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jerfer 12-19-2008 09:10 AM

Wiring a RobertShaw 9870i to Gas Furnace & Heat Pump
I am trying to wire a RobertShaw 9870i thermostat to a Luxaire FL9T two-stage high efficiency gas furnace and a Luxaire HL3A single-stage heat pump.

The thermostat supports controlling a heat pump in both Standard mode, where the compressor is left on during Aux heating, or Add-On mode where the compressor is turned off when Aux heating. The thermostat is set up with an outdoor temperature sensor in order to detect whether to use the heat pump or furnace for heating.

Although the heat pump can be wired to the thermostat, and can also control the furnace, through its Y2-out, W1-out, and W2-out connections I would like to instead use the thermostat for all control.

Can anyone offer any help in wiring these together? Thanks in advance.


beenthere 12-19-2008 12:15 PM

First are you replacing an existing thermostat. Or is this a new install.

The Luxaires defrost board, starts the furnace 45 seconds before it goes into defrost, if you also have teh bonnet sensor installed.
This is a comfort feature. If you swtich to using a dual fuel thermostat, you will lose this feature.

Also, since its dual fuel, you would use teh add on setting, so as not to harm the heat pump by having the furnace run at the same time in heating mode.

You also still wan t teh heat pumps Y2 out to control blower spped, if you have hot heat pump enabled.

jerfer 12-19-2008 05:02 PM

More information
Thanks for the reply beenthere. Here's some more information.

This is a correction of an existing installation. The heat pump never worked satisfactorily, going into fault every time Aux was engaged. The installer didn't jumper FFUEL to ON on the defrost board, nor did he set the thermostat to Add-On mode indicating that the heat pump has been run with the furnace, which probably caused some of the faults since the HP went into LOW/HIGH pressure faults on a few occasions.

The coolant level should be fine as I had that checked and adjusted this summer but heating is still an issue.

Regarding your comments about the defrost delay and the Hot Heat feature, I have tried to get those to work by making the following connections, to no avail, as the HP doesn't even start now and the system always heats with the furnace.

Thermostat -> Furnace -> Heat Pump


C      -> C      -> C
R      -> R      -> R
O      -> nc      -> O
Y1      -> Y1      -> Y1
E/W1    -> W1/W    -> W1-out
W2      -> W2      -> W2-out
Y2      -> Y2      -> Y2-out
D      -> nc      -> nc
L      -> nc      -> X/L
G      -> G      -> nc

I don't believe that a bonnet sensor is installed as there are no connections to the BS/BSG terminals in the heat pump. The outdoor temperature sensor and humidifier are connected to the thermostat and both are working fine.

I have set the defrost board's FFUEL and HOT HEAT jumpers both to ON.

Any clue as to why this isn't working?

Kind regards,

beenthere 12-20-2008 05:23 AM

Change the thermostats W1/E wiring to go to the heat pumps W terminal, not W1 out.
Disconnect the thermostats W2 wire.
Connect the O terminal at the furnace(causes a delay for cooling only, to allow coil to get cold before starting blower).
Disconnect the stats Y2.

What are the balance point and low temp cut out jumpers set to.(LTCO jumper does nothing in FFuel mode, the BP jumper setting becomes both the BP and LTCO setting)
If the BP jumper is set higher then then the outdoor temp is, it will use the furnace only for heat when the FFuel jumper is set to on.

jerfer 12-20-2008 01:26 PM

It worked!
Thanks beenthere, your suggestion worked perfectly.

The BP setting was 35F, which explains why the heatpump wouldn't kick in - it's currently 7F outside. I set the BP to 15F. I also set the thermostat to Normal compressor mode as the heatpump is now making decisions on whether to start the furnace or not.

I couldn't find an O connection on the furnace is it labelled something else?

Thanks again for your help, take care.


beenthere 12-20-2008 06:01 PM

Ooops, sorry, your model furnace doesn't have an O terminal, I was thinking of the ECM blower model.

jerfer 12-21-2008 11:18 AM

Converted a 2H/2C thermostat to 1H/1C
Hi beenthere,

I've been thinking about how the system is wired now, and effectively what we've done to make it work is use the thermostat like a 1H/1C device and allow the heatpump to drive the furnace as required for 3H/1C.

Of what use, then, are multi-stage thermostats if they limit the usefulness and functionality of heatpump/furnace combinations? I guess what would be needed is a thermostat that not only drives the heatpump but also listens to its Y2-out/W1-out/W2-out signals in order to make decisions on how to drive the furnace and fan.

What do you think?


beenthere 12-21-2008 03:19 PM

Actually, its working as a 3 stage system.

When the HP receives a Y signal for heat. Above balance point, it uses the heat pump only.

When the HP receives a Y signal for heat. Below balance point, it uses first stage of the gas furnace. It only energizes W1 out.

When the heat pump receives a Y and a W signal for heat when its below balance point, it uses second stage heat of the furnace. It energizes W1 out, and W2 out.

The advance features of the board, make it that you don't have to use a multistaghe thermostat.

Many techs get confused on that also.

jerfer 12-21-2008 04:10 PM

That's what I thought. Do the boards function any better than a capable thermostat or am I better off installing a thermostat that would, in some way, support the features on the defrost board but with better control of temperature and more consistent comfort?


beenthere 12-21-2008 04:43 PM

6 one way, half a dozen the other.

The defrost board brings on the furnace 45 seconds before it begins defrost(also need bonnet sensor for this). So you don't get a blast of cold air, waiting for the furnace to light up. It also cycles teh burner off and on if the air temp starts to get too warm during defrost.

A dual fuel thermostat can't do that.

Using the board, you have your choice between a stat that hold a tight temp diff, or a large diff, the same as if you were using the stat to control dual fuel. So you don't really loose any options like that.

Its sort of a catch 22.

Unfortunately, your selling/installing contractor never went over all the features, or choices with you.

jerfer 12-21-2008 08:49 PM

Thanks for the very informative post beenthere. The reason the installer used this thermostat was for its humidity control feature. In hindsight, I should have done some more research and probably asked for the 1H/1C version instead of the 2H/2C.

The installer should have installed the thing correcly though. From what I understand, he hadn't ever used this therm before and bought it on recommendation from the distributor, based on my requirements for humidity control.

In any case, the system works perfectly now. I just finished testing the second furnace stage, the therm initiates it when there's a larger than 4F difference between setpoint and current temp and it worked flawlessly. The heatpump is deferring to the furnace because it's less than 15F outside so I guess I can finally put a checkmark next to this item on the to-do list.

Thanks again for all your help beenthere, I, honestly, couldn't have done it without your help and advice. Best wishes and happy holidays to you and yours.


beenthere 12-21-2008 09:09 PM

Your welcome.
Thank you, and Happy Holidays to you too.

Your furnace doesn't have the provisions to slow the blower down in cooling mode. If you were concerned about humidity in the summer.

That feature is on the VS blower models.

If your refering to controling a humidifier in the winter, that different, and the furnaces blower type doesn't make a difference.

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