On Mon, 13 Nov 2006, Christian McArdle wrote:

Is it 'normal' to have 2 zone valves on an S-Plan with an unvented
cylinder?

Yes. It is a legal requirement. It is against the law not to have it. The
thermostat has to be non-self reseting (i.e. requires a "reset" button to
turn back on).

Building Regulations Part G3
3.3 To meet the requirement a directly heating unit or package should have a
minimum of two temperature activated safety devices operating in sequence:

a. a non-self resetting thermal cut-out to BS3955:1986 Specification for
electrical controls for household and similar general purposes, or to
BS4201:1979 (1984) Specification for thermostats for gas burning appliances;
and

b. (snipped description of temperature/pressure relief valves).

3.6 Safety devices listed in paragraph 3.3 (see also 3.4) for direct heating
are also required for indirectly heated units and packages but the
non-self-resetting thermal cut-out should be wired up to a motorised valve
or some other suitable device to shut off the flow to the primary heater,
that is:

My S-plan system with system boiler and unvented DHW tank has only one
zone valve on the DHW primary circuit but surely the safety cutouts built
in to the boiler itself are sufficient as backup safety devices i.e. two
zone valves are not required.

In addition to the zone valve and associated cylinder stat, there is a
boiler thermostat which controls the maximum temperature of the output
flow of the boiler. There is also a second over-temperature cutout in the
boiler which requires manual reset and the usual over-pressure release
valve in the boiler.

Even if all these safety devices in the primary circuit failed, there are
the two safety over-temperature / over-pressure valves on the cylinder,
i.e. there are five independent safety devices in total.

Surely then it is not a requirement to have two zone valves?


--
Alistair Riddell - BOFH
Microsoft - because god hates us