On 11/11/2016 00:25, Andy Wade wrote:
On 10/11/2016 01:22, John Rumm wrote:
[...]
Yup I think that is the way I would design it if given the same choice
of components (assuming the boiler can modulate low enough to run the
UFH directly - you may need a blending valve or two in there).

Er, if the heat demand is less than the minimum modulated output of the
boiler the boiler will just cycle on and off (PWM). If you force it not
to do that it will be delivering excess energy which has to go
somewhere. Either the house will overheat, or a room 'stat will perform
the PWM function instead. Unless weather compensation is employed that
will be the main control mechanism in any case.

It will indeed.

The purpose of the blending valve in this case is to control the maximum
flow temperature into the UFH to prevent it being uncomfortably hot on
the feet, or distorting floor coverings etc.

The boiler will still need to cycle to only deliver the power level
actually needed.


[Re DHW:]

The boiler would need to be capable of (or be fooled into) split
temperature operation so that it could run an appropriate flow temp for
heating, but then use a much higher temp for store recharge.

True. Combis do this innately of course if you replace "store recharge"
with "provide hot water, but I dont know if there are any system boilers
that have two demand inputs like that. There should be though...

There are a few - I spent some time researching when looking for a
system boiler setup for my place. Some have it built in, others you can
add it or get the same effect with the right control system. Anything
that has weather compensation will normally include it.

Most of the Vaillant models seem to include separate temperature
controls on the boiler itself for the DHW and CH - even on the basic 400
series "heat only" vented range.

The posher weather compensating controls allow for the CH flow temp to
be slaved to the external temperature, modified via user selectable
response curves to model the thermal properties of the building, and
also to introduce an element of PID style control such that the current
target temperature in the building will be taken into account (i.e. girl
mode - if its well below the current set temperature it will turning it
up more for faster warm up ;-). The DHW then runs at a separate preset
maximum flow temp (with an optional weekly "anti legionella" cycle where
it recharges the DHW at a higher temperature).

Many weather compensators seem to use a NTC thermistor as their
temperature sensor. So a control system could obviously switch in
additional resistors to trick the system into running a different flow
temperature.


--
Cheers,

John.

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