Heater relays (was liquid vs direct air heaters)

["VanDerWal, Peter MSgt" <vanderwp@fhu.disa.mil>]

>I also had some notes from an OEVA meeting on heater relays--the note said
>to use magnetic blowout relays--the others are a fire hazard. However, I
>haven't really found any that have the appropriate DC voltage rating,
>especially for a higher voltage (>200V) pack. 

Sounds like a good recommendation.  Naturally I (being obstinate) did mine a
different way.  I use a double throw four pole relay and route the current
through all four poles in series.  I also use a snubber circuit (that belt &
suspenders idea).

FWIW I've watched the current (via the e-meter) with just the heater on.  In
my setup the initial current starts out at about 2 amps and takes about 10
seconds to climb to 9-10 amps, at which point it is putting out full heat.

FWIW I wired my setup so the fan can run without the element on.  I use two
switches.  The first turns on the 12V fan and supplies power to the second
switch.  The second switch is wired in series to the original thermal switch
from the heater, the original thermostat (now mounted on the dash), and the
relay that controls the HV to the element.  

When it gets warm enough the thermostat turns off the element but the fan
keeps running.  This helps get the last bit of heat out from the element and
also keeps the windshield frost free even when the element is off.  I've
noticed that under some circumstances the windshield will fog up again, even
though the cab is warm, if the fan isn't blowing.

Oh yeah, one final thing.  The HV for the heater is also routed through the
main contactors so even if somehow something failed in my heater wiring, I
can still turn it off by shutting down the whole truck.