DHW safety valve?

[Duncan62]

Anyone installed a Gen 7 Samsung A2W heat pump and found out where to connect the DHW safety shut off valve please?

 

 

ehs-r290-installation-guide.pdf

[Duncan62]

ev-ehs-r-290-mono-pjt-ubs-s.pdf

[Duncan62]

Booster heater and thermistor installed. 

It's just the cut off valve for the over temp that needs attention. 

 

I think I need to connect the valve to the tank thermal cut out  - with the BROWN wire connected in position 2 and GREY/BLUE/GREEN wired up to live.

Unsure on the ORANGE wire.

[Duncan62]

Live is fed to the immersion heater via the thermal cut out. 

 

This is where the valve can pick up it's power (?) is my guess.

[JohnMo]

Why would you be doing an S plan with a heat pump? That isn't the way to do heat pump plumbing, maybe ok for a boiler, but still wouldn't go that route.

 

You should be using a diverter valve, which is your positive energy cutout. Primary pipe from heat pump, to diverter valve, normally open route to CH and the normally closed to the cylinder. The diverter is controlled via ASHP controller on a demand for cylinder heating.  ASHP ramps flow temp up for cylinder heating.  Your ASHP is equipped with the safety features to only need that.

[Duncan62]

22 minutes ago, JohnMo said:

Why would you be doing an S plan with a heat pump? That isn't the way to do heat pump plumbing, maybe ok for a boiler, but still wouldn't go that route.

 

You should be using a diverter valve, which is your positive energy cutout. Primary pipe from heat pump, to diverter valve, normally open route to CH and the normally closed to the cylinder. The diverter is controlled via ASHP controller on a demand for cylinder heating.  ASHP ramps flow temp up for cylinder heating.  Your ASHP is equipped with the safety features to only need that.

This is exactly my set up.

 

The extra valve is only on the DHW return to act as a safety cutout.

 

Is it enough to have the diverter only?

 

[JohnMo]

51 minutes ago, Duncan62 said:

Is it enough to have the diverter only?

 

Yes

[SimonD]

46 minutes ago, Duncan62 said:

The extra valve is only on the DHW return to act as a safety cutout.

There are debates going round and round in circles in the heat pump installers forums that I'm on about this. Some still seem to think you need it, but basically you haven't got a heat source that's ever going to get close to problematic temperatures, so it's simply not required. I have a shelf full of unused 2-ports as they come with every cylinder I buy and never get used. If you're worried, you can take your switched live from the controller to 3-port through the cylinder thermal cut-out, but really not necessary - just check the three port model specific wiring diagram as some will have permanent and switched live, some will just have a switched live, for example.

[ProDave]

What is missing from this conversation is WHY it is there.

 

It is one of the many safety features of an unvented cylinder, to ensure it cannot boil.  So each heat source feeding the cylinder must have a fail safe cut off.  There is a theoretical fault possibility in that many heat pumps have a built in electric resistance heater for backup or boost reasons and if the controls operating that stuck on, then the cylinder could continue heating until it boiled.  So the valve we are talking about will shut down that flow from the heat pump if the heat cylinder is getting too hot.

 

It is okay to use your diverter valve, or a separate valve for that, as long as the valve is normally closed, i.e. no power to the valve and it is shut.

 

If using your diverter valve for this function then it is wired as normal, but with the addition that  the L feed to the valve also passes through the normally closed contact on the cylinder thermostat that will have been supplied with the cylinder.  This will open if the cylinder is getting too hot regardless of what any other controls might be telling the valve to do.

 

I hope that makes sense, as up to this point I have not seen mention of the cylinder thermostat.

[JohnMo]

2 hours ago, ProDave said:

So each heat source feeding the cylinder must have a fail safe cut off.  There is a theoretical fault possibility in that many heat pumps have a built in electric resistance heater for backup or boost reasons and if the controls operating that stuck on, then the cylinder could continue heating until it boiled.  So the valve we are talking about will shut down that flow from the heat pump if the heat cylinder is getting too hot.

