Sounds to me like your aux or backup heat strips ain't workin. If the heatpump cannot reach the desired temp then the aux heat strips is supposed to kick in and finish the job on cold nights. Read more here:
Looks good to me. You might want to add a melting type fusenear the burners in case the fan don't come on and the limit fails. Most of the time the fuse gets tied into the thermocouple circuit so that when it melts it kills the thermocouple and that shuts down the gas valve. They sell a little insert that has 2 wires on it. You remove the themocouple and stick the insert in the magnet then screw the thermocouple into it. You can then add in the fuse to the circuit.
The heat circuit and the fan circuit. The heat circuit consists of 24 volts going to the gas valve from the transformer and being operated by a simple 2 wire thermostat like in yer pix. Note: usually the pilot thermocouple has a high limit mounted in it's circuit on the the heater that melts and kills the pilot in case that klixon don't turn the fan on or the fan is bobo and that is yer built in safety.
The fan circuit consists of that Fan klixon (wired to live current always) when the temp reaches around 120f it kicks in AND a high limit which kills power to the circuit if the fan fails to come on at the correct temp. Both the LIMIT AND the FAN Klixon are identical in nature but work opposite. The fan klixon will have a F and the temp rating stamped into the metal such as F-120 while the Limit will have something like L250 stamped into it.
So what goes wrong?
If the fan does not come on the limit shuts everythang down AND their ought to be another fuse (rated 300f) mounted somewhere near the burner to kill the pilot so that the thermocouple will cool down and shut off the flow of gas before the whole shebang burns down the shop.
Symptoms of a hangin heater gone south.
Pilot working, main gas won't ignite? Bad gas valve solenoid, transformer is wank, rat chewed the transformer wires.
Note: If you have a cheap asz boss like I had their is no need for a thermostat. The only time the heat came on was when he paid the bill and manually turned it on! LOL
OK The inducer fan comes on and the glowbar fires up. Then you should hear the CLICK/WHOOSH! Do you hear click but no whoosh? or no click at all? If you hear a click but no whoosh of ignition then I would say one of them rollout switches is tripped OR the gas valve solenoid is bad. A quick check for continuity on the gas valve will verify that. If you hear no click at all I would say the board is not getting power from the pressure switch because the negative pressure from the inducer is not present. Clogged tubing from the inducer to the pressure switch OR the inducer fan ain't causing enough draft to trip the pressure switch and therefore the control board cannot get confirmation and aborts the mission. Captain we have a problem. What is is Spock i never allow yall to drink while on duty. Captain dilythium crystals won't fire off and I am freezin my aszs off up in here.
207166 usual suspect if water fills but won't go. It is called a check switch. Very common issue with these machines. It would be very easy to check, remove the switch and see if the little button pops out. If it is bad it usually won't.
The Williams wall heater uses low voltage thermostat mounted on wall. If that thermostat has no continuity get another one. The heater uses a 120 to 24 volt ac transformer to power that gas operator on the combination gas valve. It is called a combination gas valve because it combines a gas operator( to open the burner gas thermistatically) and a pressure regulator to bring down the meter pressure to specific pressure to allow for the rest of the gas appliances in the house to function properly. The transformer is located up near the top of heater where the power cord comes in. If it is bad then the heat will not come on. Remove the 2 wires on the gas valve operator and check them for 24 volts output. Never touch the 2 wires together to check for power as this will fry the heat anticipator on the wall thermostat and ruin it. Check the 2 terminals on the gas valve for continuity. If they are open the gas valve will not work(the pilot will, but not the main gas). The gas valve has a thermocouple whichs holds open the gas when the pilot is lit. The thermocouple has a ceramic fuse mounted in series which kills the thermocouple and shuts down the gas, when the heater gets too hot. The burner must be wire brushed cleaned well to prevent uneven combustion. The heat exchanger must be clean of cobwebs and rust. The chimney stack must be clear. If not, the heater fuse will shut down. The burner will fire off but the fan does not come on right away. The fan klixon mounted midways up the heat exchanger under the front cover will turn the fan on as soon as the heat exchanger is properly warmed. Another 3 wire klixon controls high and low speeds. A third one is a high limit which shuts down the heater if that fan won't come on. The heater is pretty easy to work on. The burner assembly is removable for bench testing and repairs. The gas valve will need replacing sooner or later and you can damage the burner if you are not careful. I put the valve in a vice and turn the burner near where it screws into the valve. The fan blade will tend to rattle and this can be fixed by peening the hub. I remove the fan blade and set it on a 1/2 inch shaft, using a ballpeen hammer and blount nose chisel. I dimple the hub all around the shaft to tighten it up. Hey it saves 50 bucks. I also oil the motor with Hi temp oil. I use zoom spout turbine oil. The pilot on these heaters is mounted on the cast iron burner so that when heat is called for the flame will quickly ignite the burner.
To light this heater: Move the knob to the "PILOT" position. Press down on the gas knob and light the pilot. Hold down on the button until the pilot stays lit with the button released. Move the knob to the "ON" position. If the wall thermistat is in the on position then the burner should light off. After a minute or so the fan should kick in. When the wall thermostat is satisfied the burner will shut down but the fan will continue to blow until the heat in the exchanger drops.