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We currently have a 4 bedroom converted bungalow which has 2 showers a bath and 10 radiators. We are planning to alter it so we have 5 bedrooms, 3 showers a bath and 12 radiators. At the moment we have a 24Kw Worcester combi boiler which we do have pressure issues with sometimes.
One builder who has quoted for the work has said that a pressurised system with a tank needs installing as even a bigger combi boiler could give pressure issues. Another builder said that a bigger combi would be ok.
I am not sure what to believe. Obviously just replacing our existing boiler would be the cheapest option but whatever we go for I want it to be right.

Any advice gratefully received.

I have two baths, two showers, and 14 rooms, and 15 radiators. All powered from a 24 kW oil fired system boiler and I have no problem keeping the house warm.

But to fill a bath takes around 30 gallons of water, you can do the calculations if you want, but I would say it takes around 40 minutes to reheat the water once a bath is run.

I have two showers, both electric never ever tried to use both at the same time, I could run two showers from the stored water, but they would need to be power showers to get the pressure.

Combi boiler with gas is different to oil, really the oil one is not a combi, it just has the storage tank built into the boiler. With gas the boiler can modulate, (turn down) but you are unlikely to be able to run two showers together. As to a bath, it will likely take around 40 minutes to run the bath, that was my problem in the last house, not combi but did have a direct heating boiler well two boilers one for central heating and one for DHW.

The problem is the bigger the boilers maximum output, also the bigger is the boilers minimum output. So getting an over sized combi to supply the DHW may mean it is too big for the central heating.

Oil boilers turn on/off, simple, but gas boiler turn up/down, the idea is to gain the latent heat from the flue gases, so over sized means it has to switch off/on as can't turn down low enough. So with that size house likely you will want a storage tank.

There are a few methods, mains pressure tanks need annual testing, but mains pressure hot coils inside the tank do not, so you can get mains pressure hot water, with a storage tank without annual testing using some options.

You can also use water tanks to store water for central heating, so you can use solid fuel, solar panels and gas all to heat the DHW and run CH, my brother-in-law had this, and when he visited his children in Germany his Welsh home stayed at 14ºC without costing him a penny, and would heat to 20ºC using geofencing before he got home using the stored hot water to heat it.

However it was not cheap to install.

This house I use programmable TRV heads for fast re-heat, it heats main rooms first, then bed rooms once main rooms warm.

The cost to install some of the systems is silly, so it is often a compromise. And we call the guys who install it all, heating and ventilation engineers, and engineer means to me University trained. Some of the systems are rather complex, I am an engineer but not heating and ventilating.

My heating has faults, but to cure the faults, is simply not worth the money. One has to save a lot of fuel to pay for £1000 worth of control, so most systems are a compromise, but one way or another I would guess you need a hot water store.