Line Voltage Wall Thermostat For Automotive Use

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Honeywell Thermostat https://www.homedepot.ca/product/ho...electric-baseboard-heat-thermostat/1000429052

In the Q&A for the product, company rep claims it is not suitable for 12V use


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Kenworth
· 2 months ago

Will this work on 12 volt to turn an automotive heater motor on and off.​

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  1. Resideo Team
    · 2 months ago

    No, the Honeywell Home Manual 2-Wire Electric Baseboard Heat Thermostat provides line voltage control of a radiant cable, electric baseboard, and resistive rated fan forced heaters. - Jerry
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    It raises the question - why not? Do low voltage switches need a different contact material? Back story to my question - four years ago I installed a line voltage wall stat for 24V AC control of a baseboard heater (used 24v transformer and relay).The stat just failed - it will not make contact sometimes and will therefore not turn the heater on reliably. Four years is a short life for that stat especially since it is only switching 60 ma at 24V ac. Thought I would look further into it. A google search did not turn up anything. Second pic shows the contacts and they do not look excessively burned. 000_0267.JPG000_0268.JPG
 
I do not see why this would not work in a low voltage application.

The only thing I can think of the difference when breaking a DC circuit versus an AC circuit.

Penn Controls also make the A19 series thermostats that is rated 24-600 VAC, 125VA


I also think some RV thermostats will control DC, example a 12V motor/control on a propane furnace.
 
Could be that it works fine but lacks some safety code thing intended for RVs so they don't want to endorse it.
 
There’s a difference- the contacts could be rated for 120VAC, 1A, and when you try to use it on 24VAC, what was initially 1A would now be 5A.

Electric/electronic manufacturers know what they’re doing when they put ratings on their products.
 
No manufacturer is going to tell you to use it in an unintended, untested application. It opens up a whole can of worms about liability in addition to the question of why the customer isn't using the appropriate other widget instead.

There is another possible issue. How does this thermostat work? If it uses a bimetallic sensor strip as indicated on the product page, it may trip a relay through a supply circuit that is set up for driving the coil from a 120VAC source, stepping that down to the appropriate level. Depending on the specific relay they used, it's coil trigger voltage, you might be able to reverse engineer and mod it to work off 12VDC (might even be a 12V coil so just bypass the subcircuit), or not as easily if solid state.

Then there is the issue of how much DC current the blower uses and what the real, lower, DC current it can handle is, and whether it is tolerant of back EMF from a DC motor shutting off when it was designed for a resistive load. The inductive load also induces a high current upon motor startup, and if the bimetalic strip is itself the conductor for the drive circuit, this is a slow contact method generally avoided for DC applications because it will heat and degrade the contacts, with the heat itself being an issue since bimetallic strip operates on temperature.

I agree with cricket whisperer, would see an RV thermostat designed for 12-14(.n)VDC.

 
There’s a difference- the contacts could be rated for 120VAC, 1A, and when you try to use it on 24VAC, what was initially 1A would now be 5A.

Electric/electronic manufacturers know what they’re doing when they put ratings on their products.
It is now switching just 60 ma. As mentioned by another poster, I wonder if the voltage/current is too low for these contacts and the voltage is not enough to punch through any built up oxides on the contacts.
 
No manufacturer is going to tell you to use it in an unintended, untested application. It opens up a whole can of worms about liability in addition to the question of why the customer isn't using the appropriate other widget instead.

There is another possible issue. How does this thermostat work? If it uses a bimetallic sensor strip as indicated on the product page, it may trip a relay through a supply circuit that is set up for driving the coil from a 120VAC source, stepping that down to the appropriate level. Depending on the specific relay they used, it's coil trigger voltage, you might be able to reverse engineer and mod it to work off 12VDC (might even be a 12V coil so just bypass the subcircuit), or not as easily if solid state.

Then there is the issue of how much DC current the blower uses and what the real, lower, DC current it can handle is, and whether it is tolerant of back EMF from a DC motor shutting off when it was designed for a resistive load. The inductive load also induces a high current upon motor startup, and if the bimetalic strip is itself the conductor for the drive circuit, this is a slow contact method generally avoided for DC applications because it will heat and degrade the contacts, with the heat itself being an issue since bimetallic strip operates on temperature.

I agree with cricket whisperer, would see an RV thermostat designed for 12-14(.n)VDC.

