Furnace will not start or stop running reliably

Users who are viewing this thread

PhillyRunninMan

New Member
Messages
5
Reaction score
0
Points
0
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
 
Messages
951
Reaction score
6
Points
18
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.
 

Ian Gills

Senior Robin Hood Guy
Messages
2,743
Reaction score
3
Points
0
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.
 

Thatguy

Homeowner
Messages
1,454
Reaction score
3
Points
0
Location
MD
I have replaced the 60-year old wiring & the 12-year old thermostat and still have the problem.
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:

NHmaster

Master Plumber
Messages
3,176
Reaction score
5
Points
0
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
 

Attachments

  • pressuretrol-250x250.jpg
    pressuretrol-250x250.jpg
    11.1 KB · Views: 1,585

PhillyRunninMan

New Member
Messages
5
Reaction score
0
Points
0
Location
New England
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.

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!
 

PhillyRunninMan

New Member
Messages
5
Reaction score
0
Points
0
Location
New England
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.

I was hoping that it was that !!! but no, the damper is balanced & moving smoothly

Thanks!
 

PhillyRunninMan

New Member
Messages
5
Reaction score
0
Points
0
Location
New England
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.

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!
 

PhillyRunninMan

New Member
Messages
5
Reaction score
0
Points
0
Location
New England
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

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
 

Thatguy

Homeowner
Messages
1,454
Reaction score
3
Points
0
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.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,771
Reaction score
1,191
Points
113
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.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks