Keston C25 h/w demand mod
 

For Ed Sirett and anyone else who's interested, here's the mod I made to
the control panel of my Keston Celsius 25 system boiler to create full
heat demand for hot water operation.

The circuit is not the simplest one possible but for reliability it
avoids having a relay in the high impedance signal path to the boiler
demand level. The 4k7 resistor means that there is always a small
current flow on the relay contacts which is something I prefer. The
worst that can happen if the relay contacts fail is that the mod ceases
to function rather than the whole c/h packing up.

The relay is mains powered and must have gold plated contacts (which is
rare on mains relays), I used RS model 394-9116.

The mod is straightforward to implement but requires some soldering
skill. The 10k resistor needs a cut in the boiler demand level signal
which I made by cutting the control pot wiper lead (middle connection)
and putting the 10k res in its place. The diode & the 4k7 resistor can
then be soldered to the pot terminals on the back of the board and the
relay connections wired up.

Full demand for the Celsius 25 is when the demand control level is at
0V; I have not found that the diode drop affects the operation
significantly.

Control board mod:

+5V
---
|
ctrl / 10k modified
panel \----/\/\/\----------------------- demand
pot / | level
(100k) \ +5V |
_|_ --- |
0V | |
\4k7 |
h/w / |
demand ----- \ 1N4001 | 240V~
| |--||---| ---
- | |
| | o o
RL1 | | RL1a / RL1b /
coil | | o o
- | |
| --- |
--- 0V c/h --------- boiler
240V RTN demand demand



Against my better judgement, here's a version that doesn't involve a mod
to the control board but places a cut in the loom instead. Lots simpler,
just a relay and wiring, but with the relay in the signal path so beware
of dirty contacts affecting operation in the years to come. Again, relay
contacts must be gold plated.

Loom based mod:

Control Board Loom

+5V
---
|
ctrl / RL1a modified
panel \--------------(#---------o |------ demand
pot / \o----| level
(100k) \ o
|---------------(#---------|-------------
_|_
0V

h/w
demand ----- 240V~
| ---
- |
| | o
RL1 | | RL1b /
coil | | o
- |
| |
--- c/h --------- boiler
240V RTN demand demand

I both cases, be careful to separate the wiring on the two relay
contacts as one is low voltage and one is mains, neat soldering is
required and best to sleeve the joints or gloop with hot melt glue.

I've posted a couple of pictures of the first mod on
alt.binaries.pictures.misc under the same thread name. If anyone is on a
non binary server then don't worry, you're not missing much ;-)

HTH
--
fred

On Mon, 27 Dec 2004 11:06:25 +0000, fred wrote:

For Ed Sirett and anyone else who's interested, here's the mod I made to
the control panel of my Keston Celsius 25 system boiler to create full
heat demand for hot water operation.

The circuit is not the simplest one possible but for reliability it
avoids having a relay in the high impedance signal path to the boiler
demand level. The 4k7 resistor means that there is always a small
current flow on the relay contacts which is something I prefer. The
worst that can happen if the relay contacts fail is that the mod ceases
to function rather than the whole c/h packing up.

The relay is mains powered and must have gold plated contacts (which is
rare on mains relays), I used RS model 394-9116.

The mod is straightforward to implement but requires some soldering
skill. The 10k resistor needs a cut in the boiler demand level signal
which I made by cutting the control pot wiper lead (middle connection)
and putting the 10k res in its place. The diode & the 4k7 resistor can
then be soldered to the pot terminals on the back of the board and the
relay connections wired up.

Full demand for the Celsius 25 is when the demand control level is at
0V; I have not found that the diode drop affects the operation
significantly.

Control board mod:

+5V
---
|
ctrl / 10k modified
panel \----/\/\/\----------------------- demand
pot / | level
(100k) \ +5V |
_|_ --- |
0V | |
\4k7 |
h/w / |
demand ----- \ 1N4001 | 240V~
| |--||---| ---
- | |
| | o o
RL1 | | RL1a / RL1b /
coil | | o o
- | |
| --- |
--- 0V c/h --------- boiler
240V RTN demand demand



Against my better judgement, here's a version that doesn't involve a mod
to the control board but places a cut in the loom instead. Lots simpler,
just a relay and wiring, but with the relay in the signal path so beware
of dirty contacts affecting operation in the years to come. Again, relay
contacts must be gold plated.

Loom based mod:

Control Board Loom

+5V
---
|
ctrl / RL1a modified
panel \--------------(#---------o |------ demand
pot / \o----| level
(100k) \ o
|---------------(#---------|-------------
_|_
0V

h/w
demand ----- 240V~
| ---
- |
| | o
RL1 | | RL1b /
coil | | o
- |
| |
--- c/h --------- boiler
240V RTN demand demand

I both cases, be careful to separate the wiring on the two relay
contacts as one is low voltage and one is mains, neat soldering is
required and best to sleeve the joints or gloop with hot melt glue.

