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

We have an E140 Wood Gun boiler with a Riello oil burner, installed in 1992. The Wood Gun works great on both wood and oil, but the oil burner has never fired reliably, especially after burning wood for several days/weeks/months. Despite having the burner serviced every year, the oil burner only "caught" about 50% to 75% of the time, making the system unreliable when we both travel.

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


Proposed Operation

Schematic and Major Components

See the system schematic (110KB .pdf file). The system consists of 3 timers, 6 relays, a clamp-on thermostat, and a solenoid. All devices operate at120VAC line voltage to eliminate the need to convert voltages. Here are the parts used (unless otherwise stated, all parts were purchased from MSC Industrial Supply, www.mscdirect.com):

Theory of Operation

The reset device operates in a series of "states", depending on the inputs and conditions. Each state is depicted in a pdf file and assumes that switch S1 is in the "active" position. For each state diagram, heavy blue lines indicate a previously-energized line and heavy red lines indicate a newly-energized line. Thinner black lines between devices indicate unenergized lines. Note: there is a minor wiring difference on R6 between the state diagrams and the schematic above. The change was to free up a set of contacts for another project. The change does not change system operation. Please pardon the .pdf files, but sending CAD drawings to JPEG files resulted in unreadible diagrams. The PDF files give much better quality.

Construction and Installation

Reset Device Mechanism Construction

This rearranged schematic shows how the device was actually constructed. The reset device mechanism was installed in a 12"Wx14"Hx6"D NEMA 1 enclosure (MSC P/N 54080205) with a removable mounting panel (MSC P/N 54080098). Connector C1 is a euro-style barrier strip used to connect wires from the boiler, aquastat, and solenoid to the reset device. Connector C2 (C2 Connection points are scattered across the schematic) is a 12-pin AMP/Molex connector that connects points on the reset device (mounted on the panel) to lamps L1-L6 and switches S2 and S2, that were mounted in the enclosure itself. The wiring for C2 is depicted here. Connector C3 on the C2 schematic is the outlet for the audible lockout alarm. Here is what the reset device looks like, assembled and installed in the enclosure. You can see C1 running horizontally above the orange timers and C2 toward the bottom of T1. The timers are screwed directly to the panel and the relays are in sockets clipped to a strip of DIN rail, which is screwed to the panel. All internal wiring is 16AWG stranded hookup wire, except for the black wires to S1 and through C1/C2, back to the boiler's On/Off Switch and the R5/R6 contacts handling boiler power (These are 16AWG THHN, due to the extra current and heat they need to handle). There are 3 flex conduits leaving the enclosure to connect to the boiler, aquastat, and solenoid. The black object on top of the enclosure is the audible lockout alarm (a Mallory Sonalert), wired to a "wall wart" DC power supply that plugs into C3 at the outside top right of the enclosure. It's sitting on top because I didn't find the Sonalert until after the reset was built and installed. The blue CAT 5 cable inside the lower right-hand side of the enclosure and leading up the right side of the backboard is connected to the second set of R6 contacts and senses reset events for another system.

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.


The following image shows the boiler, with the aquastat and solenoid actuator mechanism installed. The aquastat is toward the lower left of the boiler and the oil burner and solenoid are to the lower right of the boiler. The enclosure containing the reset device mechanism is located just on the other side of the wall to the right of the boiler.

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

The following picture shows the electrical connections in the boiler's main wiring box (See the first picture in this section. The main wiring box is the small blue box on the upper left side of the boiler [near the "Wood Gun" emblem], with lights and switches in it's front panel). The five wires from the reset device run in a taped bundle across the bottom of the box and have white, single-pin AMP/Molex-type connectors on them. Starting with the black wire at the lower right and counting counter-clockwise, the wires are:
  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".



I really didn't want to install the reset device on the boiler without a good shakedown test to make sure everything worked correctly. I built a test console with 3 switches and an AC power cord. One switch acted as the boiler's On/Off switch and provided power to C1 terminal 1. Another switch provided the lamp indication that the boiler was calling for heat. This switch provided power to C1 terminal 3 when turned on. The last switch simulated the aquastat and was wired between C1 terminals 6 and 7. The test console also provided neutral and ground to the reset device.

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.


Some of my assumptions on settings were better than others. My assumption of a 150 degree set point for the aquastat was way off. The thermal resistance of materials and radiation/convection losses on surfaces caused me to lower the set point to 80 degrees. Much below 80 degrees, the aquastat never opened the circuit and much above 80 degrees caused too long a wait to reach temperature.

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

Prior to installation of the reset device, it was a good question on any day as to whether or not the oil burner would work. Since the reset device was installed in late 2002, the oil burner is now a hands-off device. For the first time since we bought the boiler in 1992, we can actually use the oil burner in "backup" mode for the wood burner, so that if the wood fire goes out, the oil burner will heat the house. During the 2002-2003 heating season, the counter on the reset device showed that it reset the boiler about 15 times (an approximation because the timers were not properly adjusted).

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.


The labor was all mine, so the total cost was materials. The material cost was about $1000 (the parts listed above, plus the enclosure, conduits, wire, connectors, hardware and special tools), mainly because I was buying small quantities at retail prices and paying shipping. If I were a company, I could buy at wholesale (at least 50% off) and if I bought in quantity (like a manufacturer), I could probably get another 25% to 50% off wholesale. A bit expensive, bit worth it to have a reliable system that will help insure that neither of us come home from a business trip to frozen pipes and burst radiators.

Things I Did Wrong

The biggest mistake I made was in designing how the aquastat worked in the reset device. The way I designed it, relay R1 is energized anytime the boiler is switched "On" and the flue temperature is below the set point. Since the relays are rated for continuous operation, it's not a technical problem or a danger, but I wish I had designed it so that no relays or timers were energized when the flue temperature is below set point and the boiler isn't calling for oil heat.

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).

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Copyright 2001-2009, Tim Sharpe. You are free to use this information for personal, non-commerical use without restriction. All rights reserved for commerical, organizational, or government use. Questions or comments to tim@beaststwo.org. Flames to /dev/nul.

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