Wood Gun Oil Burner Reset

An automatic reset for Oil Burners attached to "Wood Gun" Boilers
(formerly made by Eshland Enterprises, now made by Alternative Heating Systems)

The Problem

In January, 2002, I came home from a 4-day business trip to find that the house was 45 degrees on the inside. The system had been down for days. After talking with friends and coworkers who have oil burners, I found that most them have to reset their oil burner at least once per heating season. Since oil burners "lock out" after misfiring and require human intervention to restart, it was apparent that this wasn't a workable solution for two people who travel with their jobs. There must be a way to improve overall system reliability.

Theory and Design

Design Assumptions/Constraints

• Most of the problem results from the fact that, as a wood fired boiler, we don't run the oil burner very much. The oil burner is used mostly during the fall and spring, when it's too warm to maintain a fire, and when we're both away from home on travel. During heating season, the boiler may run on wood for months at a time without firing the oil burner. When we burn wood, a certain amount of ash collects in the firing chamber. Almost no amount of maintenance can overcome this.
• Since the oil burner almost always lights on the second try, we need to give the system a few extra chances to fire the oil burner before allowing it to give up. One try just isn't enough!
• The reset button on the oil burner is a thermal breaker, rather than a switch. It must be physically pushed to reset the burner.
• Our boiler is equipped with the "oil/gas lockout" and "oil/gas delay timer" features, requiring that the boiler be powered down to reset the boiler mechanisms whenever the oil burner is reset.
• When the oil burner attempts to fire, it shoots a small amount of oil to the firing chamber. The solution must lock itself out after some number of resets so that the oil burner won't eventually pump the contents of the oil tank onto the basement floor. The solution should set off some type of alarm to indicate that the system is locked out.
• The solution must be minimally invasive to the boiler and not interfere with its mechanisms.
• The solution must not interfere with maintenance of the oil burner.
• The solution should be easy to remove from the boiler if required.

Inputs/Outputs

• The boiler is equipped with a line voltage indicator that lights when the boiler provides operating power to the oil burner. The hot side of the lamp is a reasonable place to sense that the oil burner should be running. This wire can be picked up in the boiler's main electrical box.
• By sensing the flue temperature, I should be able to determine if the oil burner is actually running. I should see a significant flue temperature rise if the oil burner is burning.
• The boiler mechanism can be reset by dropping power for a few seconds. If the wire between the boiler's On/Off switch and the boiler mechanism is routed through the reset device, it could providing operating power to the reset device, and the reset device could interrupt power to the boiler mechanism when the oil burner is reset.

Proposed Operation

• A system of line voltage timers and relays will operate as a state machine to sense the oil lamp and flue temperature, and reset the oil burner and boiler. All parts will operate at line voltage.
• When the boiler calls for oil heat, if the flue temperature doesn't rise to a certain temperature within a given time (a few minutes), the boiler will power down and the oil burner reset button will be physically pushed. The oil burner reset button will be pushed using a line voltage solenoid and a lever mechanism.
• If the flue temperature doesn't rise to the specified temperature after about 5-6 resets, it will power down the boiler and set off an alarm. The boiler will remain locked out until a human either toggles the On/Off switch or hits a reset switch on the on the reset device itself.
• Flue temperature will be sensed with a "clamp-on" line voltage thermostat.
• The wires for the boiler's oil lamp, hot, neutral and ground will be picked up in the boiler's main electrical box. Connections will be made using single-pin insulated connectors polarized so that the the reset device can be easily removed and normal boiler wiring quickly and easily restored.
• A bypass switch will allow the reset device to be bypassed, returning the boiler to "normal" operation.

