Wood Fired Boiler Electrical System

When it comes to wiring a wood fired boiler or steam generator, never forget the acronym KISS; Keep it simple, stupid. There is no need for expensive controls to monitor the flame as are required with fossil fuel burning boilers. With an oil or gas boiler, if the flame were to go out, it would soon become catastrophic if the fuel source were not also shut off. No such danger exists with a wood fired boiler, as it will simply begin to lose steam pressure as it cools down.

Our boiler electrical system serves to open and close a horizontally configured guillotine-type air shutter to maintain a constant pressure in the boiler, and also to open a solenoid dump valve to release excess pressure should it rise slightly above the set point. It should go without saying not to depend on an electrically operated solenoid for overpressure protection, so additionally there is a 5 psi mechanical relief valve installed into the system.

You'll notice on the wiring diagram to your right(click the print to view full size) power is split from L1 into a control system and a power system. The controls are connected via a 1 amp fuse to protect the mercury switches in the pressure and over pressure controllers. if you failed to do this and had the mercury switches operating the solenoids directly, the first time a coil fails, or an electrical fault occurs, it will destroy the expensive mercury switches. This system was designed and implemented in the U.S., so L2 is connected to the neutral, which is a grounded conductor. For use outside the U.S., simply connect L2 to the other power leg and then purchase solenoids and relays, etc., rated for the appropriate voltage.. No other modifications are necessary.

Wood fired Steam Boiler-Sequence of Operation and Testing Procedure

1. Connect power to the unit and verify the air intake solenoid has pulled the air shutter open.

2. Fill the boiler with water to the fill line and ensure the McDonnell Miller 247-2 has stopped the flow of water into the boiler.

3. Open one or more steam valves to the atmosphere to prevent the unit building pressure.

4. Build a fire in the firebox and wait for the boiler to begin building steam.

5. Once you've checked for leaks, close the valves to the atmosphere and open the supply valve to your load(s).

6. As pressure begins to build, observe the pressure gauges to ensure no overpressure condition occurs.

7. As pressure approaches the operating point of 1 psi, check and adjust the operating pressure controller as required so that it opens the circuit to the air shutter solenoid. A spring will close the air shutter and the pressure should begin to drop slowly.

8. When the pressure drops by approximately .2 psi, the mercury bulb will tilt the other way, re-energizing relay PC1 and re-supplying power to the intake air shutter solenoid valve.

9. Open the blowdown/drain valve and let approximately 2" to 3" of water and steam escape from the boiler. The low water cutoff switch should interrupt power to relay PC1, removing power from the intake air solenoid valve. At the same time the low water alarm light should begin flashing.

10. As the boiler fills again, the LWCO will change state, shutting off the low water alarm light and also will return power to PC1, opening the intake air shutter, provided the pressure switch is also calling for heat.

11. The air intake and steam dump valves can both be tested using the pushbuttons on the front panel.

12. To test the overpressure dump system, push and hold the intake air pushbutton until the steam pressure reaches the overpressure set point of 1.5 psi. At this time the overpressure relay OP1 will kick in, supplying power to the electrical dump solenoid. A time delay off timer will keep the coil energized for 15 to 300 seconds, depending on setting. The off timer should be adjusted to return pressure to around 1.2 psi or so. This completes the system test.

Water Feed system and piping

Our Wood fired steam boiler uses industry-standard water feed controls. We chose the McDonnell Miller 247-2 water feed valve with the optional low water cutoff switch, referred hereafter as LWCO. The LWCO is SPDT which allows us to interrupt power to the combustion air shutter solenoid in the event of a low water condition, immediately lowering the air supply and therefore the intensity of the wood fire. In addition, we wired a flashing red light into the LWCO contacts which close on a drop in water level.

Such a condition actually occurred last winter when our water supply line froze. The boiler water level dropped by about 2", triggering the light to come on which got our attention right away. It was then an easy fix to thaw the line with a torch, returning our boiler to operation.

When you connect the McDonnell Miller 247-2 feed valve, be sure to use hand operated ball valves as shown to your left above to quickly isolate and/or bypass the water feed valve should a malfunction occur. If something goes wrong in the middle of the night when it's -30F, you don't want to be taking apart the system for troubleshooting. If piped as shown you can simply fill the boiler by hand and keep it in operation while you wait for better conditions. It should also go without saying to use lots of unions so you can rapidly disassemble the system for repair or replacement.

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