Furnace will not start or stop running reliably

Discussion in 'HVAC Heating & Cooling' started by PhillyRunninMan, Oct 18, 2009.

  1. PhillyRunninMan

    PhillyRunninMan New Member

    Messages:
    5
    Location:
    New England
    Hi all,

    I've got a 60 year old steam furnace with a newer gas burner unit (~12 years old) that will not reliably start up and, when started, will continue to heat long after the target temperature has been met. I have replaced the 60-year old wiring & the 12-year old thermostat and still have the problem.

    Any thoughts? We had the local gas company's service people out--that was kind of worthless--they said it was the thermostat or the wire, but they didn't work on them, and then they sent us a bill for their 'diagnosis'.

    Thans in advance,

    Cheers,
    PhillyRunninMan
  2. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I don't know that this fits the particulars of your problem, but the problems I've had with furnces/AC not shutting off or not starting in response to set point have often fallen into the following categories:
    1. Old battery in the digital thermostat. (I've seen this on several units in different home...they get squirrely when the battery is low.)
    2. Air infiltration in the wall the thermostat was mounted on--had this happen last year. In this case there was an open channel up the wall to the attic. This was creating an offset in both summer and winter, most noticeable in winter when the displayed temp seemed to lag other measurements in the home...and the furnace would continue running well after the home was warm--to the point of becoming hot. I noticed the wall near the thermostat was very cold during a ~0 F night. The next day I checked the attic and found a gap in the insulation. After stuffing the cavity the unit has behaved predictably.
    3. Stuck or defective contactors on outside units. Sometimes a sharp rap on the box will get them functioning (at least temporarily.) Other times I've had to replace them.
  3. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,777
    Location:
    USA
    Does it have a vent damper and is that playing up?

    On my old furnace the vent damper acted like a switch. On those days it refused to open, the furnace would not start up.

    Just an idea.
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Post a schematic of the furnace.
    Closed contactor points at rated current that are good should read 30 mV or less across them, with bad being over 100 mV.
    This can't be too hard to find the problem.
    Last edited: Oct 19, 2009
  5. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Check the pig tail that runs to the pressuretrol for 3 years worth of mud and crap

    Yours probably does not have the nice clear cover like the one in the picture. Teh pigtail is that squirrly pipe it's attached to

    Attached Files:

  6. PhillyRunninMan

    PhillyRunninMan New Member

    Messages:
    5
    Location:
    New England
    Thanks for the response (& sorry for my delay). The digital thermostat (& batteries) is new & the wall is well insulated--no drafts.

    I'm thinking it has to be the relays. Interesting (with the new thermostat & batteries) the Furnace turns on on cue, but still fails to shut off. The electronics of the burner are well beyond my ability--Im going to find someone (other than the gas company) that can work on steam systems.

    Thanks!
  7. PhillyRunninMan

    PhillyRunninMan New Member

    Messages:
    5
    Location:
    New England
    I was hoping that it was that !!! but no, the damper is balanced & moving smoothly

    Thanks!
  8. PhillyRunninMan

    PhillyRunninMan New Member

    Messages:
    5
    Location:
    New England
    I'll try to dig up the schematics--that was a hectic time (my son was in the hospital when we closed on the house--he's fine now) & I'm afraid that I didn't get them when the system was converted from oil.

    I'll have a look and see what I can find.

    Thanks!
  9. PhillyRunninMan

    PhillyRunninMan New Member

    Messages:
    5
    Location:
    New England
    I don't believe my system has a pressure controller like that (with the pig tail). The valve to bleed off the sludge from the furnace works fine--I bleed it off every other month in the heating season.

    I'll have a look an see if there is another portion (pig tail-like) of the pressure controller inside the old cover (the heat exchanger & burner are in the original 1930's case which has been modified a couple of times).

    Thanks,
    Bret
  10. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Wait for the symptom and measure the relay/contactor coil voltage.
    If the voltage drops to zero but the unit continues to run it has to be the contacts.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,058
    Location:
    New England
    FWIW, one spring a few years ago I realized my a/c unit wouldn't turn on. After investigating, discovered that a mouse had electrocuted itself across the main power contactor (240vac). It happend the end of the season and over the winter the thing was mummified, but in its death throws, the fluids gummed up the contactor. A lot of contact cleaner got it free again, and it works. All sorts of things can get into contacts - ants, termites, mice, and give you all sorts of weird symptoms.
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