Isolated towel radiator question

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Hi all

Got a sealed system, boiler been losing about 0.4bar over a week. I have found a pinhole leak half way up the towel rad (where one of the horizontal bars meets the vertical frame) so am hoping that's the cause of the pressure loss. I have closed both valves to isolate the offending rad.

*daft question alert*

So how come the flow pipe to the radiator is now cold when the CH turned on? I mean, rad is cold and return pipe is cold - as you would expect - but how does the hot water "know" not to flow towards the isolated rad?
 
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If water can't flow through the rad the pipes will not retain heat for long,as no hot water flowing through them.
 
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how does the hot water "know" not to flow towards the isolated rad?

How does your cold water “know” not to flow down the pipe towards your kitchen sink tap when it is turned off?

Maybe a car analogy involving traffic jams and red traffic lights is required?
 
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Hope this isn't a Pi$$ take but ..... The CH hot water needs a path to be able to flow, the path in a radiator is from the main flow pipe, through the feed pipe, through the open feed valve, around the rad and back out the return pipe through the open return valve.

When the water in the pipe up to the valve can't move because either of the valve's are shut, then the hot water in the flow pipe can't get to it, so it stays cold. If it's really close to the flow then it can get warm due to conduction/convection but that's about it.
 
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@terryplumb and Madrab
Got yer, makes sense now, cheers.

Also, is a drop in pressure of 0.4 to 0.5 bar over a week consistent with something as innocuous as a tiny weep in a radiator? This sealed system I have is a pain in the wotsit, honestly wish the house just had an old fashioned set up.
 
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0.5 bar drop in a week is a sure sign of water loss ,either from within the boiler or on the system side. Doesn't take a lot of water leakage to drop that much.
 
D

durhamplumber

Maybe a car analogy involving traffic jams and red traffic lights is required?
I think a better traffic analogy is wondering why the traffic is not flowing when a brick wall has been built across the road.
 
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It can drop like that yes, has it stopped dropping now you have isolated the Rad?
Hi Madrab. Topped it up to 1bar 24 hours ago - system now cold and it's settled back down to 1bar, so fingers crossed that was the source of the problem. Just seems mental that a system with the volume of 7 rads + UFH can lose 0.4bar in a week over such a small loss of water.

One thing, the weep from the isolated rad has now almost stopped. Would this be because there's now no "flow" of water; ie the higher pressure of the hot water was exposing the pinhole more than idle cold water?
 
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No ,it's because the rads valves are closed and its contents are no longer under the system pressure.
 

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