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Part 1 by Julian Edgar & John Clarke This smart controller can improve the energy efficiency of your home. It can transfer warm or cool air between rooms automatically when needed. Ducted Heat Transfer Controller T his device controls a mains-­ powered fan that is used to transfer heat between rooms via ducts. The controller can be used manually, automatically, or based on a timer. The wall-mounted LED gives an indication of the temperature difference between rooms. » Powered by the 230V AC mains » Operates during all seasons without changes » Three different operating modes » Adjustable temperature difference and hysteresis » Optional adjustable timer » Optional fire alarm feature » Wall plate button with sound and LED indicators » Sensor disconnection indication » Temperature difference options: 1, 1.5, 2, 3, 4, 5, 6, 8, 10 or 11°C » Hysteresis options: 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8 or 10°C » Timer options: 15m, 30m, 1h, 2h, 3h, 4h, 5h, 6h, 8h, 12h or multiples thereof » Modes: manual, timed, or automatic » Fire alarm function: switches on RLY2 and rapidly pulses the piezo buzzer and LED when the temperature rise of either sensor is >8°C per minute or 70°C is exceeded (this does not replace a smoke alarm!) » Maximum total fan current: 10A reasons reason for this. The first is that the heater has been shut down – the damper closed to reduce the airflow. The second reason for smoke emissions is burning green wood that has high moisture levels. With current heaters that must meet emissions standards, a wood heater burning dry wood at full power produces no visible emissions. But the key point is ‘at full power’ – throttling the heater output reduces its efficiency and increases emissions. That’s where a fan-forced heat transfer duct comes in. It is much better to keep the wood heater burning furiously and transfer some of that heat to other rooms in the house than it is to shut the heater down. Since most homes using wood heating have only one heater, using a transfer duct also works to warm more than just the room where the heater is located (see Fig.1). The second reason for using a ducted heat transfer system is in houses that use passive solar heating. In southern Australia, windows facing north can be used to warm the house in winter. The sun shines in through these windows, heating the wall and floor surfaces of the room, and subsequently the air within. Because the sun is higher in the sky in summer, projecting eaves can shade these windows in summer, so Australia's electronics magazine siliconchip.com.au Advantages The most common reason for using a ducted heat transfer system is when the source of heat for the house is confined largely to one room. There are two likely situations where that would occur: a wood heating stove is located in one room, or passive solar heating occurs largely at one end of the house. While in some jurisdictions, wood heating is frowned upon (for example, the Australian Capital Territory is phasing out wood heaters), wood heaters can be environmentally acceptable and, in some areas, cost little to run. Wood heaters are effectively carbon neutral; the carbon dioxide absorbed by the trees during their growth is released when the wood is burnt. Wood heaters have a bad reputation for emissions – we’ve all seen wood heater flues emitting a stream of smoke for many hours. There are two Features & Specifications 74 Silicon Chip WARNING: Mains Voltage Air return paths are required A heat transfer duct works by moving air – that is, pushing air from one room to another. But unless the air has a return path, the duct will not be very effective. Without a return path, air pressure will rise in the destination room, slowing the transfer of air. It’s therefore best to leave some internal doors open so that good circulation can be achieved. the northern windows don’t heat the house when you don’t want them to. In the northern hemisphere, this is reversed – you want southerly windows. However, the number of rooms in a house that can face north is quite limited, so this type of heating can usually work in only one or two rooms. That’s especially the case if the house was never designed with passive solar heating in mind. In this case, a ducted heat transfer system can be added to move solar heat to other rooms. The problem with commercial options Fan-forced heat