Choosing a Water Heater to Convert into a Biodiesel Processor



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

New Water Heaters

Your local hardware or appliance store will have a water heater that will 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

Water heaters are rated "to deliver", so a 40 gallon hot water heater will deliver 40 gallons. It will "contain" 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.


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 will only hold 35 gallons making a 50 gallon water heater the biggest you can use with one 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 will sell you a damaged or returned water heater at reduced cost. These are a good way to save money. Call all the hardware stores in your area to find one. It may not be exactly what you want, but it will probably work for you.

Used Water Heaters

You can save several hundred dollars by using a used water heater. They are typically removed when they are filled 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 will also give them away since they have to pay someone to dispose of the old ones. Some plumbers will have them stacked up out back like cordwood. The disadvantage to a used water heater is that it can take a day or two to unseize the plumbing and clean it out. Then once you have cleaned it up, it may leak and not be usable. This is why I personally will only use a new water heater. I don't want to risk one or more days preping a water heater that leaks.

Once you find a supply it's time to go shopping. First, only look at electric water heaters. In some parts of the country they are rare used because only apartment complexes will use them. 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 will 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 a the end of a garden hose or pressure washer into. 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. Screw the elements into place. Plug all the holes with pipe plugs, or anything handy. Two holes on the top need to be fitted with ball valves. You will need a garden hose to pipe thread adapter to attach a garden hose to one of the valves. 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 the hose is attached on.

Let it sit pressurized over night. A water puddle on the ground will tell you that you have a leak 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 you have a leak. 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 you have a leak you need to tighten all your plugs and fittings and try again. I have never seen a leaking hot water heater, only leaking fittings. This process takes about a day to find, clean and test the water heater and will save you the cost of buying a new one.

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