On Wed, 16 Feb 2005 08:00:10 +0000, Tim S wrote:

On Wed, 16 Feb 2005 06:08:37 +0000, John Rumm wrote:

Doctor Evil wrote:

circulation. Each pump can have its speed set to the needs of the zone.
There will be no parasitic circulation if each zone has its own flow and
return tapping off the store.

except you don't want to be running your heating off the store with a
modern boiler....

Hi John,

Why not - I thought thermal stores assisted the boiler by ensure long
efficient burns?

Tim

This depends on the type of boiler in use.


If you are talking about a conventional one operating at 82 degree
flow and 70 return, then there *could* be a benefit in the approach of
running the heating circuit from the store on the long efficient burn
argument.

However, with a condensing boiler, the greatest efficiencies occur
when the burner is modulated down rather than on/off and when the
operating temperature is lower.

In the intended set up with a HW cylinder (or store for that matter),
the boiler, in HW mode, operates balls-out to replenish the cylinder
quickly and at high temperature. Although this is one long burn, it
is not in the most efficient operating range. The heating circuit is
intended to be connected directly to the boiler and the boiler will
modulate down as the load reduces when TRVs begin to close. Ideally,
the system should reach an equilibrium with boiler running
continuously at lowish power to match the radiator load. This avoids
cycling and runs the boiler in a more efficient temperature range.

If you insert a store in between boiler and radiators, you effectively
defeat this arrangement and force a cycling situation. The radiators
gradually drain heat from the store, and at a certain point the boiler
is brought on to replenish it using a high temperature burn as for hot
water.

There *may* be exceptions to this if you want to bring in additional
heat sources such as solar, or possibly if you want a source of lowish
temperature water for UFH (although that can be done with a blending
valve)

Other than that, it is better to keep the store for DHW and let the
boiler drive the radiators directly if it's a modulating condensing
model.







--

..andy

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