First I'll give my understanding on how I think it works, haha bear with me people. Say if I had a simple combination system and I wanted to make sure the boiler can heat my radiators across a flat that has an area of 50m2. The height of the flat is about 10m high. Would about 1-1.5 bar suffice in pressure to pump water to all the appliances? My question is about meters head, I believe that bar is the amount of pressure needed to lift water perse by so much meters head (1 bar == around 10.2m head). But if I had a swimming pool that is 20m2 and 5m deep, is that 5m head? And if I have a cup that is about 8cm in diameter but 5m tall and filled with water is that also 5m head? Sorry about the wording of the question, it was actually really difficult to try explain what I want haha Also if I have gotten it completely wrong I apologise in advance, maths and science were never my strong points haha. Cheers to everyone who helps!