Mark

If I understand your test results correctly it seems that fitting the bypass only improves the situation by a couple of degrees. That is to say that with the fan on and the heater off, with the bypass valve closed the cabin temperature is only about 2°C less than when the bypass is open (feeding hot water to the heater element).
Also you have arrived at the conclusion that the major source of un desired heat is hot engine bay air????

This would seem to indicate that it is most important to limit the entry of hot engine bay air. A second slight improvement can be made using the bypass valve.

Having said all that the reduction of only 2°C on a hot day is still significant.

I still remain somewhat undecided as to whether or not it is worth fitting the bypass valve.

Peter

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