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Old 12-25-2010, 04:14 PM   #1
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240v thermostat on 120v baseboard?


I have a 6 pack of Honeywell RLV4300 programmable thermostats. Specs are 3000w max load at 240VAC and they work fine for the 240v baseboard and fan forced heaters.

Would it be possible to use these on a 120V 750w baseboard heater?

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Old 12-25-2010, 04:26 PM   #2
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240v thermostat on 120v baseboard?


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Originally Posted by DragonFire View Post
I have a 6 pack of Honeywell RLV4300 programmable thermostats. Specs are 3000w max load at 240VAC and they work fine for the 240v baseboard and fan forced heaters.

Would it be possible to use these on a 120V 750w baseboard heater?
Yes, at half the max rated power (same maximum current). Just wire the hot through one pole of the thermostat and leave the other pole disconnected. Alternatively, wire both poles in parallel to divide the load between the two halves.

The only catch is if these thermostats are powered by the line. They may require 240V to operate. You can check the manual on this and see what i says. If it's battery powered then this will not be a problem.

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Old 12-25-2010, 05:57 PM   #3
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240v thermostat on 120v baseboard?


This particular model seems to depend on connection to a 240 Volt circuit, with at least a 500 watt heater connected in series.

I doubt it will operate properly on a 120 Volt circuit. You can try, but no guarantees here.

Edit to add:

I found a more recently updated owners manual for this unit, and it says 120/240 Volts. If this is the case, it should work fine with your setup.
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Old 12-25-2010, 07:14 PM   #4
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240v thermostat on 120v baseboard?


Thanks kbsparky,
There indeed seems to be additional information in the revised manual regarding 120v.

I am going to email the manufacturer to see what they say. Hopefully these would work on 120v as it would save me buying another separate programmable thermostat.
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Old 12-27-2010, 04:13 PM   #5
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240v thermostat on 120v baseboard?


I received a confirmation email from Honeywell, and yes these do definitely work on both 240v and 120v at 3000w and 1500w respectively.
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Old 09-28-2014, 11:22 PM   #6
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240v thermostat on 120v baseboard?


I have tested this on a 120v fan forced wall heater, and it does not function correctly. The fan heater is working perfectly, and the RLV4300A claims to work on 120v heaters of this type. However, (*even with fan mode set*) the t-stat appears to attempt to run the fan at half power (bad for the fan???) and the heater goes into overheat mode and shuts down.
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Old 09-29-2014, 07:58 AM   #7
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240v thermostat on 120v baseboard?


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I have tested this on a 120v fan forced wall heater, and it does not function correctly. The fan heater is working perfectly, and the RLV4300A claims to work on 120v heaters of this type. However, (*even with fan mode set*) the t-stat appears to attempt to run the fan at half power (bad for the fan???) and the heater goes into overheat mode and shuts down.
the RLV4300A appears to be a 240v only device from what little I could find online. read the mfr literature to verify.
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Old 09-29-2014, 10:36 AM   #8
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240v thermostat on 120v baseboard?


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Old 09-29-2014, 11:16 AM   #9
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240v thermostat on 120v baseboard?


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Originally Posted by obscenic View Post
I have tested this on a 120v fan forced wall heater, and it does not function correctly. The fan heater is working perfectly, and the RLV4300A claims to work on 120v heaters of this type. However, (*even with fan mode set*) the t-stat appears to attempt to run the fan at half power (bad for the fan???) and the heater goes into overheat mode and shuts down.

The thermostat is designed for 120 or 240 V heaters. It is typically used for passive baseboard heaters but seems to be fine for those with fans.

I guess if you want to control a fan space heater with the RLV4300 you need to set any thermostat in your fan heater to be always on (or wire it so it is bypassed) as the Honeywell needs to see the resistive load of the heating element to work properly.
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Old 09-29-2014, 12:11 PM   #10
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240v thermostat on 120v baseboard?


old thread with new question, just trying to help out... thanks for the alert!

"The thermostat is designed for 120 or 240 V heaters. It is typically used for passive but seems to be fine for those with fans."

I respect that the OP t-stat RLV4300 was a 120/240 v device. but the "A" suffix may indicate that it is a 240v device only. see link below.


http://yourhome.honeywell.com/home/P...e/RLV4300A.htm
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Old 09-29-2014, 03:43 PM   #11
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240v thermostat on 120v baseboard?


Apparently this was not the problem. The t-stat should work with 120v systems; however EVEN in fan mode, the t-stat still tries to reduce the voltage which thereby reduces the torque and therefore the speed of the fan motor. This is both harmful to the fan, AND a fire hazard, both via the fan and the heating coil. The manual clearly says "compatible with... fan forced-heater", but I confirmed with a tech support supervisor on the phone that there is not a single thermostat in honeywell's inventory which is safely compatible with a heater where the fan is on the same hot wire as the heating coil.
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Old 09-29-2014, 11:31 PM   #12
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240v thermostat on 120v baseboard?


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Apparently this was not the problem. The t-stat should work with 120v systems; however EVEN in fan mode, the t-stat still tries to reduce the voltage which thereby reduces the torque and therefore the speed of the fan motor. This is both harmful to the fan, AND a fire hazard, both via the fan and the heating coil. The manual clearly says "compatible with... fan forced-heater", but I confirmed with a tech support supervisor on the phone that there is not a single thermostat in honeywell's inventory which is safely compatible with a heater where the fan is on the same hot wire as the heating coil.
The Tstat doesn't "change" voltage. It merely switches power on and off. If you have a 240V heater that has a 240V fan motor it won't do well on a 120V circuit. That has nothing to do with the Tstat.
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Old 09-30-2014, 05:00 PM   #13
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240v thermostat on 120v baseboard?


