Converting from a combi to a unvented cylinder

OK taking the plunge to change from a Combi to a new system boiler and unvented tank.

Based on what I've seen on here a 210 litre tank will be sufficient. We have one bathroom (bath and separate shower cubicle) and an ensuite which is about to be refitted with a shower with those body jets hence the "upgrade". 2 adults and 2 children so highly likely both showers in use at the same time. Mains pressure I'm told is good at 1.5 bar.

According to the calculations the house needs just over 16kw for the heating.

I have two areas of question about changing system types:

How do you know what size boiler to go for? I know the heating requirement but presumably if I go for one with the biggest output I can afford then the reheat time on the cylinder would reduce. My theory is that the boiler would modulate down for the CH demand and modulate up for the DHW as and when necessary.

Also obviously I'm used to a combi supplying the DHW and this move means getting used to setting the timing for the DHW. Are there any simple rules to follow? Is it as simple as setting it for an hour in the morning and a hour in the evening? What happens if one of the kids decides to have an unplanned shower? I want to avoid not having hot water available when I want it, or is that the trade off for for moving from Combi to unvented?

I know the unvented installation isn't a job for me and requires the right qualifications. Just want to get a level of understanding before talking to CH Engineers.

War and Peace over!

Your views / experience greatly appreciated

Why bother getting a new boiler? the combi can heat the unvented cylinder easily enough.

The boiler is 13 years old and really needs to be changed. New boiler with greater efficiency helping to offset cost of the new system is another of my theories!

Size of boiler can be calculated using the "Whole of house" method. You can find the formula in John Prescots old departmental website.

I would assume that the unvented cylinder is high recovery and can leech up to about 24kW of heat, so the cylinders demand may be the deciding factor when choosing the boiler size.

How about an hour before you get up in the morning and an hour later in the afternoon? as soon as the desired temperture is reached the boiler will shut down anyway so it won't cost you anything.

Depending on the model it may well be cheaper and more efficient to run if you have an 18 kW boiler.

The power input into most cylinders is only at a maximum when the water is totally cold and fairly quickly reduces as it starts to warm up.

The mains water pressure is irrelevant! You need to measure the flow before buying an unvented. I always say an open pipe flow of 22 li/min is the minimum.

Tony Glazier