transfer ducts are commercially available for installation in new or existing builds (see the photo overleaf). Typically, they comprise flexible ducting and one or two mains-powered fans. Common duct and fan diameters are 150mm, 200mm, 250mm and 300mm. The fan and duct are usually mounted in the roof space with the inlet and outlet grilles located in the ceiling. Generally, these require you to switch them on manually when desired. You can certainly do that, but it’s a little trickier than it first appears. One thing that makes it tricky is that the temperature differences can be very small. For example, in a house that uses passive solar heating, the temperature difference from the ‘warm’ northern room to the southern ‘cool’ part of the house may initially be only 2°C. That difference may increase quite slowly – over hours. Without either walking back and forth to feel the temperatures, or consulting room thermometers, the best time to turn on the fan isn’t at all obvious. That’s if you’re even home at the time! Luckily, this Transfer Controller can do the work for you. Also, you may want the fan to operate for some time after you go to bed – you’re no longer in the heated lounge room, and you want that residual heat distributed through the house. Or you want to be manually in charge of when the fan operates, but with a monitoring LED showing when the heated room is warmer by, say, 3°C than the room at the other end of the duct. Our Controller can perform all these functions. In long ducts, more than one fan may be needed. The controller can run fans up to a total power consumption of 2300W (10A at 230V). Since most This Direct Heat Transfer Controller operates directly from the 230V AC mains supply; contact with any live component is potentially lethal. Do not build it unless you are experienced working with mains voltages. duct fans are quite low in power, it can likely drive however many fans you need. If running multiple fans in the duct, ensure they both blow in the same direction! Operating modes The main function of the Ducted Heat Transfer Controller is to switch on the fan in the duct – the output is simply on or off. However, when it activates that fan depends on the mode. Each mode is selected by switch BCD4 on the printed circuit board (PCB) – as with the other set-up features, it is expected that this will be set and then not frequently changed. In all modes, the user interface is a neat wall-mounted, spring-return rocker-type pushbutton with a white monitoring LED visible around the periphery of the button, and a beeper mounted behind. The other two inputs are temperature sensors – one in the room at each end of the duct. Mode 0 is manual mode. In this case, the pushbutton is used to switch the fan on and off. Mode 1 provides manually triggered timed operation. Pressing the pushbutton switches the fan on for a specified period. Each quick press of the button adds (for example) one hour of operation, so one press gives one Fig.1: a Ducted Heat Transfer System takes the heat from one room and distributes it to one or more other rooms. A fan in the duct is used to move the air, and our controller determines when the fan switches on. Source: Vent-Axia. siliconchip.com.au Australia's electronics magazine August 2025  75 This 150mm Ducted Heat Transfer System uses a single fan to distribute the air to two other rooms. Note that this duct is uninsulated – not a good idea. Source: JPM Brands Switch BCD1 (temp. difference) BCD2 (hysteresis) BCD3 (timer period) BCD4 (mode) 0 1°C 0.5°C 15 minutes Manual 1 1.5°C 1.0°C 30 minutes Timed 2 2°C 1.5°C 1 hour Automatic 3 3°C 2°C 2 hours Automatic 4 4°C 3°C 3 hours Automatic 5 5°C 4°C 4 hours Automatic 6 6°C 5°C 5 hours Automatic 7 8°C 6°C 6 hours Automatic 8 10°C 8°C 8 hours Automatic hour, two presses gives two hours etc, up to a maximum of five presses. A BCD switch preset determines the base period, from 15 minutes to 12 hours. Mode 2 is fully automatic. In this mode, the fan operates when the temperature difference between the two ends of the duct exceeds a preset threshold. In addition to mode selector switch BCD4, the PCB has three more adjustments. BCD1 is used to set the temperature difference that needs to occur before the fully automatic mode (Mode 2) switches on the fan. This can be set to 1, 1.5, 2, 3, 4, 5, 6, 8, 10 or 11°C. BCD2 is used to set the hysteresis. This is the difference between the