On 12/01/2019 10:03, wrote:
On 12/01/2019 06:56, John Rumm wrote:
On 11/01/2019 22:25, wrote:
On 11/01/2019 21:30, John Rumm wrote:
On 11/01/2019 18:52, tim... wrote:

I had a new Combi fitted today (in the to-be-moved-to house).

[snip]

Anyhow, at my current house it is 55 and works perfectly well

what do you guys/galls do

I have weather compensation on mine, and so it chooses its own
[snip]

Currently the external temp is 6.5 deg C, and the flow temp is
running at 54 deg. If it were to go well below 0, then it might push
the flow temp up into the 70s. When its milder it might run flow
temps down in the 40s.


I thought the advice was to heat DHW to 60 degrees to avoid the risk
of legionella, and that's not going to happen if the boiler's running
at a lower temp.

Oddly enough when I typed the reply above, I originally included
discussion of DHW, however deleted it since I thought it was detail
not required to answer the OP.

Yup, the system runs higher flow temperatures for DHW reheat (or at
least the later stages of them). It will modulate the flow temp during
the process to achieve maximum condensation efficiency, while also
ensuring good heat transfer rates). The coil in the cylinder will
allow a recharge at a maximum rate of around 20 - 22kW - just a little
bit less than the maximum output of the boiler.

Does the boiler have an input to tell it that it's heating the DHW, or
is the flow temperature adjusted automagically based on flow and return
characteristics?

Yes ;-)

The boiler knows what it is heating - you get a Tap logo pop up on its
LCD rather than a radiator logo. The information is conveyed over its
eBus rather than with a discrete input.

Also like many boilers, it can adjust its power output based on a number
of criteria including flow and return temps etc.

I'm fairly certain that weather compensation isn't available for my W-B
Greenstar 32/50 boiler, according to the book the manual adjustment
range is 60-82C.

ISTR that when I was looking for system boiler's that could do split
temp, there were some options for some of the WB boilers.

(I also considered paring a combi with an unvented cylinder, so one
could have fast potable DHW to the kitchen which is on a long pipe run)


Its a S plan+ system, but never actually runs he rads and DHW at the
same time.
My system is S+, but with Honeywell Evohome controls. Each room is a
unique zone (2 zones in the bathrooms), with its own thermostat and time
schedule. There is no "house" stat; the rads and DHW all operate
independently of one another.

I have it set to heat to 60 degrees for six days a week, and then it
runs an anti legionella cycle one day a week, where it heats it to 70.
Raising the temp to 70C for one day per week, and keeping it at 50C
(say) at other times, seems like a damned good idea. I'll make the
changes next time I'm fiddling with the settings.

There is a TMV on the output of the cylinder to limit the maximum DHW
temperature delivered to the points of use.

I thought about a TMV when I planned the system but decided not to
bother - perhaps an error.

I was not that keen on the idea that some times it would otherwise
deliver 70 degree water to the taps.

Having a TMV on the cylinder output is a bit of a compromise, since it
means you don't have the benefit of running the very hot water through
all the pipe work like you would with multiple TMVs at every point of use.

In the end I went for a high flow TMV, and set it at max temperature
(about 55 IIRC). That allows for some heat loss in the pipes and still
delivers water hot enough you can mix in some cold at the point of use -
giving shower valves etc a bit more control.


--
Cheers,

John.

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