Dimmer as backup to Thermostat?

[ChrisBowsman]

First, if the thermostat is set to heat to 92 degrees, you will never see 95 degrees regardless of whether there is a rheostat inline or not.

I think the idea for the rheostat being there was in case the thermostat shorted.

I was just thinking about it from a really basic standpoint, and will digress to those with more knowledge of the inner workings of thermostats

[ctrlfreq]

I think the idea for the rheostat being there was in case the thermostat shorted.

I can understand that, but the problem is, if the rheostat is inline with a thermostat, and the thermostat fails, with any decent thermostat, there will be no current on the line after the failure. If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

[ChrisBowsman]

If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

I didn't know that either. If I had known that they shut down, I'd never have thought it would work in the first place (obviously).

[SatanicIntention]

Just as an example.

I have one of the older model Herpstat II's, and it came with the original white, flimsy probes instead of the heavy-duty grey probes. One night, one of the probes failed(at 2am no less), either by being bent or it just failed.. Either way, that thermostat shut that section off and made this high pitched wailing alarm. It was quite obvious that it didn't need help shutting itself off or regulating itself. After about a month, the other probe failed too, but the unit shut that section of the thermostat off, and beeped at me. At least now those t-stats come with the heavy-duty probes and it's unlikely that the probes will fail again. If they do, I trust the unit to take care of itself.

If I did want to back it up, all I would have to do is get a Johnson Controls or Ranco, plug it in to the wall, set it about 5 degrees higher than what you want your hotspot at. Then plug your Helix or Herpstat into the JC or Ranco, set it to what you want your hotspot to be, and that's it.

[td30]

I can understand that, but the problem is, if the rheostat is inline with a thermostat, and the thermostat fails, with any decent thermostat, there will be no current on the line after the failure. If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

Thanks everyone for the replys. This makes the mose sense to me. Glad I ordered the Herpstat

[JeffFlanagan]

The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.

[ChrisBowsman]

If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.

That's kind of what I was thinking.

[Gloryhound]

The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.

The rheostat is a resister which limits current by using power! This means it will require the proportional T-stat to push more power to achieve the proper temps. This could result in overloading the proportional T-stat as most are only rated for between 400 Watts to 500 Watts. That equals a little less/over 4 amps at 120 Volts AC. In order to create a limiting point you will have to create a situation that limits based on its max Wattage. Running at max wattage is taxing on SCR's and transistors as they run hotter. This reduces the life of the proportional T-stat and also will create problems with the overload circuitry shutting down and no heat being created until it is reset. The only way I would do it is use a On/Off T-stat before the proportional controller if a back up was needed!

You can't hook over a 400 Watt heating element to a herpstat Pro! That is the most it will push!
Watts = Amps X Voltage
Voltage = Amps X Resistance

If you increase the resistance in the circuit voltage will go up to maintain a given amount of Amps through the circuit. If Voltage goes up to keep Amps the same then Watts go up! More Watts a proportional controller pushes out the hotter the internal circuitry will get. The hotter the internal circuitry gets the more likely failure becomes.

[JeffFlanagan]

If you increase the resistance in the circuit voltage will go up to maintain a given amount of Amps through the circuit. If Voltage goes up to keep Amps the same then Watts go up! More Watts a proportional controller pushes out the hotter the internal circuitry will get. The hotter the internal circuitry gets the more likely failure becomes.

If someone put a rheostat in parallel with the heat tape, a bad mistake, then your concerns would be well-founded.
In series, it's limiting the amount of current that could go through the thermostat. For example if you had 10W tape, with no rheostat and the probe came loose, you'd be pulling 10W through the thermostat at 110v. If you reduce the current 50% with a rheostat, the tape would only be able to pull 5W at 110v. As far as the thermostat knows you just replaced your 10w tape with 5w

The thermostat is going to run at a higher % at all times with the rheostat in place, but that current is going to be limited by the resistor in the circuit, and will use less power at max output than the tape alone. I'm not aware of any problem with a herpstat running around 100% for extended periods as long as you're not drawing too much current.

I don't think it's as good a plan as just using lower wattage heat tape, but it could work. A non-proportional thermostat before the proportional one may be a better idea, but that would increase your odds of a no-heat failure state.

I recommend 4" flexwatt + a herpstat, and no screwing around with rheostats or second thermostats at all.

[Gloryhound]

If someone put a rheostat in parallel with the heat tape, a bad mistake, then your concerns would be well-founded.
In series, it's limiting the amount of current that could go through the thermostat. For example if you had 10W tape, with no rheostat and the probe came loose, you'd be pulling 10W through the thermostat at 110v. If you reduce the current 50% with a rheostat, the tape would only be able to pull 5W at 110v. As far as the thermostat knows you just replaced your 10w tape with 5w

The thermostat is going to run at a higher % at all times with the rheostat in place, but that current is going to be limited by the resistor in the circuit, and will use less power at max output than the tape alone. I'm not aware of any problem with a herpstat running around 100% for extended periods as long as you're not drawing too much current.

I don't think it's as good a plan as just using lower wattage heat tape, but it could work. A non-proportional thermostat before the proportional one may be a better idea, but that would increase your odds of a no-heat failure state.

I recommend 4" flexwatt + a herpstat, and no screwing around with rheostats or second thermostats at all.

Current is a constant in a series circuit and voltage is divided by the various loads.

Voltage is a constant in a Parallel circuit and the current is divided by the various loads.

This is why all household recepticles and lights are wired in parallel. If not each divice would not see 110V AC.