wierd one for the plumbers

On or around Sun, 6 Nov 2005 19:39:37 -0000, "homer" mused:

Sounds like the stat in the immersion has failed closed and the resonating is the water in the system boiling.

but if that were so would it not continue after I turned off the cold main valve?

Is there a backup stat?

Are there time switches involved?

DG

Just the stat on the actaual immersion ellement

The Ecconomy 7 control timer

Ah, but recent BS immersion stats have the conventional thermostat PLUS a manual-reset overheat thermostat. This change was deemed to be necessary after someone's cold water storage tank filled up with boiling water from a defective immersion heater stat and collapsed. They got a scalding shower in bed. It killed them & seriously injured their partner.

I'd replace the stat with a new one, even if it is not defective.

It's probably just at the right pressure to causing it to resonate, steam will be escaping from the vent, and cold water topping up the header tank, by turning off the valve, you lower the pressure in the hot water cylinder.

I presume the hot water must be scalding in the morning, a definite sign the thermostat has gone.

Check the cold water storage tank when it's happening. Make sure the tank doesn't get hot, and that the water isn't overflowing. There must be an open vent pipe from the top of the DHW cylinder; make sure it's open end is not immersed in the cold water.

Homer wrote: my water pipes seem to start resonating.

What do you mean by resonating and how long after 1am does it start?

But it stops if immediately if I shut the water off at the mains so I don't see the connection with the stat.

It hums quite loudly. A little like wind in telegraph wires but a more constant note. Not sure about the time. I became subconsciously aware of it for some time before actually waking at about 3 o-clock

Thanks Dave that seems to tie in all the facts. Can the stat be removed without replacing the element?

Have a heart Steve. It was 3 AM and my roof space is not a fun place at the best of times. I'll check the vent pipe though.

Could it be unconnected with the economy 7 immersion heater, just something that happens in the middle of the night _anyway_? If so it might be a cistern valve (e.g. on the tank in the attic) that's only just holding back mains pressure at normal daytime pressures, but in the night when demand is less and pressure is higher lets by causing the noises you hear.

Who's Steve? One other thing, the demand for water goes down at night and the mains pressure goes up (less flow rate = less pressure loss due to friction), often it's 2x the daytime pressure. It might not be connected with Economy 7, it may be the increased mains pressure. This might be forcing open a defective float valve on the tank, or lifting a pressure relief valve on a water heater. It would account for the noise & the noise stopping when the stopcock is shut.

I can't see a connection with the immersion stat (not saying that there isn't one) but I'd be happier with a new thermostat anyway.

Have you got a double check valve in your cold water supply? These sometimes make a wining/groaning noise when there is a very small flow through them. If so, can you reproduce the noise in the daytime by cracking a tap open ever so slightly?

If so, you need to ascertain *why* there is a small flow from the mains in the middle of the night. It may or may not have anything to do with water heating. For example, it could be a leaking ball valve in a cistern - where the water only gets up to a certain level after it hasn't been flushed for a few hours.

Yes just pulls out.

'Tis a weird one. What happens if you don't let the immersion come on, does the hum still happen? Unless the expansion is causing an overflow and thus when the tank cools a bit opening the cold water header tank ball valve I can't see any connection between the HW system and the mains stop tap.

The mains water pressure does vary, at night it might be such that it triggers resonance in the pipes but during the day is sufficiently different so that resonance doesn't occur.

On Sun, 6 Nov 2005 22:48:09 -0000,it is alleged that "Dave Jones" spake thusly in uk.d-i-y:

In case it helps,

On Sun, 06 Nov 2005 23:11:26 GMT,it is alleged that Chip spake thusly in uk.d-i-y:

Just an addition to that, it _may_ be an idea to look at changing the heater if it's old, and doesn't have the modern safety cutout. That would prevent this kind of overheating thing occurring in the future. (if indeed that is what it is)