Need help with ignition on gas floor furnace
Anthony Cook
We have lived there for 10 years. Over the past 4 years, the furnace
has been getting harder and harder to get to operate in the fall. The
control unit under the house in line with the furnace was replaced
several years before we moved in 10 years ago.
1996: Crawl under house and light pilot. Come in house and turn up
thermostat, but furnace doesn't fire up. Thinking maybe there is some
sort of oxidation inside the mercury vial, I short the terminal on the
outside of the mercury vial and the furnace kicks on. Let the furnace
run for a few minutes, and the whole system worked good the rest of
the winter.
1997: Pretty much the same thing as in 1996.
1998: Pretty much the same thing as in 1996.
1999: After doing the same thing as previous years, still couldn't
get furnace to ignite. Replaced the thermostat on the wall with a new
one. Still no luck. Went under the house and shorted across the
thermostat lugs on the control unit with a wire, and the furnace
ignited. Thinking that the terminals or the thermostat wires were
oxidized, I undid the thermostat wires and scraped them along with the
lug faces and screwed it all back together. System worked well from
there (might have had to short across thermostat lugs one more time
before I got it running...can't remember exactly).
2000: This year I had to repeat everything that I did in 1999, but
there is nothing I can do to get the system to work with the
thermostat on the wall. The only way I can get the furnace to fire up
is to short across the thermostat wire lug connections on the control
unit under the house. If the thermostat is on up in the house, then
when I short across the lugs under the house, then it will stay lit
when I remove the short. However, once I shut the furnace off via the
thermostat in the house, then I cannot get it to re-ignite via the
thermostat.
Where could the problem be. Any suggestions? I would love to be able
to fix this myself and save on a big repair bill. I have multimeter,
so if I need to check voltages or anything I can do that, but just
don't know what to look for. By the way the there are three lugs on
the control unit. The thermostat wires go to the outer two lugs and
the thermopile wires go to the center lug and an outer lug.
Thanks,
Anthony
Speedy Jim
by the tiny voltage generated from the flame on the
thermoplie unit. IIRC, it is about 350mv but there
may have been several standards.
More than likely, the thermoplie is weak or the pilot
flame is not striking it in such a way to maximize the output voltage.
Jim
Larry Kapigian
for heat ) If your generator is putting ot say less than 300-400
millivolts you may need a new generator, or simply clean the pilot... The
gas valve needs at least 250 millivolts to hold, and your going to get
25-75 millivolt drop across the stat.
For what its worth
Larry
ftwhd
To the original poster; what are the letters designations on the gas
valve? Are there 4 wire terminals that say P and P and TH and Th? If
so they you have a power pile system which generates either 750, 500,
or 250mv depending on which power pile you have.
Given the age of the furnace it may very well be a power pile system.
In which case the replacement thermostat will have to be a millivolt
stat and not a low voltage stat which is what I suspect you replaced
the original one with.
Millivolt stats will have 3 terminals R, W, and Y. R is always used
and W is used on a 750mv system while Y is used for the other two.
You have multiple problems if you have a millivolt system, the first
being the original problem which could have been the old stat or it
could be that the pilot is dirty or the power pile weak. The second
most probable problem is that you installed the incorrect stat which
wont work properly on a power pile system.
The original stat could have been dirty or you have poor connections
from the stat to the gas valve. Millivolt systems are very sensitive
to connection weakness. ie. corrosion, dirt, loose, ect.
If you don't have a power pile and it is a 24v system you can tell
this simply by the fact that the burners will light with out any 120v
supplied to the furnace. Most of the power pile systems didn't have
forced air and used natural convection as a means of heat transfer.
In short if you see a transformer on your furnace you more than likely
have a 24v system and other problems If no transformer then its a
millivolt system.
Unless you are able to provide detailed exact information on which
type of system you have its impossible to tell from here exactly what
is wrong. If you can't provide the needed info then your not savvy
enough to attempt a DIY repair and need to call a tech to diagnose the
problem on site. Its not rocket science but there are a lot of
variables that affect furnace operation that require an on site
analysis.
Mike
UA local 370
Anthony Cook
>If so they you have a power pile system which generates either 750, 500, or 250mv depending on which power pile you have.
>Given the age of the furnace it may very well be a power pile system.
