Megaflo & Unvented Systems

What actually decides the cylinder size

Not the number of bedrooms, which is the shorthand most quotes use. Two things decide it: peak simultaneous demand, and reheat time. Peak demand is what happens on the worst morning — two showers and a basin at once — measured in litres, not in optimism. Reheat time is how quickly the boiler can put that heat back through the cylinder's coil, and it depends on the coil surface area and the boiler output, which is why an undersized boiler makes even a large cylinder disappointing on a busy morning.

The third number is the one that caps everything: the flow rate of your incoming water main. An unvented cylinder delivers hot water at mains pressure, so it delivers exactly what the main can bring in and no more. If your main gives twelve litres a minute, no cylinder on the market gives you a twenty-litre shower. That is measured at the survey, and if it comes back low the honest conversation is about an accumulator or a pump, not about a bigger cylinder.

The legal side: this is notifiable work, and not every plumber may do it

An unvented hot water cylinder stores a large volume of water above atmospheric pressure. Installing, servicing or replacing one requires a specific competence — the G3 qualification, named for the part of the Building Regulations that covers it — and the work is notifiable to building control. A plumber who is not G3 qualified may not lawfully install or service one, whatever else they are qualified for.

The safety chain is a sequence: an expansion vessel to absorb the water's expansion as it heats, a temperature and pressure relief valve, and a discharge route through a tundish to somewhere the discharge can be seen and can safely go. That discharge pipe has rules about its size, its fall and where it may terminate, and it is the part most often got wrong on a cheap installation — a discharge that ends somewhere nobody would notice defeats the point of having it.

Unvented cylinders also need an annual check: the expansion vessel loses its charge over time, and a flat vessel is the usual reason a system starts discharging water out of the tundish for no apparent reason.

The secondary return, and why the shower runs cold for twenty seconds

In a large house the cylinder may be a long way from the furthest bathroom, and every metre of pipe between them holds water that has gone cold since the last time anyone ran the tap. That is the delay, and it wastes both water and patience. A secondary return circulates hot water in a loop so it is present at the outlet, at the cost of a small pump and a little standing heat loss.

It is worth it in a four-storey house with an en-suite at the top and pointless in a flat where nothing is more than a few metres from the cylinder. Deciding which you have is part of the design, and it is far cheaper to install with the rest of the pipework than to add later.