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Part 2 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 L ast month, we introduced the Ducted Heat Transfer Controller that switches a fan used to move heat between rooms that are at different temperatures. This month, we describe how to build it and set it up. We will also show an example installation in detail. Device layout The Ducted Heat Transfer Controller is made using three different PCBs. The main PCB holds most of the components and is installed within a 171 × 121 × 55mm polycarbonate IP65 waterproof enclosure. The second PCB is for the control panel. This mounts at the rear of a Clipsal rocker switch plate and hosts the switch, LED and piezo buzzer. The final PCB is for the temperature sensor. You will need two of these – one for each sensor. This PCB simply provides a connection between the 8P8C (RJ45) socket and the DS18B20 temperature sensor. These boards can be housed within small vented enclosures, such as Jaycar’s HB6116, which has room for the sensor end of the PCB. The larger HB6114 allows the whole PCB to fit. Alternatively, you can use the probe version of the DS18B20 and install the PCB within the wall cavity, with the probe exposed to the room air. Both the control panel and the 68 Silicon Chip temperature sensor boards connect to the main PCB using 8P8C (RJ45) plug-terminated Cat 5, Cat 5E or Cat 6 cables. You can also have two control panels, with one in each room. In this case, they connect to the main PCB using an 8P8C (RJ45) double adaptor and extra Cat 5/5E/6 leads. You can use pre-made Cat 5/5E/6 cables in fixed lengths with connectors already fitted at each end, or make your own using suitable cable, connectors and a crimping tool. Main PCB construction The main PCB is coded 17101251 and measures 151 × 112mm. Fig.4 shows the parts layout on this board. Begin by installing the resistors. Their colour codes were shown in the parts list last month, but you should also use a digital multimeter to check each resistor before mounting it in position (sometimes the colour bands are hard to distinguish). Diodes D1-D19 are next on the list. Make sure these are orientated correctly and that the correct diodes are installed at the right location before soldering their leads. D1-D16 are the smaller 1N4148 signal diodes, while D17-D19 are larger 1N4004 power diodes. In each case, the cathode end is indicated by a band, so match those up to the PCB silkscreen and Fig.4. Bridge rectifier BR1 (containing four Australia's electronics magazine power diodes) can then be installed, taking care to orientate it with the correct polarity. We used a socket for IC1. However, this IC could be soldered in place, assuming it has already been programmed with the Ducted Heat Transfer Controller firmware (it’s available as a download from siliconchip.au/ Shop/6/1835). As mentioned last month, the PCB is designed to use either BCD switches for BCD1 to BCD4, or alternatively, a 2×8-pin header instead of each switch. Install the BCD switches or the DIL headers that go in the middle of their footprints, depending on which you prefer. Also fit the two-pin header for JP1 now. The capacitors can now be fitted. Two types are used: electrolytic and MKT (polyester). The electrolytic capacitors need to be orientated correctly since they are polarised (the longer leads are positive), while the MKT capacitors can be installed either way around. REG1 is installed horizontally and secured with an M3 screw and nut. Bend the leads to insert them into the pads before soldering the leads in place. Q1-Q3 can also be installed now; they are all the same type and orientated identically. Connectors CON1 through to CON4 can now be installed. Note that the siliconchip.com.au Fig.4: the main PCB is straightforward to assemble. If you don’t want to install the BCD switches, instead solder a 2×4 pin header into the eight pads in the centre of the switch locations and use jumpers. Watch the orientations of IC1, the diodes, BR1, electrolytic capacitors and BCD switches. wire entry for CON3 is toward REG1, while for CON4, it is towards the nearest edge of the PCB. Then fit the three 8P8C RJ45 connectors (CON5CON7). The next step is to mount relay RLY1 on the PCB with its coil terminals toward CON3. The relay is secured in position using M4 screws and nuts, with each screw inserted from the underside of the PCB. RLY2 is soldered directly to the PCB. Transformer T1 is a PCB-mounting type. A cable tie that wraps around the transformer and is tied to the PCB by passing it through the slots provided. The cable tie is necessary to prevent the transformer body from being pulled off the PCB when only supported via the soldered pins, so make sure it’s tight. Once it’s firmly anchored, solder its leads. Case preparation The main PCB is secured to the enclosure base using M3 screws into the integral brass inserts. However, before