"John Aston" wrote in message
...

In your case with having three
differing functions of different
temperatures a heat bank is the
ideal way to supply those temperatures
promoting maximum
efficiency from the boiler.

The most efficient, and easiest,
is having two boilers as described above.

IMM, leaving my particular case to one side,
may I ask in what application would you
recommend the use of a weather-compensated
modulating boiler?

Firstly, having a dedicated boiler for distinct functions is the ideal
method: DHW, UFH, rads. Separate dedicated functions easy to control and
design. That was the way in ye olden times. But in ye olden days boilers
were expensive and very large, so quickly they found ways of joining up all
the various circuits: DHW, rads, fan coil unit circuits etc. They devised
methods using headers, buffers (thermal store) etc. All this was a
compromise to tap into one source of heat, the boiler. It worked well
because the boilers had to operate at high temperature because they were
non-condensing. High temperatures water was on tap and the varios circuits
tapped off it blending it down when necessary for the various circuit
functions.

In commercial applications they could then have two sequenced boilers,
bringing in both or just one, or none, depending on heat demand, and a spare
boiler if one is down. All was fine and dandy and things went along like
that for decades. These sort of systems are still in the mindset of many
designers today, designing the total system to the highest temp required in
a sub-system.

Then condensing boilers came along offering high efficinencies the lower the
return temp. This required a re-think. With a simple two function system
of say: DHW and UFH you could have the boiler on a weather compensator
serving the UFH to the lowest temps for high efficient operation. If DHW is
called, a 3-way diverter sends all the boilers heat to the cylinder (must be
quick recovery) while ramping up the boiler temp to maximum for the high
temp DHW requies and rapid DHW re-heat. When more than one function is
thrown in, with all three, or more, requiring different operating temps, as
in the rads of your system, matters become a little complicated when the aim
is to keep the boiler running at the lowest return temperature for maximum
efficiency.

Also, boilers became smaller and smaller and cheaper and cheaper. This then
made the dedicated function boiler a cost effective reality. To implement a
condensing boiler system to maxiumum efficiency, running it at the lowest
temps for most of the time, it becomes more expensive in ancilliry equipment
and controls, and complicated, as you have seen in attempting to get a
solution. The best solution was thermal store/heat bank with dedicated
sections, or zones, for the various functions (DHW, UFH, rads) down the
store with these sections providing exactly the the right temperature for
the functions. The boiler only heating one of these sections at a time to
the lowest temperature for that function - ideal, and sorted.

Back to your question. "what application would you recommend the use of a
weather-compensated modulating boiler?"

If you have one function, say just rads or UFH then it is ideal. It is also
ideal for two functions, but a priorty system is required, with the highest
temp having priority with boiler cycling eliminated. A thermal store can
supply a buffer of water to eliminate this, as you don't want the system
cycling from one function (UFH) to the other (rads or UFH) every couple of
minutes.

The answer:

1. One dedicated function (like rads or UFH)

2. Two functions like UFH and DHW (only if the system is designed properly
to a priority system).

To go further.........

As I have said, in your case it is worth costing up:

1. Low Temp Circuit: One dedicated condensing boiler serving the UFH only
on a weather compensator. Simple, separate and sorted.

A number of makers supply integrated weather compensated boilers, as an
extra. Viessmann, MAN, etc, are very expesnive and will probably have far
more control functionallity than what you need. They are very good and the
RRs of boilers, with price to match. Very good boilers are available from
reputable makers much cheaper.

2. High temp circuit. Another boiler using a 3-way diverter valve serving
DHW and rads with DHW priority. You may want a weather compensator
maintaining boiler to the lowest temp the rads require. When DHW is called
the boiler runs up to max temp. This is similar to normal domestic setup.

3. A controller staging in the UFH and rads to give precise seamless
control of room temps. UFH 1st stage, rads 2nd.

4. Backup: Now you have heating backup if one boiler drops out. Electrical
backup for DHW in the cylidner or heat bank cylinder.

6. The cylinder can be:
a) An unvented version,

b) A DHW only heat bank, such the DPS Pandora, which doe not require an
overflow so can be fitted anywhere in the house and will take high pressures
way above 3.5 bar.


You will find that two condensing boilers can be had for less than the price
of the Viessmann, and lots of change too.