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  #11   Report Post  
Old January 12th 19, 07:56 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

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.* 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.* ****

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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Old January 12th 19, 08:12 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On Friday, 11 January 2019 22:35:12 UTC, Tim+ wrote:
wrote:


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.


That may be true, but in practice I don’t think Legionella has ever proved
to be a problem in domestic systems.



Oh yes it has. Plenty of people have died.
https://www.dailymail.co.uk/news/art...ish-homes.html
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Old January 12th 19, 08:16 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On Friday, 11 January 2019 23:33:44 UTC, Max Demian 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

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

I tried to explain this but was met with

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


The instruction manual for my Ideal says that the 'e' setting is best
for condensing, which is quite hot. I don't understand this as I would
have thought that the cooler the returning water temperature the more
condensing.

I use a much lower temperature as I don't have a room thermostat and the
TRVs are fiddly and don't seem to keep the air temperature constant.

--
Max Demian


TRVs don't work. They are more influenced by the nearby radiator temperature than the room air temp.
You can get them with remote sensors. They work.
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Old January 12th 19, 08:30 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On 12/01/2019 00:55, RJH 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.

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
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).


I don't follow how that can work properly, as for most homes different
rooms will have a different 'curve'. Or does tweaking the TRV compensate?


It seems to work well enough in practice. Each room also has a TRV, and
I have the place split into two zones; upstairs and downstairs, so that
will account for some variation.

It needs a little bit of experimentation to set the profile the first
time - basically waiting for colder days and seeing if the system still
heats the place comfortably and quickly enough. If it doesn't, then you
just tweak it up to a steeper curve. It also helps if you have
appropriate rad sizes for the rooms, and the system is balanced.

The response curves look like:

http://wiki.diyfaq.org.uk/index.php/...tingCurve1.png

IIRC the system defaults to the 1.2 line. If you live in a super
insulted place / particularly sheltered location then you would tweak
down. In my case (exposed location - solid wall construction), I needed
to go up. I found the 1.8 curve worked well.

The system is also smart enough to automatically shift the response
curve vertically based on the currently demanded internal target
temperature[1]:

http://wiki.diyfaq.org.uk/index.php/...tingCurve2.png

So if you tweak the room temp up or down during the day (or have
different times programmed with different set point temps), then it can
vary the flow temperature to match the requirement.

[1] Note that all the system temperatures sensors (internal downstairs,
internal upstairs, DHW cylinder, and external) are digitized and
processed as actual temperatures, not just as on/off "call for heat"
style demands.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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Old January 12th 19, 08:35 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On 11/01/2019 22:42, wrote:
On Friday, 11 January 2019 18:54:03 UTC, 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

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

I tried to explain this but was met with

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

and the completely bogus "the temperature of the water in the
radiators is set by the TRVs not the boiler temp. 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.
****

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

what do you guys/galls do

tim


The only realistic answer is to run it as hot as is necessary to
provide enough heating. Cooler means more boiler efficiency, hotter
means more rad output. The required target temp varies from one house
& install to another, and varies with the weather.


yup, spot on.

It leaves three options:

set and forget - chose a high flow temp that meets the worst case, and
accept that you will be running with less efficiency / comfort for most
of the year.

manual - adjust the flow temperature yourself in response to
particularly cold days - probably works quite well for users who are
clued up enough to understand the requirement.

automatic - use weather compensation to "close the loop".


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


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Old January 12th 19, 08:51 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On 11/01/2019 23:33, Max Demian 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

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

I tried to explain this but was met with

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


The instruction manual for my Ideal says that the 'e' setting is best
for condensing, which is quite hot. I don't understand this as I would
have thought that the cooler the returning water temperature the more
condensing.

I use a much lower temperature as I don't have a room thermostat and the
TRVs are fiddly and don't seem to keep the air temperature constant.

If your boiler allows external timers consider getting a TADO or Drayton
Wiser (Or just wireless TRVs and a Reaspberry PI if you are the
adventurous type). Lets you see what temp the TRVs think the room is at.

Dave
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Old January 12th 19, 08:54 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

On 11/01/2019 23:48, Harry Bloomfield wrote:
tim... pretended :
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


I have an opened vented system, with stored HW and the house is heated
by the one boiler, installed new last March. A Vailant ECOfit Pure 418.
We like the stored water to be good and hot, the boiler only has one
temperature setting available for its output temperature - so I have it
set at its max, of 75C, in order to meet our HW needs. The house heating
would be satisfied by a lower boiler temperature around 60C and it would
be more gas efficient, but unless I manually manage the temperature
setting I am stuck with the less efficient 75C.

I even tried to find a way to fool the boiler into working at a lower
output temperature, by adding a separate bi-metal stat on the boiler
output pipe to CH only, set at 60C, to shut it down at that temperature
- as heating satisfied. That was a miserable failure, in that the boiler
constantly cycled on/off via the stat, whilst ever the room stat
indicated a demand, rather than modulating its output down - as it would
on its own as it neared its 75C setting.

If anyone knows a workaround, to allow split boiler output temperatures
on this boiler - do let me know please.


IME many of the Vaillant boilers do support split temperature operation,
but it does depends on what controls you have them paired with.

The 400 series Vaillant I did (probably previous model range, since this
was = 12 years ago), actually had separate knobs on the front for CH
and DHW flow temperatures. Although that was retrofitted into system
with traditional controls that could not distinguish between the source
of the call for heat - hence the DHW control never came into play and it
ran like your system does.

The blurb on their web site:

https://www.vaillant.co.uk/for-insta...ler-26116.html

Seems to suggest its compatible with the VRC 700 weather compensating
controls, so that ought to allow split temperature as well.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
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Old January 12th 19, 09:28 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

Max Demian explained :
I use a much lower temperature as I don't have a room thermostat and the TRVs
are fiddly and don't seem to keep the air temperature constant.


That is at odds with my own findings, since installing TRV's last
March, but I do have a room stat, in the hall and no TRV on just that
one rad.

I tried setting the TRV's up in March for each room, but there wasn't
much need for heat at all then. I found it better to set them once the
cold weather arrived this winter. Having gradually tweaked each to a
comfortable temperature, they work pretty well. Working on just a room
stat, the temperature variation was quite noticeable as the boiler
kicked in and out on just the room stat..
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Old January 12th 19, 09:38 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

John Rumm pretended :
It needs a little bit of experimentation to set the profile the first time -
basically waiting for colder days and seeing if the system still heats the
place comfortably and quickly enough. If it doesn't, then you just tweak it
up to a steeper curve. It also helps if you have appropriate rad sizes for
the rooms, and the system is balanced.


Balanced?

I have always assumed there was no need to balance a system, where
TRV's are installed.
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Old January 12th 19, 09:49 AM posted to uk.d-i-y
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Default how hot do you run you CH boiler

It happens that John Rumm formulated :
set and forget - chose a high flow temp that meets the worst case, and accept
that you will be running with less efficiency / comfort for most of the year.


Which is what I have done. The TRV's prevent the rads getting hotter
than needed for each room, except when the heating is first put on with
the house 'cold'. In quotes, because it never gets truly cold.

To mitigate turning the heating on from cold and the room temperature
over shooting, I tend to nudge the room stat up in a series of steps
over several minutes.


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