Go.Fish
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Is Raspberry Pi capable of replacing 2 wire thermostat

Fri Feb 08, 2019 3:41 pm

Hi
First post here. Also a noob

Im wondering if there’s a simple Raspberry Pi setup to build and mount to the wall at home to control an Oil burning furnace. I currently have 2 wires coming out of the wall which are controlled by a manual dial (bi-metal type thermostat). It’s either open or closed based on temperature. 24volts AC with low amps. Sounds simple enough to be controlled with a relay using Raspberry?

I don’t need a smart “nest” type. But wouldn’t complain if it could be done. It needs to have an LCD

So basically I’m asking around for suggestions on which setup to look into. Raspberry pi....Arduino...Sparkfun etc?

The only experience I have is adding a heated bed to an Arduino based 3D printer, 40amp relay, updated firmware...

Advice appreciated, thanks!

drgeoff
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Fri Feb 08, 2019 4:40 pm

Possibly but be aware that the RPi + relay will need power. That makes a drop-in replacement on the existing two wires more "challenging".

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ptimlin
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Fri Feb 08, 2019 11:24 pm

Ya the problem is you typically do not have 24Vac AND the return for that transformer and without both you can not power the Pi (or whatever board you wanted to use) off just those two wires. You get 24Vac coming into the thermostat and when it closed it simply sends that 24Vac back down to the heating system controls.

That said, if you had another way to power the Pi, then yes you could make a thermostat with one. Very simply you would need power for the board, a thermistor or other type of temperature sensing device (DS18x20 type one-wire temp sensor), and then a relay to send the 24Vac signal to the furnace.

Since it is AC voltage a solid state relay (SSR) may be a good choice since they typically work with AC outputs and are easily found with control inputs of 3.3V compatible and little current. If you went with a mechanical relay then you may need some sort of interface between the Pi and the relay itself to drive the typically higher voltage and current requirements of the relay's coil. That interface would be a simple transistor, an opto-isolator, or something like the ULN2003LV relay driver.

uilfut
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Fri Feb 08, 2019 11:55 pm

You could consider two pis (Pi0Ws would do).

One in the boiler cupboard where there's presumably power, one out in the hallway or wherever you want to measure temperature.

This way they could both be located near power outlets.

The one next to the boiler could have an extra wire run from the thermostat terminals on the boiler, so the old wiring could be left in place for a future home owner.

This would require a working network connection, more code, more points of failure, more learning opportunities :)

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ptimlin
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Sat Feb 09, 2019 6:38 am

uilfut wrote:
Fri Feb 08, 2019 11:55 pm
You could consider two pis (Pi0Ws would do).

One in the boiler cupboard where there's presumably power, one out in the hallway or wherever you want to measure temperature.

This way they could both be located near power outlets.
What for? If you are going to put one up where the thermostat is and plug it into a power source, the two wires are right there to close or open and control the furnace. What advantage would a second Pi down at the furnace do for you?

Paul Hutch
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Mon Feb 11, 2019 3:28 pm

This is one of those projects that will take far more time and cost far more money than any plan will estimate. Plus it may cause grief if bugs in your custom system leave the heat off and you get real cold, leave it on wasting oil, or off while your away and the house pipes freeze and your insurance won't pay up because your custom device caused the problem.

Considering a good quality Honeywell LCD readout battery powered thermostat can be bought off the shelf for less than the cost of a RasPi 3 alone, I'd have to be desperate for a project to do before I'd tackle this project.

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rpdom
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Mon Feb 11, 2019 4:19 pm

Paul Hutch wrote:
Mon Feb 11, 2019 3:28 pm
This is one of those projects that will take far more time and cost far more money than any plan will estimate. Plus it may cause grief if bugs in your custom system leave the heat off and you get real cold, leave it on wasting oil, or off while your away and the house pipes freeze and your insurance won't pay up because your custom device caused the problem.

Considering a good quality Honeywell LCD readout battery powered thermostat can be bought off the shelf for less than the cost of a RasPi 3 alone, I'd have to be desperate for a project to do before I'd tackle this project.
I don't know about that. My Pi has been controlling my heating 24/7 for years now and has saved me a lot of money by only turning on the heating when needed based on the temperature and time of day.

The default for the heating was to be run from a timer and use thermostatic valves on radiators to control the temperature in the rooms. That meant that it spent a lot of time keeping the heating system up to minimum temperature and pumping water around during the day time. Now it only does that when the temperature in the main room drops below a certain temperature for that time of day. The rest of the time it is off completely and using no gas or electricity other than the tiny amount required by the Pi. I don't have to worry about "Oh, it's winter. I'll turn the heating on" or "Too warm, turn heat off". It just works all year around at minimal cost.

Control of the heating is just a simple relay across one switch.

Earlton2
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Thu Feb 14, 2019 1:31 am

then again you could use a standard thermostat in the control circuit and use a relay closure or ac switching device across it. the thermostat could be set at a safe default minimum e.g. 10c while the new circuit runs the weekly sequence. the pi needs to get time from either the web or an rtc.

buteman
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Re: Is Raspberry Pi capable of replacing 2 wire thermostat

Sat Feb 16, 2019 8:50 pm

If the 2 wires can be extended to somewhere that would allow power to be run to the Pi then it should be a straightforward matter of setting things up for the Pi to control a relay. Or the Pi could be mounted near the 2 wires to connect them to a relay and power run to it from wherever it's available then I see no problem. So just a few cables to connect, getting accurate time as has been mentioned and the rest is just programming. For that I would personally use python.
There are few rewards greater than accomplishing a difficult task. Just because something doesn't cost money doesn't mean there is no cost. Every success makes the next one easier. Of course that takes determination and patience.

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