switch-on and switch-off temperatures. This can be set to 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8 or 10°C. It must be set lower than the temperature difference. Let’s imagine the temperature difference is set to 4°C and the hysteresis is set to 1°C. If the heated room is at 20°C and the unheated room is at 16°C (a difference of exactly 4°C), the fan will switch on. It will stay on until the difference in temperature decreases to 3°C; eg, the unheated room warms to 17°C. In use, if the fan switches on and off too frequently, increase the hysteresis setting. On the other hand, if the temperature of the room at the other end of the duct varies up and down too much, decrease the hysteresis. BCD3 sets the timed period that occurs in Mode 1 with each button press. In the example above, I suggested that each press gives a onehour extension of the on-time. However, each button press can actually be set to be 15m, 30m, 1h, 2h, 3h, 4h, 5h, 6h, 8h or 12h. Refer to Table 1 for all the BCD switch settings. 9 11°C 10°C 12 hours Automatic Monitoring LED and beeper Table 1 – BCD switch settings While we have described the function of the controller as operating a fan-forced duct that transfers warm air to a cooler room, the system can also transfer cool air to a warmer room. In fact, no changes are needed to do this because the system operates based on the temperature difference between the two rooms, rather than how much cooler the room is at the far end of the duct. For example, say you have the difference in room temperature set to 3°C and the Mode set to 2 (Automatic). When the room at the end of the duct is 3°C warmer than the room at the beginning of the duct, the fan will switch on, transferring cooler air to the hotter room. Of course, the source room needs to be the same room in both winter and summer. In addition to the pushbutton switch, the wall-mounted indicator is equipped with one LED and a beeper. The beeper operates in the same way in all modes: a single beep indicates switch on (a short press of the button) and a triple beep indicates switch off (achieved by a longer press of the button). The triple beep comprises a single beep followed by a quick double beep. The LED can show different information in each mode. In manual mode, if the fan is off, the Australia's electronics magazine siliconchip.com.au What about transferring cool air? 76 Silicon Chip LED is off, possibly flashing on briefly. If the fan is on, the LED is on, possibly flashing off briefly. If it’s flashing briefly every two seconds when the fan is off, that indicates the measured temperature difference is greater than or equal to the set temperature difference, so you might want to switch it on. Similarly, if it’s briefly flickering off while the fan is on, that means the temperature difference has fallen below the set difference (including hysteresis), indicating you may want to switch it off. Manual Timed operation (BCD4 position 1) has LED behaviour that is the same as the manual mode. Automatic mode (BCD4 position 2) has different LED behaviour. If the system has been disabled, the LED flashes. If the fan is on, so is the LED. If the fan is off, again, so is the LED. A summary of these modes is shown in Table 2. Other potential uses This device can also control a powered ventilator or fan; for example, one that ventilates a hot roof cavity in summer. In this use, one temperature sensor is placed in the roof cavity (or other hot area needing ventilation) and the other outside in an area protected from the weather (eg, under the eaves). In this application, the best settings will probably be Mode 2 (automatic), with the temperature difference set higher than you would use for internal house use (eg, 10°C with 5°C of hysteresis). Another use is for solar air heaters. While uncommon in Australia, these have been widely used in solar homes in the United States. In this approach, air is heated by a flat plate collector – a little like a traditional solar water heater but with air rather than water heated through contact with the plate. When the air in the heater rises sufficiently in temperature, a fan can be used to move that heated air into the house through conventional air-­ conditioning ducts. In this application, one sensor would be placed so that it is exposed to the air in the heater (but shielded from direct sunlight), while the other would be placed inside the house. The