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How I connected my millivolt Hearthstone Modena gas stove to a Nest thermostat

How I connected my millivolt Hearthstone Modena gas stove to a Nest thermostat


Old 10-14-16, 12:17 PM
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How I connected my millivolt Hearthstone Modena gas stove to a Nest thermostat

Web research

This thread describes the general electronics

More useful information here

This comment on Amazon was actually immensely helpful as well

I had a Hearthstone Modena 8140 gas stove professionally installed about a year ago. My condo is 980 sqft, and the Modena easily heats the whole place, so I've permanently disabled my 1970's forced air gas heater. The Modena comes out of the box with the Hearthstone remote control thermostat (3 AA batteries) that talks to a control box (4 AA batteries), which can manually be switched ON/OFF or to remote mode, and which is wired to the gas control valve.

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The installer wired the control box's white TPTH wire to TPTH terminal on the gas valve, and the green TH wire to the TH terminal, as expected.

This remote thermostat setup had a few deficiencies:
  • The thermostat is in the remote, so for consistent heating I had a specific spot for the remote to live, away from drafts. So my morning ritual was to wake up to 62 degrees, walk down the stairs to where the remote lived, bump it up to 68, and walk back upstairs to go to bed until it warmed up. Not really convenient.
  • Two points of failure: if the batteries in the remote, OR the batteries in the control box, got low -- then my heat effectively was disabled. By Murphy's Law, this happened exactly twice, and during sub-zero cold snaps when I was out of town. I was lucky my pipes didn't freeze.
  • I can't program it, remotely monitor and turn the heat on/off, and other problems that the Nest thermostat solves nicely

* Honeywell R8845U-1003 Universal Switching Relay with Internal Transformer $65.73 from Amazon

* Coleman Cable 18/3 Thermostat Wire, 30 foot $15.95 from Amazon

Ace Hardware, $9.62:
* 22-16AWG ring terminal
* 22-16AWG male spade terminal
* grounded 8' 16AWG power cord

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Local electronics place, $7:
* 2 female spade terminals
* cable clamp (that you see pictured below on the grey power cable going into the box)
* 12' 2-terminal 16AWG speaker cable

Total: $98.30


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Brown 3 conductor cable attached to transformer terminals:
R(T) = Red wire
W(T) = White wire
C = Green wire

Then, this attached to my Nest as follows:

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White 2 conductor cable attached to transformer low-voltage outs:
X1 = Red wire
X2 = Black wire

Then, this attached to my Modena's gas control valve as follows:

TH terminal = black wire
TPTH terminal = red wire

In other words, Transformer X1 --> gas control TPTH; Transformer X2 --> gas control TH.

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Finally, the power cord as expected, which you can see a few pictures up:
L1 (hot) = Black wire
L2 = White wire
Ground screw on case = Green wire

That's it. Here's the outside of the case showing the (included) rubber grommet and the cable clamp I bought:

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Pretty straightforward. The one flaw to this system is a power outage; if this happens, I have no heat unless I crawl under the stove to unplug the two terminals from the transformer and replace them with the old remote control box, and use that until the power comes back on. Which is a huge pain in the neck.

Does anyone have any ideas about a battery backup I can add on to this? Or some clever way to 'splice in' the remote control box so I don't have to crawl around and wire it up, or worse off, explain to a cold houseguest how to do this?

Hope this helps somebody!

PS: Here's a random unneeded picture that I can't figure out how to delete:
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Old 10-14-16, 12:30 PM
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Connect your remote directly across the terminals where the red and black are connected in the red circle. (there is no polarity)

NOTE: This puts both switching sources in parallel. If either one is on.... you'll get heat.

The NM connector is in backwards. (pink arrow)

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Old 01-03-17, 03:25 PM
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How to run a millivolt B60 gas valve on 24V thermostat

I know this is long after the original thread, but I'd like to contribute. My house has an ancient (1970s or earlier) forced air furnace designed as a 'drop in' replacement for a 1950s floor furnace. When I moved in, it still used an old millivolt thermostat to control a B60 gas valve. The fan was heat actuated. As the furnace warmed up, a thermostrip tilted a mercury tube switch to electrify the fan. Believe it or not, that part still works.

The problem was the distance between the downstairs furnace and the upstairs thermostat. Flaky, iffy, unreliable operation. I finally replaced the millivolt thermostat with a standard 24V model. Went to Radio Shack (sigh) and bought a small transformer, relay, rectifier, and a large capacitor. Installed all into a small box. I wired the B60 to the relay secondary, distance a few inches. Relay primary runs to the 24V thermostat circuit. Thermostat thinks its controlling a modern furnace unit. B60 thinks its being controlled by a millivolt thermostat. This kluge has worked pretty reliably for decades. I turn off the pilot from spring to autumn. Easy to live with in SoCal.

Recently I wanted to install a Nest thermostat, but gave up when faced with the maze of connections on the box. My configuration was too bizarre to explain to anyone. Returned the unit without opening the package. As to why I still run obsolete technology in 2016: furnace is not very efficient, but it's on only half the year in an LA winter. Gas prices now are especially low, while the price of a new furnace with updated ducting would buy a century worth of natural gas. Or so I tell myself
Old 01-03-17, 03:44 PM
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Welcome to the forums.

So........ is there a question here ?
You already have all the parts to make the nest work.
Old 01-06-17, 12:14 PM
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As I said, I did not pursue the Nest. Not clear that I have what I need in the right configuration. Current thermostat is presented with only two wires that carry 24VDC (I could easily change to AC). B60 needs millivolt current to work. I need to actuate the furnace-resident relay from the Nest, which has approximately one gillion contacts. Seemingly designed for turnkey replacement of a 'modern' thermostat installed in a standard way.
Old 01-06-17, 04:34 PM
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With a millivolt system like most fireplaces..... you are switching under 1VDC.
Typically 500 mv or so.

ANY battery operated thermostat will work for your fireplace as a battery operated stat uses relays for switching. The nest and many other of that type use semiconductor switching which won't work on a millivolt system. Those stats need at least 15vac to switch the output on.
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