This is a very basic mechanical thermostat, with a bimetallic strip mechanicaly closing and opening snap-action contacts. Strip does not conduct current in this thermostat. There is no heat anticipator since it is meant for electrical baseboard heaters which do not have much thermal mass.
 
This is a very basic mechanical thermostat, with a bimetallic strip mechanicaly closing and opening snap-action contacts. Strip does not conduct current in this thermostat. There is no heat anticipator since it is meant for electrical baseboard heaters which do not have much thermal mass.
Maybe a cap between the contacts to cut down on the DC arc..
 
Why are people talking about DC ? The OP's question was about a 24 volt AC circuit .
 
It raises the question - why not? Do low voltage switches need a different contact material? Back story to my question - four years ago I installed a line voltage wall stat for 24V AC control of a baseboard heater (used 24v transformer and relay).The stat just failed - it will not make contact sometimes and will therefore not turn the heater on reliably. Four years is a short life for that stat especially since it is only switching 60 ma at 24V ac.

Maybe the voltage has nothing to do with it and the stat just failed because it's junk?

Perhaps after so many cycles of on and off the contacts are no longer pushed together with enough force to make a reliable connection?
 
All good answers - but how did the unit fail? Was designed to be mounted on the wall, so vibration may have played a role in the failure?
 
-------------------------------------------------------------------------------------------------------------------------- four years ago I installed a line voltage wall stat for 24V AC control of a baseboard heater (used 24v transformer and relay)

This may be why the thermostat failed. The contacts are designed for a resistive load. A relay is an inductive load. When the magnetic field in a relay coil collapses because the power to it has been turned off, it can generate very high voltages.

In a DC relay, a resistor or reverse-biased diode across the relay coil is used to "snub" or "shunt" these high voltages to prevent arcing on the upstream switch contacts. You often see diodes used in automotive relay circuits for this purpose. Sometimes the relays have these resistors or diodes built right in.

For an AC relay, a diode can't be used for obvious reasons.

Here's some examples of "snubber" circuits to use with relay coils:

 
All good answers - but how did the unit fail? Was designed to be mounted on the wall, so vibration may have played a role in the failure?
The stat was wall mounted. I used a 240-24v transformer, a 24v relay and 22/4 telephone cable to do a wired remote mount of the stat. I since found out units are available for just this purpose
 
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Why are people talking about DC ? The OP's question was about a 24 volt AC circuit .
Confusion very understandable. My original question was about the Q&A at Home Depot for the stat application for heat control in a car. I then switched over to my own 24v ac application.

I have decided to use a line voltage stat again, this time a Honeywell instead of a Ouellet stat which I used the first time. Hopefully I will get more than four years out of the Honeywell. The Honeywell seems to be of better construction than the Ouellet.

On another note, the stat is a Honeywell in name only. Resideo just licenses use of the Honeywell name. I don't like it. A long time ago I bought a Snap On branded flashlight at Costco and it was garbage. Snap On was just prostituting itself, allowing its name on substandard products for monetary gain. No doubt a brilliant idea from the geniuses in the Snap On marketing dept.
 
For around $6 you can order a simple digital DC thermostat on Amazon. It will be powered by 5v usb power with programmable NO/NC contacts.

That said, the Honeywell should work fine electrically. Element exposure could be a thing if it’s it a damp location.

ps. - you might have to get used to reading it in Celsius
 
For around $6 you can order a simple digital DC thermostat on Amazon. It will be powered by 5v usb power with programmable NO/NC contacts.

That said, the Honeywell should work fine electrically. Element exposure could be a thing if it’s it a damp location.

ps. - you might have to get used to reading it in Celsius
I will have to replace the 24vac transformer with a 5v dc power supply to use it. I am going ahead with the Honeywell stat.

It is located in a dining room. I moved the stat for an elderly couple who did not want to kneel down to operate the baseboard heater stat. They have good central heating and only use this BB heater on very cold days.

We do Celsius in Canada! I have to do some mental calculations when the temp is in Fahrenheit.
 
It should work but it's working on temperature alone for on and off. Like a 24 vac stat will have a heat anticipator that brings the heat on a little faster or slower. The reason the thermostat failed is because the contacts are not made for 60ma their made for 22 amps. It would have to have gold contacts to last. Might be able to clean the contacts with a business card. Found a honeywell stat made for 12vdc and good for .75 amp.
 
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