furnace does not start in morning

[gigihsu]

Hi,

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 furnace.

Thanks for any advice.

Gigi

[RP]

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.

hvacrmedic

[hvactech2]

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.

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[Joseph Meehan]

Well the OP did write forced air gas furnace. However I agree that more information would help.

[Joseph Meehan]

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 wrong.

[maradcliff]

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 the thermostat.

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...

Mark

[gigihsu]

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: VS820 A1054 Oper. VS824A Reg. V5306A

3.5 ADJ

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 as described.

Thanks,

-Gigi

[e-mail] wrote:

[RP]

It cannot possibly be the thermocouple, period! The thermocouple only holds in the pilot valve, if it gets weak the pilot light will drop out.

hvacrmedic

[HeatMan]

20 years old? Its at the end of its life. It's also inefficient as he**.

Replace it.

[maradcliff]

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.

Mark

[Stretch]

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.

Stretch

[RP]

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.

hvacrmedic

[maradcliff]

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?????

Mark

[Greg]

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.

[HeatMan]

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.

[HeatMan]

Not quite. The "C" designation denotes the 'stat was sold by a home center. The "C" could stand for consumer or some such. My VisionPro can run on 24v from the heating unit or batteries.