Pipe sizing-mains pressure DHW

[oldbutnotdead]

Had a quick search but no answers...
Plan is to use a plate heat exchanger on the thermal store to provide mains pressure hot water to 2 bathrooms, objective is that both bathrooms can be used simultaneously (one is bath and shower, the other is shower only) without massive temperature/pressure changes when one room switches on or off. Bathroom is about 5 metres from the hex, shower room is 6 metres from the bathroom (so 11 pipe metres from hex).
Supply flow/pressure is good (2.8bar at 30 litres/minute). 22mm cold supply to bottom of heat exchanger.
Questions are;
Will 22mm hot to the bathroom (with a prv to the shower/bath/sink leg) then 15mm to the shower room achieve my objective?
Do I need the same arrangement with cold (22mm and prv to bathroom, 15mm onward to shower room)
Do I need a backcheck valve in the cold supply to the hex (I'm assuming yes to this but not certain)
What size expansion vessel will be appropriate for the dhw system (the hex suppliers just said 'a small one'. Is there a % of system volume commonly used- on my plan I'll have dhw tube volume of about 4 litres)
Ta all
PS the thermal store is NOT pressurised so no G3 requirements. Ta

 

[bernardgreen]

Use a heat store with an internal "shower" coil. This method does not require any pumps or flow switch etc etc

Bathroom Cylinder

• bernardgreen
• 25 Jul 2020

 

[oldbutnotdead]

Arr, i could do that but the store i acquired cheap is direct rather than indirect & has the plate hex, pump, switchgear etc already attached.
The same q with pipe sizing would apply to your setup running 2 bathrooms...

 

[bernardgreen]

Bathroom is about 5 metres from the hex, shower room is 6 metres from the bathroom (so 11 pipe metres from hex)

without massive temperature/pressure changes when one room switches on or off.

Fluctuations in pressure will be minimised by having separate pipe runs from heat exchanger, one to the shower room and the second one to the bath room.

When there is a length of pipe that is common to both outlets the pressure drop along that pipe due to flow to one room will reduce the pressure of the supply to the other room .

HEX
|
---common pipe------BATH ROOM----------SHOWER

HEX
|
------------------------BATHROOM
|
-------------------------SHOWER

Using a large diameter pipe for the common run from heat exchanger to bath room will reduce but not eliminate pressure drops along that pipe when water is flowing into the bathroom. Reducing the pressure drop along the common pipe will reduce the pressure fluctuations in the shower room

A large diameter pipe has a larger water volume hence more water will be wasted before hot water arrives in the bathroom.

The resistance to flow through the heat exchanger will be common to both bathroom and shower room outlets and will result in flow to one room affecting the flow to the other room.

The heat transfer capacity of the heat exchanger will also need to be large enough for the heat requirement of both rooms to be met otherwise temperatures will drop when water flow through the heat exchanger increases.

 

[oldbutnotdead]

Ta for that lot @bernardgreen .
You've kind of mirrored my choices there. If I run separate supplies to the 2 rooms the bathroom (presenting 6m resistance to flow) will receive better flow than the shower room (11 metres).
To balance the rooms i'll need a prv at the bathroom.
My thinking re 22mm from hex to bathroom was to minimize the pressure drop in that section, volume per metre of 22mm is about double that of 15mm so the 22mm section should be able to sustain 2 x 15mm outlets at the same pressure/flow (the pressure/flow available at the end of the 22mm)?. The prv at the bathroom is to balance the extra 6m run to the shower room, theoretically keeping the demand from both rooms within what can be supplied by the 22mm. And yes re the water waste in the 22 but it is 1.9 litres as against 1 litre in equivalent run of 15 so not the crime of the century.
I know the only way to completely eliminate pressure variations would be by using tanked cold, indirect cylinder, separate drops from tank and cylinder to each room and pumps in each run (low roof here so not much head on the tank). What i'm trying to achieve is the best result with what i've got- so acceptable rather than dramatic pressure drops if that makes sense

 

[oldbutnotdead]

Addendum- logically whatever i do with hot mains i'll need to mirror with the cold. Makes sense.....one question answered (tho there's no pressure drop from hex to worry about. Might do some experiments this afternoon, think i've got enough pushfit to do something bodgetastic

 

[picasso]

Don't use 22mm pipe, do everything off a manifold with 15mm to bath and showers and 10mm to wash hand basins and toilets.

 

[oldbutnotdead]

Don't use 22mm pipe, do everything off a manifold with 15mm to bath and showers and 10mm to wash hand basins and toilets.

That's fairly straightforward, ta. Cold as well? (Yr answer implies that unless you have a posh hot flushing loo )
Will i hit problems if i have about 1500mm of 22 out of the hex- that'll get the manifold up into the (accessible) roof void where the rest of the pipework is. Or would 28 work better (still got the bender & enough tube to do that)

 

[picasso]

Nah 22mm from the heat ex to the manifold then 15 and/or 10 to the fixtures, same with the cold.

 

[oldbutnotdead]

Perfect. Glad my neighbours are out, time for some SDS action . Ta both

EDIT And finally a use for a roll of 12mm i bought about 29 years ago, only needed 2 metres (this was pre me having a decent drill & tubing a rad through 3 skins of solid brick), came in 10 metre coil