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Old 08-04-2012, 09:59 AM   #1
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Thermostat to control two systems?


Our house has both an oil-fired furnace for hot water and baseboard heat in the winter, and a mini-split heat pump for spring/summer/fall heating and cooling.

Is there a thermostat that can control both systems simultaneously? It's easy enough to turn the mini-split off in the winter and the furnace thermostats in the summer, but I'd like something simpler for spring and fall. Especially for when the outside temperature is right near the point where (I'm told) the mini-split becomes more efficient than the furnace.

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Sim

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Old 08-04-2012, 10:07 AM   #2
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Thermostat to control two systems?


What sort of thermostat is used on the minisplit now? Some have their own proprietary stats, while others use standard 24v stats.

If yours uses a 24v stat, you could perhaps rig up a multistage/multitransformer stat with outdoor sensor that would run the mini split if temps are above 40 degs (or whatever temp) outside, and then change over to run the other when the outside temp drops to near freezing.

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Old 08-04-2012, 11:28 AM   #3
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Thermostat to control two systems?


The minisplit is a Mitsubishi Mr. Slim. The only thermostats we have for the evaporators (MSZ-GE12NA, MSZ-GE09NA, 2x MSZ-GE06NA) are handheld wireless remotes.

Your reply pointed me to a few Google search terms, which turned up the Mitsubish MAC-397IF-E wired interface adapter. Not a sure thing, but it's beginning to look possible.

Thanks!

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Old 08-04-2012, 03:56 PM   #4
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Thermostat to control two systems?


it is not possible sorry.
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Old 08-04-2012, 05:58 PM   #5
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Thermostat to control two systems?


Sorry to hear it. Can you tell me more - do the Mr. Slim units not support it, or... ?
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Old 08-04-2012, 06:06 PM   #6
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Thermostat to control two systems?


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Originally Posted by hvactech126 View Post
it is not possible sorry.
Nothing is impossible, This would just take a lot more controls to make it work.... would probably be silly in the end....

Actually, this would be pretty simple to do...

Here is a wiring diagram that would work, not sure your skill level on wiring with relays and such, but it is pretty simple.




As you can see in the diagram, any temp above 40* is going to run the heat pump, I just threw in a 40 amp contactor, you can use what ever size your split unit is sized for, this can be mounted right at the panel location to simplify the installation, when the temp drops below 40* the heat pump is de-energized, and the W from the t-stat on the furnace will close and fire off the furnace when the stat calls for heat, I chose to only break "W" because i wanted your tstat to stay active and still be able to use the "G" terminal on your furnace if so desired.

The only bad thing I can think of, is that when the heat pump gets re-energized after a cold night, but warm day, is what temperature the heat pump will resume too???? Does it go back to the last set point or a factory one?

Last edited by stickboy1375; 08-04-2012 at 06:40 PM.
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Old 08-04-2012, 07:42 PM   #7
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Won't work... a Mr slim used a proprietary thermostat that is wireless. Their wired t-stat still is proprietary and does not use 24v.
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Old 08-04-2012, 07:44 PM   #8
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Thermostat to control two systems?


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Won't work... a Mr slim used a proprietary thermostat that is wireless. Their wired t-stat still is proprietary and does not use 24v.
Yes it will, im breaking the 240v going to the unit itself. look at the diagram again....
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Old 08-04-2012, 07:44 PM   #9
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Thermostat to control two systems?


Also Mr slim units are efficient to at least 0F
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Old 08-04-2012, 07:46 PM   #10
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Yes it will, im breaking the 240v going to the unit itself. look at the diagram again....
You are still having to use 2 controls. The HP has to use the wireless control and you are using a 24v thermostat.
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Old 08-04-2012, 07:49 PM   #11
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Thermostat to control two systems?


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You are still having to use 2 controls. The HP has to use the wireless control and you are using a 24v thermostat.
Well, you are stuck with that of course, but IMO, the OP wanted to be able to run the heatpump and Boiler without having to turn one unit off and turn the other on... my diagram provides that function....

What my proposed diagram does will run the heat pump at a set temperture, and the boiler at another, without any intervention... which is a nice feature...
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Old 08-04-2012, 08:14 PM   #12
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Thermostat to control two systems?


Hvactech126, I don't have the time or (who knows?) the skill to RE Mitsubishi's protocol. Too bad it's proprietary. Regarding efficiency, can the Mr. Slim units reasonably be expected to be more efficient than an oil-fired furnace all the way down to 0F? That sounds too good to be true.

Stickboy1375, I get your diagram perfectly, and it does exactly what I need. I did my own electrical for the minisplit, and this isn't a huge step past that. Just need to make sure that when the condenser powers up, the evaporators start back up at their powerdown settings.
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Old 08-04-2012, 08:19 PM   #13
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Thermostat to control two systems?


Don't know of any heat pump that isn't more efficient then oil even at 0 degrees outside temp.
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Old 08-04-2012, 08:20 PM   #14
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Thermostat to control two systems?


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Originally Posted by Simbilis View Post
Hvactech126, I don't have the time or (who knows?) the skill to RE Mitsubishi's protocol. Too bad it's proprietary. Regarding efficiency, can the Mr. Slim units reasonably be expected to be more efficient than an oil-fired furnace all the way down to 0F? That sounds too good to be true.

Stickboy1375, I get your diagram perfectly, and it does exactly what I need. I did my own electrical for the minisplit, and this isn't a huge step past that. Just need to make sure that when the condenser powers up, the evaporators start back up at their powerdown settings.
Cool, I have a similar setup as to yours, except I have an actual furnace, so I can't run the fossil fuel and heat pump at the same time, that out door t-stat has a temp differential on it, so I set mine at about 32*, seems to work flawlessly, oil runs at night, heat pump during the day in the dead of winter. Im very happy with it...
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Old 08-04-2012, 08:20 PM   #15
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Don't know of any heat pump that isn't more efficient then oil even at 0 degrees outside temp.
but if your heat loss is too great, you'll never heat your house either. I've had my heat pump running at 18* during the day and it did seem to heat the house no problem, I have a 2 stage unit as well....

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