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Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

June 24, 2018, 07:41:57 am

Two projects requiring new engineering:
1. FURNACE
Replace the 20 year old Atwood 8940iii (Nickname "Boeing 767 on takeoff") with the Suburban SHD-2542Q quiet two-stage furnace.
I will be modifying the original access cover and flange.

Improvements:
a. Stop water intrusion with a new and better seal. See photo.
b. Stop cold air intrusion. Insulate the access cover. Old access cover had no insulation. May need high-temperature insulation from an aircraft firewall.
c. Reduce noise substantially. At the 25,000BTU setting, flame noise in negligible. Fan noise is about 10db lower, in my estimation.

2. THERMOSTAT
Replace Duotherm thermostat with the Ecobee4 Smart Thermostat. See photo of back panel connections.
Why would I install a thermostat meant for a house into a U270?

- Voice command: Alexa, set temperature to 72 degrees. Alexa, tell me a joke!
- Timed furnace control. Save propane.
- It's cool!

Challenges: Smart thermostats require a constant source of power (a few watts). The Suburban RV furnace supplies a constant 12VDC power through the R wire. However, I need to determine if this 12V power will run the Ecobee4 thermostat. Here are my connections so far:

Table 1 - Suburban SHD-2542Q furnace to Ecobee4 thermostat connections
FURNACE  THERMOSTAT    COMMENT
R                  RH                      Connected to 12VDC coach power in the furnace.
W1                W1                      Stage 1, low-speed heat, 25,000BTU, connects to R when heat is called for.
W2                W2                      Stage 2, high-speed heat, 40,000BTU, connects to R. Hope the Ecobee does this properly.
Ground        C                        Common wire. New connection. Need to connect to motorhome ground. To be confirmed.
                                                House furnaces have 24VAC neutral connected to this wire. This may confound my design.

Will the 12VDC between RH and C reliably power the thermostat? Any advice would be appreciated.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #1 – June 24, 2018, 10:12:02 am

It looks like your new furnace is also takes a millivolt thermostat like the old one. I'm not sure your Ecobee4 with 24V needed to power it will work. I changed ours to a millivolt model with digital display/programmable, etc and it works well. It's powered by one or two AAA batteries. I just have to remember to check batteries each year.

Here is an article from someone installing an Ecobee4: Ecobee3 and millivolt system (transformer + relay setup) question : ecobee

I also hate our loud furnace. Plus, about 50% of the heat goes out the exhaust.

Pierce

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #2 – June 24, 2018, 10:25:35 am

Can't wait to see how this goes.. interested in the ecobee also. Seems it needs 110v and it's a winner.. need the inverter on at all times for it to function .. large Battery bank with solar for the off grid applications

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #3 – June 24, 2018, 10:50:09 am

Do you have always available WiFi internet got the Ecobee?  Needed for the smart functions.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #4 – June 24, 2018, 12:01:28 pm

Seems like two ways of doing this.  Here's a 12Vdc to 24Vac psw inverter:
24VAC DC/AC power inverter with 12VDC input, how to convert 12 volts dc to...

Might also check the board on thermostat, see if you can identify a rectifier input  and voltage trimmer.  Likely the t-stat does a ac-dc conversion, if so might be able to bypass this and feed with dc.  With fewer and fewer discrete components it's getting harder to do these type things.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #5 – June 24, 2018, 12:43:05 pm

Just seems like a lot of incredibly needless complications in an already complicated vehicle. If it turns the heater on when you want it to, what more do you need? Lots of millivolt, battery powered, programmable thermostats on this page. Just make sure it illuminates when you touch the controls.

millivolt thermostat digital | eBay

Pierce

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #6 – June 24, 2018, 06:10:28 pm

Most thermostats are simply s contact 2, 5, 22 or 48 volts they don't care.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #7 – June 25, 2018, 08:36:27 am

Thanks for your advice. I spent hours researching this and determined:

- QUACKERY - There is lot of misunderstanding about heater and thermostat systems. Ecobee and Nest should offer the adapter I describe below. They could sell 1000's of thermostats to RVers if they sold this adapter.
- MILLIVOLT - Our RV hot air heaters are called the Millivolt type, from the steam boiler age and need an adapter to operate safely. Why? Modern furnaces and air conditioners have built-in 24VAC. For more information, see: Reduce installation costs without compromising safety when dealing with...
- SIMPLE - Two or three parts are required to adapt modern thermostats to a simple Millivolt furnace

See the schematic below for my proposed solution. I have ordered two 24VAC relays and am certain this will work with the Suburban SHD-2542Q furnace. By the way, W1 and W2 are the two stages of heat. W1 is the 25,000BTU quiet stage and W2 is the 40,000BTU "jet takeoff" stage.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #8 – June 25, 2018, 09:18:37 am

I never bragged about my grades in school... hands on (I have that covered)... I want to hook the Ecobee up and be able to run off the grid and with the inverter off if needed and the ecobee still get power.. if that can not be done without the inverter the plan is to have enough battery bank to run the inverter 24/7. Solar should take care of the rest with 1200w on the roof.

