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I have a boiling chamber that contains about 2.12 qt (2 l) of water and a stainless steel-sheathed immersion heater with 800 W using 240 V of electricity. The heater is 7.09" (180 mm) long and looks like an elongated J shape, so the total length is about 25.6" (650 mm) long. The heaters usually are run 12 to 16 hours per day. Water is added automatically when the water level dips 0.25" (6.35 mm).

The heaters are made so they have low watt density, but nonetheless, most of the heaters last only a few weeks. I am at a loss as to why they do not last. Spots form on the heater, and they get bigger over time. Eventually, the heater cracks. Why do the heaters burn out so quickly, and how can I overcome this problem? - A.N.

Several key pieces of information are missing. Is the heater a cartridge heater or a tubular heater? Tubular heaters generally are more durable than cartridge heaters. What is the outer diameter of the heater? One cannot calculate the watt density without knowing this, and hence one cannot verify whether the reader's low watt density claim is correct. What is the purity of the water being boiled? Is the reader getting scale buildup on the element? What is the nature of the spots? Are they areas of corrosion or hot spots from the element wire being too close to the sheath wall?

Normally, stainless steel sheaths are perfectly adequate for water immersion unless there are unusual chemicals present (plating acids and the like). However, one can upgrade to an Incoloy or Monel sheath for increased corrosion resistance. Watt density can be reduced further by increasing the heater diameter as well as by increasing its length. If the reader is using a cartridge heater, he can switch to a tubular heater for longer life. - Michael E. Ontko, P.E., Wahlco Air Systems, (714) 979-7300 or e-mail montko@wahlco.com