Thermostat wiring Hybrid dual fuel heat pump added to existing hydronic oil

Reply

  #1  
Old 12-15-19, 01:20 PM
R
Member
Thread Starter
Join Date: Dec 2019
Posts: 3
Received 0 Votes on 0 Posts
Thermostat wiring Hybrid dual fuel heat pump added to existing hydronic oil

Hi there first post here so thanks in advance for all your help.

First a snapshot overview of what is existing.

I have an existing forced hot water baseboard hydronic heating system that operates with a heating oil boiler. This system works perfectly for the record. I currently have no central ducted a/c in the home.

What I want to do:
  1. Add a central ducted a/c to the home for summertime cooling.
  2. Take advantage of the fact that I am adding a ducted system to the home to add a second or backup gas based heating system to the home in case my oil boiler ever quits or dies on me.
  3. While I'm at it, why not save money by installing a hybrid dual fuel heat pump, that way I can save on oil during the winter shoulder months.
​​What is the problem?
Typically you would install the new ducted hybrid heat pump system by wiring up the thermostat so that once the odt (outdoor temp) drops below some preset temp, the system will switch from heat pump mode to gas furnace.

What I want is for the thermostat to switch to the existing oil boiler system instead of switching the forced hot air system to propane when Temps drop say below 32 degrees.

I only want to have the propane based furnace operate during an emergency situation, the only emergency would be my existing hydronic oil boiler forced hot water system going down.

I would still like to be able to run the furnace / heat pump using gas, but only when the oil boiler system goes down. In other words have some kind of switch or override that I can toggle manually to select.

Because the oil system is forced hot water, the thermostat doesn't need to run the fan at all when the system cuts over from heat pump mode to the oil boiler forced hot water system.

I know there are many concepts out there like one / two stage systems, aux/ emergency heat thermostat wiring.

Because of this, I am somewhat confused as to how I could take advantage of some of these thermostat features to wire the system in a way that could at the very least approximate the use case I am looking for.

So basically, when odt drops below 32, thermostat switches from heat pump automatically to oil heat. If I flip a switch, thermostat switches from heat pump to propane furnace.

I have seen this thread and think it is a similar problem they are using some relays to trick the thermostat. You guys have any ideas?

https://www.doityourself.com/forum/heat-pumps-electric-home-heating/512730-trane-tcont802-oil-hydronic-furnace-heat-pump-electric-coil-c.html

 

Popular Reply

 
12-18-19, 09:40 PM
Houston204's Avatar
Houston204
Houston204 is offline
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
What do you want to do about defrost mode?
The gas furnace will be slow but the boiler would not work for defrost cycles.
I'd wire something like this and use the Redlink wireless outdoor temperature sensor with a Prestige IAQ.
 
Attached Images  
  #2  
Old 12-18-19, 09:40 PM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
What do you want to do about defrost mode?
The gas furnace will be slow but the boiler would not work for defrost cycles.
I'd wire something like this and use the Redlink wireless outdoor temperature sensor with a Prestige IAQ.
 
Attached Images  
PJmax, RonG2029 voted this post useful.
  #3  
Old 12-23-19, 07:17 AM
R
Member
Thread Starter
Join Date: Dec 2019
Posts: 3
Received 0 Votes on 0 Posts
Thanks so much for your reply.
Your Diagram makes total sense, I can definitely see how you are switching between heat pump and gas furnace using a simple switch on the W terminal. I can also see the problem with the defrost call from the heat pump to the furnace would be an issue.

I don't know if there's a right way to do this, as there are many ways to probably design this control system. Right now I am trying to
  • avoid using a switch as I had originally proposed,
  • having only 1 thermostat control both systems.
  • I also want to get forced hot air (gas or heatpump source) WITH radiant forced hot water OIL when there's a +3 differential call for heat (staged heat, which is a feature of this sensi thermostat).
Updated Use Case

I came up with a slightly different solution after thinking about this for about a week straight and doing some testing with my new sensi touch thermostat. The use case slightly changed, because I wanted to be able to activate both oil / furnace and heat pump based on the thermostats +3 differential multi staging. So in this configuration, I am using a single stage Heat Pump thermostat setting, but a 2 stage Gas configuration. So HP1/GA2.