Just need someone that understands this stuff to point me in the right direction.. I want to be able to run the thermostat from my house/ job or wherever. I have my mobley for internet in the coach.. its dedicated

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #9 – June 25, 2018, 12:25:29 pm

How's this idea? A backup old-school thermostat:

PROS
- When the inverter is off, and the smart thermostat is down, the backup thermostat regulates cabin temperature
- Avoids having to buy expensive 12VDC to 24VAC inverter, which costs $100
- If the complex smart thermostat fails, the simple old-school thermostat takes over
- Cheap. Could use the old Duotherm thermostat that came with the rig

CONS
- A little complicated. However, only two wires are needed. One for R and one for W.The furnace is started by connecting these two wires together.

Our rig has big solar, so the inverter is on 24x7. But on several occasions, long story, we were on 12VDC power only for up to six hours until the sun came up.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #10 – June 30, 2018, 06:51:31 am

Status report
1. It works on my test bench.
  a. Saying "Alexa, set temperature to 73 degrees" (one degree above ambient) activates the Stage 1 heat relay.
      This is 25,000 BTU on the Suburban furnace a SHD-2542Q.
  b. Setting for five degrees above ambient activates both Stage 1 and 2 relays, the 40,000 BTU setting.

2. When the new furnace works, I will publish a video. The new furnace, still under warranty, has a cracked blower housing and fan, delaying the project. Damage was probably due to shipping.

3. Here is a detailed schematic and wiring diagram of my Tim-1 Thermostat to RV Furnace Converter. I used a breadboard to hold the relays and DB9 connector. The relays are from eBay, SCHRACK part number PT270524 24VAC 12A.

4. A thick aluminum sheet metal plate has been built to seal the furnace and adapt to the old Atwood flange and access door. EPDM 1/4 rubber seal is used throughout. High-temperature silicon seal will seal the exhaust port to the adapter plate.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #11 – July 06, 2018, 05:00:35 pm

I made a video about how the Ecobee4 thermostat now works with the actual Suburban SHD-2542Q furnace. See:

Tim's RV Tips #10 - Relay box to use Ecobee4 Thermostat in an RV - July 6,...

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #12 – July 06, 2018, 08:30:11 pm

Do you think you can run the 2 ac and the aquahot with this

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #13 – July 06, 2018, 08:50:37 pm

Yes, assuming they have on-off controls. A relay is needed for each function including:

- Low heat
- High heat
- Low cool
- High cool
- Heat strip
- Heat pump
- Fan-only
- Circulator pump 1 for hydronic system
- Circulator pump 2
etc...

The relay costs $5.90 each on eBay : SCHRACK PT270524 RELAY 24VAC 12A | eBay

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #14 – July 06, 2018, 09:16:07 pm

I am nothing close to an engineer or electrician ... would you just need 1 relay for fans.. low and hi for either ac or heat? or separate for each?

9 total? what about aquahot zones?

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #15 – July 06, 2018, 09:17:20 pm

side note.. if I did a 48v battery and stepped down and say battery was 100ah at 48v would that make it 200ah at 12v?

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #16 – July 06, 2018, 09:56:53 pm

As far as I can see the EcoBee4 Is a single point thermostat.  My AC system has two AC units and two AC thermostats that can be reversed. The AquaHot has four thermostats.

You can add extra sensors but I don't think they don't add zones.

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #17 – July 07, 2018, 05:53:58 am

The following may need to be added for Aquahot:

- Zone Control board(s)
- Multiple Ecobee4s
- Wireless remote sensors

Ecobee4 and Alexa can control multiple zones in multiple homes. Based on my research, here are some interesting features:

PROS
- "Alexa, set HOME temperature to 72 degrees"
- "Alexa, set U270 bedroom temperature to 78 degrees"
- "Alexa, set Aquahot to 68 degrees"
- Occupancy detection. Room temperatures change if humans are present
- Works with Alexa anywhere on the globe, or through an app

CONS
- Complicated, as was previously mentioned. May need an engineer to design a custom system. Time consuming.
- Needs 120VAC. Needs an internet connection for voice control. Has a touch screen for manual control.
  Hence the need for a simple backup thermostat, especially in the winter to keep the pipes from freezing.

Excerpt from Ecobee support:
'...The common connections for each zone input are as follows: "R", "G", "Y/Y1" & "W/W1" and should be mostly standard on the various brands of zone-control systems. If they are using a multi-stage heating/cooling or heat-pump systems then additional inputs may be used such as "W2", "Y2", "O/B". What sets the ecobee different from other thermostats is the capability to access each zone remotely from one ecobee web portal and we provide a helpful grouping feature so that when they make a change on one thermostat, it can automatically be propagated to all the thermostats...."

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Re: Replacing Atwood furnace with Suburban and Installing Ecobee4 Smart Thermostat

Reply #18 – July 07, 2018, 09:07:39 am

In their instructions, ecobee says ... "The ecobee is compatible with zone control systems and can be used to replace an existing zone thermostat be it a "master or slave" thermostat.". Each zone needs its own ecobee4 thermostat.  They can be controlled individually or as a group remotely from their app. The add on sensors tell the single ecobee4 thermostat that it is colder in one place where someone is sensed that the ecobee set point.  The single point control turns on heat (or cool) for everywhere.  At more than $200 each this gets pretty expensive.  We have 11 independent heat zones in our home with infloor hydronic heat.  These are set and left alone for the season for the most part. 

Interesting idea though.  Let us know how it has worked for you after a complete heat/cool season.

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