temperature difference would be set quite low (eg, 3-4°C, with perhaps 2°C of hysteresis). Parts List – Ducted Heat Transfer Controller 1 polycarbonate IP65 enclosure, 171 × 121 × 55mm [Altronics H0478, Jaycar HB6218] 1 double-sided, plated-through PCB coded 17101251, 151 × 112mm 1 lid panel label (84 × 65mm) and side panel label (64 × 10.5mm) 1 3VA 9+9V PCB-mounting mains transformer (T1) [Altronics M7018A] 1 FRA4 250V 30A AC SPST relay with 12V DC coil (RLY1) [Jaycar SY4040] 1 PCB-mounting 250V 10A AC SPDT relay with 12V DC coil (RLY2) [Altronics S4160C, Jaycar SY4066] 4 PCB-mounting 10-position BCD switches (BCD1-BCD4) [Altronics S3001] OR 4 2×4-pin headers and 12 jumper shunts 1 2-way header, 2.54mm pitch (JP1) 1 jumper shunt (JP1) 2 15A 300V 2-way screw terminals, 8.25mm pitch (CON1, CON2) [Altronics P2101] 1 2-way screw terminal, 5/5.08mm pitch (CON3) 1 3-way screw terminal, 5/5.08mm pitch (CON4) 3 8P8C RJ45 PCB-mounting horizontal sockets (CON5-CON7) [Altronics P1448A] 1 IEC mains input socket with integral fuse [Altronics P8324, Jaycar PP4004] 1 mains lead with IEC plug 1 surface-mounting mains socket (GPO) [Altronics P8241, Jaycar PS4094] 1 20-pin DIL IC socket (optional, for IC1) 1 fast-blow 10A M205 fuse (F1) Hardware 2 M4 × 10mm panhead machine screws with matching hex nuts 2 M3 × 15mm panhead nylon machine screws 5 M3 × 6mm panhead machine screws 3 M3 brass hex nuts 1 200mm cable tie and 8 100mm cable ties 1 3-6.5mm diameter wire entry cable gland Wire & cable 1 200mm length of black 7.5A hookup wire 1 50mm length of light-duty red hookup wire and light-duty black hookup wire assorted lengths of 10A mains-rated green/yellow striped wire (150mm length); brown wire (200mm length); and blue wire (100mm length) 3 Cat 5, Cat 5E or Cat 6 patch leads, lengths to suit installation assorted lengths of clear heatshrink tubing (70mm length, 5mm diameter; 30mm length, 4mm diameter; and 50mm length, 1mm diameter) Semiconductors 1 PIC16F1459-I/P microcontroller programmed with 1710125A.HEX, DIP-20 (IC1) 1 7805 1A 5V linear regulator, TO-220 (REG1) 3 BC337 NPN transistors, TO-92 (Q1-Q3) 1 W02(M) or W04(M) 1.5A 200V/400V bridge rectifier (BR1) 16 1N4148 200mA 75V diodes (D1-D16) 3 1N4004 1A 400V diodes (D17-D19) Capacitors (16V PC radial electrolytic, unless specified) 2 470μF 1 100μF 2 100nF 63V/100V MKT polyester Resistors (all ¼W, 1%) 5 10kW 2 2.2kW 4 1kW 1 470W Control panel parts (per panel) 1 double-sided, plated-through PCB coded 17101253, 51 × 67mm 1 Clipsal Iconic 3041G single Gang Switch Grid Plate ● 1 Clipsal Iconic 3041C-VW single Gang Switch Plate Cover (Skin Only) ● 1 Clipsal Iconic 40FR-VW Fan Dolly Rocker Vivid White ● 1 Clipsal Iconic 40MBPRL-VW 10A Momentary Bell Press Switch Mechanism with LED ● 1 panel label, 45 × 30.5mm 1 top-entry 8P8C vertical RJ45 socket (CON10) [Altronics P1468] 1 3-16V self-oscillating piezo buzzer [Altronics S6104] 1 2-way vertical polarised header, 2.54mm pitch, with matching plug and pins (CON11) 1 2-way terminal block, 5/5.08mm pitch (CON12) 1 8P8C double adaptor (only required if using two control panels) [Altronics P7052A] ● available from electrical wholesalers, including www.sparkydirect.com.au The complete circuit for the Ducted Heat Transfer Controller is shown in Temperature sensor parts 2 60 × 60 × 20mm vented enclosures or similar [Jaycar HB6116] 2 double-sided, plated-through PCBs coded 17101252, 20 × 37.5mm 2 8P8C RJ45 PCB sockets (CON8, CON9) [Altronics P1448A] 2 DS18B20 temperature sensors (TS1, TS2) [Altronics Z7280 or Z6386] siliconchip.com.au Australia's electronics magazine Circuit details August 2025  77 Fig.2. Microcontroller IC1 monitors the temperatures via sensors TS1 & TS2, which connect to the main board via 8-way Cat 5 (or similar) cables and RJ45 plugs/sockets. In each case, pin 4 carries the digital signal, pin 8 the 5V supply for the sensor and pins 5 & 7 are grounds. TS1 & TS2 are Maxim DS18B20 1-wire digital thermometers. Just one data line (DQ) is required for serial communications. A minimum of one extra connection for the common ground connection is also required. Power for the sensor can be derived from the data line, but we include a Enabling the fire alarm feature The Ducted Heat Transfer Controller can also be configured as a fire alarm. Because the system has temperature sensors that would normally be placed at divergent ends of the house, monitoring of these sensors provides a