accumulator

A plumber suggested the addition of an accumulator to improve the pressure of the domestic hot water when showering, there are two showers.
The boiler is a WB Heatsalve 12/14 Combi, max flow rate is 15l/min.
Shower 1 is a Mira excel
Shower 2 is a Hansgrohe ecostat S
Measured cold water pressure is 1.9 bar.
15m copper pipe.

How do you calculate accumulator sizing?

I have read that only 50% of capacity of an accumulator is available.

Don't think an accumulator is going to work well on the dhw side of the combi- a limiting factor is how many litres per minute can the combi heat.
Have you tested the hot water flow rate with a cold tap on full as well (it might drop, it might not).
And have you tested cold water flow rate (ideally at an outside tap and measure the pressure at the same time so you get dynamic rather than static pressure)

Do you mean pressure or quantity? The pressure of supply on this combi should be equivalent to the cold pressure, but a scaled DHW heat exchanger will reduce flow rate which can appear to be low pressure. Test as above.

Thanks for your help.
I spoke to a boiler engineer who advised to use an unvented cylinder by the side of the boiler to heat the hot water and set the boiler to heat the CH and cylinder, or just one shower and retain the combi facility.
The hot water flow rate of the boiler is not sufficient to heat two showers.

jackg said:

Thanks for your help.
I spoke to a boiler engineer who advised to use an unvented cylinder by the side of the boiler to heat the hot water and set the boiler to heat the CH and cylinder, or just one shower and retain the combi facility.
The hot water flow rate of the boiler is not sufficient to heat two showers.

Click to expand...

Yeah thought that might be the case. Just be wary- the combi and the unvented cylinder will both be relying on mains pressure and flow to deliver simultaneous showers (By the way, I'd suggest combi for the shower nearest the boiler & cylinder for the other one) so worth checking that your water supply is up to the job.
Useful test- run one shower at good shower temperature & flow rate (whatever you think is adequate here, if anything like my combi then full hot and a bit of cold) & measure flow rate (bucket, stopwatch) then turn the other shower as well (on cold only with a good flow rate) and check delivery rates of both. If the hot shower flow rate drops significantly then you may have to explore other options (is your supply from the street lead? Any scope to increase the size of the cold water pipe- unvented cylinder will probably need 22mm as a minimum to give you any worthwhile gains)