[SGVLUG] HAK group question: How to remote control a millivolt thermostat (circuit)

[Scott Packard]

What you don't see by removing the cover is the hysteresis built into the
existing thermostat.
You must build that into your remote thermostat, or you'll burn out your
heater.

In HVAC terminology, you're trying to do zoning, where either zone can call
for the heater to be lit.
Probably the easiest way is just to acquire another thermostat for the
upstairs, and run a wire downstairs.

As far as the magnet holding the heater on longer than you think it needs
to, the hysteresis is usually adjustable,
but, is usually adjusted right for the unit, though maybe not right for
your taste.

You may find happiness by using a ceiling fan instead, and run it so it
pulls air upward to the ceiling.
It'll allow mixing of hot air close to the ceiling with cold air at bed
level, without blowing air across your body which cools
you by evaporating perspiration.

Regards, Scott

On Sat, Dec 8, 2012 at 10:59 AM, Homan Chou <homanchou at gmail.com> wrote:

> I haven't been able to attend the HAK meetings yet, let me know if there
> is a more appropriate forum to ask my question.
>
> My home (built in 1945), has an old "gravity" heater, which is basically a
> gas furnace under one central vent located in the middle of house under the
> floor.  There is no forced air distribution at all, the hot air simply
> rises up the vent, and pretty much all of it goes up stairs and not into
> other rooms downstairs.  That's actually fine because we only need the
> heater in the middle of the night for a few hours, and all our bedrooms are
> upstairs.  However, the thermostat is located downstairs and is too far
> from the vent in order to operate as an effective switch (it's in the
> dining room which pretty much stays chilly even if even if it's blazing hot
> upstairs.)
>
> I would like to turn the heater on or off based on the ambient temperature
> upstairs.
>
> I removed the thermostat cover and it is just a metal coil that expands
> and contracts, with a little magnet at the end that snaps onto another
> piece of metal to close the millivolt circuit.  That explains why the
> thermostat is not very precise at switching on or off, the magnet holds
> onto the connection longer than it needs to.
>
> The way I see it, there are 2 problems that need to be solved:
>
> 1) I need to be able to remote control the opening or closing of the
> millivolt circuit.  a) I can do this manually by attaching an arduino +
> servo arm to move the existing thermostat temperature lever.  or b) bypass
> the thermostat entirely using some kind of relay?  But I don't know enough
> about relays ... what kind, how would it be wired up?
>
> 2) I need to be able to forward the ambient temperature upstairs to the
> arduino controlling thermostat downstairs.  How do I do that?  Is there a
> wireless temperature sensing product I can purchase and hack?  Or do I just
> get another arduino upstairs, get ambient temperature using a thermistor,
> then wirelessly forward the measurement to the other arduino using a pair
> of xbees?  I have never used xbees before.  Should I consider wifi internet
> shields instead?  That way I can potentially also control the heater with
> my phone/browser?
>
> Thanks for any advice.
>
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