On 21 Dec, 13:42, John Rumm wrote:
Mark wrote:
On Thu, 20 Dec 2007 19:40:10 +0000, John Rumm
wrote:

wrote:

The boiler I'm planning to get is a Vaillant ecoTEC system boiler, in
order for it to be in condensing mode most of the time I'm oversizing
radiators a little and using a quick recovery cylinder.
Will this make a difference?
Some - in the coldest weather. The boiler will do a reasonably good job
of keeping the return temp down where it can anyway though by modulating
the heat input.

One of my relatives has a Vaillant ecoMAX system boiler and on the
inside of the control flap it advises to set the boiler flow temp to
maximum if it is connected to a thermostat, I thought condensing
boilers should be set as low as possible.
Shall I tell him to ignore this advise?
Depends on the type of thermostat. If the boiler is the type that
supports a external stat with proportional control, then that makes some
sense. It would in effect leave the internal boiler flow temp control
set at maximum and hence "out of the picture" and the actual flow temp
would be controlled based on the actual heating demand.

If however the external stat is the more typical "on / off" style
device, then the advise in the manual does not seem correct.

I can't see how else it could work in the latter case. I always
believed that condensing boilers modulate down when the return
temperature rises too high (unless a weather compensator is fitted).

Please correct me if I am wrong ;-)

That is my understanding also... The most logical control setup for a
condenser is treat the internal water temp stat as a maximum limit, and
then modulate for optimal (or at least better) return temperature.

For a non condensing, modulating boiler, maintaining the flow temp at or
near the stat temperature would make as much sense.

This does open up the question of how the boilers are not true "designed
from the ground up" condensers will behave, since they are based on a
re-engineering of an original design.

--
Cheers,

John.

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Thank you for all useful information.