Where to find info about furnace/thermostat operation?

4493 Views 53 Replies 6 Participants Last post by beenthere, Feb 10, 2016

Killavolt

Discussion Starter · Feb 8, 2016

I'm playing around with an Arduino microcontroller and a touch screen and wanted to turn it into a thermostat that would control heat/cool cycles and humidity.

However I'm a little uncertain of how a furnace is controlled by a thermostat.
From what I've read and found so far it looks like the thermostat just redirects power to the different contacts.

So for heat, the thermostat makes a connection between R and W(?), and for cooling it makes a connection between R and C(?), so the thermostat basically acts as a sort of relay or switch? I believe I have a 5 wire system right now as it controls heat and cool, and if I wanted to control humidity as well I would need a 7 wire system?

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yuri

· #2 · Feb 8, 2016 (Edited)

Heat is R to W

Cool is R to Y

Fan ON is R to G

R is 24 volts hot and C is the Common side of the transformer

Humidity would depend on the type of humidifier. If it is a 24 volt humidifier then the humidistat takes power from W and sends it to the humidifier. You want the heat ON when humidifying as it needs to vaporize the water.

Here is a link to a site with a bunch of manuals. Click on the Lennox G51 as I have one and it is a basic furnace. Go to page 42 and it has a sequence of operation. Most furnaces follow the same sequence.

http://www.hvacpartsshop.com/manuals/lennox-furnace-service-manuals.html

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Killavolt

Discussion Starter · #3 · Feb 8, 2016

yuri said:
Heat is R to W

Cool is R to Y

Fan ON is R to G

R is 24 volts hot and C is the Common side of the transformer

Humidity would depend on the type of humidifier. If it is a 24 volt humidifier then the humidistat takes power from W and sends it to the humidifier. You want the heat ON when humidifying as it needs to vaporize the water.

Here is a link to a site with a bunch of manuals. Click on the Lennox G51 as I have one and it is a basic furnace. Go to page 42 and it has a sequence of operation. Most furnaces follow the same sequence.

http://www.hvacpartsshop.com/manuals/lennox-furnace-service-manuals.html

Ya I knew humidity would need the heat on to maintain the temperature and prevent the coils from freezing.
What I couldn't find is if furnaces allow heat and cool to run at the same time, but I'm working with an electric furnace so it's much simpler. I should be able to kick the heating coils on for a few minutes at a time, or just run one of the coils or something.

yuri

· #4 · Feb 8, 2016

I don't know but we have a guy here named Bob Sanders who loves playing with his electric furnace elements and staging it etc so I am sure he will help you. And he is from Winnipeg.

Bob Sanders

· #5 · Feb 8, 2016

Killavolt said:
What I couldn't find is if furnaces allow heat and cool to run at the same time, but I'm working with an electric furnace so it's much simpler.

With residential systems either the heat is on or the cool is on but not both. The only time a heating element is on in cool mode is with a heat pump while in defrost mode.

Some humidifiers require heat to operate efficiently and some (like the old drum style) don't. They just need the fan to run.

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user_12345a

· #6 · Feb 8, 2016 (Edited)

yuri said:
I don't know but we have a guy here named Bob Sanders who loves playing with his electric furnace elements and staging it etc so I am sure he will help you. And he is from Winnipeg.

There's a big divide between the general public and most techs (who just want the stuff heating and cooling) and the enthusiast/"tinkerer"/energy conservation guy who wants to maximize efficiency/comfort.

Glad to see some stuff from people who care more about this stuff than just "if it works" or "good enough" or are only motivated by money, not that there's anything wrong with that. :smile:

user_12345a

· #7 · Feb 8, 2016

Killavolt said:
Ya I knew humidity would need the heat on to maintain the temperature and prevent the coils from freezing.
What I couldn't find is if furnaces allow heat and cool to run at the same time, but I'm working with an electric furnace so it's much simpler. I should be able to kick the heating coils on for a few minutes at a time, or just run one of the coils or something.

Check your product literature for more detailed info.

The way most electric heat is set up a lot of that time is that the elements are timed.

There's a sequencer for each stage, each stage could be 1+ element(s).

Can prevent the whole heat kit from coming on unless you need it by getting rid of the jumper between W1 and W2, stage W2 by other means.

supers05

· #8 · Feb 8, 2016 (Edited)

Bloody hell, I hate hitting the back button just as I finish writing the whole post.... Anyways.

Some systems will lock out heat if they detect power on y, but most don't. It's very possible to run both on many systems. Do you have a heat pump or straight a/c? Don't forget that most systems require the thermostat to call for g each and every time y is called. You can damage your equipment if you're not careful.

I'd use common low power 5v DC coil relays. They seem to stand up better on older systems then scr's or similar solid state stuff. (contactor coils can draw over an amp each depending on model) Don't forget your driving and suppression circuits.