Schematic and Major Components

• Timers. All 3 timers are solid state cube timers made by National Controls Corporation.
  • Timers T1 and T3 are Q1T type "delay on make" devices (The spec sheet on the National Controls web site shows a different device from what I ordered, but the part numbers shown here will get the correct timers). These are two terminal timers, with an input and output terminal (I haven't figured out how they get neutral, unless they use a leakage current to operate). Operation is as follows: power is applied to the input terminal, and after a time delay, the output terminal is energized and stays energized until power is removed from the input terminal. A potentiometer determines the time delay. Here are the devices I bought (they were about $20-$30 each):
    • Timer T1. NCC P/N Q1T-00600-311 (MSC P/N 54022645). The delay is adjustable between 0.5 and 10 minutes.
    • Timer T3. NCC P/N Q1T-03600-311. The delay is adjustable between 3 and 60 minutes. MSC Industrial Supply doesn't stock this part, so you can't order it on the Web site. I called their Customer Service 800 number and they special ordered it for me. It took a few weeks and cost no more than the ones they stock. They are really nice folks and a true pleasure to deal with!
  • Timer T2 is a Q2T type interval timer. When the two "input leads" (6 and 7) are shorted, the timer provides a path from pin 1 to neutral (through pin 2), for a length of time determined by a potentiometer. Operating power for the timer connects to pin 3. I bought NCC P/N Q2T-00010-321 (MSC P/N 54022512, about $25.00), which provides 0.5-10 seconds of ontime per activation.
• Relays (R1-R6). I used 6 Omron 120VAC/10A DPDT relays, P/N MK2PNSAC120 (MSC P/N 54046479, about $13 each). I could have used SPDT relays in some places to reduce the cost, but I wanted to keep down the number of different parts needed and keep the wiring as standard as possible. These are 8-pin octal-base relays, with a red LED indicator that lights when the relay is activated. To connect to the relays, I used Omron P/N PF083AE (MSC P/N 54046354, about $5.00 each) Octal sockets. The sockets clamp onto standard DIN rail, so I bought a 3-foot stick (MSC P/N 54046370, about $9.00).
• Thermostat. I used a Honeywell Strap-on Aquastat, Honeywell P/N L6006C1018 (MSC P/N 01033968, about $90). It is made to strap onto hot water pipes to sense temperature. I purchased some longer clamps (MSC P/N 48558308, about $2.00 each) to reach around the flue. The aquastat has SPDT contacts and is adjustable to sense temperatures between 65 and 200 degrees F.
• Solenoid. I purchased the solenoid from ElectroMechanicsOnline.com, P/N SOODL030052120P (spec page), for about $41 including shipping. It's an open-frame, short-duration "pulse" model, capable of generating about 4 pounds of pull at about 1/2 of it's 1 inch pull range. It's a DC solenoid (my research indicated that AC solenoids are fraught with problems, from overheating to premature failure), so I placed a full-wave bridge rectifier 20A/250PIV in series with the solenoid and placed a 1uF/250V metal film capacitor (both from Radio Shack) across the solenoid to reduce vibration from the 60Hz voltage (I initially started out with a 1000uF capacitor, but the charging current caused relay contacts to fuse). I used a pair of 3-prong Hubble Twistlock connectors (from a local home center) between the solenoid assembly and the cable assembly going back to the reset device, so that the entire solenoid assembly could be easily disconnected from the boiler during oil burner maintenance.
• Indicators (L1-L6). I used line voltage neon panel lamps to indicate the state at several points in the circuit. They're about $3 each.
  • Red, MSC P/N 06045934.
  • Amber, MSC P/N 06045850.
  • Green, MSC P/N 06045892.
• Switches.
  • Bypass Switch S1, MSC P/N 06489256, about $5. This is a heavy-duty 20A DPDT Toggle switch. It must be a serious switch (not some little 2A job) as it must handle startup and running current of the boiler.
  • Lockout Reset Switch S2, MSC P/N 54075734, about $5. This is a pretty basic single pole, normally closed pushbutton switch. The one I bought is much bigger than the job at hand, as it only needs to break the coil current of a relay (about 35mA at 120VAC).
• Connectors. Since all connectors were inside electrical boxes, I used plastic AMP/Molex type connectors to create wiring assemblies. They're cheap and work nicely.