widespread back-up system to the legally required smoke detectors. When this function is enabled by shorting the pins of JP1, each temperature sensor is monitored for both the temperature and the rate of temperature change. If the temperature exceeds 70°C and/or the rate of temperature change exceeds 8°C per minute, the beeper and LED rapidly pulse. Relay RLY2 is also energised, which can power a low-voltage warning siren, switch on low-voltage lights etc. If the fire alarm goes off, a short press of the wall-mounted button will switch off the buzzer, but the LED will continue to flash. A long press will switch off the buzzer, LED and RLY2, and the system will be re-armed to monitor again for fire. Note that this is a mains-powered system with no battery back-up. It should always be used in conjunction with traditional battery-powered or battery-­ backed smoke detectors. We suggest that this function be activated in all installations since it’s unlikely to ever be triggered unless there is a fire. 78 Silicon Chip Australia's electronics magazine direct 5V supply connection (Vdd/V+) since we have enough wires and this makes signalling easier. Two-way communication between the microcontroller and temperature sensor is possible since the DQ pin is an open drain with a pull-up resistance of 2.2kW. Open drain means that the drain of a Mosfet connects to this pin, so when the Mosfet is on, the pin is pulled to 0V, while if it is off, it is pulled up by the resistor. A Mosfet at either end of the wire can be used to pull it down to 0V, so a signal can be sent by the device at either end of the wire. The microcontroller uses its RC2 and RB4 I/O pins to request temperature readings and get them from the sensors. The DS18B20 has a temperature reading accuracy of ±0.5°C from -10°C to +85°C. Temperature readings are available in 0.125°C steps, but for this project, we measure the temperature in 0.5°C increments. BCD switches The four BCD switches that select the various mode, temperature and timer features have internal contacts siliconchip.com.au Fig.2: microcontroller IC1 reads the positions of BCD switches 1-4 (or the alternative jumper sets) to determine is jobs. It then reads the temperatures from sensors TS1 & TS2 connected via Cat 5/5E/6 cables and determines when to energise relay RLY1 to connect mains power to the fan(s). that connect the “1”, “2”, “4” and “8” terminals to ground in a combination that totals to the switch setting. For example, if the switch is set to the 9 position, the “1” and “8” terminals will be connected to ground but the other two won’t. This allows IC1 to sense 16 possible positions for each switch using four wires (although these switches only have 10 positions). Rather than the common (C) terminal of each switch being connected to ground, they are connected to a separate pin on microcontroller IC1. This way, the micro can pull them high one at a time, and use the same four lines (RA1, RC5, RA0 & RC4) to read the position of the selected switch. Isolation diodes D1-D16 are required because, while the other switches can be set to have their common terminals floating while one switch is sensed, those switches could still end up effectively shorting two or more of the sense lines together, depending on their positions. We need the diodes to ensure the switches don’t affect each other during the sensing procedure. siliconchip.com.au During switch sensing, any open BCD switch will be pulled low to 0V via one of the 10kW pull-down resistors. BCD switches can be expensive, so we have provided an alternative system using a 2×4-pin header with up to four jumpers placed on it to replace each BCD switch. Fig.3 shows how the jumper settings equate to BCD settings. Since these settings are rarely (if ever) changed, there’s little disadvantage in using jumpers on headers instead. Control Panel The wall-mounted control panel for the Ducted Heat Transfer Controller comprises switch S1, LED1 and a piezo buzzer. This is all incorporated in a Clipsal sprung-return rocker switch plate that includes an indicating LED. The piezo buzzer is an addition to the switch Fig.3: this shows the simple binary codes you need if using jumpers instead of the BCD switches. IC1 also monitors switch S1 and the four selection switches, BCD1 to BCD4. In response to these settings and temperature readings, the microcontroller can sound the piezo buzzer, light LED1 and switch on