It is going to decrease it some.
Water expands when it is heated, this expansion increases the pressure. The bladder tank absorbs the extra pressure, protecting the hot water heater.
A high pressure will be able to push the coolants into the heater cores.
The only way is to run 3/4 to the heater. If the house is 3/4 this should have been done when it was installed. You should only reduce the size at the fixtures.
no pressure release valve
Older style tubs have a pressure switch usually mounted directly on the heater manifold, or attached to the plumbing. The pressure switch is connected to the spa pack/circuit board with 2 wires and shuts the heater off if there is not enough pressure in the heater tube/manifold to protect the heater from becoming damaged or even exploding or causing a fire in a dry heater condition. Newer spas that use Balboa controls have sensors built right into the heater so a pressure switch is no longer needed as the safety function of monitoring the heater for constant water pressure is now all performed within the heater on newer quality spas.
My guess is that the hot water heater is installed downstream from a pressure reducer to limit the maximum water pressure that the water heater is exposed to.
You would need to specify the size of the heater
Water pressure from a garden hose is sufficient.
It's the coolant control valve for the heater core that's leaking. It's under more pressure when the engine is running and that why the leak is more noticable.
Provided the heater is of a low wattage it should not affect normal running of the car
I disagree. I was having issues with my heater taking a long time to heat up. I had to have my battery replaced and my heater functioned normally after that.
A superheater increases the temperature of the steam for use in a turbine or an engine. Without the additional heat, a multi-stage turbine or engine will get moisture (water) in the lower pressure stages as the steam cools, causing erosion of the blades or cylinders.