I've posted a couple of pictures of the first mod on
alt.binaries.pictures.misc under the same thread name. If anyone is on a
non binary server then don't worry, you're not missing much ;-)


Thanks a lot. The latter arrangement I had worked out for myself, but I
can see your first design is less invasive.

After thinking about this I'm not yet inclined to implement the mod.
At present the bathroom rad is also a simple restricted lock-shield
bypass, so it would become lethal at 80C during water heating.
It would also grossly over heat the 1.8m x 1.6m room.


I might eventually put in a smart bypass and put a TRV on the bathroom rad.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

In article .uk, Ed
Sirett writes

Thanks a lot. The latter arrangement I had worked out for myself, but I
can see your first design is less invasive.
You're welcome.

After thinking about this I'm not yet inclined to implement the mod.
At present the bathroom rad is also a simple restricted lock-shield
bypass, so it would become lethal at 80C during water heating.
It would also grossly over heat the 1.8m x 1.6m room.


I might eventually put in a smart bypass and put a TRV on the bathroom rad.

I often run the flow at 50-60 so just a little low to have a cyl stat
set to 60. The following mod would let you set the h/w demand flow to 65
or so to avoid the superheated bathroom rad. There's plenty of room on
the panel for an extra pot, perhaps with the spindle cut short and a
screwdriver slot cut to avoid accidental movement.

Just another one of many variations that could be applied but if your
system is working I understand it is hard to work up the enthusiasm to
tear it apart.

Variable h/w demand control board mod:

+5V
---
|
ctrl / 10k modified
panel \----/\/\/\----------------------- demand
pot / | level
(100k) \ +5V |
_|_ --- |
0V | |
1k \ 1N4001 |
h/w h/w /--||---|
demand ----- demand \ 240V~
| pot / ---
- | |
| | o o
RL1 | | RL1a / RL1b /
coil | | o o
- | |
| --- |
--- 0V c/h --------- boiler
240V RTN demand demand

Regards,
--
fred

In article , Fred wrote:
Just another one of many variations that could be applied but if
your system is working I understand it is hard to work up the
enthusiasm to tear it apart.

The two in our church are working just fine and I think my fellow
members would rather they stayed that way g. I am interested to
know though what the basic control algorithm is. I assume that the
knob sets the flow temp, and the control board looks at the return
and flow temps to decide how much burner power is needed. But
obviously somewhere it's also deciding how fast to run the pump. Any
insights would be welcome.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm

In article , Tony Bryer
writes
In article , Fred wrote:
Just another one of many variations that could be applied but if
your system is working I understand it is hard to work up the
enthusiasm to tear it apart.

The two in our church are working just fine and I think my fellow
members would rather they stayed that way g. I am interested to
know though what the basic control algorithm is. I assume that the
knob sets the flow temp, and the control board looks at the return
and flow temps to decide how much burner power is needed. But
obviously somewhere it's also deciding how fast to run the pump. Any
insights would be welcome.

No great insights I'm afraid, at least no more than I am sure you have
gleaned from just watching the boiler in operation, as you have described
above. I've guessed that the pump speed increases in stages to match the
power output but that is just based on what would seem sensible to get
the increasing heat away.

Andy Gabriel seems further ahead with his control system than I am and I
am hoping he will publish new insights as he comes across them, much
as he did for his recent slime beast solution ;-). I've picked up a lot from his
posts on this boiler and his system, also quite a bit from yours, thanks.
--
fred

In article ,
fred writes:

No great insights I'm afraid, at least no more than I am sure you have
gleaned from just watching the boiler in operation, as you have described
above. I've guessed that the pump speed increases in stages to match the
power output but that is just based on what would seem sensible to get
the increasing heat away.

Andy Gabriel seems further ahead with his control system than I am and I
am hoping he will publish new insights as he comes across them, much
as he did for his recent slime beast solution ;-). I've picked up a lot from his
posts on this boiler and his system, also quite a bit from yours, thanks.

Cheers. Some things I have gleened -- may not all be true...

I think the knob sets the flow temperature -- using a clip-on pipe
thermometer (not accurate), I calibrated it from 30C at the lowest
setting through to 85C (IIRC) at the highest.

The pump seems to run at full speed until the temperature differential
across the flow and return starts getting small, at which point it
slows down in a number of steps. This is presumably to achieve a better
temperature differential.

