Central heater will not heat up to set temp on thermostac

Induced fan motors on older units are wired through the high limit safety on the stack - in series. When the furnace calls for heat, the induced fan motor comes on and after a time the furnace will light.

Older units have a temp switch in the stack that rotates to bring on the burners and blower motor. . When there is a call for heat, the induced draft fan motor comes on and then the burners will light.

After the heat exchanger is warmed up the limit switch will bring the blower motor on. That's basically how it works - there are two circuits - one for the induced draft motor and the burners and the other for the blower and high limit.

New units have a plug on the board you just plug in.

A

I assume this is gas . induced draft blower or ignitor.

I would suggest that you have someone turn thermostat up to call for heat while you are watching. Being that old it may or may not have a induced draft motor on it. It it does the draft blower should come on first, then spark igniter,then pilot ,next the main burner,and after a short period the main blower. If you do not have induced draft motor the spark igniter should come on as soon as thermostat calls for heat. If it doesn't I would check all electrical connections making sure they are all clean and tight fitting. Just follow it through step by step . You can use volt meter to check across red and white wires from thermostat to where they hook into furnace. When you are calling for heat you should read about 24 volts. I would be inclined to replace spark igniter but if it does what it's is suppose to then I would go to gas valve. It might be worth your time to check connections on t-stat. I have occasionally seen wires that were broke under screw and some time make connection and other times not. You can also remove red and white wires from t-stat and temporairly touch them together. this should turn heater on. Hope this helps. Thanks.

yeah long as he got abit about him

A draft inducer blower is used to create a proper draft in a chimney or exaust pipe from your water heater or furnace. In a high efficiency furnace or water heater, there is not enough heat expelled from the unit to the chimney to create a proper draft. The toxic carbon monoxide gas which is expelled through the chimney from your appliance is normally carried upward due to the rising hot air in the chimney. When appliances got more efficient (putting more heat into the water tank and less going out the chimney), manufacturers had to add this blower to help get the gases out the chimney since there wasn't enough hot air to create a draft.
Hope this helps.

flame senser

sounds like the filter inside of the unit may be dirty and or the vent cap could be partially blocked causing the flue gas to build upo until the unit tripos on the high limit saftey