Chris,
I spent a good six months on an electric heater project for a 911 without heat exchangers. I was aiming to launch this as a product. Here is the system:
The heater units (1 per side) fit in the kick panels and consist of ducting, a fan and a 420W PTC heater element. Fits behind the carpet and is unseen.
Installed:
A control board replaces the ashtray (but you could place this anywhere) :
Installed:
Here are my observations:
1) The cabin heaters on ebay (as per Russ' post) are a waste of time. They typically have 300W heater elements and have weak fans ( ~20 CFM ). I had the PTC heater elements custom made for my prototype, they are 420W. The fans are 106 CFM and are the most powerful on the market.
2) In use, each heater takes 30A to run, e.g 60A both on. I have an SC with the 75A alternator but this is still not man enough once you consider lights, wipers, rear screen demist etc. The early cars only have 55A alternators. Because of this, my controller monitors the 12V line - if it dips below a threshold, the controller starts a sequencing mode where each side gets pulsed on-off in turn to conserve power. This usually happens when engine idling, lights on etc. When you get the RPMs up again, both heaters resume normal operation.
3) So, does it work? Kinda. The bottom line is that if is below ~10 degrees outside, you really won't get the cabin up to temperature in a sensible amount of time, however, you will still benefit from screen demist. If you have really damp conditions, there is no hope really. If you have a car that lives in a garage and isn't used in the winter much then the system as it stands could be a goer.
4) If the incoming air can be pre-heated just a bit, then the system can provide the rest of the heat required. I did experiment with scavenging heat from the engine bay and also from the oil cooler with some reasonable results. I think the best solution would be a small exhaust clamp type heat exchanger as used on light aircraft ( google "aircraft heat muff"). This could provide enough pre-heat, with the electrics doing the rest.
So there you go. I probably have enough parts to build you a system to play with if you fancy. PM me.
Great car btw!