Of course all of the above assumes conventional control. Some newer systems use proprietary comms, usually on rs484 or rs422. (Don't know any that use rs232) They are usually touch screen stats, so you probably don't have one.

Good luck and have fun. Keep us updated.

Cheers!

PS. A raspberry pi or Android hobby board might be easier to make a higher level UI, with network connectivity, especially on the TS. Of course it's been a while since I've looked at the ardrino community libraries.

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Killavolt

Discussion Starter · #9 · Feb 8, 2016

Wow, lots of good replies here. The electric furnace is a Supreme 23kW, but I have to double check specs and model number because I think it's a Heil but I can't find any product manuals for a Heil model. I did at one time have a manual somewhere.

It looks pretty simple inside, and being an electromechanical engineering technician, the wiring is fairly simple to figure out. I won't need all of the heating power so I'm going to remove some of the elements, they're designed to be removed or added.

This model doesn't have the cooling system built in so I'll need to add that, though it looks like there's a spot for a second transformer for cooling and places to add the stuff for a cooling system so I should be able to run both for a dehumidifying cycle.

One of the problems is that I'm not doing the programming myself since it would take me years to learn it and figure it all out.....so I am open to other options for programming this if they're easier and can do what I need.

yuri

· #10 · Feb 8, 2016

Supreme was made by ICP, International Comfort Products who make Heil. Now it is owned by United Technologies who owns Carrier etc. Pretty basic common furnace and it should have a wiring diagram on the back of one of the doors. Doubt there is a manual for it. Basic cheap Builders grade no frills furnace and I sold a few.

Killavolt

Discussion Starter · #11 · Feb 8, 2016

yuri said:
Basic cheap Builders grade no frills furnace and I sold a few.

Exactly what I needed for this little project of mine. nice and simple inside, I wish I could find out where I could buy another one, or parts f or it.

supers05

· #12 · Feb 8, 2016

Parts will be fairly generic parts, should be easy to find online or local suppliers. (locals usual only sell to hvac companies though)

If you're not into programming, the ardino might not be the best solution. Lol. Along with the hobby boards require quite a bit if scripting at the very least. (building the program via copy and paste and tinkering)

Cheers!

Bob Sanders

· #13 · Feb 8, 2016 (Edited)

Killavolt said:
This model doesn't have the cooling system built in so I'll need to add that, though it looks like there's a spot for a second transformer for cooling and places to add the stuff for a cooling system so I should be able to run both for a dehumidifying cycle.

The average thermostat can handle switching involving up to two transformers but it might be worth it to know that heating and cooling can be done with a single transformer. Most thermostats are convertible to handle the switching from pone power supply... or two.

The timing sequence of the heating element and the fan is important. The fan must be the first thing on (or at least at the same time) and the last thing off otherwise you risk burning an element or one of the element's protection devices.

Killavolt

Discussion Starter · #14 · Feb 8, 2016

yuri said:
Heat is R to W

Cool is R to Y

Fan ON is R to G

R is 24 volts hot and C is the Common side of the transformer

http://www.hvacpartsshop.com/manuals/lennox-furnace-service-manuals.html

So the basics of it would just be using a relay to send power between R/W or R/Y or R/G depending on what I want.... the rest all comes down to programming.

supers05

· #15 · Feb 8, 2016

Killavolt said:
So the basics of it would just be using a relay to send power between R/W or R/Y or R/G depending on what I want.... the rest all comes down to programming.

Yes and r-humidistat if it's a 24v version. (using the furnace transformer, otherwise it'll use it's own 24v transformer and it'll be 2 separate wires.)

Cheers!

Bob Sanders

· #16 · Feb 8, 2016

Killavolt said:
So the basics of it would just be using a relay to send power between R/W or R/Y or R/G depending on what I want.... the rest all comes down to programming.

Not sure if you're clear on this?

For heat you need R to W and G
For cool you need R to Y and G

yuri

· #17 · Feb 8, 2016 (Edited)

With an electric furnace on cool you need to energize G if it has no circuit board. If it does then the board does it 4 U.

With heat the sequencers turn on the fan but if you want to eliminate them then yeah you need to energize G and you need a fan relay.

If you plan to eliminate the sequencers then you need heavy duty contactors not cheap relays as you will be drawing 20 amps. They may clang or make a loud click when cutting in. There are special contactors with better contacts as the large number of cycles they do will burn the contacts on a regular one. Honeywell makes them, not sure of the part #.

Why do you want to do this project anyway?

From what I hear elec is crazy expensive in Ontario. If you are serious about heating your house cheaper then a heat pump may be a better idea and just use the furnace for backup/supplemental heat.

Killavolt

Discussion Starter · #18 · Feb 8, 2016 (Edited)

4

Bob Sanders said:
Not sure if you're clear on this?

