Furnace issue. [Archive] - Glock Talk

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Davegrave
11-27-2012, 19:14
My thermostat is set at 70. Every few hours I get cold and go check and the temp in the house is closer to 60. If I switch the thermostat off and back on, the furnace kicks on and the house gets up to temp. Then later, it's back down.

This is just a bad thermostat right? I'm gonna switch it out tomorrow and hope that fixes it. But it's a newer thermostat, only 2 years old. So I'm just checking here hoping everyone agrees with me. I sure don't want to have to pay for a service call.

Man, oh man, I'm hoping one of our HVAC guys doesn't say it sounds like a way more complicated problem.

jakebrake
11-27-2012, 19:26
is the thermonstadt battery powered (really...some of the rockwell units are.) and it's a furnace, not a boiler, correct?

HollowHead
11-27-2012, 19:42
Has anything about the location of the thermostat changed? Years ago, I had an iguana in an aquarium with a heat lamp cover and I moved it against a different interior wall. The thermostat was directly on the other side in the dining room and the heat from the tank heated the shared wall just enough to make the thermostat read high. HH

Davegrave
11-27-2012, 19:51
Yes, a furnace. Yes, the thermostat is battery powered and the batteries are fresh.

No, nothing else around the thermostat to falsely affect the reading. The reading on the thermostat shows that the temp is lower, and it show's what it's set at, it just doesn't kick on unless I switch the thermostat to off and then back to heat.

davew83
11-27-2012, 19:52
Stop buying cheap thermostats at Lowes. Get a Honeywell pro 5000 or 6000 if you want a programmable one.

Davegrave
11-27-2012, 19:58
It's a Honeywell. I don't need a lot of features. I have a tiny 4 room home. I just need it to kick the furnace on when it's supposed to.

The $30 or so for a new one isn't a big deal, I'm just hoping general concensus is that that's the only likely problem.

HollowHead
11-27-2012, 20:13
Yes, a furnace. Yes, the thermostat is battery powered and the batteries are fresh.

No, nothing else around the thermostat to falsely affect the reading. The reading on the thermostat shows that the temp is lower, and it show's what it's set at, it just doesn't kick on unless I switch the thermostat to off and then back to heat.

Is the existing old thermostat matched to the new furnace in terms of line voltage, setback and cycle rate? I'm guessing by your description that the setback is wrong. HH

KenInColorado
11-27-2012, 20:25
I'd replace the thermocouple on the furnace. It sounds to me like the furnace gets the initial signal to make heat, but once it reaches temperature it is not receiving the signal to make heat again. It's an easy fix.

Make sure you get one that's long enough, they generally come in length increments like 12", 15", 18", 21", etc.

Dan_ntx
11-27-2012, 20:29
Gas or electric?
I had a gas one that did the same trick as yours...turned out the flame bar (flame sensor) was fouled and causing the burners to not stay lit. Cheap fix.

JBG30
11-27-2012, 20:34
I'd replace the thermocouple on the furnace.

Betting he doesn't have one. Plus if he did and the thermocouple was bad the furnace would never come on unless something was dangerously wrong.

Hopefully it's a newer furnace. 10 years or so this should apply.
The next time the furnace won't come on go look at it. Take the front panel off, there should be a clear window on the blower compartment, you will see a red LED inside flashing a fault code most likely. The inside of the front cover should have the flash codes listed.

When you turn the stat off and on I think you may be resetting the fault only to have it repeat.

rgb03
11-27-2012, 20:52
Another thing to look at is the flame sensor. most times it just needs a quick wipe to remove the ash.

closetoreality
11-27-2012, 21:20
I love how everyone's an hvac tech now....

It's not a thermocouple...a thermocouple detects a flame which keeps the gas valve open...flame goes out, gas valve closes.

On stand alone pilot you hold the gas valve open and light the pilot, while continuing to hold the gas valve open for 20-30 seconds which allows the thermocouple to heat up and create enough millivolts to hold the gas valve open. After that 30 seconds it should stay lit on its own and you can walk away.

A bad thermocouple would not allow the furnace to light, as the gas valve would have no way of opening.


Long story short, most likely your thermostat went bad....I stick to the original honey wells...keep it simple.