 

It is okay to use your diverter valve, or a separate valve for that, as long as the valve is normally closed, i.e. no power to the valve and it is shut.

 

If using your diverter valve for this function then it is wired as normal, but with the addition that  the L feed to the valve also passes through the normally closed contact on the cylinder thermostat that will have been supplied with the cylinder.  This will open if the cylinder is getting too hot regardless of what any other controls might be telling the valve to do.

 

If doing as above the normal thermostat doesn't comply as it has to be non self resetting, a dual function cylinder thermostat would comply or a stand alone non- self resetting one complies

 

That is indeed one way to achieve compliance, but not the only way.

 

From G3 Regs 

The selection of safety devices should take account of the physical location of the devices, and the design, confguration, location of components and performance characteristics of the system to which they are attached.

3.18 An acceptable approach might consist of: a. a non self-resetting energy cut-out to disconnect the supply of heat to the storage vessel in the event of the storage system over-heating; and b. a temperature relief valve or a combined temperature and pressure relief valve to safely discharge the water in the event of serious over-heating. Alternative approaches to this are acceptable provided that they provide an equivalent degree of safety.

 

Unvented hot water storage systems – systems up to 500 litres capacity and 45kW power input

3.21 If an indirect supply of heat to an unvented hot water storage system incorporates a boiler, the energy cut-out may be on the boiler.

Narrative from me. As a heat pump cannot achieve high enough temps to allow boiling of the water (even R290 can't get hot enough) a primary control is there by default.  If the heat pump uses an inbuilt immersion that immersion has to have a non- self reset thermal cutoff to comply with regs

 

For the regs a heat pump is a boiler.

 

Diverter valves - To comply with G3 Building Regulations. When installing with an unvented cylinder, Port A must be used for the hot water connection to the cylinder heat exchanger and Port B for the central heating connection.  This means the valve when de-powered shuts the flow to the cylinder.

[Nickfromwales]

8 hours ago, Duncan62 said:

This is exactly my set up.

 

The extra valve is only on the DHW return to act as a safety cutout.

 

Is it enough to have the diverter only?

 

Have you bought a heat pump cylinder, or a regular cylinder?

[Duncan62]

On 18/06/2026 at 19:43, Nickfromwales said:

Have you bought a heat pump cylinder, or a regular cylinder?

Heat pump cylinder

[Nickfromwales]

9 hours ago, Duncan62 said:

Heat pump cylinder

Odd to see any requirement for overheat protection beyond the usual 3 port diverter and the stock cylinder stats, as it’s an ASHP so really it’s a low temp device / low risk install.

 

It’s only the immersion that could boil your tank afaik, and that’s got its own protection built in.

[JohnMo]

4 hours ago, Nickfromwales said:

Odd to see any requirement for overheat protection beyond the usual 3 port diverter

He seems to be an S plan install, which is even odder on an ASHP 

[Nickfromwales]

4 hours ago, JohnMo said:

He seems to be an S plan install, which is even odder on an ASHP 

I’ve done a few S plan, and prefer the redundancy tbh.

[JohnMo]

45 minutes ago, Nickfromwales said:

I’ve done a few S plan, and prefer the redundancy tbh.

Two normally closed valves or X plan one normally open the other normally closed for PDHW?

[SimonD]

5 hours ago, JohnMo said:

He seems to be an S plan install, which is even odder on an ASHP 

Loads of installs still use an s-plan setup and manufacturers still provide schematics for this with a bypass for over-run with both valves closed. 🤷‍♂️

[Nickfromwales]

25 minutes ago, JohnMo said:

Two normally closed valves or X plan one normally open the other normally closed for PDHW?

MI’s show wiring / plumbing options for most common setups, and I’m a big fan of 2 x 2-port S plan, which can be wired up to do whatever end functionality; they are connected to the HP PCB and the CPU does the thinking.