PRV valve on heat pump install

[blankton]

I am currently fitting a Grant 6Kw heat pump. According to the schematic, there should be a 3 bar PRV and tundish on the primary heating circuit. Obviously there is one on the unvented cylinder and the reducing valve. However, this is showing one on the primary circuit. 

 

The exploded diagram / schematic of the heat pump shows an internal PRV as well. 

 

Why two? 

[JohnMo]

Just had a look at the Chofu manual (original manufacturers) only the expansion vessel shown on the heating circuit, as you say the rest is internal on the ASHP. Sound like a bit of cut and paste engineering from Grant.

 

 

I would install per Chofu schematics and use the controller that Grant hide away as the controller for everything.

[crispy_wafer]

I'm working to this but removing the volumiser and extra pumps.  Don't think it could get much simpler.

[JamesPa]

13 hours ago, blankton said:

I am currently fitting a Grant 6Kw heat pump. According to the schematic, there should be a 3 bar PRV and tundish on the primary heating circuit. Obviously there is one on the unvented cylinder and the reducing valve. However, this is showing one on the primary circuit. 

 

 

13 hours ago, blankton said:

The exploded diagram / schematic of the heat pump shows an internal PRV as well. 

 

Why two? 

Two because the two circuits are totally separate.

 

The reason for the one on the primary circuit is to prevent overpressure due for example to forgetting to turn off the filling loop properly.  I actually did that with my old boiler (where the PRV was) isolated.  It blew a radiator quite spectacularly, fortunately it didn't blow anything under the floor otherwise it would have been an expensive mistake.

 

If your mains pressure is low then its perhaps less important although a fault in the immersion could conceivably cause boiling in the primary (through heat transfer to coil) so there is (just) a way to get overpressure even with low mains water pressure, albeit very unlikely.

 

If there is one in the heat pump then I cant see you need another.

Edited Tuesday at 11:26 by JamesPa

[sharpener]

3 hours ago, JamesPa said:

If there is one in the heat pump then I cant see you need another.

 

Many (?most) HP installations include isolating valves for the outdoor unit in case it needs to be drained down or even swapped out. So better if that is not the only place a PRV is fitted on the primary side. I think from various pix I have seen it is usually sited with the filling loop and pressure gauge. Typically all teed into the return pipework before it goes back out to the HP via the filter, which should be the last thing indoors.

 

 

Edited Tuesday at 14:50 by sharpener

[JamesPa]

 

20 hours ago, sharpener said:

 

Many (?most) HP installations include isolating valves for the outdoor unit in case it needs to be drained down or even swapped out. So better if that is not the only place a PRV is fitted on the primary side. I think from various pix I have seen it is usually sited with the filling loop and pressure gauge. Typically all teed into the return pipework before it goes back out to the HP via the filter, which should be the last thing indoors.

 

 

fair point, particularly given my experience referred to above with my gas boiler, albeit down to my stupidity!

[blankton]

I'll post up the schematic later. 

 

The filling loop comes off the cold after it's already gone through the 3 bar reducing valve (that they also stipulate), so I'm limited on what pressure I can achieve via overfilling. 

 

I appreciate that if the external isolating valves were shut off to the hp, I'd then be isolated from the prv within the hp. But I'd also be isolated from the source of heat 😂. We would then be down the road of sharpeners suggestion that the immersion boils the cylinder and transfers heat to the coil, but the length of time this would take, would need the cylinder start on the immersion to have failed. 

 

It all seems highly unlikely, but I guess I better fit it to comply with the manufacturers instructions. It's just another series of pipework joints and a component that will need maintainance.  I plan to fit it next to the cylinder (on the return), it can have its own tundish, but share the 28mm drain pipe to outside that the cylinder prv would use. The convinient place that it is shown on the manufacturers schematic, (on inside wall next to hp), isn't so convinient in real life.