FYI, I emailed Triton to get their thoughts on the rig up after all the comments. This is their response.
Thank you for your recent enquiry.
Potentially there is no problem with the arrangement you have suggested.
The key point is to ensure that the pump chosen is capable of delivering the minimum flow and pressure to the T300si Shower unit. You would also need to ensure that you have the minimum site conditions for the pump with regards to the min head and storage capacity.
I hope that this information is of help.
With regards the noise, I will look to encase the unit the best i can with rockwool and an antivibration matt to make sure noise is kept to a minimum.
Some possible options with an electric shower supplied via a separate pump:
Most electric showers require a minimum pressure before the solenoid opens the valve to allow water to flow.
Most pumps are activated by water flow, so the only pressure available before the pump starts is that due to gravity.
Result - shower will not allow water to flow due to no pressure. Pump cannot be started due to no water flowing. End of line.
Negative head pumps keep the output under pressure all the time, so the shower would allow water to flow. However those pumps are activated by the output pressure dropping, which the electric shower may detect as a fault and switch off.
And if you've got a hot water cylinder already then do what someone else has already suggested & put a power shower motor on tanked hot and tanked cold. Electrically heated showers are at best disappointing
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