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The Tstat doesn't "change" voltage. It merely switches power on and off. If you have a 240V heater that has a 240V fan motor it won't do well on a 120V circuit. That has nothing to do with the Tstat.
I'm not totally sure that you're correct. These new programmable t-stats have an automatically controlled heat output which is indicated by a display with 1 to 5 bars, with 5 bars being highest heat output. Whilst it's true that they could be rapidly cycling the TRIAC on and off (essentially pulse width modulation heating) it seems more likely that being designed for ONLY resistive loads, that they are actually reducing the voltage resulting in a lower output from the heating coil.

At any rate, they are NOT (as advertised) compatible with fan forced heaters. The only time they'll work with a fan forced heater is if the fan is on a totally different circuit which is being controlled by.... magic? Or the fan is ALWAYS on.
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Old 09-30-2014, 06:30 PM   #14
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240v thermostat on 120v baseboard?


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Originally Posted by obscenic View Post
I'm not totally sure that you're correct. These new programmable t-stats have an automatically controlled heat output which is indicated by a display with 1 to 5 bars, with 5 bars being highest heat output. Whilst it's true that they could be rapidly cycling the TRIAC on and off (essentially pulse width modulation heating) it seems more likely that being designed for ONLY resistive loads, that they are actually reducing the voltage resulting in a lower output from the heating coil.

At any rate, they are NOT (as advertised) compatible with fan forced heaters. The only time they'll work with a fan forced heater is if the fan is on a totally different circuit which is being controlled by.... magic? Or the fan is ALWAYS on.

They are compatible with fan based heaters. I dug a little deeper and understand better. Honeywell documentation is very poor.

The bars (1-5) are an after the fact indicator of how much the heater is running in %. One bar is 20% or less, 3 bars 60% or less, 5 bars 100%. I guess a feature to let you know how hard your heater is working. It isn't a triac modulated power input to the heater (a-la a dimmer switch). The triac is either on or off so the heater is either on or off.

Now the next part. The default mode of operation is a series of 15 second "turn on's" to satisfy heat demand. I suppose their are algorithms in the software such that it will guide the heat to the set point with a lesser duty cycle than 100% to reach the set temp. This isn't a cycle to cycle algorithm, rather chunks of 15 second "on" blocks to build to the desire temp. So I suspect the algorithm will look at past history and say if I am 1 degree away from set point I'll back off to say one on chunk, two off, one on, three off or something like that. They seem to suggest tighter temperature control s possible with this fine tuning.

The advanced features have a setting to disable the 15 second chunks to 5 minute cycles so you don't damage the fan by turning it on and off so often. Makes sense, if you have a fan run the cycles longer so you aren't stressing the motor windings.

So if you have a 240V forced air heater, just set the advanced setting to FAN and it should work fine.
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Old 09-30-2014, 07:13 PM   #15
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240v thermostat on 120v baseboard?


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They are compatible with fan based heaters. I dug a little deeper and understand better. Honeywell documentation is very poor.

The bars (1-5) are an after the fact indicator of how much the heater is running in %. One bar is 20% or less, 3 bars 60% or less, 5 bars 100%. I guess a feature to let you know how hard your heater is working. It isn't a triac modulated power input to the heater (a-la a dimmer switch). The triac is either on or off so the heater is either on or off.

Now the next part. The default mode of operation is a series of 15 second "turn on's" to satisfy heat demand. I suppose their are algorithms in the software such that it will guide the heat to the set point with a lesser duty cycle than 100% to reach the set temp. This isn't a cycle to cycle algorithm, rather chunks of 15 second "on" blocks to build to the desire temp. So I suspect the algorithm will look at past history and say if I am 1 degree away from set point I'll back off to say one on chunk, two off, one on, three off or something like that. They seem to suggest tighter temperature control s possible with this fine tuning.

The advanced features have a setting to disable the 15 second chunks to 5 minute cycles so you don't damage the fan by turning it on and off so often. Makes sense, if you have a fan run the cycles longer so you aren't stressing the motor windings.

So if you have a 240V forced air heater, just set the advanced setting to FAN and it should work fine.
That sounds totally well and good in theory, except that in practise, I'm watching my fan spin (at a consistent) slower speed (seems roughly in line with the number of bars shown on the t-stat), and I'm watching the overheat sensor light up, click, and shut down the heater. The t-stat has always been set to fan mode. When I disconnect the thermostat and hard wire the hot to the heater so it's getting line voltage all the time, it spins perfectly at full speed and doesn't overheat. So the issue is absolutely with the t-stat. Having spoken with a "technician" on the phone at Honeywell yesterday, he confirmed that this thermostat is NOT compatible with a heater which contains a fan on a common hot with the heating coil. He implied that the t-stat did in fact lower the voltage and therefore was not compatible with a fan motor. Whilst your answer still seems the most correct, my experience is contradicting that. FYI, the heater I'm using is 120vac, though the user manual suggests that running at 120vac should be completely non-problematic - so I've pretty much ruled that out as well.

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