How to system boilers heat water already in the cylinder?

[jimmyhorns]

I have also posted this in the screwfix forum, so apologies if you look at both!

 

I'm currently in the process of mapping out how and when our current heating and hot water installation gets changed over to the new system as part of a house extension.

Currently we have a regular boiler, feed and expansion tanks for both the central heating circulating system and the conventional vented hot water cylinder.

We are moving to a system boiler, with an unvented cylinder.


I understand that with a system boiler, the water that is sent to the unvented cylinder is heated on its way there by the system boiler.

What happens once it has gone cold in the cylinder? (is there still a loop with a 3 way diverter valve taken off the central heating circuit like the current Y plan design.....? ) Surely it cant rely on the water having been heated in the past and not getting too cold?....

Thanks

James

[PeterW]

20 minutes ago, jimmyhorns said:

Surely it cant rely on the water having been heated in the past and not getting too cold?....

Yes - UVC will hold it’s heat for 24 hours 

 

With a system boiler and a UVC install, the things you will lose are the feed and header tanks in the attic, and the things you will gain are expansion vessel on the mains to the UVC, filling loop from mains to boiler circuit, and potentially an expansion vessel on the boiler / heating circuit if the boiler doesn’t have one built in. Everything else stays the same. 

[jimmyhorns]

47 minutes ago, PeterW said:

Yes - UVC will hold it’s heat for 24 hours 

 

With a system boiler and a UVC install, the things you will lose are the feed and header tanks in the attic, and the things you will gain are expansion vessel on the mains to the UVC, filling loop from mains to boiler circuit, and potentially an expansion vessel on the boiler / heating circuit if the boiler doesn’t have one built in. Everything else stays the same. 

 

Thanks for explaining - most appreciated.

 

So effectively the UVC just 'slots' in place of the normal cylinder (with an expansion vessel of course)?

[Temp]

14 hours ago, jimmyhorns said:

I understand that with a system boiler, the water that is sent to the unvented cylinder is heated on its way there by the system boiler.

What happens once it has gone cold in the cylinder? (is there still a loop with a 3 way diverter valve taken off the central heating circuit like the current Y plan design.....? ) 

 

Essentially yes.

 

Sorry if this is obvious but... The cylinder is always full of water. The boiler can't send more water to it unless there is somewhere for the water already in it to go, and that's back to the boiler. Eg there is a closed loop similar to what you currently have.

 

The difference  between a regular/vented and unvented/system set up is explained here..

https://www.plumbnation.co.uk/blog/the-difference-between-vented-and-unvented-cylinders/

 

 

 

 

 

 

[MJNewton]

21 hours ago, jimmyhorns said:

We are moving to a system boiler, with an unvented cylinderI understand that with a system boiler, the water that is sent to the unvented cylinder is heated on its way there by the system boiler.

 

When you say 'the water' which water do you have in mind? If it's the hot water that comes of the tap (known as Domestic Hot Water (DHW)) that is *not* heated directly by the boiler. The DHW sits in the cylinder and is heated indirectly via a coil of pipe through which water heated by the boiler is circulated. It's the same water that's circulated through the radiators, and indeed it is essentially a radiator submerged inside the cylinder. 

 

Apologies if I've completely misunderstood your post and you knew all this! 

Edited December 12, 2020 by MJNewton