For heat you need R to W and G
For cool you need R to Y and G

Thank you, I wasn't clear on that.

yuri said:
With an electric furnace on cool you need to energize G if it has no circuit board. If it does then the board does it 4 U.

With heat the sequencers turn on the fan but if you want to eliminate them then yeah you need to energize G and you need a fan relay.

If you plan to eliminate the sequencers then you need heavy duty contactors not cheap relays as you will be drawing 20 amps. They may clang or make a loud click when cutting in. There are special contactors with better contacts as the large number of cycles they do will burn the contacts on a regular one. Honeywell makes them, not sure of the part #.

Why do you want to do this project anyway?

From what I hear elec is crazy expensive in Ontario. If you are serious about heating your house cheaper then a heat pump may be a better idea and just use the furnace for backup/supplemental heat.

So the furnace I'm using is a Heil Supreme fem23-m2401am-b I've got pics of the inside, but this thing is simple, it has no control board that I can see and basically just uses time delay relays of some sort to control everything.

I'm not doing this to heat a house, in fact I'm going to end up disabling most of the heating elements as heat will really only be needed to maintain a temperature during a dehumidify cycle which I think I will handle by running the AC and cycling the heat on and off while the AC is running.

I'm building a climate control system for a small green house. Much of the project is extremely simple and consists of basically making a furnace an all in one HVAC unit. The complicated part is the programming of the arduino to control the system instead of a thermostat.

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yuri

· #19 · Feb 8, 2016 (Edited)

Cool. I actually like that furnace.

The time delay relays are actually called sequencers in the HVAC biz. They start the elements in a sequence which varies in time so they all don't start at once and overload the circuits or dim lights. Has a bi-metal warp switch at the bottom pushing on a rod on contacts.

You probably want some magnetic contactors rated for 20 amps and I am not sure about how they rate them for duty cycle etc and full load. There are differences in quality so I would not go cheap. Each element is 5000w/240 volts approx 20 amp draw.

If it is very humid in there I would be concerned about corrosion of the motor etc as it may rust to pieces quick. I worked in greenhouses and we had huge 275,000 BTU furnaces running w/o chimneys to provide CO2 and heat for the plants. Not sure what you need if your are growing "medicinal" marywanna or "tomatoes".:biggrin2:

Once you alter the factory wiring any UL or CSA rating goes away. If it starts on fire you may not have insurance or could be liable if it damages anybody or anything. It may be possible to get a special electrical permit? Would need a licensed electrician to take it out and get it inspected?

There are other issues depending on when you plan to use it. You cannot run a AC unless it is 50F or 10C and if it does not have enough load in the other times it can freezeup and damage the compressor. Not necessarily as simple as it sounds.

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Killavolt

Discussion Starter · #20 · Feb 8, 2016 (Edited)

yuri said:
Cool. I actually like that furnace.

The time delay relays are actually called sequencers in the HVAC biz. They start the elements in a sequence which varies in time so they all don't start at once and overload the circuits or dim lights. Has a bi-metal warp switch at the bottom pushing on a rod on contacts.

You probably want some magnetic contactors rated for 20 amps and I am not sure about how they rate them for duty cycle etc and full load. There are differences in quality so I would not go cheap. Each element is 5000w/240 volts approx 20 amp draw.

If it is very humid in there I would be concerned about corrosion of the motor etc as it may rust to pieces quick. I worked in greenhouses and we had huge 275,000 BTU furnaces running w/o chimneys to provide CO2 and heat for the plants. Not sure what you need if your are growing "medicinal" marywanna or "tomatoes".:biggrin2:

Once you alter the factory wiring any UL or CSA rating goes away. If it starts on fire you may not have insurance or could be liable if it damages anybody or anything. It may be possible to get a special electrical permit? Would need a licensed electrician to take it out and get it inspected?

There are other issues depending on when you plan to use it. You cannot run a AC unless it is 50F or 10C and if it does not have enough load in the other times it can freezeup and damage the compressor. Not necessarily as simple as it sounds.

I don't think I need anything, I'm not really changing the way the furnace works aside from disabling a few heating elements since heat really won't be needed except to dehumidify and maybe for night time temperature control.

I'm not actually growing anything, I'm just a guy with too much time and skill on my hands. I saw some similar units and thought to myself "self, I'm sure I could build one of those for much cheaper and probably better"
I wasn't really going to bother with it until I saw this electric furnace come up for sale nearby like 2 days later, so I took it as a sign that I should probably give'er a whirl.

In this furnace those really are just time delay relays, it even says so in the manual, there is no bimetallic disc in those or anything like a sequencer. It appears the time delay isn't adjustable by any means though.

The unit I saw was basically just a furnace without heating elements, and with a quick disconnect style of AC unit controlled by thermostats. there might be some custom programming involved, but the basic unit sold for like $5000, then you could buy additional components like filters sterilizers, dampers all for an additional cost.

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