attaching the PCB, you will need to make cutouts for the IEC connector at one end of the enclosure and the 8P8C sockets at the other, as shown in Fig.5. You also need to drill and shape holes for the GPO socket in the lid. The large cutouts for the mains GPO and IEC connector can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing the edges to a smooth finish. Alternatively, use a speed bore drill to remove the bulk of the area before filing it to shape. If you are using the Fire Alarm function, you will also need a hole for a cable gland to allow wiring to pass through and connect to RLY2 via CON4. Once the drilling and filing is complete, move on to the IEC connector. Cover the Active busbar metal strip on the rear with a layer of neutral-cure Fig.5: use these diagrams to mark and then cut out the required holes in the enclosure. siliconchip.com.au Australia's electronics magazine September 2025  69 Fig.6: these labels can be printed out and stuck on the switch plate and main enclosure. If you choose not to use the labels, ensure you mark the sockets for the switch plate and two temperature sensors. silicone sealant (eg, roof and gutter silicone) to prevent it from being a shock hazard, then mount the connector to the case. The IEC connector must be attached using 10mm-long nylon M3 screws, although metal nuts can be used. Using nylon screws means they cannot become live should a mains wire inside the enclosure come adrift and contact the screw. The PCB can then be placed inside without securing it into the integral brass inserts just yet. You can download the panel label artwork shown in Fig.6 (siliconchip. au/Shop/11/1844) and print it out at actual size to make the panel labels. Details on making an adhesive front panel can be found on our website at siliconchip.au/Help/FrontPanels Now wire it up as shown in Fig.7. All wiring must be run using mainsrated cable. Be sure to use 10A cable for all connections except those to CON3 or CON4, where you can use either 10A or 7.5A mains-rated wire. Note that brown wire is used for the Active wiring, while blue (ideally light blue) is used for the Neutral leads. The green/yellow-striped wire must be used for Earth wiring (only), and the Earth lead from the IEC connector goes straight to the GPO. Be sure to insulate all the connections with heatshrink tubing for safety, and cable tie the wires where shown to prevent any wire breakages coming adrift. The Active and Neutral leads are secured to the GPO using a cable tie that passes through the hole in its moulding. 70 Silicon Chip Take great care when making the connections to the mains socket (GPO). In particular, be sure to run the leads to their correct terminals; the GPO is marked A or L for Active or Live, the Neutral terminal is marked N and the Earth terminal E. Do the screws up tightly so that the leads are held securely. Similarly, make sure that the leads to the screw terminals are firmly secured. Control Panel assembly The Control Panel PCB is coded 17101253 and measures 51 × 67mm, as shown in Fig.8. Solder the vertical 8P8C connector, polarised header and terminal block on the top side. Make The temperature sensor PCB is placed through a hole suitably drilled and filed in the rear wall of the enclosure. The RJ45 socket in accessible from the rear. In use, the socket and cable protrude into the wall cavity. Australia's electronics magazine sure the terminal block wire entries face away from CON11. The piezo buzzer can then be soldered on the other side, with its + terminal orientated as shown. The LED will be supplied already wired with current limiting resistors and a diode suitable for being powered via the mains voltage, with all exposed connections heatshrink wrapped – see the photo at the bottom right corner of the page. Slit the heatshrink tubing down one side and remove it to expose the two LED leads. Remove and discard the original diode and resistor. Solder short lengths of hookup wire to the LED and cover the joints with 1mm diameter heatshrink tubing. These wires can then be crimped to pins and inserted into the plastic block to plug into 2-way header CON11. Two wires are also required for the switch terminals to CON12. Make those connections using 7.5A mains-rated wire or similar. This wire size works best for the switch terminals that are designed for heaver gauge wire compared to light-duty hookup wire. A 14mm hole needs to be drilled in the 3041G single Gang Switch Grid Plate for the piezo buzzer, while a 2mm hole should be drilled in the 3041C-VW cover plate for the buzzer sound to exit. Temperature sensors The temperature sensor adaptor PCB is coded 17101252 and measures 20 × 37.5mm. It is shown in Fig.9. Assembling the temperature sensor PCBs involves installing the siliconchip.com.au WARNING: Mains Voltage 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. Fig.7: take care when doing the mains wiring. Use the correctly coloured