Is your hydronic loop pressurized?
Umm, I'm not sure how to answer this.
The coolant is not under pressure while the system is at rest. But, as with most coolant-loop systems, there is an expansion tank at the very upper-most part of the system, which is, itself, a pressure vessel. It has a pressure cap on it similar to the one on an automotive radiator. There is also about a 6' height difference between the top level of coolant in the tank, and the lowest point of the system, which means that, even at rest, the low point is at a static pressure of roughly 17psi.
When the system is in operation, the "boiler" heats the coolant to somewhere around 200° Fahrenheit. I put quotes around the term, because, while hydronic heaters are often called diesel-fired boilers, the intent is not actually to "boil" the coolant, meaning to cause it to change state from liquid to gas. (That being said, if the circulating pump stops, this is exactly what might happen. The boiler, whether Webasto or some other make, will have high-temp cut-out safeties to protect itself from overheating in this case.) One reason you should use "antifreeze" coolant mix in the system, as opposed to plain water, is that the antifreeze raises the boiling point of the coolant, from 212° to around 265°, making actual boiling of the coolant less likely in the event, for example, of low circulation in the system. The actual temperature your system will reach is one of the design parameters, controlled by a thermostat (sometimes called an "aqua-stat") in the coolant loop which causes the boiler to switch on or off.
As the temperature of the entire system rises, the "partial pressure" of water vapor and even antifreeze vapor in the expansion tank will rise, as well as the air pressure in the tank as the air, eventually, will also heat up to nearly the design temperature of the system. At this point, the Ideal Gas Law takes over -- as long as the pressure cap stays on the tank and does not open, the pressure of the air/vapors in the tank will rise as the temperature rises. Because it is, at this point, a closed system, that gas pressure will be transmitted to the coolant and will increase the operating pressure of the whole system. To protect the system from hose ruptures or worse, the pressure cap on the expansion tank has a relief valve that opens at a specific pressure, just like an automotive radiator.
Even if the system were open to atmosphere at all times (which would decrease the thermal efficiency), the circulation pump also causes a pressure difference. In addition to the static pressure mentioned above, the operation of the pump will cause a dynamically lower pressure on the input or "suction" side of the pump, and a dynamically higher pressure on the output side. Calculation this pressure difference is a matter of fluid dynamics, the math for which is way more than I want to do on a bulletin-board post.
Long-winded, but I hope I've answered your question.
Incidentally, contributing to the materials discussion, I think PEX is a bad choice because it is not rated for the temperatures and pressures that the system MAY encounter should there be any kind of malfunction. Our system is entirely plumbed with 1" diameter silicone "heater" hoses with solid (non-perforated) hose clamps. All valves, nipples, lead-ins, etc. are copper or brass rated for hot water. While copper pipe would also work, long runs of copper pipe will be subject to significant length changes due to thermal expansion, and the thermal conductivity of the copper will mean you will lose a lot of heat to the environment as compared to hoses, unless you insulate them. Plus, hose is generally easier to run. It used to be more expensive than copper, but, with copper skyrocketing, I'm not sure any more. FWIW.