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One Boiler Zone Not Working?

2010 November 4

Is One Of Your Boiler Zones Not Working?

If your home has hot water heat, one zone could fail while the rest of the house still has heat.  There are several things that could cause this problem.  Here are some of the common causes and their solutions.

One Zone Has Air In The Pipes.

Bleed each radiator to purge the air out of the system.  Cast iron radiators have bleeders near the top of one side.  Baseboard radiators usually have bleeders under the metal trim.  Most bleeder valves require a special key or a flat head screwdriver.  Bleed each radiator until a steady stream of water comes out.  Once you have bled each of the radiators, repeat the process again.

If there are no bleeders on your system, it will have to be purged.  This process will take place at your boiler.  Determine which pipe is the supply pipe to the cold zone.  Next locate the return pipe for the cold zone.  Isolate the cold zone by shutting down all of the other zones.  If ball or gate valves are available to close the working zones, use them.  If your boiler has zone valves, they can be closed as well.  Interrupting power to the zone valve transformer closes all common residential zone valves.  What you are trying to accomplish is to make the cold zone an open loop of pipe while rendering all of the other zone loops impassible.  You should also isolate the boiler if valves are available to do so.

Force water up the cold zone supply pipe.  This water will enter the boiler system through the pipe that connects your boiler to your house water supply.  If there is a motorized zone valve, be sure that it is in the “manual/open” position.  Add water slowly.  Never bring a boiler’s pressure above 25 lbs of pressure.  The pressure relief valve will blow if the pressure reaches 30 psi.

Drain the water from the cold zone’s return.  The installer should have provided you with a hose spigot at that location.  If there is a shutoff valve between the spigot and the boiler, it should be in the off position for this process.

Drain the water through a hose and into a bucket.  Once all of the air is out of the pipe, make sure that all of the valves are put back the way that you found them and that the boiler pressure is at 15 psi.

Try your heat again.  Give it 30 minutes to heat up before deciding if your repair has worked.  Remember:  NEVER add cold water to a hot boiler.  Doing so could destroy a boiler.

The Thermostat is Broken.

Testing for a bad thermostat is fairly easy.  First, turn off power to the boiler.  There should be a switch near the boiler.  If not, use the circuit breaker and test the 120v power at the boiler to make sure that it is off.

Remove the thermostat from its backplate to expose the thermostat wire.  Many thermostats simply pop off, while some require a small screwdriver.  Locate the “W” and “R” wire terminals of the thermostat.  Temporarily jumper them together using an alligator clip or a scrap of wire.

Restore power to the boiler.  If the cold zone works with the jumper attached, the thermostat should be replaced.  Turn the power back off and replace the thermostat.

NOTE:  “R” and “W” often correspond with the red and white wires of the thermostat wire.  Be careful, because this is not always the case!  Make sure that you see the terminals and confirm that they are marked “R” and “W.”

Broken Zone Valve.

If neither of the solutions above restored heat to your broken boiler zone and your system has motorized zone valves, it is likely that one of the zone valves is malfunctioning.  Replacing a zone valve is difficult and should be done by a professional. You can restore heat to the cold zone temporarily by moving the lever or dial on the valve to the position marked “open” or “manual.”  While the valve is in the manual position, the broken zone will heat whenever ANY of the other thermostats call for heat.  The rooms heated by the broke zone may be uncomfortably warm or cold, but this solution could help to prevent a freeze-up while help is on the way.

If you decide to tackle the project yourself, you should confirm that the valve is broken by using an electrical multimeter.  When the thermostat calls for heat, the boiler should send 24V to the zone valve.  The valve should then open.  You can usually hear the motor turn and see a small lever or dial move across the valve body.

Replace the valve with an exact match.  Label the wires when you remove them and replace them in exactly the same way.

First turn off water to the boiler and zone valves.  Then drain the water from the cold zone.   If you are able to isolate the working zones while they are still full of water, it will help you later.  The water may be extremely hot.

Remove the old zone valve.  Once the pipe is empty, you should be able to do this by heating each socket with a torch and pulling the copper pipe out of it with large pliers.

Solder the new valve in place.  Be sure that the inlet and outlet side of the zone valve are not reversed.  In other words, there should be a small arrow located somewhere on the zone valve.  That arrow should point in the direction that the water will flow.  Look at the other zone valves to make sure that yours is correct.

Fill the system with water until it reaches 15psi.  Put all of the valves, including the motorized zone valves, back to the positions where you found them.

Bleed or purge the system completely, as described above.

Restore power to the boiler and be sure that the thermostat is calling for heat.  Once the boiler has reached temperature, be sure that all zones are heating properly.  Re-bleed and purge if necessary.

This post was written by Matthew Klausner, who has been a lead HVAC technician and crew foreman since 1998.

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