Five-minute Guide to… Hot Wa­ter Stor­age

We may think that hot wa­ter stor­age is sim­ple, but there’s more to the sub­ject than meets the eye, says Tim Pullen

Homebuilding & Renovating - - Contents -

Do you need a ther­mal store or un­vented cylin­der in your home? There’s more to stor­ing hot wa­ter than meets the eye, as en­ergy-ef­fi­ciency ex­pert Tim Pullen ex­plains

Ap­pear­ances can be de­cep­tive. The ad­vent of more ef­fi­cient boil­ers, mul­ti­ple heat sources and the need to lose as lit­tle heat as pos­si­ble from the cylin­der means that a hot wa­ter stor­age sys­tem is no longer just a cop­per cylin­der in the air­ing cupboard.

Herein lies the prob­lem for the home­owner: the ter­mi­nol­ogy and func­tion­al­ity of dif­fer­ent op­tions. We have sim­ple cylin­ders, di­rect or in­di­rect, com­bi­na­tion cylin­ders, buf­fer tanks, vented and un­vented tanks, ther­mal stores, calori­fiers, ac­cu­mu­la­tors, and most of the above in hor­i­zon­tal ver­sions, too. In some cases, where the boiler is a long way from the bath­room, a buf­fer tank will be used to pre­vent long de­lays in get­ting hot wa­ter to the tap. These are sim­ply a rel­a­tively small tank with no heat­ing ca­pa­bil­ity. When huge vol­umes of hot wa­ter are needed, an ac­cu­mu­la­tor (the same as a calori­fier) will be used. These are like a buf­fer tank, but big, usu­ally many thou­sands of litres.

In re­al­ity there are only two types of cylin­der in com­mon use: the in­di­rect, vented cylin­der or the ther­mal store (both of which are the sub­ject of this ar­ti­cle).

In­di­rect, Vented Cylin­der or Ther­mal Store?

The tra­di­tional hot wa­ter stor­age is an in­di­rect, vented cylin­der — a cop­per or stain­less steel cylin­der with a coil in it that is con­nected to the boiler in a closed loop. Cold wa­ter is sup­plied to the cylin­der from a tank, usu­ally in the loft. The boiler heats the wa­ter in the coil and that, in turn, heats the wa­ter in the cylin­der. This in­di­rect heat­ing pre­vents con­tam­i­na­tion of the wa­ter from the taps. The cylin­der will have a pipe lead­ing to the tank in the loft, so that if the wa­ter in the cylin­der over­heats it can overflow safely to the tank. The wa­ter pres­sure at the tap is de­ter­mined by the dif­fer­ence in height be­tween the tap and the tank in the loft. There are many un­vented op­tions for in­di­rect cylin­ders avail­able. These are typ­i­cally used where wa­ter pres­sure is an is­sue, ei­ther be­cause mains pres­sure is poor or a header tank can­not be in­stalled. These en­sure good pres­sure at the tap by pres­suris­ing the sys­tem. They work well, but come with a main­te­nance over­head as they need an­nual main­te­nance to en­sure they are safe.

A ther­mal store, mean­while, is a means of stor­ing heat. Heat is sup­plied di­rect from the boiler (or an­other heat source, such as a heat pump), which elim­i­nates ef­fi­ciency losses. The ther­mal store typ­i­cally con­tains one or more heat ex­changes (which may be coils or ex­ter­nal flat-plates), which are heated by the sur­round­ing wa­ter.

There are two fur­ther main dif­fer­ences be­tween a tra­di­tional in­di­rect cylin­der and a ther­mal store. The tra­di­tional in­di­rect cylin­der will main­tain the wa­ter in it at broadly the same tem­per­a­ture through­out. A ther­mal store can ‘strat­ify’ — that is, main­tain wa­ter at dif­fer­ent tem­per­a­tures, top to bot­tom. This makes a ther­mal store ideal in a home which re­quires higher tem­per­a­ture out­puts for do­mes­tic hot wa­ter, and lower tem­per­a­ture out­puts for un­der­floor heat­ing, for in­stance. And, while an in­di­rect cylin­der will deal with a sin­gle heat source (plus an im­mer­sion heater), a ther­mal store can deal with mul­ti­ple heat sources (for ex­am­ple, boiler and so­lar pan­els or heat pump and wood­burn­ing stove).

In these sit­u­a­tions, where there are mul­ti­ple heat sources, a ther­mal store comes into its own. A good ther­mal store will se­lect the cheap­est heat source to call on when heat is needed. It be­comes the ‘ heart’ of the sys­tem, manag­ing the sup­ply and dis­tri­bu­tion of heat as re­quired.

Size and Costs

In­di­rect cylin­ders tend to be smaller than ther­mal stores as they are not, pri­mar­ily, stor­ing and manag­ing heat. A ther­mal store will gen­er­ally be in ex­cess of 200litres and in­di­rect cylin­der less than 150litres. The ac­tual size will vary with the house size and the num­ber of oc­cu­pants. It needs to be cal­cu­lated prop­erly by an ex­pert, to en­sure that it is big enough to meet de­mands, but not so big that the heat source can­not get it up to tem­per­a­ture.

An in­di­rect cylin­der will be around £150 to £300, depend­ing on size and qual­ity. A good ther­mal store, like the Chelmer Eco­cat or the Gled­hill Tor­rent, could be over £2,000, again depend­ing on size — re­flect­ing the fact that a ther­mal store is an en­ergy man­age­ment sys­tem, and with­out it your re­new­able en­ergy sys­tem will just not work prop­erly.

Which is Best for Your Home?

In­di­rect cylin­ders have been around for many years do­ing a per­fectly good job. Plenty of sim­ple in­di­rect and mains-pres­sure sys­tems are still sold, and where there is a sin­gle heat source and broadly sim­i­lar heat out­put de­mands (ie ra­di­a­tors and do­mes­tic hot wa­ter), that is the right op­tion. But they are prob­a­bly not best where there is un­der­floor heat­ing and some form of re­new­able en­ergy.

The ad­vent of con­dens­ing boil­ers, the move­ment away from combi boil­ers and the in­creas­ing use of low tem­per­a­ture heat sources like heat pumps means the ther­mal store is be­com­ing in­creas­ingly ne­c­es­sary. Add to that po­ten­tially high tem­per­a­ture in­puts like so­lar ther­mal sys­tems and wood­burn­ing stoves and the low tem­per­a­ture out­put needed by un­der­floor heat­ing, and pre­cise heat­ing man­age­ment be­comes a re­quire­ment. H

Tim Pullen Tim is Home­build­ing & Ren­o­vat­ing’s ex­pert in sus­tain­able build­ing and en­ergy ef­fi­ciency. He is the au­thor of Sim­ply Sus­tain­able Homes.

THER­MAL Store Cylin­der The Eco­cat ther­mal store ( be­low) is de­signed to work with heat pumps and boil­ers and can also be op­ti­mised to work with so­lar ther­mal sys­tems, ranges and wood­burn­ing stoves. Here’s how it works…

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.