Boiler control questions

So far the RTC is running, as are the Dallas 1-wire thermometers for room
stat, cylinder stat and outside temp, and web server.

Anyway, I have an idea I'd like to pass by the experts (Drivel need not
apply)....

Application is a Y-plan system. As it stands, during the winter when CH is
required, boiler runs for CH rarely seem to coincide with boiler runs for
HW. Boiler (situated in garage) has to heat up from cold each time.
Naturally both CH and HW controls will incorporate (configurable) hysteresis
and impose minimum boiler run times. The idea I have is this: Suppose we
have CH demand, but HW temp is above HW minimum. When CH demand is
satisfied, then I would 'top up' the HW to HW max, thus possibly eliminating
an unnecessary boiler start. Vice-versa, of course, for HW demand and CH
top-up.

Does this idea make any kind of sense, or am I totally out of my tree?

Steve S
--

Yes, it makes sense. Whether it'd save much fuel is debatable (imho)

I wonder if it's possible (Using a system of valves) to divert hot
water from the HW cylinder to the radiators instead of firing up the
boiler?

Surely it's more efficient to 'lose' heat from the cylinder to heating
the house than have it lost to the airing cuboard, iyswim?

sponix

But then you've got the wasted heat in the boiler popes to the coil,
when time comes to reheat, and the boiler goes off with warm pipes, and
the hot pipes from the cylinder to the heating.
It may be in many cases that these are outside the habitable space,
and so the heat is wasted.

If you have the room, 3-4400mm of roof insulation round the CH tank will
greatly reduce loss.

I expect heat exchanger ventilation would make a fair bit of difference
too, assuming that the insulation elsewhere is adequate.
Seal all leaks, and actually design the ventialtion rather than letting
it happen.

If your boiler can cope with heating the CH and HW at the same time (if it
can't, maybe you should have a W-plan rather than Y-Plan) *and* if the HW
cylinder is well lagged, wouldn't it make more sense to - in effect - have
HW on constant so that, whenever CH is on, you also heat the HW to the max
setting?

On a more general note, it is really worth putting a fancy control control
system on a Y-Plan system? Wouldn't it be better to try to convert it to an
S-Plan++++, with a separate zone for each room? It would presumably be easy
enough to have a temperature sensor in each room. You would then need to
control the flow to each radiator - possibly using remote control radiator
valves, some of which have micro-switches to feed back their status. You can
then have a different heating profile for each room to suit your lifestyle -
but still have global over-rides for holiday/party settings etc.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

It would have to be a bl**dy big room! <g>

Yup, I see what you are saying.... much of the answer may depend on
specifics of your boiler however. A modern modulating condensor will
have a different closed loop response than for example a large cast iron
lump with all or nothing control.

In the latter case there is probably good argument to save an additional
heat exchanger cool down/reheat cycle if this can be achieved with
tacking on a HW reheat at the point where the room stat is satisfied.

--
Cheers,

John.

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S-Plan++++ would be great, but would be very expensive to implement. Floors
to be lifted, cables to be routed, redecoration, etc. etc., not to mention
the remote control valves. My solution is essentially free except for my
time in hacking a bit of C. Boiler copes with CH and HW together except in
the coldest weather.

Steve S

--

You're doing all this (inc a webserver) on a humble PIC?

Have you considered a Rabbit core instead? Loads of i/o and ethernet built
in....

www.rabbitsemiconductor.com

True..

I'm thinking of the situation where no hot water is going to be used
but the water in the cylinder is hot.

The water in the cylinder could be pumped round the rads, thus
ensuring the heat isn't wasted.

sponix

It's a PIC18F6621 running at 40MHz with the free Microchip tcpip stack.
Already most of the way there with only 40% of the rom, 47% of the ram used,
and oodles of space in the external eeprom for web pages/config info.
Webserver is already running. Will also add smtp for outage/fault
notification later. Rabbit core was *much* more expensive.

Steve S

It's the latter currently. Looking to replace probably spring/summer 2007.
Updating the firmware accordingly will be no problem :-)

Steve S

--

I think the point I was making was that I personally wouldn't be prepared to
put a lot of time and effort into re-inventing more or less what could
already be achieved with conventional controls. If I were going to program
my own system, it would have to do something dramatically better than
anything I could buy off the shelf.

:)
Seriously though - once you get over a diameter or so of extra
insulation - 400mm on a 400mm tank, the gains from further insulation
get quite small.
At the wall of the tank, the circumference is 1.2m or so.
At the outside of a 400mm tank with 400mm of insulation, it's around 5m,
so the effect of an extra mm of insulation is down to 1/4 of that next
to the tank.

I see where you are coming from, but I'm partly doing this for the
enjoyment. There's plenty of scope for incremental enhancements later on,
without hardware mods to the controller.

Steve S

I think this is nice project. You can also add features later like:

Fire boiler for 30 seconds each day it's never used to stop the pump
jamming while you're away.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://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
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

Thanks, Ed. Daily boiler run is already planned, in addition to occasional
exercising of the 3-port valve when CH not in use. Also thinking about
using NTP (or possibly something simpler) for time updates -- needn't worry
about the clocks going forward/back then.

Cheers,

Steve S

Actually you don't have to anyway as the heating season more or less
coincides with the GMT season.

--
geoff

Nah! The charter for that group quite clearly states it's for pictures of
British Gas salescritters. :-)

Steve S

Agreed, from a control point of view it's not necessary, but from a user
interface perspective it is desirable not to have to remember that the
controller is always on UTC.

Steve S