Why does my hot water flow rate drop?

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Hi,

For the past month or so, the flow rate in our upstairs shower has been a bit rubbish. I put it down to the mains pressure having a problem (unvented system with an ariston cylinder) but I then noticed the down stairs kitchen taps shows similar behaviour - when I run the hot water the flow seems fine, but then noticeably drops ( to the point that I don't get any splashes when I put my hand under it) after 30-40 secs. Cold water is fine. Any thoughts on what could be going on, and what needs to done to fix it?
 
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Any thoughts, anyone? Or is this something that can't be diagnosed on t'internet ?
 
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Clogged filter at the PRV?
Any cold taps downstream of the PRV which are misbehaving?
 
H

Hot&Cold

unvented system with an ariston cylinder

When is the next service visit due ?

In the mean time you can check all isolation valves on the incoming main are fully open and as a test try the pressure in the early hours of the morning when there is less draw on the external water mains supply pipe,see if the pressure is higher.

Have you actually measured the flow rate ?
 
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Hi - I checked the flow rate this afternoon - the outside tap gives me 18 L a minute. The kitchen mixer tap is 10 L/min for cold, 9 L/min for hot, but the latter drops to 5.5 L/min after 30 secs or so. I see no drop in the cold.

If the filter is blocked, why would I see a drop after 30 secs (and this behaviour is pretty consistent). Is there anything in the system that could be giving a temporary pressure boost - eg an expansion vessel?
 
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If the filter is blocked, why would I see a drop after 30 secs (and this behaviour is pretty consistent). Is there anything in the system that could be giving a temporary pressure boost - eg an expansion vessel?
Unvented system will have a bubble or some other means of permitting expansion.
 

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