Why? because the thermostat engages W and W2 when you hit +3 on the thermostat. So that means that I could have radiant OIL boiler and Forced Hot Air (whether it be HP or Gas Furnace) running during recovery when temps are way lower than the set point, e.g. indoor temp is 54 and set point is 64. When ODT is above freezing, I would get Heat Pump + W/E (which is oil) on a +3 call.

After reading some reviews about the goodman control board, I decided that the new modern sensi thermostat would allow me to switch between HP / Furnace directly, and that I didn't need the dual fuel control board at all. So instead I could wire direct to thermostat and let the thermostat activate each stage.

My only problem based on the Thermostats logic table (the logic table (tested with with a multimeter) is that during a +3 call with GA2/HP1, if the ODT is above freezing, I could run into a situation where I have the heat pump Y + (W) and the furnace (W2) running, which is obviously a no-no.

Name:  20191223_084525.jpg
Views: 754
Size:  3.21 MB

I have an email out to Sensi support right now asking the following:
How does the sensi thermostat decide when to switch the heat pump from Y to W/E (in dual fuel mode, first stage W/E wire is taken up). What I want to know specifically is the algorithm the thermostat uses to decide when to activate Y and when to instead switch to W/E, as this this thermostat does not require a dual fuel control board, i.e. you can wire Y and W/E directly and the thermostat decides the most efficient heat source in dual fuel mode. The other note I made is that even with HP1 / GA2, the thermostat still outputs 24vac on the Y terminal. The Note in the Thermostats wiring diagram states clearly (DUAL FUEL MODE REMOVES W1 OUTPUT). But there's no mention how to activate dual fuel mode in the thermostat's options.

SensiThermostat Terminal Wiring Guide
Name:  2019-12-16_9-46-16.jpg
Views: 896
Size:  188.0 KB

GoodMan Dual Fuel Control Board Wiring Diagram
Name:  2019-12-20_9-02-13.jpg
Views: 927
Size:  490.1 KB

CONCLUSION

I can either:
  1. use the thermostat functionality to switch Dual Fuel based on it's own internal algorithm and Outdoor Temperature
  2. use the Goodman control board with a physical ODT in combination with the sensi.
Option 1 -> tricky with defrost, not sure what mode I would configure on the tstat: e.g. HP2/GA2, could I get all the functionality I want out of this setup?
Option 2 -> most complicated wiring setup involving relays, interaction between thermostat and control board could be troublesome.

It might be simpler to just let the goodman control board decide between Y and W (heat pump / gas), . I think the problem then is i can't put both the furnace and oil on the thermostats W/E terminal, meaning I would need to go HP1/GA2 and put oil on W2. The problem then is I can't switch to oil by manually putting the system into Aux Mode (W1/E terminal).
 
Attached Images  

Last edited by RonG2029; 12-23-19 at 10:06 AM.
  #4  
Old 12-23-19, 07:46 AM
R
Member
Thread Starter
Join Date: Dec 2019
Posts: 3
Received 0 Votes on 0 Posts
Logical Switching Scenarios with Goodman Control Board and physical ODT, board switches Y demand on freezing temps to W1 (W/E (W1).

Thermostat set to single stage heat pump, two stage indoor equipment for W2 +3 Differential activation: HP1/GA2

Wiring Scenario 1
GAS WIRED TO FIRST STAGE
OIL WIRED AS SECOND STAGE

Y -> control board -> ODT -> Switches HP / and Gas Furnace
W/E -> Gas Furnace
W2 -> OIL +3 demand

Problem
Using Gas instead of oil as primary fuel source when ODT is below freezing.
I want to normally use oil as the Primary Fossil Fuel Heat Source when temps are below freezing, and gas as Second Stage. This is except in Aux Mode, where I want switch to back to Gas as primary source when Y is off and emergency heat is called on W/E.

