new-fangled "thermal stores"

[richardclay09]

I was all set to replace the gravity-fed HW system in the house I am about to move in to with a megaflo (or some other unvented system), but research on the net on "mains pressure hot water" brings up lots of mentions of thermal stores (aka heat banks), and these would appear to be a recent development, with many advantages over megaflos (as described in

these are so good, and a recent development, have they made megaflos obsolete (at least for domestic use)? Would somebody like to argue the case for a megaflo and against a thermal store?

[John Stumbles]

For a system with a sealed primary circuit (as is usual with new boiler installations these days) an unvented cylinder only involves one heat exchanger between the primary (boiler) circuit and the DHW, whereas a thermal store system requires two (one from primary to thermal store, another from thermal store to DHW). This results in higher temperature drops or bigger, more expensive heat exchangers. Bigger temperature drops is worse news for high efficiency (condensing) boilers which work more efficiently at lower return temperatures.

However this difference is slight in the overall context of domestic Ch & DHW systems, and the choice for or against unvented is usually based on other factors such as the specifier's or installer's familiarity with one or other technology.

[richard]

Contrary to John's note, in my experience the water in the thermal store is part of the primary circuit and is heated directly by the boiler. Claimed advantages include the boiler being fully loaded while heating the store.

[John Stumbles]

In the bit of my post that you clipped (i.e. all of it) I said; "For a system with a sealed primary circuit...". What you're describing is a system with a vented primary. Some thermal stores are like this, including the one I have built for myself. However since CH installations increasingly tend to have sealed primary circuits these days thermal stores for use with them have to have two heat exchangers.

This is less of an advantage with modern boilers which can modulate their outputs to match the demand so the use of a thermal store as an efficiency-improving buffer between the boiler and the heating system is perhaps less relevant.

However re-heat times with vented primary systems are very good and are IMO a good argument for such systems in the right situations.