"Ed Sirett" wrote in message
news
Thanks to all - this group may have saved me from making an expensive
mistake!
I'd heard of heat banks but didn't know
what they were. They look the ideal
solution, what's the downside?
In the unvented cylinder the boiler heats the
HW indirectly (just like a
vented conventional copper cylinder).
With a thermal store the boiler directly
heats all the water in the
cylinder.
Some, e.g., the DPS Pandora and my Systemate heat the store indirectly.
Some are actually pressurised, like an unvented cylinder, but these are
rare.
HW is produce when mains cold flows
through a heat exchanger in the
cylinder and comes out hot. Boilermate is
a brand I know of.
Most now have an external plate heat exchanger.
There is not a lot to choose between the methods on cost.
Pros for store [heat bank]:
Can be legally fitted without a building notice by uncertified installers.
Can be fitted with extra heat exchangers to provide lower temp water for
underfloor heating or to connect to a solar panel.
Cons for store [heat bank]:
Probably doubles the ammount of primary circuit water,
That is not a con?
so you will need a larger or additional expansion vessel and two lots of
corrosion inhibitor and any other chemicals as needed.
Larger expansion vessel? only if the whole cylinder is pressurised, which is
rare. With the Pandora and Systemate having their own heat transfer coils,
this is not a problem. It is also not a problem with an open vented heat
bank too. It is true that a couple of 1 litre cans of inhibitor has to be
used in the heat banks stored water. Which is a "very" minor cost not worth
considering.
A better assessement:
Advantages of Heat-Banks
§ Instant high pressure hot water - When the thermal-store has
reached the set temperature, water is delivered instantly at the taps.
§ Usable hot water in a matter of minutes - when using a boiler
flow/return blending valve.
§ Very high water flow-rate - The high-end heat-banks have a
flow-rate up to 45 litres/min.
§ Operates on low incoming pressures - Requires a supply pressure of
1 bar to obtain a good flow-rate.
§ Long efficient boiler burn - Reduces boiler on-off cycling
increasing efficiency. Inefficient boiler cycling is no longer a major
problem with boilers with forced flues.
§ Maintains optimum boiler temperature range - If a blending valve
on the boilers flow and return is used, it will deliver only the required
high temperature to the heat banks, but also maintain the flow/return
temperature differential.
§ Combines the output of the stored water and the boiler - Some
versions can do this.
§ Cylinder may be smaller for a similar performance
§ Cylinder at low pressure - Unlike an unvented cylinder it does not
store water at high pressures. Although some version do.
§ Fast cylinder recovery rate - When the boiler is connected
directly to the heat-bank or an indirect coil, the recovery rate is rapid.
§ Legionella bacteria eliminated - The Legionella bacteria cannot
survive in the high temperature sealed conditions of a heat-bank.
§ No scale build-up in heat-bank - Containing primary and not
secondary fresh water, scale does not build-up inside the heat-bank.
§ Cold water storage eliminated - No need for cold water storage
tanks.
§ Solar heating storage - Water heated via solar panels may be
stored in the heat-bank via a solar coil.
§ Easy maintenance - If an external plate heat exchanger requires
cleaning or replacing it is a matter of draining down the heat-bank, or
closing isolating vales, and unscrewing the plate heat-exchanger. In some
rare instances plate heat-exchangers are fitted directly inside the
heat-bank preventing on-site maintenance.
§ Easy to improve hot water flowrates - By simply adding additional
plate heat-exchangers in parallel, hot water flow rates may be improved.
Retrofit additions are possible if extra bathrooms or showers are installed.
§ Stored water vessel need not be cylindrical - As no internal coil
is used for hot water heat transfer the stored water vessel may be any
shape, as opposed to a thermal store which has to be cylindrical for maximum
efficiency. This has advantages where space is limited, promoting excellent
packaging.
Disadvantages of Heat-Banks
§ The store needs be fully up to temperature to supply baths -
Before any hot water is drawn off, the heat-bank must be up to temperature.
Many later versions use a blending valve on the return to the boiler to
ensure only up to temperature water is pumped into the store by the boiler.
This prevents agitation of the stored water, and aiding heat stratification
within the store giving useful water at the top of the store within a very
shot time.
§ May not take full advantage of a condensing boiler - Maintaining
the stored water at 75o to 80oC results in a generally high boiler return
temperature. This will not take full advantage of a condensing boiler,
which increases in efficiency with lower return temperatures. With the
superior heat stratification of taller cylinders this problem will be
reduced. Condensing boilers with a high operational flow and return
temperature differential are best suited to thermal stores and heat banks.
Fortunately most have a wide temperature differential.
§ Lower water temperatures with fast flow-rates - As with Combi
boilers, fast flow-rates through the plate heat-exchanger results in lower
water temperatures. This is not so pronounced with heat-banks as with
thermal-stores.
§ More controls - An extra pump, thermostatic blending valve, flow
switch and thermostatic controls are required over a conventional cylinder.
An unvented cylinder system requires extra pressure controls.
§ Heat loss - Storing water at high temperatures is not efficient as
heat loss is more pronounced. Heat-banks are more efficient when constantly
used being less efficient when used in houses of infrequent occupation. Some
Gledhill heat banks have super high levels of insulation, even under the
cylinders, negating this point.
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