Theory of Operation

• State 0. This is the initial state of the reset. It is assumed that boiler is turned on and set for oil heat, and that the flue temperature is less than the set point for the aquastat (my initial estimate was about 150 degrees F). The boiler supplies power through S1, which supplies power back to the boiler mechanism via R5, R6, and again, S1. L2 is illuminated indicating power to the boiler mechanism, and L3 is illuminated indicating that the flue temperature is below the set point. Note that if switch S1 is toggled to the "Bypass" position, power is sent back to the boiler and the reset device is deenergized. S1 also provides power through the aquastat contacts to the following:
  • R1, which permits power from the Oil Burner lamp to start the reset device into action.
  • R3, which permits the safety lockout timer S3 to operate independently of timers T1 and T2.
• State 1.0.0. In this state, the boiler has called for oil heat and provided power to the boiler's oil indicator. This sends power through the R1 contacts (which are closed because the aquastat is below the set point), illuminates L1 and causes the following actions:
  • T1 is energized and starts it's wait state (my initial estimate was 3 minutes).
  • R2 is energized, causing T2 to power up and wait for a trigger event. Power is supplied to the coil of R6, but T2 blocks the path to neutral, causing R6 to remain in its "rest" state. R2 also energizes R3, which energizes T3, starting it on a wait state (my plan was about 5 T1 cycles, or about 15 minutes) before locking out the system for safety and illuminating L4. After R2 energizes R3, the top set of contacts on R3 provide a latching mechanism that keeps R3 energized until power is interrupted by either the boiler's On/Off switch, the aquastat, or by S2, so that the T3 will continue it's safety lockout wait state.
• State 1.1.0. If the boiler lights, the flue temperature rises, eventually causing the aquastat to open. This drops power to all relays and timers. Later, after the oil burner shuts down and the flue temperature cools to the point where the aquastat closes, the reset device will again be in State 0.
• State 1.2.1. If the boiler fails to light, the flue temperature will not rise and the aquastat will remain closed. The wait state for T1 will expire, causing R4 to switch, which shorts the input terminals of T2. This results in State 1.2.2.
• State 1.2.2. When T1 causes the input terminals of T2 to be shorted, T2 completes the path to neutral for R6, causing R6 to switch. This interrupts power to the boiler mechanism so that it can reset, as well as energizing the solenoid that pushes the oil burner's "reset" button and illuminating L5. When power is dropped to the boiler, power to the oil indicator is lost, deenergizing T1 (which permits it to operate again later), R2 and R4 (which removes the short from T2's input contacts so that it can operate again later). Note that R3 remains energized due to the latching mechanism, so that T3 keeps timing during the reset event. After about 5 seconds, the T2 "ontime" expires, causing R6 to return to its "rest" state. This returns power to the boiler mechanism, releases the solenoid, and returns the reset device to State 1.0.0 to start the cycle again.
• State 1.3.0. If reset actions fail to light the oil burner, the flue temperature will fail to rise above the set point before timer T3 expires (I expect 5-6 resets before the timer expires). T3 energizes R5, dropping power to the boiler mechanism and illuminating L6. As with state 1.2.2, oil indicator power is lost, dropping power to T1, R2, and R4. R3 remains latched, providing power to T3 which holds R5 closed, keeping the boiler powered down. The result is State 1.3.1. The system remains in this state until either the boiler's On/Off switch is cycled, or until S2 is pressed. I wired an electrical outlet in parallel with L6 to power an audible alarm that sounds when the system is in lockout. Pushing S2 breaks the latching mechanism for R3, which causes T3 and R5 to drop, returning the reset mechanism to State 1.0.0 to start the cycle again.

Construction and Installation

Reset Device Mechanism Construction

If you look at the enclosure lid (bottom of the picture), you'll see a gray strip on each side running to the bottom of the picture. I installed some self-adhesive foam weatherstipping on the sides and top of the inside of the door to reduce convection through the enclosure due to heat, reducing airfow and dirt intake (we have a wet, dirty basement).

The picture below shows the left side of the enclosure and shows the indicators and switches. It's a bit blurry as I had to take it without a flash to cut down on glare. You can see the Sonalert on top of the box with the flex conduits. Switch S1 (at the top of the enclosure face) indicates that the reset device is "Active" and not "Bypassed ". The lamps are L1 through L6, in order, with L1 at the top (Boiler Power) and L6 at the bottom (Safety Lockout). The lamps indicate that the boiler's On/Off switch is "On", the boiler is not calling for oil heat, and the flue temperature is below the set point (i.e., the reset device is in State 0). Switch S2 is at the bottom of the enclosure face.

Installation

I had originally planned to clamp the aquastat directly to the stainless steel fluepipe, but the temperature of the fluepipe didn't rise fast enough for my purposes and was variable depending on how clean the fluepipe was at any given time. After a few weeks of fiddling, I finally mounted it to the pipe where hot gases leave the boiler for the cyclone ash collector (the blue drum object at the lower right of the previous picture. This pipe gets as hot as 350 degrees F and the aquastat has some gray foam rubber protecting the sensing bulb. I tested the foam with a 700+ degree soldering iron and the foam didn't melt or even scorch, so I felt safe with the location. I cleaned off the rust with a wire brush (our basement is damp during the summer) and mounted the aquastat to the pipe using the longer clamps I mentioned above.

The solenoid actuator mechanism mounts to the oil burner case as shown in the picture below. The full-wave bridge rectifier and capacitor are inside the electrical box behind the solenoid.

In the following pictures, you can see that a steel plate was mounted to the oil burner case, using spacers to leave room under the plate for screw heads and nuts as required (I used threaded holes and screws as much as possible, then used nuts to secure the screws against vibration). Another piece of plate stock mounts the solenoid vertically above the plate. You can't see it in the pictures, but a long screw through the solenoid bracket gives the solenoid plunger something to rest on and keeps it from falling out of the solenoid (The plunger slides into the hole in the solenoid and nothing holds it in place).

