The Kombi Files

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10/05/2005 15:21   Dash Heater   [Interior]

We failed our rego inspection last week for a number of reasons. One of these was that we don't have a working heater in the van. This is to ensure that the vehicle can de-mist a foggy windscreen, which is probably sensible, but a pain for us.

The old ducted heaters on the kombi were really really crappy so we took out the ducting (spider homes) and heat boxes (raises engine temp) way back in late last year. I seem to have a scrap of memory about someone questioning this decision based on rego requirements, but maybe I'm making that up.

We have built a heater for our van. The biggest problem with an electric vehicle heater (not a heater for an electric vehicle) is managing to deliver the power requirements. A car electric system is going to run at a nominal 13V. Most alternators can deliver 20A (or so, this does vary a lot!) which puts a theoretical limit on the acceptable power load -
13 x 20 = 260W. This is nothing compared to the 2600W you'll find pumping through a small home blower-style heater.

Realising this limitation it is a simple matter to choose a sensible power draw (we're choosing 200W as we don't want to overload our alternator) - measure the voltage supply at load (thus turn on all the lights and turn up the stereo and measure the voltage available at the connection points for the heater electrics) and find the current drain:

P = 200W = VI => I = 15.3 A @ 13 V

To get this current drain at that voltage we need a specific resistance. This is calculated from Ohm's Law and will give:

V = IR => R = V/I = 13 / 15.3 = 0.65 Ω

If we can make a resistor of that value that can handle the high temperature and current then we have our heater! It's that simple. There is one more point I would like to mention and that is cooling effects on our heater. Suppose we make a heater element that will melt at 2000°C. We could overload that heater element but ensure that air is blowing over it to cool it below that melt down temp. This is where everything gets really complicated and we don't have enough technical data (or motivation) to get involved.

I should also mention that our basic assumption is that all the heat from the heater element is going to go into the air. This is a reasonable assumption but not 100% correct, various amounts of heat will radiate or conduct through the element and into the chassis of the car, warming the air outside every so slightly and being lost to us for ever.

There are two types of high temperature wire available to us: Cuprothal and Nichrome. The nichrome wire has a much higher melting temperature but a greater resistance per metre. We initially used a single strand of cuprothal (because that's what was on hand) but there was insufficient heating effect, mainly due to the low surface area of a single wire strand.

We settled on a nichrome design that used five heater elements wired in parallel. Why? Wiring resistors in parallel will reduce the effective resistance of the circuit. Since the resistance of the wire is high and we want to get it down to 0.65 Ω then we need more than one element in parallel. We picked 5 since we can reasonably expect to fit them around the element. Assuming 5 elements, that requires each to have a resistance 5 times that of the total - or 3.25 Ω each. The nichrome wire is 13.4 Ω per metre, so that requires 24cm of wire. Too long to fit on a 7cm insulator, but if we wind each strand up then it will fit. We did this by cutting the wire to length and wrapping it tightly around a single strand of coat-hanger wire (2-3mm ∅). The five curly elements were then arranged on the insulator and bound in place.

Nichrome wire can not be soldered, which is good since once it's at temp the solder would melt anyway! We used mechanical connections (small bolts) to join our wires together.

You can see the finished element in the picture below. It was very fiddly to make but fits very neatly into the venting on the van. The attached cartoon shows the desired operation.

Last night Nick and I fired this baby up and were very pleased with the results.

Current drain = 14A
Voltage at drain = 12.5V
Total Heater Power = 175W
Effective temperature change on air = 20°C.

So that's the bomb! You could actually feel a faint warm breeze coming out of the vents. Let's home that it passes rego!

 
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