Using a 24v thermostat on 12v line?


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Old 08-30-14, 05:05 PM
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Using a 24v thermostat on 12v line?

I can only find 24v as the lowest voltage in the hardware store. I measured the existing thermostat with a meter and it reads 12v. Can I just connect the new thermostat to this? It's battery powered made by aube and it runs 3 baseboards... noir sur off total wattage.
 
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Old 08-30-14, 07:05 PM
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You can not select a thermostat based on a voltage reading at the thermostat.
There is not enough info to be able to help.

Some baseboard thermostats directly switch the elements and others switch relays.
We need to know exactly how yours is connected and what the make and model of the old one is.
 
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Old 08-30-14, 07:57 PM
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Old one is a chromalox from 22 years ago. Connects to a remote box that controls the baseboards. All others in the house are direct 240v.
Connections coming out from the wall are one black wire and one white both finger than normal electrical wire.
 
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Old 08-30-14, 08:40 PM
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Old one is a chromalox from 22 years ago. Connects to a remote box that controls the baseboards.
Most any battery operated thermostat will work for that system. You would only use the R and W connections on most units.
 
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Old 08-31-14, 03:27 PM
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You need to find something electronic, battery powered with relays - the lower voltage might cause problems with triacs found in cheaper t-stats.

Honeywell is a safe bet: rth 6xxx series and up definitely have relays. (if it doesn't click, return it)

Mechanical t-stats with a resistive heat anticipator aren't suitable for 12v.
 
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Old 08-31-14, 03:40 PM
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I connected it but it won't turn on the heaters. The heating icon comes on. The thermostat is battery operated but I guess the control box is expecting a12v signal?
I tried swapping the black and white between Rh and W and nothing.
Any ideas?
 

Last edited by qwertyjjj; 08-31-14 at 04:19 PM.
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Old 08-31-14, 05:32 PM
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There is no "signal" coming from a common residential thermostat.
By signal I assume you mean some kind of voltage to close a relay.

Common thermostats only close a set of contacts that pass voltage from the power supply located at the furnace or air handler to the relay.

Most of the old high voltage baseboard relays I am familiar with are 24 volt, ac.
If you measured and got 12 volts it is possible the voltage was lower because it was in series with the relay coil.

You first need to investigate to check the transformer and relay coil to determine exactly what you have.
 
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Old 09-01-14, 09:33 AM
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It's a chromalox LR56555.
Looks like it's powering 2 circuits as there are 2 red wires coming out of the top going to the baseboards powering 3 baseboards totalling 4250W.
The wires at the thermostat measure 12V DC (which is odd because upstairs at another remote thermostat I can only get a 22V A-c reading).
I presume the transformer is combined in the box? https://drive.google.com/folder/d/0B...dyY0ZjY1E/edit

Could I just replace this box with a new 24v one?
 

Last edited by qwertyjjj; 09-01-14 at 10:36 AM.
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Old 09-01-14, 10:21 AM
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Try shorting the two wires together that connect to the thermostat..... what happens ?
Do you get heat ?

If you get heat then you need to get a relay type thermostat like muggle mentioned.

It's a Chromalox PM2/240B controller but I can't find useful info on it.
You could replace it with a 24vac one if you wanted to.
It would need to be rated for at least 4500 watts at 240vac.
 
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Old 09-01-14, 03:24 PM
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The transformer is part of the relay box or are they separate?
The current one I have is for relays. Is 24v and clicks. Is an aube TH147-p
Strange thing is I am getting 28v a-c on this one dc but 12v dc - on the same wires?
 

Last edited by qwertyjjj; 09-01-14 at 03:55 PM.
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Old 09-01-14, 03:37 PM
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It's a self contained control box.... the relay is inside.
 
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Old 09-01-14, 03:47 PM
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Can you find what you have here.
I can't find info on any of the numbers posted.
 
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Old 09-01-14, 04:03 PM
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Can't find it there at all but it's probably 30 years old. The current thermostat I have is for relays, is 24v and clicks. It's an aube TH147-p
I played around with the electrical meter again... strange thing is I am getting 28v a-c on the wires but 12v dc - on the same wires?
 

Last edited by qwertyjjj; 09-01-14 at 04:25 PM.
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Old 09-02-14, 07:02 AM
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Any reason why a 24v stat wouldn't work on a 28v line?
The DC reading of 12v is strange as well. Shouldn't the meter only be picking up 28vac?
 
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Old 09-02-14, 03:10 PM
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I don't mean to be rude but you should put away your voltage tester and give us the exact specs for the relay/control box.
If a person understood circuits they could surmise that a relay coil in the circuit you are measuring could cause a "strange" reading.
I am familiar with heating relays similar to what you show in the pic but you will need to determine the correct specs for what you have.

When you say the control box is "expecting a 12v signal" is not what a standard heating thermostat can provide.
A standard thermostat has only a set of contacts that close or open and what this provides is not considered a signal.
A signal when it relates to a control would be something more that a switched wire.
A signal is normally a variable voltage or ma current to control something where you would have a C wire at the control to power a circuit.

So, put away the meter and give us as many CLEAR photos of what you have, including if you have a regular relay or electronics inside the box.
 
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Old 09-09-14, 09:21 AM
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Originally Posted by PJmax View Post
Try shorting the two wires together that connect to the thermostat..... what happens ?
Do you get heat ?

If you get heat then you need to get a relay type thermostat like muggle mentioned.

It's a Chromalox PM2/240B controller but I can't find useful info on it.
You could replace it with a 24vac one if you wanted to.
It would need to be rated for at least 4500 watts at 240vac.
Touched wires together no heat.
Installed old mechanical stat with 2 connections it then gives heat.
Installed new Digital stat to Rh and W no heat.

Doesn't really make sense.
 
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Old 09-09-14, 10:33 PM
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That makes absolutely no sense at all.
 
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Old 09-11-14, 03:38 PM
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either way something must be wrong with the transformer.
Guy at the hardware store said if it's reading 28v then it means the transformer is old as they increase in voltage over time.
To replace the existing transformer and coil, I can just buy a new small integrated coil/transformer?
 
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Old 09-11-14, 03:51 PM
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That is not necessarily correct.
Many transformers are designed to be tested with a load and yours could be perfectly fine.
What you are measuring would be referred to as "open circuit voltage".
 
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Old 09-11-14, 04:18 PM
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well...possible but either way it will not work with the 24v thermostat I bought.
I know the thermostat is ok because I tried it upstairs on another 24v circuit and it works fine.
I know the downstairs circuit is "functional" as it works with a mechanical themrostat.
So, that leaves some issue with the relay/transformer?
 
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Old 09-11-14, 05:32 PM
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What do you mean "well...possible"???
You were proposing to throw a transformer at it based on that a "Guy at the hardware store said".

I really don't think we can help you.
There is some technical info here that should help and I have asked for some info and didn't get it.

Although this is a DIY forum you would do well to call a competent service tech to solve this for you.
I don't think the final solution will be that complicated or expensive.
 
 

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