Is it really bad to leave the engine heater plugged in 24/7?

From Reiff's web page.

We encouraged Aviation Consumer for years to conduct a study on this issue. Here it is...

PREHEAT AND CORROSION: NO SMOKING GUN

Aviation Consumer, March 2007, pg. 23

We can't print the article due to their copyright, but you can buy a copy off their web site. Here are a few excerpts:

They instrumented a Continental 520 on a Bonanza that was equipped with our Standard System. The preheat system was plugged in and they recorded internal crankcase temperature and %RH (relative humidity) data over a 7 day span. During that period ambient temps ranged from teens to 30F.

Results... The internal RH started at 60%RH at ambient temperature. After about 12 hrs of heating it stabilized at about 120F and 15%RH and it held pretty constant for 7 days.

Conclusion... "These findings don't support the view that preheaters cause corrosion."

"Clearly, a preheater seems to dry the engine out, not cause it to behave like a terrarium."

"While our experiment is far from definitive and doesn't consider all possible ambient conditions, it does seem to show that far from causing corrosion, an always-on preheater actually appears to benefit the engine."
 
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