I just installed a 12x17ft elevated addition to my hillside ranch style house. Instead of running new air ducts from the primary house HVAC unit I opted for a forced air 3000w 240v wall heater. Problem is that the contractor placed the thermostat (mains powered and passed through) directly above the heater (appx 4 ft up). Moving it is not an option now.
When the heater is running everything is fine, and when the thermostat shuts the heater off (again, a simple mains power cutoff), both the fan and the heating elements shut off.
When the mains are cut off, the fan stops but the heating elements are still radiating a lot of heat. This heat raises the temp of the wall directly above and also the thermostat temp...basically a lot of wasted heat and inaccurate readings on the thermostat until the heat dissipates.
I am somewhat experienced in programming an Uno R3. Here's what I want to do:
Detect when the mains power to the heater is shut off
Using a relay, bypass the thermostat and then supply power directly to the fan for appx 2 minutes (to cool the elements so they don't just sit there radiating heat)
I would really appreciate any advice given about the following questions:
Heater is wired on 12/2, two hots, one ground, no neutral. What is the best way to give my project box power on this wiring type? Running it on a different power source isn't possible.
What recommendations do you have regarding proper hardware between the mains and the UNO when it comes to detecting the cutoff?
Appreciate the reply. Maybe you misread. I wasn't asking how to code it or what relay to use. Actually, an OFF delay isn't what I would need to write. It would end up being an "if poweroff then relay on for 2 min" type of thing.
Was curious how to power the Arduino and what hardware to detect the current flow. Thanks though!
Yes I have direct access to the wiring both inside the heater and on the existing line level thermostat. It is on a dedicated 240v double pole breaker. The only current that would be needed is for the fan inside the heater (and the power supply for the arduino of course).
So what you want to do is run the fan independently of the heater. Understood. You'll want to think through your design carefully, because any failure mode that allows the heater to run with the fan disabled could set you up for a dangerous condition, as well as an insurance liability.
Free advice, FWIW. When my friends want to mess with their wired appliances beyond like-for-like repairs, or modify their house wiring directly, I always ask them, "Are you comfortable with DIY changes that could result in a fire that burns down your house, with your wife and children asleep in it?" The answer is almost always, "No, hadn't thought of that".
Pick off 24VAC from the thermostat into a 12VDC converter? Could be an option, or a programmable thermostat, or my first choice would be to call the contractor and have him fix it. Why should you even have to consider this? He installed it and common practice is for the fan to keep going for a couple minutes after the heater shuts off.
You will need to build some simple electronic circuits.
Do you have experience with soldering and making circuits on a protoboard?
Have you ever wired-up a wall switch or installed a mains power outlet?
And I do sincerely appreciate the cautions and warnings from the other poster as well! Very dangerous stuff if you dont do it correctly. (Edit: In fact, safety is exactly why I want to do this project)
To clarify a few things:
Existing thermostat is line-level - L1 and L2 in and out wires only, no 24v
I understand common practice is for the fan to keep going but it doesnt...that is exactly why I need to make this project.
I do NOT want my addition to "completely" control the fan. When the power to the entire heating unit is killed by the thermostat, I want my circuit to activate a relay that will turn the fan back on for 120 seconds. In real-time it would seem like the fan never shut off, but effectively thats what is happening.
OK
The safest way to detect the current on/off is with a current transformer. With the transformer there is no direct connection to the mains voltage. You simply pass one of the live wires (L1 or L2) through a hole in the transformer.
To turn the fan on, you will need a DPDT relay. It should have a at least 20A 240VAC contact rating. I think most of the ones you find will have coil voltages of 12VDC or 24VDC, so you will need a transistor or MOSFET to control the relay with an Uno.
Awesome! Thats part of what I was looking for, the current transformer. Thanks for that! And yes a DPDT relay was part of my design idea.
Curious, why such a high amperage contact rating on the fan relay? Im not supplying any current for the heating element, and by my calcs the entire thing (heating element and fan) only uses appx 12.5 amps per leg.
I see what you are getting at. Similar to the concept of a startup capacitor on an HVAC unit? My only question about that is.. since it would be a nearly instant transition the startup power should be minimal? The fan wont be at a dead stop and then suddenly kick on. Primary power drops, relay switches, power back on..a few milliseconds.
Thanks Jim. I will take it apart and find the rating on the fan. In the meantime, any ideas on how to power this circuit? There's no neutral so not sure how to go about this yet. I do have a few nanos laying around so could probably go with battery power if need be.
Since there is no neutral it just means you need a 240V AC to xxVDC power supply. Many have a universal input that will accept a 90VAC to 240VAC input range.
xxVDC is to be determined. If you use a relay with a 12VDC coil then a12VDC supply would make sense.