Re: liquid vs direct air heaters

[Lee Hart <leeahart@earthlink.net>]

David Roden wrote:
>> If you are going to heat the air in the vehicle, the more efficient
>> way is to heat the air directly with a heating element of some kind.

Though, a heat pump would be more efficient yet. Heating the occupants
and glass instead of the whole car would also be more efficient (like
heated seats and windows). Insulating the car would greatly reduce the
amount of heat needed in any case (most cars are uninsulated tin cans).

Gary Graunke wrote:
> I noted that some of the EV suppliers sell 1500W ceramic heater cores
> as well.

This can be a good solution if it fits in your car, and if it isn't too
difficult to get the old heater core out and the new one in.

Minnesota winters are more severe than most, but I have found that 1500w
is only adequate for a small car in a mild winter. For most cars, you'll
need more like 3000w to match the stock heater's performance.

Many ceramic heaters deliver significantly less than their rated heat
output. As they heat up, their resistance goes up, so their heat output
goes down. A typical 120vac "1500w" ceramic heater actually draws about
900 watts once it is warmed up. If you force more air through them with
a bigger fan, they produce more watts of heat but the temperature of the
air coming out of them drops to where it won't effectively melt ice on a
windshield. So, it may make sense to design to allow TWO ceramic
heaters.

> 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.

This is very true! If you try to switch high voltage DC with a standard
AC-rated switch, the contacts will quickly be destroyed from arcing.

The two basic ways to get a relay to safely switch high voltage DC is
with magnetic blowouts, or large contact spacings (i.e. several contacts
in series). The Potter & Brumfield (now owned by Tyco) PRD series relay
is commonly used because it can be ordered with magnetic blowouts. For
example, the PRD-7DHO-12 is a DPST-NO relay with a 12vdc coil and can
switch up to 25a and 125vdc per contact. With the two contacts in
series, it can switch up to 250vdc.

The P&B KUP series is also available with magnetic blowouts, and you can
get up to 3-pole versions; they will switch 10a at up to 375vdc.

Kilovac has some small contactors that will switch high voltage DC. I
don't have their rating handy, but the smallest would certainly work for
heaters.

One last point. Switches and relays that switch DC benefit considerably
by having a "snubber" circuit across their contacts. This is a series
network consisting of a capacitor and resistor. For the ceramic elements
we are talking about here, a 0.1uf to 1.0uf capacitor and 10-100 ohm
resistor is adequate. The higher the load current, the higher the
capacitance and less the resistance, but values are not critical.

> Wayland recommended that, due to the complexity of the Insight
> climate control, I use a small liquid system rather than the easier
> replacement ceramic heater under the dash as he normally does.

I'd say this depends on how hard it is to get at the existing heater
core. The ceramic heaters are self-limiting (even zero airflow won't
burn them out); thus, a variable speed fan or an air proportioning valve
still works as it would with the hot water system.

It seems like kind of a waste to convert such a lightweight efficient
vehicle by adding heavy bulky heater compononts. The water, pump,
heater, etc. are going to weigh considerably more than a ceramic
element.

> Do you need a pump to circulate the water?

It might be possible to build a "gravity" or steam system. The box with
the resistive heater would be below the heater core. The heater core's
hose (or hoses) must come out the bottom. A large hose would connect the
heater to the heater core. Here's how it works:

- The resistor boils water in the heater
- Steam goes up the pipe to the heater core
- Steam condenses back into water, giving up its heat
- water runs back down the same pipe to the heater

No pump is needed. The system is sealed. Warm-up is extremely fast,
because only a tiny amount of water is used. A pressure switch cycles
the heating element on/off.

> Is the electrical isolation more of a problem, given the liquid
> conductor vs air?

It can be. Most consumer-grade immersion heaters have a thin outside
metal case which is grounded. Inside is a length of resistive wire,
insulated by ceramic. The ceramic is pretty effective for preventing
outright shorts to ground, but at high temperatures, the ceramic has a
lot of leakage current to ground, which can trip GFCIs. The key to
preventing this is to not run the heaters at too high a watt density, so
they don't run their heating wires red hot.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen