Home automation Zero Carbon

EcoBee 3 Dual Fuel

Dismal Manor denizens awoke to 68 degrees with churning (heat pump running) indicated. Below 50 so burning (furnace on) expected. Ecobee was not set up correctly for dual fuel. There’s a manual for that but not in the box. You have to ask!

In the spring of 2022, we discovered that the Nest Learning Thermostat was not operating the reversing valve. It had been fine in the fall. The Nest energizes the reversing valve to cool. Come May and time to cool it wasn’t. Oh, and the fall update stomped on the heat pump lockout setting. So the proprietor fired Nest and hired Ecobee.

Church uses a large number of Ecobee 3 Lite devices with extra area sensors and has found them to work well. So Dismal Dave bought one and installed it. It set up easily enough for heat pump operation with automatically configured staging and automatic heat pump lockout and easy configuration of the associated gas furnace. But we had some drama anyway.


  1. 2022-10-20 Original




Dismal Wizard made a wire diagram, tagged wires, and removed the incumbent Nest thermostat. Following the wire diagram, the Wizard connected and mounted the Ecobee 3 Lite. When the power was restored, the Ecobee started and entered the configuration wizard after doing power on self test.

The configuration wizard leads the installer through connection checks, configuration of cooling, configuration of heating by heat pump, and configuration of auxiliary heat that backs up the heat pump. Everything went well and there was spring cooling.

Our particular heat pump (Lennox Elite) energizes the reversing valve to cool and deenergizes to heat. Should the valve solenoid fail, it fails to heat. That’s how we caught the Nest. It was heating when calling for cooling. Our technician confirmed that the valve was working and that no cooling signal was present.

So all went well and the machine cooled brilliantly during the summer cooling season.

Surprise, it’s fall

So fall comes. It is September, October, and November as our climate recons things. Our first frost date is normally in November except when it is not. This year, it was October 20 at Dismal Manor. and October 19 was cool also. So the air temperature dropped. The house began to cool, and in the wee hours of Wednesday, the Ecobee 3 called for stage 1 heating. Then stage 2 heating. The heat pump should have started. And maybe it did. The devil is in the details. But house temperature (the white line) continued to fall. I’m guessing it never started.

The chart shows the Ecobee 3 performance data collected. At about 0230, the thermostat called for stage 1 heat. The building continued to cool. The thermostat called for stage 2 heat. The building continued to cool. Aux heat was never tried.
Ecobee 3 Historical Data for October 19

The chart shows the Ecobee 3 performance data collected. At about 0230, the thermostat called for stage 1 heat. The building continued to cool. The thermostat called for stage 2 heat. The building continued to cool. Aux heat was never tried.

The heat pump can heat the house down to about 50F. Below about 45F, it cannot replace the heat lost to the environment and the building will cool. So at 50 F, the system is supposed to stage to the furnace (aux heat in this system). It didn’t.

I checked the outdoor unit. It just sat there. I checked the outdoor unit circuit breaker. It was in the shut position. I did not trip and reset it as it was not in trip-free position. And the breaker was young. So I placed a service call.

The Missing Manual

The setup wizard does not address the less frequent uses cases where aux heat is something other than strip heaters beyond connection of the aux heat relay. I first looked for the heat pump lockout temperature and didn’t find it. Ecobee uses different terminology a bit closer to that taught to HVAC techs (why HVAC techs prefer Ecobee to Nest).

I did a search, and yes, I consulted the Duck (Duck Duck Go) for Ecobee dual fuel setup. The Duck found the missing manual. Ecobee automatic staging configuration is not recommended for dual fuel but we missed that detail while following the wizard. So on Wednesday, following the configuration guide [1], I used the manual staging configuration procedure described to set up staging from Stage 2 heat pump to the furnace. This setup will shift heating to the furnace if the heat pump is indisposed.

The configuration procedure is somewhat abbreviated as Ecobee expects HVAC trades to be doing the configuration. To have access to all of the settings mentioned in the setup guide, you must choose manual staging configuration. That gives you access to the transition temperatures, restart time delays etc mentioned in the procedure.

Some Voodoo is Needed

There’s one piece of information that needs to be determined empirically, the temperature at which heating should shift from the heat pump to the furnace. The heat loss from the building is proportional to the difference between indoor and outdoor temperatures. The proportionality constant results from the exposed surface area of the building and the way the building is constructed, how well the building envelope is air sealed and insulated. This is one of those unknowns that is estimated when doing the HVAC sizing calculation per Manual J. But it is a an assumption, especially on an older building like my 1955 hip roof ranch with its ginger bread sheathing and plank roof deck and sub-flooring. More recent buildings are designed to code mandated targets.

From experience back in 2006, the heat pump starts loosing authority at about 50 F and below 45 F, the building begins to cool as heat loss is exceeding the heat moved by the heat pump. I have these numbers from experience. It is likely that your HVAC system provider will have a feel for them from installing and configuring many systems similar to yours in buildings similar to yours. This is an important number to note and should be recorded somewhere in your service provider’s records.

Meanwhile, Something Broke

While I was drafting this article, my service technician came in from checking the compressor to report that he had found the relays associated with the defrost valving were on the fritz. This caused the machine to think that it was frosty and could not be defrosted. Had the machine actually been running, it would have been wearing a nice coat of frost or ice. So it went into protective lockout. The compressor logic board will require replacement for about $600.

While he was at it, my tech reviewed and verified that the staging was configured appropriately. Nothing missed or strangely set. On Thursday, the aux heat (gas furnace) carried our heating load.

Aux Heat only working

By davehamby

A modern Merlin, hell bent for glory, he shot the works and nothing worked.