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I have a natural gas hot water furnace and I can't get heat. First I turn up the temperature at the thermostat and I hear the furnace turn on, the supply line gets hot and half way down the line it stays cold. I tried getting the air out of the line but now I have no water pressure. I don't know if the pressure was at 0 before I even started.

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    I do not know enough about these types of systems to give an answer. But based on what you describe, I expect a pump somewhere to circulate the water while heating. The pump would also generate the pressure you seem to be missing. With some luck, maybe the pump exists, does not have power and has a reset button on it somewhere. Hope this can get you started on diagnosing the problem. Would be helpful if you add more details about your unit. Maybe some pictures. – zerpsed Jan 20 '16 at 18:38
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    What do you mean "no water pressure"? Do you have a pressure meter on the system? Do you have any air bleed valves near the highest point of the system? – A. I. Breveleri Jan 20 '16 at 23:15
  • This might either be helpful, or it's a near-duplicate: diy.stackexchange.com/questions/84366/… – Ecnerwal Jan 26 '17 at 1:55
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It sounds like you fill valve is not properly set, or has failed. Do not run the furnace until you are sure the system (boiler shell, circulating pipes, and radiators) are completely filled with water.

If the water level in the boiler gets too low the system could be damaged. Boilers usually have a fuse plug that limits the damage if the water level fails, but that plug cannot be reset and is expensive to replace.

A typical circulating hot-water heating system has an automatic fill valve connected to the house water supply, which admits water to the system until the pressure rises to a preset level. (Usually the level is set to match the total height of the system.) This valve can become blocked by a tiny amount of shmutz in the supply line.

Sometimes you can clean and reset the automatic fill valve but sometimes you have to replace it.

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This picture shows a typical gas-fired, hot water furnace system:

enter image description here

The black thing in the center is the recirculator pump. When the furnace runs that recirculator is supposed to run. If you touch it or put your ear to it, you should detect it running. If it is making no noise, or is buzzing, then it needs to be replaced.

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