Creating the Preheat/Constant Level Tank

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1. Tank

I now use ammo type boxes as they have a lid seal which is a good feature. At 160F, some gases/fumes can be produced from the waste oil in the box and you don't want these going into the room. They may not smell so good. Anyway, I started using ammo boxes for this feature, they are liquid tight, and used ones are inexpensive. I get them from a local guy for $10. My tanks are approximately 5 3/4" wide, 7 1/4" tall and about 12" long. I dont think I'd get a tank any smaller or it might be difficult to get everything to work out well. Notice the hinged lid. I will refer to the FRONT of the box as the side with the lids hinge (5 3/4" wide side) and the BACK of the box as the opposite side that has the lids latch. When looking at the FRONT of the tank, I will refer to the SIDE O (outside) as the left side and SIDE F (furnace) as the right side. SIDE F will be closest to the furnace when mounted. The pictures shown is of 5.56 MM ammo cans I buy.

sides of a 556 millimeter ammo box picture

Boxes with lid open

ammo can lid

Diagram of FRONT drilled holes

Drilled hole layout for tank drawing

2. Drain

On the bottom of the can I install a 1/4" NPT petcock drain valve. I don't know of a 1/4" NPT nut so I created one by taking an 1/8" thick by 1" by 1.5" piece of flat scrap metal I had, drilling a 9/16" hole, then tapping a 1/4" NPT thread. Next I drill a hole near the BACK and SIDE O corner on the bottom of the box, being sure the "nut" I just made will jam against the inside wall of the box when tightening the petcock. Drill the hole a little less than 1.5" from SIDE O and a little more than 1.5" from the BACK. Before installing the petcock I took a washer about 1 1/4" diameter and drill out the center hole to 9/16. Then I placed the washer on the petcock and then placed the assembly into the hole. Inside I put an o-ring that fit tightly on the threads, then I screwed on the homemade nut and tightened the assembly from the bottom using a wrench on the petcock. This way the o-ring doesnt spin or twist while being tightened - it just compresses. Do not over tighten or it may leak. You could also use some RTV silicone to help the seal.

petcock drain valve picture

Petcock drain installed on bottom on box. Notice the washer under the drain. Under the washer is a rubber ring to help seal it.

3. Constant Level Float

For the constant level valve I used a Dial 4164 3/8" compression fitting. Similar float valves will work. This one has a threaded connection for a 3/8" compression fitting that will be put through a hole located on the FRONT, in the center, and up 5 inches from the bottom of the tank. When adjusting the float level in the tank, also be sure to adjust the length so the float will not hit the BACK wall when in the fully up position.

compression float valve picture

This usually comes complete with a compression fitting that will fit 3/8" OD copper tubing as well as a washer and nut that is used to mount and tighten the float to the FRONT side. I chose the 3/8" OD as this is normally the size of the copper lines used in the fuel return line which we will be connecting this to later on.

The purpose for the float is to maintain a constant level in the tank. The float will need adjusting so that the level is around 3/4 inch above the fuel outlet. Once this is determined, put a piece of tape or mark on the tank. When this whole assembly is mounted to the side of the furnace, the tank then needs to be placed so the level of oil is from 2 - 8 inches below the center of the burners nozzle (center of flame tube). I aim for about 4 inches. Keeping a constant distance between the nozzle hole and the waste oil when siphoning is one of the variables to maintain an even burn rate/flame inside the furnace. Another factor is the air pressure.

4. Heating element

The heating element is a 1500 Watt 120V screw in water heating element. This heating element will be controlled by a 2nd PID controller to heat the waste oil to a set temperature of 160F. The heating element requires a rather large hole and this is easiest to make using a step drill bit. The hole is located on the FRONT, in the center, and up 1 1/4". The heating element comes with a rubber seal but you will need to also buy a 1" conduit locknut to fit the threads and tighten it to the box. You can find these conduit locknuts in the electrical dept. at Home Depot or similar hardware stores. They are tightened using a screw driver and tapping the ridges with a hammer as most people don't have a special wrench for this and its not necessary.

Water Heater Element - Home Depot Model 15896

water heater element picture

Conduit Locknut - Home Depot Model 96193

conduit locknut picture

5. Temperature probe

For the preheat tank, a PT100 type thermocouple is used. Notice that it has a thread and a nut which will allow it to be attached on the FRONT of the ammo box. On the FRONT, it is located 2 3/8 inches up from the bottom and .8 inches from SIDE F. The thermocouples I puchase have a max thread width of .302 so I use an "N" size drill bit. A 5/16 inch works OK too. You will need to measure the thread width for any of these parts you are drilling and make the holes as close to that as you can so that they are easier to seal. I also used two washers and a tight fitting o-ring when sealing this to the ammo box. Again, it might be well to add some RTV silicone when assembling.

