Manifold Heater for Cold Climates

I would connect it, try it, and run it. It can only get as hot as your cooling system will allow, which means temperature should be 'governed' by the thermostat and radiator pressure cap to about 110*C. I live in Australia, on older cars we do occasionally fit a 'heater tap' which is something similar to the item shown in Reply #9 by slantsix64, the purpose being for cars where the heater has water flowing (and heating) at all times, it can be isolated during summer to prevent unwanted escalating cabin temperatures (usually caused by leaking/cracked heater boxes!!!). That said, most of our 60's and 70's models with water-heated inlets flow water continually, with no temperature control. If after trying it, you really think you need 'on the move' control of the heating, look for a cable-operated heater/water control valve off an older model car.