Do Isolator/Service valves reduce flow rate?

[andy.budgell]

Morning Gentlemen,

When I install my shower, MIRA Excel running from a Combi, will service valves reduce the flow rate significantly?

Are there special isolator/service valves that dont ?

Regards
Andy

 

[chrishutt]

In practice isolating valves, even with a restricted bore through the ball, will have no detrimental effect on a mains pressure combi system. The hot side supply will be severely restricted anyway by the combi boiler, so with a flow rate of maybe 12 litres/min a 15mm isolating valve isn't going to present much resistance. On the cold side anything which increases resistance to flow will improve the balance between hot and cold and will be a good thing.

 

[andy.budgell]

Thanks for that Chris.

On the cold side anything which increases resistance to flow will improve the balance between hot and cold and will be a good thing.

I guess you mean if for example someone flushes the toilet it wont take all the pressure away from my shower?

 

[chrishutt]

Not quite that simple. Let's say you have a mains static pressure of 2 bar. When the shower operates taking in say 18 litres/min - 12 l/min from hot and 6 l/m from cold, the dynamic pressure on the hot side will drop very low, say 0.5 bar, due to the high resistance of the combi boiler, whereas the cold side dynamic pressure will only drop slightly, say to 1.5 bar.

In this example you would therefore end up with 3 times the pressure on the cold side compared to the hot, which would make it harder for the shower to maintain a constant temperature. It follows that anything that reduces the dynamic pressure of the cold side to be closer to the hot would tend to improve matters, so isolating valves would actually help if anything, although a proper flow restrictor might be better.

 

[gas4you]

I would fit full bore lever type isolators. No restriction to worry about then.

 

[Agile]

It also rather depends on the flow rate of the shower.

If its less than 8-10 litres per minute then a restricted iso valve will have little effect but if its a high flow then it would be better to use full flow valves.

On a cold supply inside a house I would only use full flow valves but iso valves at the point of use can be restricted if the flow rate allows.

Tony

 

[andy.budgell]

Thanks for the comments Gents.

Lever ones could be used for hot and normal ones for cold by the sounds of it.

In theory the shower should be savvy enough to control the pressure and maintain temperature, why else would you buy a Mira Excel at £370!

Interesting debate though, thanks.

BTW, whats the official name of "Full Bore" lever type valves and how do you know they're full bore?

 

[htgeng]

BTW, whats the official name of "Full Bore" lever type valves and how do you know they're full bore?

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=101600&ts=68905&id=11085

 

[gas4you]

Lever ones could be used for hot and normal ones for cold by the sounds of it.

Why?

Use the lever ones on hot & cold