Home HVAC / Heater question - I'm willing but lacking in knowledge

I was struggling with this and trying to think if I knew any hvac guys.  Then I realized that if anyplace can help, GRM might be able to help!   Our home hvac (gas heater only) will not fire up.   Emerson electronic thermostat, Day & Night heating unit that is about 6 years old. Not run that often.

Realized the heater isn't coming on.   You can set the temp high and the thermostat clicks like it normally would and indicates that it is supposed to be heating, but nothing happens. 

If you turn the fan on "On" it comes on.

The hvac unit has a steady flashing green light which seems to indicate that it is powered up and ok. It doesn't change no matter what we set the heater to.

A week ago, the thermostat went dead and my wife took off the wall, cleaned the slightly corroded contacts, put new batteries in it and it came back to life.   But I do not think the problem started at this point - I am pretty confident that the heater was not working before the thermostat died.

I don't know how to check or what to check.  Is the thermostat not sending a signal to the unit?  does it make sense that it would turn the fan on but not the heater, even though it clicks and everything seems to indicate that it was working properly?

Appreciate any help!

There are several sensors that can stop the burner from lighting, or shut it off immediately after it turns on.

There is a flame sensor that needs to see the flame.  If it doesn't, it will shut down the burner.

There is a "roll out" sensor that detects flame coming out of the burn area.  This will also shut off the gas valve.

Finally, I believe there is a way to just jumper the thermostat so that it calls for heat.  This would test the thermostat circuit separately and determine whether you had a burner issue or a signal issue.

I would think that the indicator light would signal a problem with any of those, yes? 
 

Great call on the jumper.   I did some looking and was able to figure which wires to jump. 
 

the good news is that it turned on the heater.  The bad news was it just blew air. 
 

I also jumpered it for the fan and it worked as well. 
 

so it SEEMS that my thermostat is not signaling the heater AND the heater is not firing on. 
 

but rarely do two separate issues pop up. So either something happened to one that effected the other or my test is inconclusive.   
 

the green light is still doing a heartbeat    At one point I saw it change to short, pause, long , short, pause, repeat

Any thoughts?

I'm not familiar with your heater brand, but my furnace won't fire some years when we start the heating system. Everything appears normal, but no fire. It's always caused by some build up of trash in the air intake pipe, right at the furnace , some 10 feet of pipe from the wall. There's a sensor that measures combustion air flow and if there isn't enough it won't fire. Now I just remove a couple of rubber collars and suction out any crap with the vacuum cleaner before heating season. I've put a finer mesh screen on the outside of the house to keep stuff out of the pipe and it happens less frequently now.

We are going through the same thing. Tech came out (since they just installed it 16 mos ago) pressure switch on flue fan was being messed up by condensate rolling back down the flue. As noted still being addressed , but check the pressure switch?

Bad thermocoupler?

https://youtu.be/3EKbSiAJyiI

In reply to jfryjfry :

There should be a tag inside the furnace section with a listing of trouble codes.  The long / short pulses are likely a “code” that will indicate the problem.   If you can share the model number I’ll see if I can dig into it if you like.  I think Day and Night is a Carrier brand and their literature is usually available.  

The normal sequence of operation for a gas warm air furnace is to have the igniter light and prove the pilot then fire the main burner.  Once the burner has heated up the furnace either an internal thermostat or a time delay relay will start the fan.  Most manufacturers now combine all this (ignition, flame safety circuit and  time delay) into a main control board as opposed to individual components.   The gas valve itself could be defective as well. 

One more dumb question, is this the only gas appliance in the house?  You mentioned it’s not often used, if there’s been any interruptions to your gas service it’s possible that your piping is air bound.   Carefully cracking a union to see if you get a whiff of the odorant that’s put in gas will eliminate that question. Just be sure to tighten the union back up.  

On my gas furnace, it was always either the ignitior not igniting (the gas would turn on, then off again in a few seconds without the flame being sensed) or the flame sensor (the gas will ignite, then shut off after a few seconds because it doesn't sense the flame that's actually there.) I'm not a professional though... so if your house blows up... it's on you. :p

I'll see about finding those trouble codes.  Unfortunately the unit is in the attic and hard to reach. 
 

regardless, after reading above about blocked inlet I noticed our filter was disgusting.  So I changed it and jumpered the correct wires and this time it lit off. 
 

so I think I've narrowed it down to the thermostat and I'll get a new one tomorrow to test the theory

Easy stuff first.  If you open the access panel on the furnace, there should be a fuse on the circuit board.  Check that first.  If it's blown, it could also mean that the transformer is bad as well.  The transformer takes 120v and steps it down to low voltage for the thermostat and signals back to the furnace.

From there, it's a series of sensors.  There is likely a furnace blower to inject air into the flame for a hotter burn.  That is the first thing that comes on.  Once a sensor indicates that air is blowing, it will energize a solenoid to open the gas valve and start the ignition source.  Then there is a temp sensor that verifies things are getting hot.  If it doesn't see heat, it assumes the flame didn't ignite and starts over.

Also verify that the thermostat is sending voltage on the heat wire when it clicks.  Just because the relay in the stat clicks doesn't mean it's sending juice.  Verify voltage on the appropriate pin of the thermostat and at the circuit board's appropriate post when it is signalling for heat.

There are also sensors for airflow in the intake and exhaust.  Any restriction (bee's nest, leaf, bird's nest, dead squirrel) will tell it to shut down.

The flashing green light is also a thing.  Most of them only flash if there is an error code.  Find your model number and download the repair/owner's manual to be sure.  Since it is a steady flash, maybe it's flashing code #1 or #11 or #111.

Great help - thanks!

you nailed it on the click but no power from the thermostat.  A $30 smart (read: programmable) replacement later and we are back in business!

Glad you're not an ice cube. :)