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Where does the pressure go?

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Discuss Where does the pressure go? in the Plumbing Jobs | The Job-board area at Plumbers Forums

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Lecky

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Hi, Could anyone explain what might be happening in my system? I have a sealed gas central heating sysyem (Baxi Solo 3) and Megaflo unvented hot water tank. 4 Hot water taps in the house and a shower off the HW tank. 8 rads in total, small 3 bed terrace. If I turn off the cold water mains and leave over night (with the heating and HW off as well), there is a 'rush' of water through the pipework when i subsequently switch the mains back on in the morning. No tap is turned on during the period that the mains is off. As far as I can tell there is no leak in the system anywhere. I just dont understand where the water can be going. My thought is evaporation from the HW tank but can't see how it would loose much over night? There will be some warmth left in the tank from the previous evenings heating but thats it. Another thought might be the megaflo's internal expansion system being re pressurised but surly this can't be the case as where can the pressure inside the tank go when the mains is turned off?

Thanks
 
Hi, thanks for the quick reply. I don't think that there is a problem as such as the system works fine with no loss of pressure (as shown on the pressure vessel) or leaks. I'm just curious as to where the water is going when I turn the mains back on?
 
water gets absorbed by the expansion of the megaflow , dont panic and stop playing with your system you could damage some components by doing this every day
 
Water contracts when it cools & therefore the air gap will expand, - this will drop the mains pressure a little & then when you turn it back on it will bring the pressure up again. Do also check that the discharge pipe is not dripping.
 
Last edited:
Thanks again for all of the replies. I'm not doing this every night. My little test was to make sure that turning off the mains didn't affect my CH as I intend to do this when leaving theproperty unoccupied during winter periods.

Toilet ruled out as not used while mains was off.

3 Scenarios:

1. If the tank was warm when the mains was switched off I'd expect the water to contract slightly as it cooled. This would allow the air 'bubble' to expand thus taking up the extra room created. In this scenario I dont see where the extra space would be to allow fresh water to enter when I open the Stopcock.

2. If the tank was cold and not heated, there would be no change in water volume thus no room again for fresh water to enter.

3. If the tank was cold then warmed up, the water would expand. Expansion taken up by the 'bubble' with slight increase in pressure. Again, no extra room created to allow fresh water to enter.


Think I've covered all scenarios there so still cant see where the extra space is being created to allow extra water to enter - other than an unknown leak.

If my theories laid out in scenarios 1-3 above are correct (please tell me if not!!) then I might be looking for a hidden leak?
 
What?

If theres no evidence of leaking leave it alone, I never turn the water off when I go on holiday, I have a room stat at 10 degrees which kicks in if the temp drops below that. I have never had a problem. If you like turn the water off when your on holiday, and make sure the hot water is off at the timer as well.

Stop worrying,
 
Not a wind up, he has concerns when he goes on holiday, which to me is fair enough.

But possibly over analysing the issue. Mains cold water pressure is variable, who's to say it won't be greater in the morning than when isolated later at night.

Have a good holiday and don't worry about your water.
 
Ok. How long is this rush of water. That could establish how much the system is recharging by.
 
Unless it is discharging because the internal air gap has gone and you can see this through tundish, a small rush is common. but if your that concerned and you keep posting on the website call somebody out who is qualified in unvented systems. simple.
 
Thanks again for the replies. No its not a wind up question, just i've never lived in a house with an unvented HW cylinder or had anything to do with them. I am an Electrician but within the aircraft industry. This may go some way to explain why I try to understand everything down to the finest detail before I'm happy. You may be pleased to know that I wouldn't let your holiday flight take off if I thought one of its systems had a problem.
I've got no problem reading up and undertsanding these things but theres no substitute for discussing it with people who are hands on with these systems every day!!
 
Lecky said:
Thanks again for the replies. No its not a wind up question, just i've never lived in a house with an unvented HW cylinder or had anything to do with them. I am an Electrician but within the aircraft industry. This may go some way to explain why I try to understand everything down to the finest detail before I'm happy. You may be pleased to know that I wouldn't let your holiday flight take off if I thought one of its systems had a problem.
I've got no problem reading up and undertsanding these things but theres no substitute for discussing it with people who are hands on with these systems every day!!
Click to expand...

I honestly think the only thing you need to worry about is enjoying your holiday :)

Have a good one.
 
Lecky, you didn't say how long the refill takes.
And how do you disable the smoke alarms on the inflight bogs?
 
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