ACV Smartline tank-in-tank cylinder technology question?

5 Sep 2008
Reaction score
United Kingdom
This seems an interesting idea but does it work well in practise?

From what I can gather the boiler heats a secondary tank in which is immersed a smaller primary water tank thus providing a larger surface area to heat than a coil type cylinder – according to ACV this means faster re-heat times and can also allow a good continuous flow of DHW?

1, Is this (much) better than say a full Vaillant or similar unvented cylinder system and is it a cost effective form of supplying good DHW?

2, Are these cylinders relatively small for their performance compared to other cylinders?

Any thoughts appreciated

Sponsored Links
This seems an interesting idea but does it work well in practise?
They aren't foolproof like a cylinder with a coil. If the two side aren't depressurised correctly, they can be damaged meaning a complete replacement. I think Doitall has talked about this before.
They work very well and can take 40-50Kw off a boiler during the reheat time.

A Vaillant or similar unvented would only be able to lose a fraction of this into the heat exchanger.

We use them where there is limited space, the ACV will store water at 80C and has a faster recharge time, so the latent heat stored is greater for the volume, and the recharge is shorter.

It is true that you have to be careful draining down, but this doesn't mean the product is no good, just different.
Tks for your replies.

It does seem a way of having a good performing cylinder without loosing the whole airing cupboard!
Sponsored Links
Some of them have CH flow and return connections and the CH loop can be run off the primary side of the cylinder.

They are a hybrid of thermal store + cylinder. The DHW cylinder can be unvented or vented. They do not need to be equal pressure. Well not that I am aware of. Best check.

They do reheat quickly for sure. I can't see the need for them when a heat bank does the same with a plate heat exchanger that can be easily exchanged, or cleaned if scaled up. Heat bank give more usable water from cold in a very quick time. Heat banks are cheaper for what they offer, and no G3.
If you have a pressurised boiler system you must lower this before emptying the inner tank.

The pressures do not have to equalised but damage will occur if they are too diverse.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local