OK to leave electric engine pre-heater on for extended periods?

[Joel Severinghaus]

Here's a question for you A&Ps and engineers:

Is it OK to leave an electric engine-mounted pre-heater on for extended periods? Like days at a time?

Back in a 2005 thread here about pre-heating, Mike Horowitz posted some pages from Teledyne-Continental Service Information Letter SIL 03-1, which under "Application of preheat using engine mounted preheater system" says: "Begin preheating of the engine at least 5 hours prior to expected departure. However, do not leave the engine preheating system in operation more than 24 hours." Well, why not?

I really like my Reiff oil tank pad and cylinder base rings, which, with my Kennon covers, get and keep the whole engine uniformly warm. But it's a hassle to drive out to the airport to only plug it in the night before I plan to fly.

Some guys at my airport use electric outlet timers to shut off their heater if they don't go flying the next day after all. And Reiff sells a nifty remote on/off control box activated by a beeper or cell phone (http://www.reiffpreheat.com/Beeper%20Box%20pg%201.htm), but it would take a lot of 15-minute trips to the airport to justify its $369 cost (says my wife). And my hangar rent includes unmetered electricity, so that's not an issue for me.

I do, based on advice I probably read somewhere here, leave my oil filler cap open when the pre-heater is on (see photo), so moisture can escape there as well as from the crankcase breather tube.

So, any harm in leaving the engine plugged in and warm all winter?

[Robert Lees]

Re: OK to leave electric engine pre-heater on for extended periods?

Can't see any harm myself.

What's the difference between a 100F pre-heater and 100F summer days?

[Hank Jarrett]

Re: OK to leave electric engine pre-heater on for extended periods?

The first thing that came to mind was an untended heater element and a fan that aren't designed for LOOOONG extended continuous operation.
I wouldn't leave my plane untended with a heater running for days, much less weeks or months. I have been looking at the engine dryer on one of the threads and really like the idea, but don't like the idea of leaving it untended.
Now for the plus side. I WOULD be willing to use a heater and a dryer with the right sensors to kill the power if there was any rise in temperature, current draw or smoke.
I would REALLY feel better if the system also released a fire extinguisher and sent an alarm signal by a cheap cell phone (ever get the feeling I get carried away by safety some times?).
Hank

I do use a battery monitor on my cars, but I see them every day and look to make sure they are working properly. When I had a battery in my Taylorcraft, I took it home (to a battery tender) after any flight where I used it, which I rarely did.

[3Dreaming]

Re: OK to leave electric engine pre-heater on for extended periods?

When I did my IA renewal a fellow from G&N engine shop told the story of the customer who had his engine overhauled one summer. The next spring he calls and says I have a bunch of metal in my screen at my first oil change, so they tear down the engine and go through it again. Same thing happens the next spring. Comes to find out he was parking the plane in September with the pre heat on and going to Florida for the winter. What they determined is the pre heat is creating a terrarium effect in the engine causing lots of corrosion. I would assume these guys who are dealing with engines on a daily basis should know what they are talking about. Tom

[mohawktipi]

Re: OK to leave electric engine pre-heater on for extended periods?

One problem I see with leaving a heater on full time is critters. Warmth attracts all kinds, four, six or eight legged. In addition to clogging up passages, they may also chew on wires and hoses. That could lead to more serious problems.

[d.grimm]

Re: OK to leave electric engine pre-heater on for extended periods?

I'm with Tom. Heard and saw pictures and know someone it happened to.
I would NEVER leave my preheater on for more than 24 hours.
Dave

[Hank Jarrett]

Re: OK to leave electric engine pre-heater on for extended periods?

