Ed's excellent sealed system FAQ covers the principles of operation plus what to do when something goes wrong, but it doesn't tell you how to design a system from scratch or how to convert a vented system (apart from mentioning a couple of pre-requisites).
Is there a FAQ or similar anywhere which goes into things like:
- How to determine the volume of a system
- How to decide on the size of pressure vessel required
- Any rules about the location of the pressure vessel relative to boiler, pump, zone valves (if it matters)
According to my calculations, you need a pressure vessel whose capacity is about 10% of the water volume. Does that sound reasonable? [I'll expand(!) on that below]
So you need to be able to estimate the water volume. If you know how much pipe there is of each size, that bit's not too difficult. Then there's the radiators. Are there any rules of thumb for litres per square metre for typical single-panel radiators? [I'm sure I could make a stab at it by estimating the cross-sectional area of all the 'tubes' - but it would be nice if I didn't have to]. That leaves the boiler and the heating coil in the HW cylinder. Are there any typical values for these?
I'm sure there's scope for a FAQ here if anyone cares to write one (if there isn't one already).
[Back to my calcs. How does this grab you? Assume you have a 10 litre pressure vessel, and prime it with air at 0.7 bar (1.7 bar absolute). Then you pressurise the system with water to 1 bar (2 bar absolute). ISTR from the dim and distance past about the Law of either Boyle or Charles or somebody along the lines of the volume of a gas at constant temperature being inversely preportional to the absolute pressure. If this is correct, our 10 litres of air at 1.7 bar will reduce to 8.5 litres at 2 bar. If we now heat up the system so that the pressure rises to 2 bar (3 bar absolute) the volume of air will reduce further to 5.67 litres. The difference between 8.5 and 5.67 (2.83 litres) represents the amount of expansion water which has entered the vessel. So, for a pressure rise from 1 bar to 2 bar, a 10 litre vessel can accommodate about 2.8 litres of water.My research on the volumetric expansion of (tap) water suggests that it expands by about 2.5% between 4 decC and 80 degC. So a 100 litre system would expand by about 2.5 litres - which could be nicely accommodated by a
10 litre vessel, giving a pressure rise of just under a bar. (Actually, the temperature rise will be less than this since the system will be a room temp rather than 4 decC when cold. But the air in the vessel probably warms up up a bit, which I have ignored - so maybe the two tend to cancel.) Are there any major flaws in my reasoning?]