PT100 thermocouple picture

6. Fuel outlet

The fuel outlet is also located on the FRONT. It is up 2.9 inches from the bottom and 1.6 inches from SIDE F. For this connector, use a 1/8" NPT x 3/16" OD compression fitting. When attaching the 1/8" NPT fitting, I again used washers and an o-ring as well as an 1/8" NPT brass cap with a 3/16 hole drilled in the middle. This brass cap is what tightens the connector to the FRONT and it must allow the heated waste oil to pass through to the compression fitting side. Later, the 3/16" OD copper tubing from the burner modification will be connected here. Notice that the fuel outlet is higher than the temperature probe. This is for safety reasons. If the fuel level goes lower than the fuel output then the Control Unit of the burner will trip and shut down the furnace since there will be no flame. Since the temperature probe is still in oil, the PID will continue to maintain a constant temperature and since evaporation is somewhat slow, it should be safe for some time. However, if the temperature probe were higher than the fuel outlet and fuel got below the temperature probe, then the PID controller would think the temperature is too low and overheating of the remain oil would occur and potentially get way too hot and worse case create a fire by overheating the oil which could also melt the plastic float. This is why the fuel output needs to be higher than the temperature probe.

1/8" NPT x 3/16 OD compression fitting

1/8 NPT to 3/16 inch compression fitting

Compression fittings consist of a compression cap/but and sleeve.

Cap/Nut Sleeve
compression nut picture compression sleeve picture

To assemble a compression fitting, first slide the nut over the copper tubing then push the sleeve onto the very end of the copper tubing with about 1/8 inch of copper tubing sticking out. Now insert the end of the copper tubing into the connector and tighen the nut. The sleeve fits inside the nut and inside the connector end and is compressed when tightened, forming a seal. The copper pipe should be clean and smooth where the sleeve is in contact with it. Fine steel wool is good for cleaning the copper tubing if needed. Once a sleeve has been tightened on a copper tubing it will not come on/off anymore and must be cut off if there is a problem. New sleeves can be purchased separately at many hardware stores.


7. Safety Vent

The safety vent actually serves two purposes. It is used to vent any fumes to the outside of a building, and, in case the internal float should fail in the open position, oil will be pumped outside of the building and into an overflow tank. Normally this overflow tank is also the main fuel tank. The vent is made by using a 1/4" NPT by 3/8" ID hose barb fitting. Thus a tank that has a sealed lid is important. 3/8" ID hose barb means that this side of the fitting will fit onto a flexible hose that has a 3/8 inch Inside Diameter. I use 3/8" clear poly tubing so I can see if an overflow ever occurs. If it does, the furnace should be turned off immediately and the problem corrected. You should also remove the burner unit and check to see if any oil is inside the combustion chamber since the level of the overflow oil is usually higher than the level of the siphon nozzle output during overflow. The location of my vent is on the lid about directly over the petcock drain, but it can be placed anywhere you like. A better location might be near the top of the BACK wall so that when removing the lid, you wont have to move the vent line.

1/4" NPT x 3/8" ID Hose Barb

1/4 NPT to 3/8 inch hose barb picture

8. PID Controller & Solid State Relay

As with the burner modification, a PID controller and SSR is used to control the temperature of the tank. Power for the tank PID comes from the main burner's power lines as can be seen in the following diagram.

preheat tamnk wiring diagra

[pictures of actual electrical connections to PID and heating element]

9. Getting Everything Connected

Here are pictures of the assembled tank. In this picture, notice the vent fitting on top. Between the vent fitting and the lid is a small square piece of scrap metal I drill out plus a rubber washer between it and the lid to seal it.

Here you can see the piece of square metal better.

On the underside of the lid I made another small square piece of metal with the correct size drilled and tapped hole so the fitting could be tightened.

Here's the inside showing the float valve, heating element, temperature probe and oil outlet fitting. Again, notice the outlet fitting is screwed into a small piece of scrap metal that has been drilled and tapped so that it can be tightened. Also, a rubber washer was used under it to seal it from leaking. Similarly a metal washer and rubber o-ring(or rubber washer) was used with the temperature probe.

Notice the compression fittings - ready to attach copper lines.

[Details about connecting the PID controller and mounting go here]