>In which case the replacement thermostat will have to be a millivolt stat and not a low voltage stat which is what I suspect you replaced the original one with.
>Millivolt stats will have 3 terminals R, W, and Y. R is always used and W is used on a 750mv system while Y is used for the other two.
There are only 3 terminals. Recalling from memory, one terminal reads
Th, another reads Th & Tp, and the other reads Tp.
There is no transformer and no AC power applied to this system, so I
am assuming it is self-generation (millivolt) and not a 24 V system.
>You have multiple problems if you have a millivolt system, the first being the original problem which could have been the old stat
>or it could be that the pilot is dirty or the power pile weak.
>The second most probable problem is that you installed the incorrect stat which wont work properly on a power pile system.
>The original stat could have been dirty or you have poor connections from the stat to the gas valve. Millivolt systems are very sensitive to connection weakness. ie. corrosion, dirt, loose, ect.
I will next make sure that the thermopile has not been bumped out of
the region of the pilot flame. I will also check the output voltage
from the thermopile.
From memory, there is also a dial inside the wall thermostat unit that
is set for 1.2 or 1.7 or something like that.
If I understand correctly from info given so far, the main things I
need to check first are:
a) Check to see if thermopile is in the pilot flame.
b) Check the output voltage of the thermopile.
c) Check to see if I have a millivolt or a 24v thermostat.
d) Cleaning the pilot, but I'm not sure what is involve here.
Thanks,
Anthony
Larry Kapigian
Larry Kapigian
Anthony Cook wrote:
> >To the original poster; what are the letters designations on the gas valve? Are there 4 wire terminals that say P and P and TH and Th?
> >If so they you have a power pile system which generates either 750, 500, or 250mv depending on which power pile you have.
> >Given the age of the furnace it may very well be a power pile system.
> >In which case the replacement thermostat will have to be a millivolt stat and not a low voltage stat which is what I suspect you replaced the original one with.
> >Millivolt stats will have 3 terminals R, W, and Y. R is always used and W is used on a 750mv system while Y is used for the other two.
>
> There are only 3 terminals. Recalling from memory, one terminal reads
> Th, another reads Th & Tp, and the other reads Tp.
>
> There is no transformer and no AC power applied to this system, so I
> am assuming it is self-generation (millivolt) and not a 24 V system.
>
> >You have multiple problems if you have a millivolt system, the first being the original problem which could have been the old stat
> >or it could be that the pilot is dirty or the power pile weak.
> >The second most probable problem is that you installed the incorrect stat which wont work properly on a power pile system.
> >The original stat could have been dirty or you have poor connections from the stat to the gas valve. Millivolt systems are very sensitive to connection weakness. ie. corrosion, dirt, loose, ect.
>
Johnl45
blow the place up.
Anthony Cook wrote in message <3a01f9f7...@news.larc.nasa.gov>...
Anthony Cook
I have a Lux T10-1141 Thermostat. The package and the website both
say that it can be used with:
Low Voltage (24 VAC) Systems (Gas, Oil or Electric Systems), and
Millivolt (Wall Furnace) Systems
Anthony
Jack Swedberg
You may also check the continuity of the thermostat
wires.
Occasionly, modifications to the house either break
these or there are additional loads after the system was
initially designed..
Did you check the transformer connections?....Are
they used in your doorbell...does your doorbell work?
Sounds like maybe a wiring (connection) problem.
Good luck!
Jack
Anthony Cook
>Occasionly, modifications to the house either break
>these or there are additional loads after the system was
>initially designed..
>Did you check the transformer connections?....Are
>they used in your doorbell...does your doorbell work?
>Sounds like maybe a wiring (connection) problem.
external voltages applied to this gas heating system. It is a
self-generating system.
As far as the latest results, I got the gas to fire up as described
earlier by shorting the thermostat wires together at the control valve
terminals. But this time I decided to let it run for about 20 minutes
before shutting it off. It works like a champ now. The furnace now
responds to the wall thermostat and can now be turned on and off with
the thermostat. Not sure why a 20 minute break-in caused it to work,
but it did.
Anthony
rdho...@gmail.com
Can you use a heat only t-state for a replacement milivolt tstat?
And
If you jump the two wires that were wired to the existiting milivolt thermostat (someone removed the existing one a long time ago) on a milivolt system should that open the gas valve?