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closetoreality
11-27-2012, 21:20
[quote=closetoreality;19676361]I love how everyone's an hvac tech now....

It's not a thermocouple...a thermocouple detects a flame which keeps the gas valve open...flame goes out, gas valve closes.

On stand alone pilot for instance you hold the gas valve open and light the pilot, while continuing to hold the gas valve open for 20-30 seconds which allows the thermocouple to heat up and create enough millivolts to hold the gas valve open. After that 30 seconds it should stay lit on its own and you can walk away.

A bad thermocouple would not allow the furnace to light, as the gas valve would have no way of opening.


Long story short, most likely your thermostat went bad....I stick to the original honey wells...keep it simple.


Sorry for the double post

HollowHead
11-27-2012, 21:26
I love how everyone's an hvac tech now....


Free advice is just that. Don't be a ****. HH

JDennis
11-27-2012, 21:34
Another thing to look at is the flame sensor. most times it just needs a quick wipe to remove the ash.


If its gas, flame sensor flame sensor flame sensor! I ended up screaming that in my sleep a couple years ago when a similar situation happened to us. I changed the thermostat, vacuumed the furnace, even cleaned the flame sensor. Nothing helped till i called a hvac buddy. The flame sensor went bad. I now keep a couple on hand.

JBG30
11-27-2012, 21:57
I love how everyone's an hvac tech now....


I know there is one in this thread:whistling:

While it could be his stat, I can count on one hand the ones I've seen fail. Most of those were the Honeywell T87 and the heat anticipator was burned up.

Simple enough to find out, expose the thermostat sub base jumper R to W if the furnace works every time, condemn the stat.

I still think the furnace is locking out. Like the filter is filthy and the airflow is low across the heat exchanger = high limit trip. Normally it would require a line voltage reset, but I have seen units where a removal of the call for heat was enough to reset the fault.

Flame sensor is a possible lockout also. Check how many micro amps it's producing to be sure.

davew83
11-27-2012, 22:06
I know there is one in this thread:whistling:

While it could be his stat, I can count on one hand the ones I've seen fail. Most of those were the Honeywell T87 and the heat anticipator was burned up.

Simple enough to find out, expose the thermostat sub base jumper R to W if the furnace works every time, condemn the stat.

I still think the furnace is locking out. Like the filter is filthy and the airflow is low across the heat exchanger = high limit trip. Normally it would require a line voltage reset, but I have seen units where a removal of the call for heat was enough to reset the fault.

Flame sensor is a possible lockout also. Check how many micro amps it's producing to be sure.

Seen quite a few stats fail, mostly cheap ones from Lowes/ Home Depot.

I agree could be a high limit switch from dirty filter but there are several possibilities without being able to look.

w30olds
11-27-2012, 22:23
I know there is one in this thread:whistling:

While it could be his stat, I can count on one hand the ones I've seen fail. Most of those were the Honeywell T87 and the heat anticipator was burned up.

Simple enough to find out, expose the thermostat sub base jumper R to W if the furnace works every time, condemn the stat.

I still think the furnace is locking out. Like the filter is filthy and the airflow is low across the heat exchanger = high limit trip. Normally it would require a line voltage reset, but I have seen units where a removal of the call for heat was enough to reset the fault.

Flame sensor is a possible lockout also. Check how many micro amps it's producing to be sure.

There's another also:whistling:

I'm gonna agree with JBG30. Sounds like the furnace is getting locking out. Either a safety, limit switch, or flame sensor. Check the filter. If it's clogged then the high limit could be tripping. If a safety is tripping the fan will still run until the safety is closed even without heating. Have to say nope for the thermocouple. If that were the case the furnace wouldn't even fire because the gas valve wouldn't open due to a faulty thermocouple. Most newer furnaces have hot surface ignitors, or the dreaded spark style like Lennox uses.

If your using a programmable t-stat then check the program. Could be something wrong there.

aspartz
11-27-2012, 22:46
If it's a forced induction furnace, there could be a leak in the condensate drain. The system doesn't detect a vacuum and won't turn on the gas.
In any furnace, even if the burner lights, if the main blower fan is dying and not starting, the heat exchanger will overheat and shut down.

ARS