A homemade L-shaped lever (1/2" steel tube stock had wedges removed, was hammered into shape, then the joints were brazed) and a homemade gimbal bracket was used to transfer vertical solenoid motion to push the oil burner's reset button on the front face of the oil burner. Screw eyes, S-hooks, and hitch pin clips from a local hardware store connect the solenoid pin to the lever. As you can see, they all lay loosely when the solenoid is relaxed, but when it pulls, the slack comes out and it provides a nice snap action. The spring attached to the S-hook connects to a cheap mechanical ratchet counter (MSC P/N 00208017, about $5), that counts reset events. Adjustment of the counter lever and spring was ticklish, but it works reliably. The "business end" of the lever uses a 1/4-20 bolt with a rubber cap (MSC P/N 99061368, about $1) that fits over the bolt head to push the oil burner's reset button. The black ball on the other end is a phenolic knob with 1/4-20 threading I bought at a local hardware store, after I got tired of scraping my leg on the end of the bolt. It also provides a nice handle to manually reset the oil burner (and thanks to the previously-mentioned looseness of the connecting hardware, manual resets don't increment the counter).

The overall construction allows the oil burner cover to be easily removed for servicing by removing the cover screws as normal, disconnecting the Hubble Twistlock connector (look toward the lower right of the second picture), and lifting the cover (and mechanism) off. Our oil burner technician has no problem servicing the oil burner with the mechanism installed. Since all the mechanical adjustments are on the oil burner cover, they don't need to be readjusted when the cover is replaced. The mechanical fit of the lever is rather loose and it doesn't move very far nor very often, so lubrication is not an issue. OK, so maybe someday I should build a cover so it won't get so dirty!

Wiring into the Boiler

1. Black wire from the "boiler side" of the boiler's On/Off switch. This connects back to C1 terminal 1 of the reset device.
2. Red wire from the "Black" terminal on the boiler terminal strip. This connects back to C1 terminal 2 of the reset device. Before the reset was installed, a wire ran from the "boiler side" of the On/Off switch to the "Black" terminal of the boiler terminal strip.
3. Red wire from the "hot" side of the boiler's oil lamp (blue wire running back to the reset device). This wire just taps off the existing connection and didn't disconnect anything. The other end connects back to C2 terminal 3 to indicate that boiler is calling for oil heat.
4. Green wire to the boiler's ground point to pick up safety ground for the reset device. This connects back to C2 terminal 5, which grounds to the enclosure panel.
5. White wire to the "White" terminal on the boiler's terminal strip to pick up neutral to operate the reset device. This just taps neutral off the existing connection. This connects back to C2 terminal 4.

The gender of the connectors was arranged so that the reset device may be completely removed, and normal boiler operation restored, by disconnecting the wires from the reset device and plugging together the first two wires listed above. If you look closely at the Molex connectors, you can see that the "fat" part of the first two connectors (counterclockwise from the lower right) are opposite so that they may be diconnected and plugged into each other. The remaining three connectors are installed such that if the reset device is disconnected, female connector stays with the boiler for electrical safety. This is part of the design goals of "minimal invasiveness" and "ease of removal".

Other

Testing

By setting the timers to fairly short intervals and flipping the switches in various orders, I was able to force the reset device through all the states. This uncovered a wiring problem, where R6 failed to switch because I misinterpreted the datasheet for timer T2 and miswired it. The problem was fixed and testing showed that the device worked as it should, sufficiently for my spouse to agree that it could be installed on the boiler.

Adjustments

The T1 timer delay also had to be changed. I had hoped to reach the aquastat set point within 3 minutes, but the best I could get was about 4 minutes. A shorter timer setting caused the device to reset the boiler and oil burner even when the oil burner fired correctly. After a year of operation, I set the T1 timer to 5 minutes and now only misfires cause a reset.

To obtain a 5-6 cycle safety reset using the new settings, I changed to T3 timer to 30 minutes.

How It Works

A few surprises have shown up. Once, switch S1 was moved from Bypass to Active while the oil burner was running. The reset device got locked up in a weird state that I haven't seen before or since. We learned that while S1 can be switched to Bypass anytime, it should only be switched to Active when the boiler's On/Off switch is "Off", or when the boiler isn't calling for oil heat. Maybe it was a fluke, but it pays to be careful. We just leave it in Active state all the time, except for when the oil burner is serviced.

Aside from the switch S1 surprise and adjustments, the system works reliably and is carefree. We found that leaving the reset device in Active mode while burning wood has an additional benefit. Lamp L3, which monitors the state of the aquastat, also provides a good indication of whether or not the fire is burning well. If the fire has gone out, or is really weak, L3 will remain lit indicating that the fire needs attention. It provides such a useful indication that we're thinking about routing a visual indication of some reset signals up into the house so we can keep and eye on things without going to the basement.

Cost

Things I Did Wrong

I wish I had thought out the indicators a bit more. Some of the indicators provide so little unique information as to be effectively useless. For example, L4 only tells us that timer T3 is energized and L5 only lights during the 2-5 seconds that a reset event occurs (almost guaranteeing that nobody will be around to see it light up under normal circumstances). They were nice ideas, but weren't useful. I haven't thought a lot about what indicators would be more useful (and probably won't).