Dual Fuel Hybrid System Incorrectly Setup

[Brewing]

I have a dual fuel system for heating that consists of a Mitsubishi heat pump and a Bryant modulating natural gas furnace (Heat pump also being used as A/C cooling in summer months). They installed some sort of equipment interface module specifically to enable Nest-compatibility.

When the system was installed, the hvac company seemed to struggle with the Nest thermostat but ultimately felt they got it sorted. Basically, they said that the Mitsubishi control unit was handling everything.

Nest does not recognize that it’s controlling a heat pump, and (I think) does not let it run the appropriate amount of time before switching to the gas furnace as aux (usually only a few minutes). It works fine as cooling when the pump is the only option.

Here is the equipment that Nest detects:

Is there anyway this is set up correctly, and I’m confused?

Thanks in advance!

Exact models: Nest 3rd gen, Mitsubishi 14.3 SEER2 3.5-ton R410 outdoor heat pump w/ inverter compressor (PUZ-A42NKA7), Mitsubishi intelli-HEAT indoor evaporator coil (PAA42C), and Bryant 97% AFUE 100k Btu modulating gas furnace w/ variable speed motor (987M)

@CoolingWizard - you seem to be a bit of a guru in the hybrid system space.

[CoolingWizard · Accepted solution]

Ok @Brewing , this is going to take a bit of explaining. So, strap in and hang on. 

That Mitsubishi highly intelligent heat pump system you have in your home, communicates digitally using three wire  interface between the outdoor unit and the indoor unit control box. This control box is located on the air handler. 

In that same control box, there is a special interface that allows you to attach it to a conventional furnace such as your gas furnace. The Mitsubishi cannot be controlled using any conventional thermostat including the Nest thermostats.

The proprietary thermostat used by Mitsubishi is a three wire control thermostat which also uses a digital signal communicate with the indoor air handler. Now that indoor air handler along with the indoor coil has a control board which provides conventional output only control signals to a furnace, such as your gas furnace.

You cannot, and you should not, attach a conventional thermostat to the terminal block that’s designed for output only and controlling your furnace.

In order for this Mitsubishi system to operate efficiently, and as designed, you must use a Mitsubishi MA remote controller (digital thermostat) or one of their IT controllers and a cloud based app. The approved IT solutions are:

* Kumo cloud app via wireless interface 2

* MHK2 Controller

* a combination MHK2 with kumo cloud app 

I am sorry to have tell you that your contractor has incorrectly the in house thermostat controller. 

AC Cooling Wizard