The gas valve electronic control seems to be a straight on/off affair.
The gas rate seems to be varied by the fan speed, presumably by using
the pressure drop in the gas/air mixer to control the gas flow -- there
is a small hose from the mixer back to the gas valve which looks like
it might be providing pressure feedback for this.

If there are not enough radiators operating to absorb the 7kW minimum
boiler output but there is still a call for heat, the boiler seems to
let the water temperature creep up by about an extra 5C before cutting
off the burner completely. There is then quite a large hysteresis
before the burner fires up again.

Several people here have noted the temperature control is not
adiquate for ideal heating and hot water in a condensing system.
Another thing the Keston is missing is provision for connecting
remote frost stats. I believe it has an internal one which will bring
the boiler on until the return water is 30C. However, in my case the
boiler is in a room where this is unlikely to trigger, but much of
the pipework is in an unheated area under a suspended floor where
it's more likely to go below freezing. Ideally, I should be able to
trigger the boiler's frost protection operation remotely from this
location too. I have temperature monitoring in this location, and
intend to do this by causing it to run at lowest temperature setting
(30C) when this area goes below zero, with no other call for heat.

--
Andrew Gabriel

In article , Andrew Gabriel wrote:
Cheers. Some things I have gleened -- may not all be true...

Thanks to you and Fred for the info

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm

On Thu, 30 Dec 2004 15:00:14 +0000, Andrew Gabriel wrote:


Cheers. Some things I have gleened -- may not all be true...

I think the knob sets the flow temperature -- using a clip-on pipe
thermometer (not accurate), I calibrated it from 30C at the lowest
setting through to 85C (IIRC) at the highest.

That's my understanding too.
The figures are about right - the 1 o'clock I have it on is about 65C
on the flow.


The pump seems to run at full speed until the temperature differential
across the flow and return starts getting small, at which point it
slows down in a number of steps. This is presumably to achieve a better
temperature differential.

And to get the boiler into condensing mode if possible.


Several people here have noted the temperature control is not
adiquate for ideal heating and hot water in a condensing system.
Another thing the Keston is missing is provision for connecting
remote frost stats.

??
Either direct from the mains to the demand terminal in which case the
boiler will heat the bypass circuit, or direct from the mains to the
CH zone valve if the whole house needs protecting.

Or have I failed to understand what you want to do?

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

"Ed Sirett" wrote in message
n.co.uk...
On Thu, 30 Dec 2004 15:00:14 +0000, Andrew Gabriel wrote:


Cheers. Some things I have gleened -- may not all be true...

I think the knob sets the flow temperature -- using a clip-on pipe
thermometer (not accurate), I calibrated it from 30C at the lowest
setting through to 85C (IIRC) at the highest.

That's my understanding too.
The figures are about right - the 1 o'clock I have it on is about 65C
on the flow.


The pump seems to run at full speed until the temperature differential
across the flow and return starts getting small, at which point it
slows down in a number of steps. This is presumably to achieve a better
temperature differential.

And to get the boiler into condensing mode if possible.


Several people here have noted the temperature control is not
adiquate for ideal heating and hot water in a condensing system.
Another thing the Keston is missing is provision for connecting
remote frost stats.

??
Either direct from the mains to the demand terminal in which case the
boiler will heat the bypass circuit, or direct from the mains to the
CH zone valve if the whole house needs protecting.

Via or in series with a Danfoss AT/F pipe stat set to about 15/20 degrees
which will only keep the pipes just warm then shut off

Or have I failed to understand what you want to do?

In article ,
"John" writes:

"Ed Sirett" wrote in message
n.co.uk...
On Thu, 30 Dec 2004 15:00:14 +0000, Andrew Gabriel wrote:


Cheers. Some things I have gleened -- may not all be true...

Several people here have noted the temperature control is not
adiquate for ideal heating and hot water in a condensing system.
Another thing the Keston is missing is provision for connecting
remote frost stats.

??
Either direct from the mains to the demand terminal in which case the
boiler will heat the bypass circuit, or direct from the mains to the
CH zone valve if the whole house needs protecting.

Via or in series with a Danfoss AT/F pipe stat set to about 15/20 degrees
which will only keep the pipes just warm then shut off

Or have I failed to understand what you want to do?

Well, ideally have the pump running continuously when some pipework
is in sub-zero areas, and have the boiler firing up occasionally as
necessary to maintain the pipework temperature above freezing.
A similar mod to fred's which instead forced temperature to lowest
setting would do this. It was just rather annoying that the Keston
already has that logic internally, but no ability to trigger it
from a remotely positioned frost stat. John's solution would work
in a similar way but without the continuous circulation, which for
lagged pipes is probably good enough most times.

--
Andrew Gabriel