non-return valves, isolating valves and service valves

Two newbie question for the experts, but here goes...
1. Under what circumstances do you use a SINGLE non-return valve, and when do you use a DOUBLE non-return valve ? What advantages/disadvantages does one have over the other ?
2. Whats the difference, apart from a few pennies, between an ISOLATING valve and a SERVICE valve ?
Q1 is for a pumped hot-water loop, and the valve is to ensure water goes the right way round the system ie preventing the fresh cold water entering the loop from going the shortest way round the loop and bypassing the thermal-store heater.
Q2 is just idle curiosity.
Thanks in advance Ron Askew
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http://www.arrowvalves.co.uk/waterregulations/waterregstutorials.shtml
http://www.arrowvalves.co.uk/waterregulations/waterrregstutorial9-isolatingvalves.doc
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Single valves are cheaper and drop less pressure. Double valves are more reliable and required if water byelaws require a check valve, such as before a shower, or garden tap.

They are both the same, except a service valve has a tap connector one end, whilst an isolating valve has compressions both ends.

I'd always use doubles, but a single would work here, provided the schematic of the system shows that the valve isn't being used to prevent contamination of the supply.
Christian.
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reliability - I've had problems recently with my system's hot water loop not maintaining temperature properly when drawing off eg for a bath, and this was worse if the pump was switched off. When the pump was on and not drawing water, I'd then also get a rattling noise. I finally traced the problem to the single valve not closing properly. So I'm changing it for a double !! (or was I just unlucky?) Thanks again Ron
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