finished attic) and has only four 6" ducts coming off of it, so perhaps it simply moves more air than it should for so few ducts, even on low speed.

    Lastly, when the thermostat shuts the furnace off, the blower shuts off instantaneously too, rather than continuing to operate until the exchanger cools off as I understand it should. There is always a several-second delay of silence, then the blower (but not the burners) kicks back on briefly -- I assume this is the limit switch doing its job.

    So there you go. Yesterday I made a copy of the wiring diagram and traced every wire on it, crossing off each as I went (this was a suggestion of the hvac pro I chatted with). All of the connections inside the furnace are as they should be. The only discrepancy between the diagram and what I actually have is the way the thermostat is hooked up. The diagram shows a 3-wire connection to the thermostat; red, white and green. but it appears that the installer used the old 2-lead wire that was already threaded through the walls up to the thermostat; the red and white are correct, the green was omitted.

    Having acquainted myself with the various parts I intuitively wonder about the fan relay and limit switch, but what I really need is a sense of how complex/expensive this will be to fix. It's already an older unit, so I'm disinclined to pour a lot of money into service calls if it will be such a tricky diagnosis that a new unit would be smarter. Any thoughts or advice?

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,941
    Once checked by an experienced tech, they should be able to find the problem with in 20-30 minutes.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

  3. #3
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    No matter how small the furnace, only 4 outlets is going to be noisy unless they are really big. Don't know if it is a manual fan/limit (doubt it on 90%) or if fan is controlled by board. If it is controlled by a fan control there should be jumpers/dip switches to time the fan on and fan off. As far as thermostat connection the green wire is for fan control only. Not really necessary unless you want to turn on the fan for circulation in the summer months.
    "Go big or Go Home"

  4. #4
    Welcome to HVAC TALK.

    Your 50K/90% unit is going to be noisy with only 4-6" ducts!
    The other fact is that your unit is indeed operating incorrectly.

    Here's something to think about though...

    When was the last time you had that unit cleaned and inspected?

    Fuel costs are skyrocketing.

    So why not call in a professional to clean & tune up your furnace? While they are there they can inspect your unit to insure a safe living environment for you and your loved ones. Not to mention, having an efficient running system could save you more than the cost of the service.

    What's your safety worth? I'm guessing more than the repair bill.

    Thanks for your understanding.

    (we can't help DIY, due to site rules )

  5. #5
    Join Date
    May 2005
    Posts
    996
    Originally posted by Black Adder
    No matter how small the furnace, only 4 outlets is going to be noisy unless they are really big. Don't know if it is a manual fan/limit (doubt it on 90%) or if fan is controlled by board. If it is controlled by a fan control there should be jumpers/dip switches to time the fan on and fan off. As far as thermostat connection the green wire is for fan control only. Not really necessary unless you want to turn on the fan for circulation in the summer months.
    the fan switch [green wire] is used when you want the fan on as mentioned above but i want to add that you could run the fan any time you want to, not just in summer

  6. #6
    Join Date
    May 2005
    Location
    Western Kansas
    Posts
    267
    have you done a do-it-to-yourself thermostat change lately?...if so you might make sure the dip switch isn't in HP mode at the stat...other than that, it's time to call a higher order.

  7. #7
    Join Date
    Nov 2005
    Posts
    3

    Wow, all sorts of thoughts.

    Sorry I didn't keep up with this thread yesterday; guess I didn't have my email notifications set right.

    It's reassuring that the problems don't sound fatal (to the furnace).

    I dug out the furnace manuals, which were left behind by the previous owners. In the section covering the limit switch there's a big NOTE mentioning that "the fan comes on as soon as the thermostat calls for heat," so perhaps that part of the wierd behavior is really working correctly. Also, the installation instructions booklet seems to conflict a bit with the technical support manual in what it says about the on/off settings. The installation manual says it's set "for the fan to go off at 100 - 90 degrees." The Technical Support Manual says on at 120, off at 90.

    No, I haven't done any DIY thermostat changes. It's still got the Sears "Weekender" model that looks to be the same age as the furnace.

    I recently had to engineer the dust collection system for my little woodshop, so I now know just enough about flow rates and static pressure to be dangerous. Dust collection, though, deals with much higher volumes and speeds. Anyhow, the technical support manual for the furnace has blower performance data for static pressures of .1 to .5 inches W.C. I don't know what static pressure you HVAC guys usually aim to hit, but even if this was on the high end of the range described in the manual, at 0.5 in W.C., the blower would be moving 690 CFM at low speed, or 172.5 CFM through each of the four 6" ducts, requiring an airspeed of 879 feet per second. Where would you want the airspeed to be for hvac? It sounds as if the static pressure might be significantly higher than it should be, cutting down the CFMs and generating all the noise?

    Currently three of the ducts go to the old, original wall registers in this 1930s house. The fourth goes to a small floor register in the kitchen. There are no ducts going to the finished attic, though I've been thinking of adding a couple of ducts up through a wall to bring some heat up there. Undoubtedly that would improve the noise somewhat, but I'm wondering whether I should be looking to increase the size of the other ducts, too?

    I know I'm going to hear "just call in a pro," but I think I need to be prepared for the pro. A pro installed this system as it is, pros have done regular cleaning, a pro looked at this issue with me already. Probably they were nice guys who didn't want to sell me something I wasn't asking for. I need to know what to ask for.

    [Edited by jon1270 on 11-17-2005 at 11:10 AM]

  8. #8
    Join Date
    May 2005
    Posts
    996
    have somebody come out and give you a heat load / loss calculation and have them 'design' ductwork for the house. i think that if you are planning on staying here for a while, i might just want to spend some money and have everything conditioned and all the right sized ducts. it seems to me that if you just add more ducts on to the existing, it might not work quite as planned.

    take note!!!!

    i am no expert @ duct design but i do know that you dont want your furnace looking like Medusa, the mythical monster. it looks horrible and i feel it wouldnt work right!

  9. #9
    Join Date
    Feb 2002
    Posts
    3,182
    The fan shouldn't be coming on as soon as the stat calls for heat. On that old furnace, the fan should be controlled by the fan/limit control in the furnace. Is the "big note" in the manual written in ink by someone? Sounds like somebody wired the furnace/stat wrong when it was installed. Its time to call a licensed contractor in to straighten it out.

  10. #10
    Join Date
    Nov 2005
    Posts
    3
    no, the note isn't handwritten; it's printed as part of the installation instructions, as if they were alerting the installer to something unusual about the unit.

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