Suction on cold supply?

[momist]

Hi everyone, I've never encountered anything like this before and wondered if anyone can explain what's causing it?
My house has mains water pressure to all the cold taps, the shower and toilet, the cold water storage tank and the heating open system header tank. The cold storage tank seems to act only as a header for the hot water system. The heating, as I said, is an open indirect central heating/hot water system from a very old gas boiler.
The odd thing is that overnight, the cold supply seems to suck back, and empty some of the cold supply pipework in the house. I know there is a problem with the kitchen cold tap, as it sometimes drips and has a tiny dribble from the mixer pipe base, but that should be under mains pressure all the time, as it's from the cold main system. This morning, the electric shower was empty and took a few moments to fill prior to working. What can cause a system under mains pressure to suck back in this way?
Thanks for any ideas.

 

[picasso]

Pure guess but you might have a sticking non return valve that only opens with full pressure, the kitchen tap dribbling is perhaps emptying the pipes without disturbing the nrv, just a theory

 

[momist]

you might have a sticking non return valve that only opens with full pressure

Yes, that would be a good explanation. AFAIK, there isn't a NRV in the system, except the one on the outside tap. Do kitchen sink mixer taps have them internally? But even then, it would be in the wrong place to give this effect.

The supply to the shower is at the highest point on the system, as the feed goes through the roof space and down to the electric shower (over the bath) from the ceiling. The feed to the central heating header tank is off the same pipe, IIRC.

 

[doitall]

First thing would be to check with the supplier to see whether they have been working on the mains overnight.

2nd thing would be to report the issue to the supplier, and check your installation as what you discribe shouldn't, cant happen.

 

[picasso]

Do you have a water meter ?

 

[doitall]

The most likely cause, other than the missing non return valve, is one of the neighbours has fittind a booster pump on the mains. Which is why you need to contact the supplier.

 

[momist]

Ah, never thought of that one @doitall, thanks. @picasso, no water meter here, although many of the neighbours do have.
As it's a bank holiday weekend, I'll not contact the water company yet, but will do later.

 

[Onetap]

The most likely cause, other than the missing non return valve, is one of the neighbours has fittind a booster pump on the mains. Which is why you need to contact the supplier.

It could be a huge water draw off in the area.

As it's a bank holiday weekend, I'll not contact the water company yet, but will do later.

The suppliers have a legal responsibility to maintain a minimum pressure at the mains connection at all times. IF the mains pressure is being lost overnight (you're not certain that it is ), THEN there is a risk of mains contamination by back-siphonage (sucking back).

Shower hoses dangling in baths, hoses in ponds and similar could start letting some nasty stuff into the mains, assuming inadequate back-flow prevention devices ( double-check valves, air-gaps and whatever) which comes as a standard feature with installations by incompetent, cheap "plumbers".

That's why you need to contact the supplier. I'd do it now. You'll be drinking the contents of your neighbours' ponds, baths and bidets when the pressure is restored.

There are some gruesome records of back-siphonage available from the USA where mains pressure systems have always been standard.