Choosing a Water Heater to Convert into a Biodiesel Processor

By Rick Da Tech


This article covers selecting a water heater for your Appleseed Biodiesel Processor.

New Water Heaters

Your local hardware or appliance store has a water heater that can work for you.

The requirements are:

  1. Electric
  2. Hot and cold-water inlets on the top of the tank.
  3. Screw-in type water heater elements

Pick the cheapest one you can find. You do not want one with a computer or solid-state controls as it can be difficult to wire up as a processor.

Water heaters are rated "to deliver," so a 40-gallon water heater delivers 40 gallons of hot water. It can only "contain," or hold, about 10% less than it can deliver. Add to that the fact that we need to leave some room for our methoxide and we can estimate the maximum batch size you can make in a particular water heater. If you have a larger water heater than you need, you can still run smaller batches, as long as the oil covers the lower heating element.


Water Heater Size Biodiesel Batch Size
40 gallons 25 gallons
50 gallons 35 gallons
80 gallons 60 gallons
120 gallons 90 gallons


A wash tank made from a 55-gallon drum only holds about 35 gallons of biodiesel and still have room for your wash water. So, a 50-gallon water heater is the biggest you can use with a drum based wash tank. The 40-gallon water heaters are the least expensive and most commonly found. With their 25-gallon batches, you only need one 5-gallon carboy for the methoxide. 40 and 50-gallon water heaters seem to be the most commonly chosen for processors.

Sometimes the local hardware stores have damaged or returned water heaters at a reduced price. These are a good way to save money. Call all the hardware stores in your area to find one.

Solar water storage tanks have extra holes that let you create different processor designs. Some even have heating elements included, and some have an integral internal heat exchanger, making it easier to use safer indirect heating.

Used Water Heaters

You can save several hundred dollars with a used water heater. People typically remove them when they fill with mineral deposits or when they start leaking. Many times, they can be found in a special section of the local dump or waste transfer station. Plumbers are quick to give them away since they have to pay someone to dispose of the old ones. Some plumbers have them stacked up out back like cordwood. The disadvantage of a used water heater is that it can take a day or two to remove old seized up plumbing and to clean it out. Then once you have cleaned it up, it may leak and not be usable. I only use new water heaters. I do not want to risk one or more days preparing a water heater that leaks.

Once you find a supply, it is time to go shopping. First, only look at electric water heaters. In some parts of the country, only apartment complexes use them, making used electric water heaters hard to find. Make sure the hot and cold-water inlet and outlet are on the top. Open the lower thermostat housing cover and look for rust on the thermostat screws and element. Find the one with the least rust. Also, make sure you have one with screw in elements as the screw in elements are easy to find in a wide assortment of sizes.

You also want one that has a usable outer sheet metal housing. Try to avoid those with huge amounts of rust on the bottom and those with big rust holes in the sides.

Once you find one, you need to take it home, strip off all the old plumbing fittings and clean out the inside. Taking out the fittings may require a really big cheater bar or even a chisel, as time is a great glue. All those plumbing holes make good places to stick the end of a garden hose or pressure washer. There may be some big mineral deposit clumps that may need to be broken up with a metal rod.

Test for leaks by plugging all the holes and filling with water. Start by screwing the elements into place. Plug all the holes with pipe plugs, or anything handy. Mount a ball valve on each of the two holes on top of the water heater. Put a garden hose to pipe thread adapter on one of the valves and attach a garden hose. Open both valves and fill the tank to within 6 inches of the top. Close the vent valve and let the pressure build up in the tank. Once the water stops flowing, close the valve attached to the hose.

Let it sit pressurized overnight. A water puddle on the ground indicates you that your tank leaks below the water level. Open the vent valve slowly. It should release pressure or blow air out of the tank. If it does not, or instead sucks air into the tank, then your tank is leaking. Place your thumb over the hole when you open the valve and feel if the tank is blowing air out or sucking it in.

If your tank leaks, tighten all your plugs and fittings and try again. This process takes a couple of days to find, clean and test the water heater and saves you the cost of buying a new one.