# Line Voltage Programmable Thermostat - Why 1 amp Minimum?

• posted on October 12, 2005, 2:28 pm
I have a hydroponic baseboard heat system in my home (just bought home in June and house was built in 1988) that is regulated by the old Mears analog (2 wire NI 120/240 Vac) thermostat. I have 3 zones therefore 3 thermostats. I wish to replace these with digital programmable models.
I have measured 122 Volts at .68 amps at the wires coming into the thermostats. I have been unable to locate a thermostat that can run on this low power. Most have a 1 amp (250 W) minimum power rating.
Besides changing my wiring to a low-voltage system, is there a solution to my problem other than just keeping analog thermos in place?
Thanks!

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• posted on October 12, 2005, 3:09 pm
A thought: Would a thermo with batteries such as the LUX ELV1 solve this problem? I assume that the switch would be controlled by the batteries and not the line power. True?

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• posted on October 12, 2005, 4:43 pm

You are misinterpreting this. What you just measured is that your existing thermostats draw 0.68 amps of current. Your house electricity is right about 120V, as it should be, and this is irrelevant for your situation.
If you put your new 1-amp rated thermostat on, it will draw whatever current it needs to function. The 1-amp rating is telling you that it will pull something around 1-amp or less. Probably right around 0.68 amps would be my guess, actually. It is a "minimum power rating", because it is telling you that your wires had better be able to handle a minimum of 1-amp, otherwise it might be unsafe. You can't measure current capacity in the way that you are doing. What you do instead is look at the wiring and make sure it is large enough (and installed properly) to handle up to 1-amp.
[ I'd be real surprised to find any 120V wiring in a house that is rated anywhere near as low as 1-amp, but don't take my word on that. ]

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• posted on October 12, 2005, 4:58 pm
I removed the thermostat and used an amp/volt meter on the wires coming out of the wall. That is where I am measuring .68 amps and 122 volts. I had previously installed a Honeywell line-voltage thermo that was rated at 2 Amp minimum and it gave me a low power indicator and was not able to switch on the heat although the display functioned perfectly.
At any rate, the battery operated LUX ELV1 will work according to LUX tech support so I believe I have found my solution.
I appreciate very much that you took your time to try to help me. Thank you.

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• posted on October 12, 2005, 8:05 pm
If you have the ends of two wires, you can't possibly be measuring amps. So either you are completely confused, or I have completely misunderstood what you are talking about. Amps are what you get when electricity travels through things. If there is no _thing_, then there are no amps. What you said is basically the same as me saying "I walked out to the road in front of my house with a meter and measured 35mph coming out of the road." It just don't make sense.
What you likelly have is not 122 volts. What you have is 12V. There is a transformer in your basement that converts from 122 volts down to 12. Now, this transformer is only capable of putting out a certain number of amps, maybe just 1 or probably even less (I really don't know -- but it is certainly down in the range of a few tens or hundreds of milli-amp, and not more than say 1-amp). When you put your "2-amp" thermostat on, it tries to pull too much electricity through the thing. What ends up happening is that the transformer can't keep up, and could overheat and ruin itself. In these cases you might see the voltage drop below 12V, and you would inteed get a "low power" indicator. Of course, you won't see any such drop when there is no thermostat connected.
Alternatively, is it that you only have a _single_ wire, which you are measuring the current as it is travelling through the wire? In that case you could be trying to wire something in series (where I mean _really_ in series, electrically, not as many people use the word). Which is bad.
Or I could just be completely off my rocker. (Admittedly, I haven't slept in a bit.)

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• posted on October 12, 2005, 8:57 pm
kevin wrote:

The "thingy" is my TENMA Digital Amp/Voltmeter. :) So, yes, I am accurately measuring the current and voltage running through my system. One wire is the load and one is the line. I also have a copper ground wire.

No, I do not have a low voltage 12 V system. I have the "old-fashioned" 120 Vac line-voltage system. No transformers unfortunately.

I have a 2 wire with a ground. The wires are thick - much thicker than those on a 12V line.

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• posted on October 13, 2005, 10:54 am
Ok, I'm probably wrong and confused, and I don't know what you are measuring. In my defense, you don't seem to know what you are measuring either.
Obviously I don't know how these line voltage themostats work. But I suppose it is reasonable that they _are_ placed in series with the load. And you are measuring the AMPs by just touching the two probes to your two wires, and so are measuring the current that would pass through your heating elements in the full ON position. I would be a little nervous about having 10 amps or more go through the little multimeter probe wires that I would be holding, and I know my multimeter would blow its fuse if I tried that anyway.
The other poster is probably right, that your thermostat is going to borrow some power using a current division circuit, and that this requires at least a certain low resistance (and so high amps) for the heating element. But that you are measuring 0.68 amps seems odd to me, because an electric heater would use 5, 10, or more amps, no?. I don't know what a hydroponic baseboard heater is (unless you are using an electric heater to grow plants in water, that is). Is this some fancy hot water system with electronic valves (that need .68 amps to operate)?
It should be obvious that I am not going to be able to help you here, but I am curious all the same.

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• posted on October 13, 2005, 7:03 pm

The current measurement confuses me too.
I went looking on the web.
Looks to me like this is what he is talking about but it is nothing but a switch.
http://www.luxproducts.com/techsup_pdfs/lv1-2_eng.pdf
If so the ELV1 looks like a direct replacement.
http://www.luxproducts.com/techsup_pdfs/elv1_eng.pdf
As for the Honeywell, all I can think of is that it might have been for a 240 volt system.

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• posted on October 12, 2005, 7:53 pm

Learn something new every day around here.
Nick

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• posted on October 12, 2005, 10:21 pm

My best guess would be that the load the stat runs has something to do with how the stat is powered. The stat "robs" power off the line voltage when the heating load is off. In other words it runs a small amount of power through the heating element constantly to power the stat. Enough to power the stat, but not enough to produce any heat. If the resistance is too high there will not be enough electrical power trickling through the system to allow the stat to work.
Your other post backs up my theory because the stat's low power indicator was lit up when connected to a too small of load, (heating element). Greg

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