Controlling a dumb heater (bridged wires)

My Aussie Braemar heater is just a dumb heater, the battery powered controller simply bridges the wires together to turn the heater on. There is 0v on the relay when bridged.

From what i can see in the hestia, there is 24v on the relay in the US version, is this correct? Is it possible to modify the hestia so there is 0v on the relay? Can i bridge another 24v powered relay onto the existing relay that has 0v on the terminals? Forgive me if non of this makes sense, electronics is not my area.

My plan is to run a separate 24v power supply if the above is possible. What amperage is required?

Welcome to the club! :slight_smile:

Does this mean there are only 2 wires coming from the wall? You would need a 24V AC source to provide power for the thermostat apart from the 2 wires going to the furnace for the control signal.

yes only 2 wires, I will power the device with a separate power supply. looking around the forum it might be possible to power it with usb power?

my main concern is the relay, will it send 24v Ac (or any voltage other than 0v) up into the heater and blow up the heaters components?

Yes, absolutely. You can use a 5V 2A USB power supply like the ones used in phone chargers.

It will “send” whatever it is given on the R terminal as long as it is AC signal.
2 points though I need to mention here:

  1. The solid state relays used in HestiaPi are electronic relays and not your standard clickey magnetic relay that does not care about the direction of the current.
  2. The R terminal (along with the C) is actually used to power the HestiaPi unit and also supply the return lines back to the furnace. If you are going to use a phone charger, you will need to make sure the R terminal is not connected any more with the power section on the PCB and it only goes to the relay’s common pin. That means you will have to modify the PCB by cutting 1-2 copper tracks or in other ways. Make sure you know what you are doing otherwise you are risking of frying your Pi or your furnace.

I am not sure the voltage between the 2 wires is actually 0V. 0V in theory means that they are in identical voltage level, so shorting them shouldn’t make any difference.
I believe there are 2 possibilities. A. You have your multimeter in DC mode while the wires have AC voltage difference, or vice versa. B. The voltage difference between the 2 wires is very little and your meter very low tolerance showing it as 0V.

Please come back if you get more info.

An alternative to powering using USB, which can be a little challenging to get plugged/wired to the Pi because the micro USB ports are blocked I opted for a C-Wire Adapter (this is the one I bought https://www.amazon.com/gp/product/B07P3J93GQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1). It basically provides the 24v that would normally power the thermostat if you had a C wire on your unit. It works like a champ and is a little easier to wire in but most importantly, the wire is thin and very long so it might be more convenient than trying to find a USB cable that is long enough.

How come they don’t have HestiaPi under the supported Thermostats??? :thinking: :grinning:

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I’ve measured it again, 70mV DC.

Then the easiest would be to use @rlkoshak power supply and add a 24V AC relay on the other side of the wires that actually gets these 2 wires in contact (once the 24V AC signal from HestiaPi’s relay arrives). Rather complicated but I believe other solutions would require modifying the PCB and maybe the case too.