We've had our heat on for a couple of weeks now and it has been working
fine. However, two days ago we noticed that the forced-air gas furnace
does not engage in the morning when we set the programmable thermostat
to raise the temperature -- we use the programmable thermostat to turn
the heat down overnight.
Investigating the situation, we find that the pilot light is on and
appears normal. Setting the thermostat to engage the system results in
no noticeable behavior (completely silent). We also tried shorting the
two thermostat wires (RH and W) which had no effect. We were able to
restart the furnace by turning the pilot light off (disabling the gas
from the line, then turning the furnace gas switch to off) and then
relighting the pilot (enabling the gas from the light, turn the furnace
gas switch to pilot, light and hold down for a minute, then turn the
switch to "full-on"). After this "pilot light reset" the furnance will
engage under the thermostat control for the rest of the day (on/off
several times). However, the next morning we had the same problem.
Any ideas of what could be wrong? The furnace looks to be about 20
years old. We just moved in so this is our first winter with this
Thanks for any advice.
You gave enough info to determine that it's most likely a sticking gas
valve. There is no repairing the valve, it must be replaced. It's
possible that something else is occurring, but we can't see it from
here. Regardless of the cause, you should get a professional out to
check it, because a sticking valve that opens slow or not at all can
also stick in the open position, and this can be a really bad situation.
On 29 Oct 2005 21:06:50 -0700, firstname.lastname@example.org wrote:
I would need a lot more info to try to even give you a guess. what
brand, Model, type of fuel? in the end it would be best to call a
local company to check it since I can't be there to test it.
spam protection measure, Please remove the 33 to send e-mail
While a sticking gas valve is surely a good guess, I would add a weak
thermocouple and I would have to guess there are others. I suggest that
this might be a good time to have a local tech come out, check out the whole
thing and see if they can determine and fix the problem.
Note: Many modern furnaces have a computer error check. If you see a
flashing diode or other indicator it may be trying to tell you what is
I agree with all the replies, but I doubt too much computerization on
a 20 year old furnace. I suggest the OP put a jumper across the
thermostat right by the furnace (when it wont start). If it still
wont start, disregard the thermostat and thermostat wiring as the
problem. If it does start with the jumper, check or replace
thermostat, check all connections an the wires.
If it does not start, the problem is in the furnace itself.
A sticking gas valve is possible, but not real likely. Before you
spend lots of money on hiring a pro. Do as I said above, then replace
the thermocouple. They are under $10 and not that hard to change.
This is most likely the problem, knowing what you said.
If none of the above fix it, then you are only out a few bucks and
hours. At that point, you may have to either replace the gas valve
yourself, or hire a pro. Those older gas furnaces were pretty simple.
Basically the gas valve and thermocouple are all that really can go
bad on them. Just be sure the furnace is getting the "signal" from
Note: If you got an old (standard) thermostat, swap your wires to that
one and temporarily mount that for a few days. The fact that it only
does this in the morning is rather odd, and tells me your thermostat
has lost it's computerized mind...
Thanks for everyone's replies.
We have some more information on the system. It sounds like we will
start by investigating the thermocouple and then see about calling
around this week to have someone take a look at it.
Furnace Manufacturer: International Heater Company
Model: GH 75 CD
Gas Type: Natural
Input: 75000 BTU/hr
Bonnet: 60000 BTU/hr
We did some web searches and found nothing on this company or model. I
guess it may be older than we thought! Any ideas on how old this model
might be? There are no LEDs/diagnostics that we can see.
The gas valve looks more modern. We think it is a millivolt system:
We actually replaced the old analog thermostat two weeks ago with a new
digital/programmable one. Since everything had been working fine for
the last few weeks, we did not suspect it. We'll investigate further
Sorry, I missed the valve description that you posted. This system
doesn't have a thermocouple, it has a power pile...close, but not the
same device. On a millivolt system either a weak pilot, weak power pile,
or bad electrical connection can cause intermittent operation. I would
think that the t-stat would either work or it would not, but it's still
a possible source of the problem, as is a sticking valve.
The power pile that this valve takes is 750mV.
On 30 Oct 2005 11:13:20 -0800, " email@example.com"
Well, if the furnace worked fine for years and 2 weeks ago you
replaced the thermostat, that sort of indicates a strong possibility
of thermostat troubles. Just because something is new, dont make it
error-proof. Of course, you could have a wire thats broken inside the
insulation and is only making a poor connection.
Like I said, jumper the thermostat wires right at the furnace. If it
starts, you know the thermostat or it's wires are at fault.
One other thing. Where the gas pipe enters the gas valve. there is a
screen filter to catch any debris that gets into the pipes. You could
have a bunch of junk in there, remove the pipe and clean it.
Finally, if you suspect a sticking valve, turn the thermostat all the
way up, then give the gas valve a smack with a wooden mallet or chunk
of 2x4 (when furnace dont start). If that makes it start, it's time
to replace the valve.
One very slight possibility is that the gas entering your house is not
under enough pressure due to the regulartor being defective. This
part belongs to the gas company. If you have a gas range, turn on all
the burners when the furnace dont start and see if they all burn
normally. This is very rare, but not impossible. If the range is not
lighting normally, call your gas company.
If it is a millivolt system, the thermostat must be powered by
batteries alone. I had a customer install a programmable thermostat
without a common wire hooked up. The bateries needed replaced every
two to three weeks.
Can you please explain this.....
You lost me !!!
A thermostat is simply a switch. While a programable one is a
complicated device in itself, the end result is simply an ON and OFF
switch that turns the furnace on or off. Thats why a jumper wire will
start a furnace with a defective t-stat. Of course the furnace wont
shut off, so a t-stat is a must.
The batteries in a programmable t-stat just run the electronics in
them, and in the end the wires to the furnace as once again, switched
ON or OFF.
So, why the battery drain?????
Some thermostats are power stealing, which means they get power from the
system , either through a dedicated common wire or 'stealing' it through
another wire, usually the white one. Some of those 'stats use a battery
simply to keep the programming in the event of a power loss. The batteries
don't have the staying power to open and close the relays.
Another thing Stretch mentioned was a milli-volt system. They're not too
common anymore, but the MV stuff requires NO outside power source at all.
Doesn't make sense unless the thermostat wasn't designed to use battery
as primary power. I have a Honeywell CT-3600 which is solely powered
via 2 AA batteries. Despite me frequently hitting the illuminate button
the batteries always lasted a full season with plenty of power left to
put in remote controls or whatever at the end of the season.
Even if the batteries test ok, I always start the heating season with
fresh alkaline batteries so I don't have to worry about them dying in
January when I'm not home.
By the way, Honeywell thermostat models that begin with C usually are
powered by batteries, models that begin with T usually are powered by
the 'C' wire, although they may need AA batteries for a backup.
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