and current-rated wire and secure the wiring with cable ties as shown. temperature sensor and the 8P8C socket on each PCB. If you are using the temperature probe package version of the sensor, instead of the TO-92 package version, then be sure to connect the wires to the correct GND, DQ and Vcc terminals. The wire colours are black for ground (GND), yellow for data (DQ) and red for 5V Power (Vdd). In this case, we suggest you cable tie the leads to the PCB using one of the PCB corner mounting holes as an anchor point for strain relief. indication at power-up, or when a sensor is disconnected while the system is powered. One beep means TS1 is disconnected, while two beeps mean TS2 is disconnected. If both are disconnected, both sound indications will occur, one after the other. This indication will occur once only for each sensor. ◀ Fig.8: there are only four parts on the Control Panel PCB so it’s easy and quick to assemble. Testing Fig.9: the temperature sensor PCB is even simpler, with only two parts. Thoroughly test the system before installing it. Do this by first selecting the four BCD switch positions that give the mode, temperature difference and hysteresis you will likely require (see Table 2 from last month). Re-secure the lid and plug in the two temperature sensors and the wall plate control switch. For this testing, you can use short Cat 5/5E/6 leads if you have them. If one of the temperature sensors is not connected, there will be an siliconchip.com.au The Ducted Heat Transfer Controller can still be used without temperature sensors; however, without the temperature readings, the unit can only be used in modes 0 or 1, and without the fire alarm or LED temperature monitoring features. If the fire alarm sounds, a quick press of S1 will silence the buzzer, but the LED will continue to flash at 5Hz. The LED provided with the switch plate is wired for mains power. In our application, it is driven from a low voltage, so both the resistor and diode need to be removed. Australia's electronics magazine September 2025  71 A long press will clear the fire alarm. The fire alarm will sound again if the temperature rise of either temperature sensor is >8°C/min or if 70°C is exceeded. Plug a mains load (eg, a lamp) into the GPO and then connect power via the IEC socket. Warm one of the sensors (your fingers can do this if you’ve set the setpoint and hysteresis values fairly low) and check that the lamp activates as you’d expect. Also check that the wall plate control switch works correctly for the mode you’ve selected, and that the LED flashes appropriately. Table 1 last month showed the modes and other switch settings. You can also refer to the sections titled “Operating modes” and “Monitoring LED and beeper” in that article for a description of how the fan, switch and LED should behave in each mode. If you wish to check other modes, you can disconnect the power, open the lid and then alter the BCD switches appropriately. However, if the system works in one mode, it should also work in the others. If you have the fire alarm link in place, check that if you rapidly heat one of the temperature sensors (eg, using a hot air gun) that the LED and buzzer pulse quickly. If you find any problems, first disconnect power and then very carefully check your wiring, parts locations, parts orientations and soldering. doesn’t matter which sensor goes in which room. The wall plate switch also connects to the controller via Cat 5/5E/6 cables and plugs. Such cables are available readymade in a variety of lengths, or you can buy the cable, plugs and a suitable tool and make your own with custom lengths. The controller plugs into mains power via an IEC cable and the duct fan plugs into the GPO socket on the controller. All the cables should be laid without any kinks or being stretched and should be fastened into place with cable ties and/or wiring clips. Installation Temperature sensor locations The controller needs two temperature inputs, one in the source room and the other in the destination room. These connections are made by Cat 5/5E/6 cables with RJ45 plugs. It The locations of the two temperature sensors are important. When using the system to transfer heated air, in the room providing the heat Insulate the ducts! Many commercial heat transfer ducts use uninsulated ducts, but that is a poor idea. The heat transfer duct comprises four main parts: • An intake grille in the ceiling of the warm room • An outlet grille in the ceiling of the room to be warmed • A duct in the ceiling connecting the two • One or more fans located in the duct All these components are in the ceiling space, which is typically poorly insulated and so is a similar temperature to the outside air. In modern houses, a roof blanket is often using to insulate the roof and so the ceiling space, but this is usually much less effective than the ceiling insulation. The roof blanket also doesn’t