Electrical cutout for unvented cylinder?

[Niall Smart]

Hi

I am doing a DIY install of an unvented hot water cylinder (in Rep of Ireland there is no requirement to be "officially registered" to do so).

The cylinder will be heated by a Viessman Vitodens 200, with a 3KW immersion element as backup.

The Vitodens comes with a probe that measures the water temperature via a cylinder stat pocket, and the immersion element has a built in thermostat. In the unlikely event of either of these components failing in a way that results in the water being heated to an unsafe temperature, I understand the mechanical T&P safety relief valve should activate to prevent explosion.

However, it is usual and/or advisable to fit an electrical cutoff in addition to the mechanical T&P safety valve? The idea being to entirely remove electrical supply to the boiler and immersion element should the cylinder overheat, rather than just relying on the mechanical safety valve. I have not seen any references to such electrical safety circuits in the cylinder manufacturers manuals.

Thanks for any advice,

Niall

[Aidan]

No, the safety devices supplied with the kit are adequate; you could probably devise additional devices if you're competent.

The immersion heater thermostat has the normal control thermostat and a manual reset overheat thermostat. If they both fail 'ON', then the 'T' device on the T&P relief valve operates at 98 degC (I think), discharging hot water and letting in cold water.

The boiler's own high temperature thermostats should prevent it approaching 100degC. However, the cylinder thermostats controlling the indirect (boiler) heating will also have a control thermostat & high- limit manual reset thermostat. Either one will break the power supply to the 2-port valve, which should then spring-return shut.

You shouldn't use an unvented cylinder with solid fuel boilers (under UK legislation) unless there are additional safety devices to prevent overheating.

[Ed Sirett]

You should find that the immersion's thermostat has a manual reset cut out. Make sure the discharge pipe is according to the instructions/regs this is the most common fault I find on uk installations.

[adder1969]

A neighbour's discharge pipe was discharging water so that it was dripping down onto the access road underneath then they finally "fixed" it. A month or so later "Booooom!" in the middle of the night.

[Lurch]

On 22 Mar 2007 04:13:45 -0700, "adder1969" mused:

I can't see the point in building a safety device in to backup another perfectly suitable one just to protect people from their own stupidity.

[Niall Smart]

Well the one advantage of an electrical cutout is that it in case of malfunction it will remove the root cause of the problem (too much heat) by cutting out the boiler and/or immersion; as opposed to "just" vent the overpressurised cylinder. Of course you still need the T&P safety valve in case the electrical cutout breaks, or gets messed with by a "well meaning" someone or other..

Niall

[Lurch]

On 28 Mar 2007 13:24:14 -0700, "Niall Smart" mused:

But the electrical one could fail as well, so how far do you go. How many thousands of back ups do you need to backup more backup safety devices.