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).

Fitters told me that I should run this at 74 degrees.

Which I thought far too high, as

Indeed...

1) it makes the radiators too hot to touch
2) basic thermodynamics suggest that a better temperature profile
will result from having the radiators at the lower temperate for
longer period than a higher temperature for a shorter period

It might be required in particularly cold weather, but most of the
time you will be able to use less.


I tried to explain this but was met with

"The recommended temperature is required for the condenser to have
any effect"

You will most heat recovered from the condenser when the return
temperature is below about 54 degrees (the dew point of the flue gasses).

and the completely bogus "the temperature of the water in the
radiators is set by the TRVs not the boiler temp.Â* I couldn't
persuade the guy that he was taking bollox, he played the "I'm the
experience heating engineer card and I know better than you" card.Â* ****

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
temperature based on the outside temp. Basically that means its runs
as cool as it thinks it can get away with and still be able to reach
the target set point temperature in a reasonable amount of time. (the
relationship is set by choosing a mapping curve that reflects the rate
of heat loss of the building).

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.

Its a S plan+ system, but never actually runs he rads and DHW at the
same time.

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.

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



--
Cheers,

John.

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