OT - Well a bit OT but heating system related.

[Chris B]

As a personal development exercise I have been trying to teach myself how the Raspberry pi works. I have created a temperature logging system using several DS18B20 temperature probes as in the following link

As a system to log temperatures, I thought I would use my heating system.

The temperature probes are fitted to horizontal pipe runs about 1 meter below the boiler (on the floor of the airing cupboard) and are "attached" to the pipes by being held in place under a piece of foam pipe lagging about 1 foot long. I appreciate this is not perfect but when stable the boiler shows a temp of 64C - so the measured 57/58 is not far out.

The system is set up to log temperatures every 1 minute.

On the following files shares I have shown the flow and return temperatures and the "delta T" which is calculated later in a spreadsheet.

Both pictures are of the same time period its just that the second is a "zoom in". The first picture shows the repeatability of the cycle.

Would anyone like to help me understand the sudden rise in return temperatures in the minute before the flow temperature starts to decrease?

Picture 1 - 2.5 hour duration

Picture 2 - same data but zoomed in to 45 minutes.

I cannot honestly say whether the cycling is due to the thermostat being satisfied/not satisfied as I didn't think of that until looking at the data.

[Tim Lamb]

In message <rra5ef$s28$ [e-mail], Chris B [e-mail]> writes

Would this be where the CH thermostat has switched off the circulating pump and the water is being routed through a bypass circuit? Stored energy in the boiler heat exchanger raising the displayed level beyond boiler switch off setting.

[Andy Burns]

TRVs closing, leading to less heat emitted by rads, hence hotter return temp?

[Chris B]

Well as there are 11 rads on the system and 2 towel rails I thought it a bit unlikely they would all close together and so consistently repeatedly. And do they really close this suddenly?

[Another Dave]

You might also try

comp.sys.raspberry-pi

Another Dave

[Andy Burns]

How well balanced are they? i.e. could one switching off make a big difference?

[Chris B]

Thanks for the pointer, but I have the Pi working - its the heating system I am trying to understand.

[Chris B]

Well that might well be a possibility.

[David]

Just a passing thought, but the boiler should continue to run whilst there is a call for heat and there is cool water in the return.

If all radiators are finally up to temperature then the return temperature to the boiler should rise - quite how swiftly that would be I have no idea.

I assume that the pump will continue to run but the boiler will stop firing until the input temperature drops.

Cheers

Dave R

[Chris B]

So further investigation there appears to be one of these

forming a shorting link between the flow and return pipes.

So might it be Room Thermostat satisfied > closes the heating zone valve and puts out boiler flame.

(there are 2 * 2 port valves, 1 for CH other for HW)

Boiler pump run on timer generates enough pressure on this now dead headed pipe to open this valve and dump flow water direct into return pipe.

delay in flow temp reducing is due to thermal mass of heat exchanger?

[Roger Mills]

I was going to suggest something along those lines. Are you able to monitor and plot room stat on/off events, and also pump on/off events? It's clearly all stopping at one point in the cycle (the point where your graph starts) because the return temperature is higher than the flow at that point - presumably from some residual heating effect. The sudden rise in return temp is entirely consistent with the operation of the by-pass valve during punp over-run after the boiler has stopped firing and the zone vales have closed.

Is it a modulating boiler, or is it all or nothing? It seems remarkable how repeatable the cycles are - so there's very definitely a regular pattern of events occurring.

[Chris B]

Now that is a challenge currently beyond my capabilities with a raspberry Pi - but never say never :-). I may however be able to get the system running and turn down the room-stat at a known time.

Or possibly just an error introduced by the way the temps are measured. I dont think probes being held against pipes with pipe lagging would be good to 1 or 2 degrees, so the return may never be actually hotter than the flow. (This might just be a measurement error).

The

The whole system is just over 2 years old (Worcester Bosch System Boiler) so I assume that it is modulating.

[Dave Liquorice]

Easy way to interface mains with a Pi is a main coil relay and connect a set of contacts to a GPIO, suitably pulled up or down depending on active make or active break. That only tells you that some should be running not that it actually is. To do that a small current transformer feeding a half wave bridge and some smoothing to the base of transistor to acting as a switch and hence to a GPIO.

Note: Pi GPIO's are 3.3 V not 5...

That's the spirit.

ISTR you have metal encased 1-Wire senors, they might be marginly better than a normal transistor like plastic package mounted in a similar way. I've got the plastic package type held in place with a cable tie and blob of thermal compound between pipe and sensor.

Not disassembled a metal cased one to see what's inside. Is it just a normal plastic jobbie inside a tube filled with thermal compound or is the sensor chip more exposed?

I'll tell you one thing though, which you've already discovered, allsorts of weird things happen with pipe temperatures in heating systems, Possibly related to gravity loops starting and stopping, even in fully pumped systems when "off".