Another go at printing the reservoir, this time using Spiral Vase mode with increased extrusion rate and lower printing speed to keep within the extruding capability.  At last success   Watertight!  Went on to drill holes for inlet and outlet pipes 3D printed) and glue them in.  Yes, that's water in there

Cooling fan has arrived.  Now to change the Duet box design to take it.

I shall try gravity circulation for the cooling water and to tell how well it's working I shall use a thermistor in a hole in the heatsink and connect it to the temperature on the second extruder system (only using one extruder and hotend so the second is not being used and can instead of the usual function can show the heatsink temperature).  I've drilled a hole in the heatsink block and inserted a standard thermistor helped by some thermal grease and held in place be a bit of scrap extrusion.  Extended the wires and terminated in a 2 pin connector to match the Duet board connector.

I now have the printer powered up and everything connected.  The main 24v 15A PSU fan is well audible (and I can see why @tekkydave added a manual control to make his quieter).  I added code for the new thermistor to the Heaters section in config.g plus another Tool in the Tools section and now have the heatsink temperature displayed   Currently there is no water in the cooling system and the screenshot below shows the heatsink warming up with heating the hotend.  Initially the heatsink was at about 23°C.  Tool 0 is the normal hotend heater and thermistor, Tool2 is the heatsink (no heater connected) and the third item is the Bed.  The Temperature Chart shows the three temperatures graphically (Red, Green, Blue).

Here is the config.g file :-

; Configuration file for Duet WiFi (firmware version 1.20 or newer)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool on Wed Jan 31 2018 19:48:21 GMT+0000 (GMT)

; General preferences
M111 S0                                  ; Debugging off
G21                                      ; Work in millimetres
G90                                      ; Send absolute coordinates...
M83                                      ; ...but relative extruder moves
M555 P1                                  ; Set firmware compatibility to look like RepRapFirmare
; Automatic saving after power loss is not enabled

M667 S1                                  ; Select CoreXY mode
M208 X0 Y0 Z0 S1                         ; Set axis minima
M208 X450 Y450 Z800 S0                   ; Set axis maxima

; Endstops
M574 X1 Y1 Z1 S0                         ; Set active high endstops
M558 P4 H5 F200 T9000 I1                 ; Set Z probe type to unmodulated and the dive height + speeds
G31 P500 X40 Y0 Z1.23                    ; Set Z probe trigger value, offset and trigger height
M557 X50:450 Y20:400 S200:190            ; Define mesh grid

; Drives
M667 S1                                  ; Select CoreXY mode
M569 P0 S1                               ; Drive 0 goes forwards
M569 P1 S0                               ; Drive 1 goes forwards
M569 P2 S1                               ; Drive 2 goes forwards
M569 P3 S0                               ; Drive 3 goes forwards
M350 X16 Y16 Z16 E16 I1                  ; Configure microstepping with interpolation
M92 X80 Y80 Z3200 E418.5                 ; Set steps per mm
M566 X900 Y900 Z12 E120                  ; Set maximum instantaneous speed changes (mm/min)
M203 X6000 Y6000 Z600 E1200              ; Set maximum speeds (mm/min)
M201 X500 Y500 Z250 E250                 ; Set accelerations (mm/s^2)
M906 X800 Y800 Z2400 E1500 I30             ; Set motor currents (mA) and motor idle factor in per cent
M84 S30                                  ; Set idle timeout

; Heaters
M305 P0 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 0
M143 H0 S150                             ; Set temperature limit for heater 0 to 150C
M305 P1 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 1
M305 P2 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 2

M143 H1 S301                             ; Set temperature limit for heater 1 to 300C
M106 P1 T45 H1

; Tools
M563 P0 D0 H1                            ; Define tool 0
M563 P1 D1 H2                            ; Define tool 1  -  Currently used just to show heatsink temperature
G10 P0 X0 Y0 Z0                          ; Set tool 0 axis offsets
G10 P0 R0 S0                             ; Set initial tool 0 active and standby temperatures to 0C

; Network
M550 PGinaRep Giant                      ; Set machine name
M552 S1                                  ; Enable network
M586 P0 S1                               ; Enable HTTP
M586 P1 S0                               ; Disable FTP
M586 P2 S0                               ; Disable Telnet

; Fans
M106 P0 S0.3 I0 F500 H-1                 ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H1 T45                ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on
M106 P2 S1 I0 F500 H1 T45                ; Set fan 2 value, PWM signal inversion and frequency. Thermostatic control is turned on

; Custom settings are not configured

Looking good.  Here are some screenshots :-

1. Starting point with temperatures nearly back down to ambient.
2. After 5m with hotend set for 260°C but not extruding, cooling system filled with water but not enough for circulation.  Heatsink warming up.
3. After 20m with cooling system full of water and purged of air.
4. After 25m and heatsink temperature holding at just under 43°C.  This is well below the glass temperature of any known filament.

Heatsink temperature is still holding at 42.8°C to 43°C after 30m.  I conclude that the cooling water is circulating purely by gravity.  Q.E.D.

Now printing a spool holder for this printer while borrowing the one from my Titan in the meantime.  Using PLA.

A couple of photos of the top of the printer.  I still have a front for the Duet box to design and print.

I think the spool holder print is probably usable if not very good - poor perimeters and lots of whiskers.  Still needs a fair bit of tweaking!

Printed the Duet box front and fitted it.

With the front on the Duet box, the fan is blowing lots of air across the back and front of the board and it's the quietest fan I've ever heard (or not heard ).  Now if I could replace the 24v PSU fan with one of those the printer would be practically silent except for the occasional sound of the belt teeth on the pulleys of the Z drive.  I've just ordered another as I can think of several uses.

Just run a test print in Spiral Vase mode and it's printing reasonably well - maybe slightly better than the same gcode last time.  Cooling working really well.  These screenshots show the temperatures before printing and when nearly finished.

Definitely better than the last print but still not perfect.

Test print - Spiral Vase mode, bowl with 45° walls, 100mm diameter base, 200mm top and 50mm deep.  Printed in translucent yellow PETG.  These photos show that it holds water unlike my previous test prints.

Vase printing on Giant 3D printer.

Finished.  Vase is about 140mm at it's widest and nearly 340mm high.  Printed in yellow PETG.  The rails are not actually bent - it's the wide angle lens!

Printing another, much bigger vase.

Ran out of filament and couldn't get the new to follow on so I'll probably start again with a new reel.  So far the printing is nearly perfect.  This printer definitely works best with large prints.  Well, it's what it was built for

I've ordered another of those ultra-quiet 80mm PC case fans to use for the 24v PSU to replace the 60mm very noisy one.  Not only will it stop the noise but is also a much more efficient fan.  OK so it needs to go on the outside instead of inside the case but that's no problem. 

Another possibility might be to use one fan to cool both the Duet board and the PSU by putting both in the same casing (3D printed, of course!).

Been thinking about what is needed to complete this printer.

1. Method of lifting/holding water tubes and cables bundle to X carriage as Z rises towards top.
2. Add acrylic panels for back and sides.
3. Front door frame.
4. Front door.
5. Tidy and secure wiring.
6. Fume extraction.
7. Quieter fan for 24V PSU and new case top.  (Nice but not essential.)

SketchUp model of door and frame.

Designed a new, smaller cover for the end of the 24v PSU with SSR and buck converter and also a new top to accommodate the bigger ultra-quiet fan.

Attached the back acrylic panel - 18 M5 screws.  Was very careful to preserve alignment and orthogonality.

Left side panel, new front wider rail and top rail added.  The printer is laying on its right side.