24v Relay for Thermostat


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Old 10-11-13, 06:32 PM
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24v Relay for Thermostat

Hi - new to the forum, so please excuse any mistakes in etiquette.

In our basement we have a hydronic in-floor radiant heating system (Tamas), powered by an on-demand combination water boiler/heater (Navien). This was installed with a mechanical thermostat, which I want to replace with a programmable model with floor and ambient sensors (something the plumber should have done in the first place).

The Tamas system only supplies millivolt to the thermostat. Most programmable thermostats for this application require 24v. Where to get 24v from...

In the mechanical room there is this relay (fig.1). As far as I can tell, it's a 120v to 24v relay. Out of pins 6 and 8 (fig.2) is a low voltage (red) cable that runs to the furnace.

Into pins 0 and 1 are the 24v 20va output from a transformer (fig.3)

And so my question is this... can I run a second low-voltage cable from any other pins to feed the thermostat? My gut says I should be able to use pins 2 and 4...

Thanks in advance!

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Fig.1
[ATTACH=CONFIG]19135[/ATTACH]
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Fig.2
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Fig.3
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Old 10-11-13, 06:45 PM
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Welcome to the forums.

That relay you are showing activates on 24v it doesn't deliver or output 24v.

In order to use a regular 24v thermostat on a millivolt system you will need a 24vac transformer or power supply and an additional relay.

If the transformer in the pic is large enough it will run the thermostat and the relay. It's only 20watt so if you have more than one zone valve on it you will probably have to purchase an additional one for your thermostat. A 24vac 10w would be fine. I used your two pole relay picture but any single pole relay will work too.

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Last edited by PJmax; 10-11-13 at 07:08 PM. Reason: added diagram
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Old 10-11-13, 08:01 PM
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Hi PJmax. Thanks for the swift response! So, I think I'm a little confused on what a relay is supposed to do...
The existing configuration looks like this:Name:  config.png
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What does the relay do in this configuration? Also, it doesn't seem to match with the diagram you made - what am I missing...!
 
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Old 10-11-13, 08:06 PM
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what a relay is supposed to do
It is a switch. It turns power on and off.
 
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Old 10-11-13, 08:09 PM
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So the furnace has an alternative source of power beyond this? And if so, why does it need this source of power?

And if not, and if it's a switch, and there's no power until it's triggered, how does the calling device (furnace) demand power when... the switch is off.

Sorry for the newbie level of these questions - electrical isn't my strong suit!
 
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Old 10-11-13, 08:17 PM
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A thermocouple pile heated by the pilot light provides the power.
 
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Old 10-12-13, 12:25 PM
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You may be able to use that existing relay but you haven't shown us how that circuit is activated. You will need to find out where the 120v to the transformer is coming from and exactly to what are terminals 6 and 8 are connected to.

In that circuit....when you apply 120v to the transformer the relay shorts those two wires together.
 
 

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