That's another good point. If your heater uses any kind of "fuel" when you burn hydrocarbons you get water vapor as a byproduct. If you are going flying in an hour or two, running the engine will heat the inside and drive that moisture off. If you leave the heater running you are making a sweat lodge in your cowl.
I was reading about airship operations in WW-I and wondered how the Germans compensated for the weight of the fuel they burned. You could release gas, but that didn't sound very efficient (and the Germans were nothing if not efficient). Then I saw that they launched with very little water ballast on board, but were FULL on return. They had a water vapor condenser on the engine exhaust and claimed they recovered a pound of water for every pound of fuel they burned! That's what a fuel powered engine heater does to your engine compartment. For every cup of fuel you use, you are dumping a cup of water all over your engine!
If I do ANYTHING, it will be an electric heater and a dryer with safety devices to shut it down if anything goes wrong.
Hank

[NY86]

Re: OK to leave electric engine pre-heater on for extended periods?

The pre-heater would be fine if you could keep the entire engine at an even temperature. (This would emulate the 100* day). The problem is most of them heat small portions of the engine, not the whole thing, so water vaporizes from the oil and condenses on the cooler surfaces higher up in the case, like the Lycoming cam shaft. The liquid water drips back into the oil sump where it is heated and vaporized...

Only takes a small temperature difference (think dew point spread) to start the cycle.

[Hank Jarrett]

Re: OK to leave electric engine pre-heater on for extended periods?

It isn't water vapor in the air that causes the problem in a combustion heater. Combining the hydrocarbons in the fuel with oxygen creates H2O and CO2. That's why you can get a pound of water from burning a pound of fuel (OK, pretty close). It isn't atmospheric water vapor, it's combustion generated water vapor.
Hank

[Joel Severinghaus]

Re: OK to leave electric engine pre-heater on for extended periods?

Thanks for all the comments, guys. The collective wisdom of this group continues to be one of the great fringe benefits of owning a Taylorcraft!

I haven't checked various parts of the engine with a thermometer, but my Reiff system with the insulating cowl cover does seem to heat the entire engine uniformly, so I haven't worried about moisture condensing on slightly cooler parts. My hanger is relatively dry and critter-free, and my plane might sit, at most, two weeks between winter flights.

To throw some more moisture into the debate, the FAQ section on Reiff's website (http://www.reiffpreheat.com/FAQ.htm#QA3) quotes a March 2007 Aviation Consumer article:

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."

[Robert Lees]

Re: OK to leave electric engine pre-heater on for extended periods?

"terrarium effect"

What is that?

[Ray36048]

Re: OK to leave electric engine pre-heater on for extended periods?

I do have a concern about anything "plugged in" an unattended hanger, one word, FIRE. Anything could happen and no circuit breaker is going to stop it.

Today, all new homes the wiring to bedrooms require arc fault circuit breakers. This may not be practical in your hanger.

But, the next best thing is to build an extension cord with a GFI outlet. The GFI will trip with a very small current to ground or even a short between ground and neutral. This could shut off the power before a fire gets started.

[mulwyk]

Re: OK to leave electric engine pre-heater on for extended periods?

OK, I'm not going to recommend this as better than the commercial products, but as a low-cost, low-tech solution: my dad would keep his cubs over the winter with just an ordinary "trouble light" under the cowl. He had, if I remember right, a 60 watt incandesant bulb in it. The steel cage around the trouble light kept the hot parts away from any sensitive engine parts, (you could grab the light with your bare hand without burning), and the 60 watts of heat seemed just about right to keep things cozy under there. We'd go winter flying maybe every two weeks or so on skies, and the old girl always started right up.

[Joel Severinghaus]

Re: OK to leave electric engine pre-heater on for extended periods?

Mr. Ambassador, a terrarium is a glass enclosure, in which one keeps pet turtles or snakes. (An aquarium with soil and plants instead of water.) But because it's closed, warm and humid, it usually has condensation on the glass. Same as inside a greenhouse - I suppose one could just as well say "greenhouse effect." (What do you call them in England, Rob, glasshouses?) Or, "snogging in a closed automobile in winter effect."

A GFI for the heater extension cord is a good idea, Ray.

[Larry Lyons]

Re: OK to leave electric engine pre-heater on for extended periods?

A GFI for the heater extension cord is a good idea,

You can buy the outlets with the CFI built in. They are used in bathrooms and around pools too. You simply pull out the existing outlet and wire in the new one with the CFI.
Larry