cover the eaves. So we have a duct that draws warm air in, and in the transfer to the other room, potentially loses a lot of that heat to the roof space. Furthermore, even when the fan is not operating, major heat loss can occur through the duct. So instead of making your home more energy efficient, you’ve made it less! The answer to this problem is to use an insulated duct. Flexible ducts suitable for heat transfer are available in a range of insulation values, where the higher the R value, the better the insulation. Ducts can be bought with R1, R1.5 and R2 insulation. I could not find any ducts better insulated than R2 – in fact, I only saw one example of R2 insulated ducts. These are made by Bradflo and are available in a variety of diameters. The Bradflo R2 duct is available by special order through Metalflex (a sister company to Reece Plumbing). Of course, you can buy uninsulated ducts and insulate them yourself, or if using insulated ducts, add to the insulation that is already there. The neatest and easiest way of achieving this is to use roof blanket insulation, which comprises aluminium foil and a thin layer of fibreglass insulation. This can be wrapped around the duct, aluminium foil outwards, with the joins made with tape. Roof blankets are rated at R1.3. The insulation value rises with thickness, so if you added two wraps of roof blanket (offset the joins) to an uninsulated duct, you’d have a total value of about R2.6 (probably a bit less because the foil doesn’t add up in the same way as the fibreglass). Flexible ducts will lose a lot of their flexibility when wrapped in this way, so it is best to position the duct in the ceiling before wrapping it. Note that the same potential for heat loss occurs even if the duct is placed under the floor. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au (the source room), the sensor should be placed high in the room – near the ceiling. This is because hot air rises, and so once warm air is available for transfer, the controller should be able to measure it. Conversely, in the room receiving the heat (the destination room), the sensor should be placed closer to shoulder height – that is, measuring the temperature of the air that the occupants will feel. Where the system is being used to transfer cool air, or warm air in winter and cool air in summer, both sensors should ideally be at shoulder height. In all cases, the sensors should not be placed close to the duct openings – the flow through the ducts will affect local temperature readings. The enclosures in which the temperature sensors are placed should be a light colour. If they are painted a dark colour, they will absorb radiant heat, especially if exposed to direct sunlight, so the temperature reading may not reflect the true air temperature. Setting the temperature difference Setting the temperature difference to a low value will cause the fan to operate earlier as the room providing the heat warms up. However, if this value is set too low, the air may not have sufficient heat in it when it reaches the destination room. This is because even if they are insulated, all ducts will lose some heat (see the panel on insulating the ducts). For a given level of insulation, the longer the duct, the more heat loss that will occur. To put this a different way, if the temperature difference is set too low, the duct may blow cold air into the destination room! The temperature difference at which the fan will turn off is called the hysteresis. If the fan switches on and off a lot, increase the hysteresis. Conversely, if the temperature in either room varies up and down noticeably, decrease the hysteresis. Conclusion Using a heat transfer duct with our automatic controller can improve your home’s energy efficiency, comfort levels and, especially if using passive solar heating, reduce heating costs. Our controller has sufficient versatility to work in nearly all situations where heat transfer is needed and can be used in either a new house or where a heat transfer duct is being retrofitted. See overleaf for a panel on how Julian Edgar installed the Ducted Heat Transfer Controller in his house. The transformer is held in place with a cable tie. The mains power connections are insulated with heatshrink and silicone sealant is used to insulate the exposed terminals on the IEC connector. This board uses the BCD switch option. ◀ Shown to the left is one of the temperature sensor PCBs. Up to two of them can be connected to the Controller PCB. siliconchip.com.au The rear of the switch plate in assembled form (shown above). There is a hole drilled in the rear plate for the buzzer to protrude through, and a smaller hole drilled in the faceplate to allow the sound to come out. This PCB is a snug fit around the switch mechanism; it can be held in place with a little silicone. September 2025  73 The details of our installation Photo 1: Bradflo 250mm ducting, insulated to R2.0. Photo 2: another layer of insulation was wrapped around the outside of the duct. Photo 3: two Papst 24V 250mm brushless fans were used. Photo 4: one of the Papst fans taped to the inner flexible ducting. 