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Furnace relay for home thermostat

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So, I'm planning a home automation system based around wireless nodes (MSP430G2553 plus NRF24L01+ module). Part of the motivation for doing this is that the wires carrying 24VAC between the furnace in the basement and the thermostat 2 floors up have broken and it's a terrible pain to fish new wires in a 100 year old house with knob and tube wiring inside the walls.


The PCBs for the nodes are on the way from Elecrow, giving me an opportunity to experiment with the radio modules and start on some other bits and pieces. Here is a simple relay box that will be used to switch the 24VAC line to the furnace contactor. It uses a double coil latching relay bought for some long-forgotten project and is powered by a 8.5 VAC doorbell transformer. The doorbell wires run conveniently adjacent to the furnace control lines in the basement.




I made a hideous mess of soldering to the perfboard, having both omitted to breadboard the circuit and misread the relay datasheet. The saving grace was using 2x turned pin DIP8 sockets to mount the relay. These little project boxes from Rapid are very handy, but slightly too small for this job. They will be perfect for the wireless nodes though. I am hoping the aluminium lids will made great ground planes for the dipole aerials JPN told us about.



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Ow! My power is hurting! Why do you fry away 7V * Iload? Also, since the furnace connection is just a signal, not carrying power, why not use an optocoupler/optotriac instead? Also, I'd recommend replacing the doorbell transformer with a SMPS 5VDC wallwart (unless you're leeching the power from the doorbell circuit that has to stay anyway, but then still try to use an SMPS to go down to 5V or 3V)

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Every thermostat in every house I have ever lived in goes click.  :smile: AFAIK thermostats almost always use latching relays instead of triacs because no further energy is expended once they have been set/unset, and because the furnace contactor presents a small inductive load that may be hard to switch reliably with a triac. In fact it's not that small, I measured the load as 0.4A at 29VAC. It actually blew the fuse for the mA scale on my multimeter.


It's obviously a very inefficient way to power a microcontroller. The value of R1 is unnecessarily large (100R would have been more sensible) but is intended to protect the zener for maximum life. The zener/resistor combo saves me a couple of dollars on a switching power supply and wastes a fraction of 1W. It will take many years to earn that back at $0.10/kWh. That's not the kind of energy efficiency I worry about in an old house. (Putting a draft excluder on the kitchen door might pay for itself by springtime.)


Likewise I can't be bothered wiring a receptable to take a wall wart when the door bell wires are already there in the basement rafters. And what else am I going to do with that transformer once I have put in the no-press chime activated by a PIR sensor?  :smile: 

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  • 3 weeks later...

Did you consider power stealing from the furnace lines? Avoid a separate power supply all together? takes a diode bridge, ultracapacitor or pair of NiMH cells, and  resistor. The cap or cells power the control when the unit is switched on. In fact, many furnace controls sense current, and you can still steal power across the resistor when they are on. (I don;t recall what the low and high switch current are for mine. I did the install two years ago. Had to be sure the thermostats were compatible)


Or from the furnace transformer (24VAC) directly? Most have designed in connections available for aux controls, going back to the 1960's or before.

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Yes, I briefly considered stealing from the 24 VAC line being switched. I saw quite an informative forum post on the subject somewhere. The coil on contactor consumes nearly 0.5A when it is switched on so I'm sure it would have been possible to steal a few mA without igniting the furnace.


But I had no need to overthink with bell wires literally adjacent. (Red/white, tapping into furnace wires; purple, tapping into bell wires).




The circuit board dangling from a splinter bottom left is my MSP430G2553/NRF24L01+ creation powered by 3V from the relay box. It is receiving temperature broadcasts from a similar board elsewhere in the house (where there are actually radiators!) and sending set/reset signals to the latching relay coils accordingly.

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