By design the heater matrix core in the Lotus Elise/Exige hvac box is always plumbed in to the hot coolant circuit. The only means to control the amount of heat in the cabin is by adjusting an airflap in the heaterbox. Heater knob off means flap closed, heater knob fully open means heater flap fully open – and everything in between. The flap is opened by a servo drive attached to the flap, which receives instructions by a powered lead which is fed 0V (fully closed for hot air) to approx 5V (fully open for hot air).

I used this control voltage to make a logic which only opens a new by-pass valve from a Ford Fiesta, only if the input voltage is above approx. 1,5 volts. Otherwise the airflap is almost closed and the heater matrix is by-passed and remains cool. The servo is driven by a 12V lead, a ground lead and the mentioned control voltage lead (0-5v). I use the 12v and ground simultaneously for the new control.

Lotus Elise-Exige heater by-pass control

The by-pass valve is designed normally open. I made connectors to attach it directly to the 12v of the flap control servo leads (always closed), or attach the control box in between (only open above 1,5 servo voltage), or disconnect it all together (heater always open). As such it is a reversible modification, or can be overridden in case of a problem en route.

So how does it work? For me it’s primarily hear-say, as i made this sketch based on intructions from my brother. But it makes sense to me:

The 1k and LED on the righthand side of the sketch create a constant 1.6V level, irrespective the state of the 12-14V battery source. The 1k resistor creates just about the right current amps for the LED. The 50k ohm trim resistance can adjust it even further down to a desired level of approx 1 volt. This enables to adjust the exact moment at which the valve is opened. This set level will be compared to the incoming voltage from the flap control (which is ranging from 0 to 5 volts). This comparison is performed by the OP27 logic. Set input 2 is compared to servo input 3 and the difference is sent to the amplificator BTS432. This amplifies it to either zero or maximum value, i.e. full battery voltage 12v. This is connected to the valve which either closes (12v) or opens (0v). Additional LED is set in parallel to have a visual check without the valve installed.

A capacitor and a resistor around the comparator have the task to flatten out spikes and self induced voltage swings when the valve is actuated.

So far it has performed brilliantly out of the box!

left to right, 50k trim resistor, OP27, BTS432
by-pass control box install diagram
By-pass control box, it resides near the bulkhead behind the wind screen wiper.
By-pass valve plumbed in
The components of the by-pass controller