74 Silicon Chip In our installation, a long, straight duct was used to link the two end rooms in a rectangular-shaped house that is currently being built. Each of the two end rooms has a cathedral (raked) ceiling, meaning that each has an interior wall that adjoins the roof space. The duct joins vents in each of these walls. We used Bradflo 250mm R2.0 insulated ducting. This had a further layer of R1.3 foil and fibreglass insulation wrapped around it. The joins were made using 75mm-wide ProctorPassive YouRippa tape, which has excellent adhesion and is airtight. That’s in contrast to the aluminium flashing tape I used first, which did not adhere well enough to remain sealed. The duct is 14m long, so two fans have been used. They are Papst units from a Bradford Ecofan Subfloor Ventilation system. I chose them because of the brand quality, their brushless DC design and the fact they work from 24V plugpacks, removing the need for mains wiring connections to the duct. The retail price for the Ecofan Subfloor Ventilation varies a lot, so if you decide to use the same fans as I did, it pays to shop around. These fans come with control boxes that allow the selection of three fan speeds. They are usually screwed to grilles, but in our application, we want to insert the fans within the ducting so I removed the grilles. To minimise noise, the fans were placed within the duct, about 1m from each end. The fans are light enough to be supported by the ducting, and the fan shroud’s diameter and circular shape means the inner ducting can be pulled over the shroud and taped into place. Once the fan was inserted into the duct, the area was re-insulated with standard Bradflo duct insulation and the additional layer of R1.3 insulation. The metre of ducting between the fans and the vents reduces aerodynamic noise, and the double layer of insulation around the fans reduces vibration (and so noise) transmission to the house’s framework. When running, the fans are inaudible on the slowest speed setting, just audible on the medium setting and can be heard (but not at an objectionable level) on their fastest speed. If you choose a duct that’s large enough, you shouldn’t need to run them at maximum speed for sufficient heat transfer. It is difficult to work out ahead of time how much airflow will be needed to heat the destination room. Therefore, in this new house build, provision was left for the installation of a second parallel duct, should it be needed. If you find you do need to run the fans at full speed, you could consider a second duct, allowing them to run them slower for similar aiflow. The original grilles from the Bradford ventilation system were not used. Instead, 250mm cone diffusers were placed at each end of the duct. These likely provide less restriction that the more intricate Bradford grilles, and were also chosen to be a styling match for additional grilles used for other purposes. The Heat Transfer Duct Fan Controller was located in the roof space near to one of the fans. The controller is accessible in this location from a loft space. The second fan’s plugpack is fed by a long extension cable that uses a male/ female plug, allowing the cable and the first fan’s plugpack to both plug into the controller’s GPO. The temperature sensor in the source room was placed high on the wall, with the other in the destination room at shoulder height. The wall plate with the LED, buzzer and rocker switch was placed in the source room – just one Control Panel was used. Editor’s note: we have been pleased with inexpensive mains-­powered axial flow fans we purchased from AliExpress (we used one for our laser cutter exhaust). Search AliExpress for “axial fan hydroponics”. Similar fans are availSC able on eBay. Australia's electronics magazine siliconchip.com.au Photo 5: the white tape and cable show the location of the inserted fan. Another two layers of insulation were wrapped around this spot, reducing heat loss from this area and making the fan quieter. Photo 6: the duct is 14m long and rests on two added longitudinal timbers. It is stiff enough that it could probably have just been draped across the ceiling joists. Photos 7 & 8: one of the 250mm grilles, called a ‘cone diffuser’ in ventilation circles, prior to plasterboard installation. My vents are in the walls rather than the ceiling. Photo 9: one of the temperature sensors out of its box. This one is located high on a wall on a sheet of bracing plywood. Photo 10: the switch plate temporarily installed before the addition of plasterboard. Photos 11 & 12: the installed Ducted Heat Transfer Controller with the insulated duct visible behind. siliconchip.com.au Australia's electronics magazine September 2025  75