Recently I had an Aprilaire 800 installed with my HVAC. I have a dual fuel system with a heat pump and propane as my auxiliary heat, with a 3rd generation NEST. When the outdoor temp went below 30 degrees the other night, I expected the aux heat to kick in. The indication on my Nest thermostat was that it was using Alternate heat. However, the propane furnace was not running nor the fan. I called my HVAC tech. He diagnosed the problem and said the thermostat was not providing the required 24v to start the burners. He tried switching wires in case it was bad wire-no luck. Tried restarting the Nest, and also did a hard reset on the Nest. Still have the problem. HVAC tech says I need a new thermostat.

Before installing a new thermostat (an ecobee), I did some troubleshooting on my own. I cut power to the Aprilaire 800 and removed the wires (* and Rh) on the thermostat that I believe were controlling the humidifier. Ran my test, and sure enough, the burners kicked on with the fan. Replaced the wires (* & Rh) and powered up the humidifier and forced humidity into the house. It came on as expected, as well as the fan. Also confirmed that aux heat would not start with outdoor temp below the set threshold.

I'm questioning the wiring that my HVAC technician did when he installed the Aprilaire 800. Both the * & Rh wires go to the humidifier. The nest does not recognize the Rh wire. * wire is pink, Rh wire is grey. Is the Rh wire necessary/required?

Nest wiring for dual fuel (heat pump with propane aux heat) and Aprilaire 800.

 

Welcome to the forums.

The Aprilaire 800 is a standalone unit and cannot be directly connected to a heating plant. Connecting it directly usually causes damage. It MUST be relay isolated if a nest used. If an Ecobee is used.... you can use a relay or two wires directly to the Ecobee.

The diagram shows a typical installation to a nest.
If you are using an Ecobee..... you can substitute the * on the nest for ACC+ on the Ecobee.

Any 24vAC N.O. (normally open) relay can be used.

A little further clarification..... on your nest you have pink on *. That pink wire goes to the relay shown in the picture. The other relay wire goes to C (blue).

Your furnace has 24vAC on the terminals. The 800 also has 24vAC on its terminals. You have both 24v devices connected to each other. That's like a head on accident.

 

Thanks for the quick reply. A couple of questions.

1. I take it I do not need the grey wire connected to the Rh terminal on the nest?
2. Should I just power off the Aprilaire 800 until I install the relay (or ecobee) to prevent any damage?

Also, my other problem is that my aux heat is not running. Any clue why?

 

You must disconnect the 800 from the furnace immediately.

You have a dual fuel system. You have a heat pump with a gas furnace as backup.
The furnace should be supplying system 24v power which means you should only have one R connection to the nest. However there are different ways of wiring.

I'd need to see the wiring at the furnace board to comment further.

 

I cut power to the 800 at the circuit breaker. I attached word doc(I think) of the circuit board with annotations - lots of wires. Hope it is helpful.

 

I'm running Libre on this comp so I can't see that doc. I can see it in print preview.
You can post pictures here directly.

The nest needs to be programmed as dual fuel. It should shut down the heat pump and start the furnace.
When you have two R wires in the slots..... not necessarily live..... the nest thinks you have a two transformer system. You don't. You should have the R wire on the furnace board on Rc or Rh.

 

I have posted the original pic without the annotations. The pink wire (*) and grey wire (Rh) are in the lower right-hand corner of the picture. They are connected to wires that go to the Aprilaire 800 (pink to red, grey to white).

The red wire (Rc on the tstat) is connected to the R wire on the furnace board.
Sounds like the pink wire (Rh) on the tstat is causing my problems?

 

Forgot to add that the nest is programmed for dual fuel - heat pump with propane.

 

Thanks for posting the picture. I labeled it for you.

No..... the pink wire is on * and stays there.
The gray wire gets removed from Rh.

Break the red to pink and white to gray splices.
Be sure to insulate the two humidifier wires and the * wire until the relay is installed.
I updated my previous diagram for you.

 

Yes, the pink goes to the * terminal. Thanks for your help. Merry Christmas.

 

If you need "emergency" humidity you can combine the two wires from the humidifier and it will run.

There are a lot of 24vAC relays available. Pick one.
6AZU2 hd relay
90-370 relay
90-290 relay
90-380 relay

Let me know which one you get and I can tell you how it connects.....

 

Waiting for my HVAC Technician to address the issue by Thursday. I don't want to void his warranty.

 

No problem. Just explain to your tech that the 800 has its own internal transformer and just requires a contact closure to operate.

 

My HVAC technician's helper came yesterday and installed a 24Vac NO switch. I attached a picture of the switch and connections. On the coil side, the pink wire goes to the * terminal on the nest. The white wire goes to the common terminal on the control board. The red and white wires go to the humidifier. Checked things out before he left, and all seemed to work fine.


However, when I woke up this morning, I had no heat. The nest showed no equipment present. I cycled power on the furnace. The nest then showed the following:

I opened the front panels on the furnace, repositioned the mass of wires, and checked for loose wiring. I did not see anything obvious. Put the front panels back on and the furnace started its heat cycle. All seems ok again. I think I still have a loose Rc wire somewhere.

 

Any loss of 24v will show that on the display.
As a guess.... I'd say make sure the blower door is hitting and activating the safety switch correctly.

The Rc wiring at the furnace is the two red wires on the R terminal.
I don't think I see any other splices that could cause issues.

If you have the problem again.... look thru the blower door sight glass and make sure the LED on the control board is on before doing anything.