Hi @Topaz,
Your lowest cost, greenest, solution (though the one least likely to appeal to you and your family) may be to make better use of your existing setup. Ask yourself:
- Do all 5 of us have to shower every day? (E.g. would alternate days, with 3 showering on odd dates and 2 showering on even dates, be acceptable from a hygiene perspective?)
- If we must all shower every day, can I spread the demand? (E.g. 2 in the morning, 3 in the evening, to give the HP more time to re-warm the water.)
- Can the shower water requirement be reduced? (E.g. lower flow rate, shorter showers, pausing the flow whilst soaping down or shampooing.)
- Can I use less tank water by heating it a few degC warmer, then blending with cold water at the shower mixer? The cost of this extra energy – even at a lower CoP – is likely to be peanuts compared with the cost and disruption of installing a larger tank.
- Can I heat the tank to the required temperature at a lower LWT by heating it more slowly? This depends on your HP controller, but in many, LWT is not constrained during DHW heating (unlike space heating where weather compensation limits it). You can see very high LWTs – 70degC or more, at a lousy CoP – especially if the tank coil is undersized. You may be able to limit this with tricks like using Quiet mode if available, or heating your tank in 2 or more short bursts, rather than letting rip (see Daikon Altherma 3, sub 2 DHW COP - #10 by SarahH for further discussion).
Let us know what you decide…