Dumb question about cisterns

[Manyanga]

How can combination cylinders get away with just 20 or 40 litres as supply to hot water in the cylinder, and yet cold/hot cisterns in the roof space have to be 230 litres and more??

So if I have a combination cylinder, I get away with 20 litres. But if I put a separate tank just a foot above it in the airing cupboard, it needs to be bigger than the cylinder!

I know I am missing something! Tell me what.

 

[Agile]

You are missing the fact that the combi cylinder is a cheap and space saving solution for small flats.

They only work if the refill rate exceeds the usage rate!

Anyone with a loft has space for a far better storage and the cost of a large cistern is not very high but still stores enough water for a full bath even without any addition during useage.

Tony

 

[Manyanga]

Thanks - you are quick as well as Agile. I understand that mains-in has to match hot and cold water out. That would explain why the regulations require a 230 litre cistern. But in that case, why don't they also require a greater cold store in a combination unit? If I had low mains pressure/flow and I put a 3 bar pump on the hot supply from the cylinder, I would empty it.....

I can see that combination cylinders are a solution in flats, but they are advertised now as a way of achieving a 'dry' attic.

 

[Roadyz]

Thanks - you are quick as well as Agile. I understand that mains-in has to match hot and cold water out. That would explain why the regulations require a 230 litre cistern. But in that case, why don't they also require a greater cold store in a combination unit? If I had low mains pressure/flow and I put a 3 bar pump on the hot supply from the cylinder, I would empty it.....

I can see that combination cylinders are a solution in flats, but they are advertised now as a way of achieving a 'dry' attic.

or mabe use a combi boiler?

 

[Roadyz]

Thanks - you are quick as well as Agile. I understand that mains-in has to match hot and cold water out. That would explain why the regulations require a 230 litre cistern. But in that case, why don't they also require a greater cold store in a combination unit? If I had low mains pressure/flow and I put a 3 bar pump on the hot supply from the cylinder, I would empty it.....

I can see that combination cylinders are a solution in flats, but they are advertised now as a way of achieving a 'dry' attic.

or mabe use a combi boiler?