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Old 01-20-2008, 09:19 PM
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Default Two thermostats on one furnace?

I want to run a “dual heat setting” at my house. I have found dual stage thermostats but they are for dual stage furnaces (like big HVAC units?) and I definitely have just a single stage unit...

Background: House sits on top of a 3’-4’ crawlspace, where the furnace is... which is a pos and uses expensive gas I'd prefer it to just be off, most of the time. Dirt floor, 18”-2’ high foundation with 2x6 studs on top of that, good insulation on the walls, no insulation on the ceiling (floor upstairs.) Also in the crawl space are the water pipes- everything for the house above plus the one coming in from the well. there's a 2x3 door from the outside you remove to get it.


The issue: In the main level is a wood stove, and when it’s going the furnace doesn’t come on. good stove. The furnace is what also heats the crawl space (also it gets a little from the floor above) and when it’s wicked cold out for a long time, and I’m heating with wood, I’m worried the crawl space will get too cold and the pipes could freeze. This gets worse if you leave the crawl space door open.. (damhik)


The solution: Leave my current Thermostat upstairs (T1) to where it’s at now (58-62F). Wire up another thermostat inside the crawl space (T2) set to 36F… screwing both to the same terminals on the furnace. This is not unlike how I’ve wired many a car speaker, 12v accessory, even homes are wired this way- And in fact that’s all this is, how would you do a switched outlet switched by two different switches… either one (or both) could complete the same connection.

No I haven’t measured the temperature in the crawl space, and should have that data point. But what if someone leaves the crawlspace door open outside one afternoon… would this “dual thermostat” solution work without any adverse effects? And I'm thinking that the lower thermostat (T2) would probably never even get called into action, since if the furnace is already going to stay above 58, it's plenty warm in the crawlspace. Only time would be if I left the door open, or we heated with wood for a week and the temperature in the crawlspace just got down that far. I'm going to try this this weekend, I think I'll sleep better. thoughts?
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Old 01-20-2008, 09:47 PM
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This could get complicated having two thermostats controlling. What about a temp sensor with an alarm to let you know its gotten cold down there or an electric heater wired to go on at 36 and off at 40.

Just some thoughts. Having two controllers for one furnace has several different scenarios where you might overlook one and the furnace doesn't come on causing damage.

We could design a circuit and embed the logic you want to determine which controller has actual control of the furnace.
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Old 01-20-2008, 10:01 PM
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The problem seems to me to be that your one furnace is trying to heat two very different areas. Adding a second thermostat is treating the symptom (cold crawlspace) but not the cause (not having a second, independent zone). Doesn't make sense. Like Ken suggests, you need a second zone, which in this case would require a second heat source of some kind.

You could design an elaborate damper system to make one furnace heat two zones independently, but I doubt you'd like paying for it nor the results. One furnace per zone, you'd be happy.

By the way, I have a perfectly good 90% furnace we recently removed (not the furnace's fault, it was the boneheaded contractor's fault; undersized for our home). I'd be happy to part with it for dimes on the dollar, if you (or anyone else) thinks they could use it. Nat'l gas. Could be revalved/rejetted I bet.
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Old 01-20-2008, 10:17 PM
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I just don't see the problem- if it gets below 60 upstairs the furnace comes on. If it gets below 36 in the crawlspace the furnace comes on. If the furnace comes on, the crawlspace gets heated, and if one of the thermostats dies, I'm still better off than I was. The only downside I can see is it's way too hot upstairs because of the fire and the furnace comes on, making it a little hotter.

But yeah Bill I'm probably interested in your furnace, let's talk!
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Old 01-20-2008, 10:27 PM
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The control would not be tough, an XOR using two relays. Pretty straight forward. But you want to do this the Rube Goldberg way. I'm seeing an MC68HC12 microcontroller, 4 line RGB LCD monitor and several zone remote sensing here. Or just a couple of $3 relays, your choice.
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Old 01-20-2008, 10:28 PM
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The problem is you have two controllers driving one circuit, each with it's own load and impedance. What happens when both controllers come on, you will end up with double the rated current and signal on the line.

What happens with the long wires you will have to run to hook both up. Reflections, ground loops? Wasn't designed to work that way.


You will drive into the other controller circuit which is only an output circuit. To solve this problem, you will have to Diode Or both outputs which will create additional voltage drops so you will have to add a pull up circuit which with the required resistor could cause reflections and impedance mismatches.

Your trying to use something in a way it wasn't designed for. A lot of times thats OK if you understand the specs, and compensate for the added effects and sometimes this means adding more circuitry and complications than doing two separate systems would be. I again am thinking a small electric heater.

Now this can be solved by having your furnace only see one controller and you get a device (Design, build, buy) that will do the voting you want between the different controllers and switch control either via mechanical relays or interpret and create a compatible signal on its own.
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Old 01-20-2008, 10:44 PM
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I can't see how two controllers on one circuit could work efficiently. I can however see another solution. Put in a duct to the crawl space from the main zone. Put in a electric vent fan. Wire the fan to a thermostat near your plumbing. When the stat pings the fan it comes on and steals some of your warm air from the main floor and heats the crawl space to 45ish degrees then shuts off till needed again. What think ye?
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Old 01-20-2008, 11:10 PM
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Ok you guys are talking some sense, though most of it went right over my head.. but you pointed out a bunch of stuff I didn't consider... hey that's why we post up our ideas right?

Kevin that would work. And in fact I could just cut another diffuser or two in the floor upstairs by the stove and probably get some hot air- would need the fan though. So if it gets below 36 in the crawl space I start pulling air from upstairs, with the assumption it's hotter than 36 up there.. hmm....

But ok, so what if I wire up a toggle switch that decides which thermostat I'm using? So it's either one or the other, never can be both? So when I'm away from the house for a while or run out of wood or by default it's on the upstairs thermostat (just like it is today), but when I'm home and burning wood I flip it to downstairs... problem solved?
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Old 01-20-2008, 11:30 PM
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Quote:
Originally Posted by nakman View Post
Ok you guys are talking some sense, though most of it went right over my head.. but you pointed out a bunch of stuff I didn't consider... hey that's why we post up our ideas right?

Kevin that would work. And in fact I could just cut another diffuser or two in the floor upstairs by the stove and probably get some hot air- would need the fan though. So if it gets below 36 in the crawl space I start pulling air from upstairs, with the assumption it's hotter than 36 up there.. hmm....

But ok, so what if I wire up a toggle switch that decides which thermostat I'm using? So it's either one or the other, never can be both? So when I'm away from the house for a while or run out of wood or by default it's on the upstairs thermostat (just like it is today), but when I'm home and burning wood I flip it to downstairs... problem solved?
The vents have no idea where to heat because it's just one heat zone from the heater. If you have it set to heat the crawl space and the crawl space doesn't take much heat then your upstairs pipes will freeze! And if the crawl space has too many airleaks your mainfloor will be over 100 degrees and/or out of propane! You can get very high efficient inline blowers used for range vent on long runs. You can also get attic fans that have a thermostat built in but I would think you want the T-stat as close to the freeze sensitive area as possible. I wouldn't put any more than one duct in or you will pull your warm air down to the ground and exchange cold air back up!
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Old 01-21-2008, 12:01 AM
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Electric heat wrap on the pipes--plug them into an outlet that is controlled by a relay. The relay is then controlled by a thermostat. When the temp drops to 32* the wraps come on and protect the pipes from freezing. Probably the most efficient solution to protecting the plumbing as it will only come on when absolutely needed instead of trying to heat the entire crawl space.
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