12V Thermostats

26 Posts
Feb 19, 2010 07:59 pm
12V Thermostats

I've got 12VDC battery power and 12VDC "El Sid" circulator pumps for my hydronic heating.  Right now I have the pumps wired to a manual switch and they work fine, but I need to have zone thermostats.  How do I set up thermostats to drive the pumps?  All thermostats I've found which can run on 12VDC have something like 0.5A max load, so they can't run a pump directly.  Do I have to use a relay?  Has anyone built a system like this before?  I wish there was just a thermostat which I could just wire to power and to the pump and it would work.  It only needs to drive about 2A.  Any help is appreciated.
462 Posts
Feb 22, 2010 10:49 am
Re: 12V Thermostats

Thomas, most household thermostats, a basic honeywell round for example, operate on 24 volts. They are normally wired to a relay at the boiler to run the pumps at 110 volts. They should be able to handle the power going to an El Cid pump. Just install the thermostat in series on the power line coming or returning from the pump.
« Last Edit: Feb 23, 2010 10:07 am by Tom Mayrand »
220 Posts
Feb 23, 2010 02:10 pm
Re: 12V Thermostats

 hey guys,

yep, bet with such low current the standard thermostat would work a treat.

another option might be one of these no frills universal jobs our hosts have available.


some nice ratings with that one...wish they gave a link to the full spec sheet?

sounds like you are good to go..those were seperate pumps without any electronic zone valves?..also, we might want to go bigger than the standard telephone/signal wire with the hookups.

cheers, dave

edit: .."one of these no frills.." should read.. "two of these.."
« Last Edit: Feb 23, 2010 02:16 pm by david ames »
26 Posts
Feb 23, 2010 06:47 pm
Re: 12V Thermostats

The standard thermostats have a maximum load rating of between 0.3 and 1.3 Amps.  That's not quite enough to run a 20 Watt pump at 12 Volts.  Do you think it's OK to exceed their maximum ratings, or would I risk frying something?  The Wirsbo rep said I would need a relay for their thermostat, and they only sell line-voltage (120 VAC) relays.  I'm going to search for automotive relays.

The thermostat you linked to is rated for 10A @ 12VDC, so that might be my best bet.  I was really hoping for something programmable though.

Yes, they're seperate zone pumps, not valves.  They pull individually through a manifold from a solar storage tank (or back-up propane water heater which is triggered by flow).  There is no single system pump.

I'm using 10 guage wire for all of my 12 VDC systems to minimize voltage loss.  If I end up using relays, I'll probably go with 14 guage for the signal wire.

220 Posts
Feb 25, 2010 12:17 pm
Re: 12V Thermostats


 i don't think we will find anyone who will come out and say it's OK to run equipment outside of it's published ratings. but that is what i would do.

 had a brief look at those programable wirsbo thermostats and did not find one that would work..those were all hardwired "voltage stealing" units i saw.

 what we need is a battery powered job (stand alone)..lots to choose from..you are correct to question the max current rating and the relay option may be a better bet..(although we will still be running outside the ratings..ie 24vac verses 12vdc)

 here is a doodle to show the possible hookups.

 cheers, dave
462 Posts
Feb 25, 2010 12:36 pm
Re: 12V Thermostats

Thomas, the 0.3 - 1.3 is not the load rating of the thermostat. Why would the maximum amp load be variable? That range is for the heat anticipator setting for the control on a boiler. The current output from the thermostat has to match the current needed on the electronic boiler control in order for it to work correctly.
 What you are worried about when running power to your El cid, from a battery or controller, is the overall power rating of the thermostat's contacts. All a thermostat is, is a switch. You can use a 110 volt thermostat without a problem if you choose to also. 
« Last Edit: Feb 25, 2010 12:40 pm by Tom Mayrand »

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