Wiring Scenario 2
OIL WIRED TO FIRST STAGE (W/E)
GAS FURNACE WIRED AS SECOND STAGE (W2)

Y -> goodman control board -> ODT -> Switches Y and W1
W/E -> OIL
W2 -> Gas Furnace on a +3 differential Call for heat

Problem
Aux mode Activates W/E (should switch to furnace!)
Solution
Switch W/E (GAS / OIL) external to control board via relay based on Y.
Y on --> Normal Operation, W/E connects to --> OIL
Y off (aux Mode) --> W/E switches back to --> Furnace
W2 is not used in aux or emergency heat mode

Wiring Scenario 2 Temperature based control scenarios

ODT BELOW FREEZING - REGULAR DEMAND (OK):
Y on -> Demand Absorbed by control board
W1 on -> energized by Control Board which switches demand (connected to OIL)
W2 off -> FURNACE

ODT BELOW FREEZING +3 demand (OK)
Y on -> Demand Absorbed by control board
W1 on -> energized by Control Board which switches demand (connected to OIL)
W2 on -> FURNACE

ODT ABOVE FREEZING - REGULAR DEMAND (NOT OK???)
Y on -> Heat Pump Demand
W1 on -> OIL ACTIVATED INCORRECTLY BY THERMOSTAT
W2 OFF -> FURNACE

ODT ABOVE FREEZING +3 demand (NOT OK)
Y on -> Heat Pump ACTIVATED
W1 on -> OIL ACTIVATED
W2 on -> FURNACE ACTIVATED

PROBLEMS - DURING ODT ABOVE FREEZING CONDITIONS (HP DEMAND)
OIL AND FURNACE ACTIVATED INCORRECTLY BY THERMOSTAT WITH DUAL STAGE GA2 SETTING BECAUSE OF INTERACTION BETWEEN THERMOSTAT CONFIGURATION SET TO DUAL STAGE INDOOR EQUIPMENT MODE AND GOODMAN DUAL FUEL CONTROL BOARD

SOLUTION
ADD ODT BASED OIL (W1) CONTROL RELAY:
DISCONNECT W/E OUTPUT FROM THERMOSTAT ON TWO CONDITIONS: IF Y IS ON, AND ODT ABOVE FREEZING

LOGIC
(Y) HP ENERGIZED --> ODT ABOVE FREEZING --> RELAY --> DISCONNECT OIL (ABSORB W2 DEMAND)

ADD ODT BASED FURNACE (W2) CONTROL RELAY:
DISCONNECT W2 OUTPUT FROM THERMOSTAT ON TWO CONDITIONS: IF Y IS ON, AND ODT ABOVE FREEZING

LOGIC
(Y) HP ENERGIZED --> ODT ABOVE FREEZING --> RELAY --> DISCONNECT FURNACE (ABSORB W2 DEMAND)

RESULTS
Y DEMAND ON OUTDOOR CONTROL BOARD WITH ODT ABOVE FREEZING RESULTS IN GOODMAN HEAT PUMP SATISFYING DEMAND, W/E OIL ACTIVATION ABSORBED, W2 FURNACE ACTIVATION ABSORBED

FINAL PROBLEM
NO WAY TO ACTIVATE OIL ON A +3 CALL FOR DUAL RADIANT / FORCED HOT AIR WITH HEAT PUMP SOURCE

SOLUTION
ADD RELAY
IF W2 DEMAND INPUT, BYPASS W/E DEMAND OUTPUT ABSORB, ACTIVATE W/E DEMAND TO OIL.
 

Last edited by RonG2029; 12-23-19 at 09:09 AM.
  #5  
Old 01-06-20, 08:00 AM
A
Member
Join Date: Mar 2010
Location: USA
Posts: 4,344
Received 114 Votes on 105 Posts
Do you really need or want three stage operation?

In two stage operation, the thermostat for the heat pump has two wires to activate the backup (gas furnace). The gas backup furnace has two terminals to accept a thermostat.

The oil hydronic system has two terminals to accept a thermostat. An old wall thermostat had two terminals to activate the hydronic system. This all functioned as single stage prior to installation of the heat pump.

Can you find a two pole 3 way switch aka double pole double throw switch? (It's not a 4 way switch.)

Connect the backup thermostat terminals (part of the heat pump thermostat) to the center terminals of the DPDT switch. Connect the gas furnace terminals to the upper terminals of the DPDT switch. Connect the hydronic system terminals to the lower terminals of the DPDT switch.

You would not need the old wall thermostat.

Now, with the DPDT switch manually set one way the heat pump system will use the gas furnace as the backup and the hydronic system never gets a call for heat. With the switch set the other way the oil hydronic system is the backup and the gas furnace never gets a call.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: