Some basics, if you need them.
Unusual to find an expansion tank on domestic hot water system, but see @Tony Brooks comments.
Most engines cooling systems have some form of integral header/expansion, usually part of the exhaust manifold. Additional header/expansion capacity is however very common as it helps with the additional expansion when you have a large coolant capacity.
Radiator and calorifier coil (effectively another radiator sitting inside the calorifier tank) are plumbed into the engines primary cooling system (same as car heaters); and subject to pressure as dictated by the pressure (radiator) cap. Opinions vary, but using any "push fit" plastic plumbing on systems subject to pressure from the engine cooling system can create a weak point.