RLY1 to drive the duct fan. IC1 can also switch on RLY2 if the fire alarm feature is selected with JP1 and is then activated. Australia's electronics magazine August 2025  79 The Ducted Heat Transfer Controller is housed in a polycarbonate IP65 enclosure (upper right photo). An IEC mains cord supplies power and the duct’s fan plugs into the power outlet on top. The temperature sensor and control panel connections are made using RJ45 sockets and Cat 5/5E/6 cables. The Controller is easy to build, with only through-hole components used. Care must be taken with the mains voltage wiring, though. The faceplate (upper left photo) incorporates a momentary rocker switch, piezo buzzer and a white LED that lights the periphery of the switch. The wall plate can be mounted vertically or horizontally – this one is configured for vertical mounting. The ‘floppy ears’ can be easily removed (they’re not needed for normal mounting). The room temperature sensors are each located in small, ventilated wall enclosures (photo shown at right). 80 Silicon Chip Australia's electronics magazine plate to complete the control panel. The control panel connects to the main board via another Cat 5/5E/6 cable and RJ45 plugs/sockets. LED1 is driven from the RB6 output of IC1 through a 470W resistor to ground. The LED current is around 4.25mA, assuming a voltage drop of 3V across the white LED. Switch S1 is connected between GND and the RB5 input of IC1, with this input pulled to 5V via a 1kW resistor when the switch is open. If the switch is closed, RB5 will be pulled to GND and IC1 can detect that. The piezo buzzer is powered from 12V using transistor Q3 to switch the negative side to ground. When the buzzer is required to sound, the RC7 output of IC1 is driven high to switch on Q3 by delivering current to its base through a 1kW resistor. Relays RLY1 & RLY2 are switched on via the RC3 and RC6 outputs of IC1, respectively. Both use a 1kW base resistor to drive a transistor to power the relay coil. Transistor Q1 is used for RLY1 and Q2 for RLY2. Diode D17, across RLY1’s coil, and D18, across RLY2’s coil, quench the back-EMF voltage from the coil when these are switched off. RLY2 is uncommitted and is intended to drive a low-voltage siren for the optional fire alarm function. RLY1 connects the incoming mains Active to the fan socket when the fan should be powered. The output socket’s Neutral and Earth pins are permanently wired to the input socket. Power for the circuit is derived via a mains transformer that produces a 9V AC output. This is rectified by bridge rectifier BR1 and filtered by two 470μF capacitors, giving close to 12V DC. This is used to power the two relays and the piezo buzzer. REG1 is a 5V regulator that drops its 12V input to 5V to supply IC1 and the DS18B20 temperature sensors. Next month That’s all we have space for this issue. Next issue, we’ll cover building the unit and setting SC it up. siliconchip.com.au Table 2 – smart remote push button/LED/buzzer Mode Push button/buzzer Fan status Faceplate LED ‘0’ Manual fan on/off Short press – beep – on Runs when fan manually switched on Fan off Temp difference < set point LED off Longer press – double beep – off Fan off Temp difference > set point LED flashes momentarily on once every 2s Fan on Temp difference < set point LED flashes momentarily off once every 2s Fan on Temp difference > set point LED fully on ‘1’ Manual fan timed operation Quick press or presses = on for set Runs for period of operation, e.g. when timer timer period is set for 30m, 1 quick press runs when set fan for 30m, 5 quick presses sets ‘on’ period at 150m (2.5h) Longer press – double beep – off Fan off Temp difference < set point LED off Fan off Temp difference > set point LED flashes momentarily on once every 2s Fan on Temp difference < set point LED flashes momentarily off once every 2s Fan on Temp difference > set point LED fully on ‘2’ or more Automatic Short press – beep – system on Runs when System disabled temperature LED flashing difference exceeds Fan off preset level LED off when system activated Fan on LED on Longer press – double beep – system switched off Fire alarm activated (JP1 shorted and fire detected) Buzzer sounds rapidly and LED flashes rapidly at 5Hz Fan off N/A Short press, buzzer sound is off, LED flashes rapidly Long Press, LED and buzzer off and retests for fire siliconchip.com.au Australia's electronics magazine August 2025  81