Choosing the right thermostat

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Joel Schoenborn

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I have a dual fual heat pump and gas furnace. The heat pump is a 2 stage unit and the furnace is a 2 stage unit as well. Am I correct in thinking I need a 4 heat and 2 cool stage thermostat. Such as a Honeywell prestige iaq
 

WorthFlorida

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What is now running these two units? What makes the gas furnace two stage? Do both systems run together or heat/cool two different parts of the home or business? The Prestiage works with RedLINK products by Honeywell. It’s installation manual is quite good and explains how to add relay switches to control multiple systems.

It will allow through a relay blocks to turn on one or more heating or cooling systems. When a second stage is required it will turn on both systems second stage through relay blocks.

There is not a thermostat that can run two system and walk through each of the four stages independently if that is what you’re looking for. Reading briefly through the specs it can only handle three conventional heating stages, not four.
 
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Joel Schoenborn

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What is now running these two units? What makes the gas furnace two stage? Do both systems run together or heat/cool two different parts of the home or business? The Prestiage works with RedLINK products by Honeywell. It’s installation manual is quite good and explains how to add relay switches to control multiple systems.

It will allow through a relay blocks to turn on one or more heating or cooling systems. When a second stage is required it will turn on both systems second stage through relay blocks.

There is not a thermostat that can run two system and walk through each of the four stages independently if that is what you’re looking for. Reading briefly through the specs it can only handle three conventional heating stages, not four.

Its new construction I don't have a thermostat yet. The furnace has a 2 stage gas valve. The heat pump runs until the outside temperature gets low enough it becomes less efficient then a gas furnace runs.
 

WorthFlorida

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So it is all one unit? Two stage heat, two stage cool? Without a model number, for heat mode the electronics of the furnace that may control the two stage heat. Or you have a furnace and a A/C heat pump independent of each other and a smart thermostat determines what runs.

Normally the thermostat will determine when to kick in the second stage. Smart thermostats learn how long it takes to raise the temp to the set point along with the outside temperature and the current room temp. Heat pumps are always the first stage and the second stage is AUX electric heating or in your case a gas furnace is turned on. Usually, both will run unless the outside temp gets too cold and the heat pump is turned off. If the outside temp is moderate and you turn up the heat four degrees above the room temp the second stage heat will be turned on.

Since you stated much of this in your response from the information of your heating system it sounds to me that the controller on the furnace may do this determination. The manual may suggest a thermostat type. Is it two stage cool? It usually means a two speed compressor or dual motor compressor.
 

Dana

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I thought with dual-fuel the gas burner & heat pump were never running at the same time, since the incoming air from the heat pump coil into the gas burner's heat exchanger would be too high(?). The higher incoming air temp leads to lower heat exchange efficiency on the gas burner (especially if condensing) and going overtemp on the heat exchanger. This isn't a problem for resistance auxiliary heat strips (which can operate safely and at the same efficiency at any incoming air temperature and can easily be modulated), but a complicated control issue for combustion equipment.

Many 2- stage gas furnaces use a simple (sometimes programmable, sometimes not) timer to determine when to kick up to the high stage, with no direct thermostat control of the second stage.

Without so much as a model number it's impossible to take more than a WAG at the control scheme and thermostat compatibility. It's sort of like asking "What type of fuel injection controls do I need for my hybrid car?".
 

Joel Schoenborn

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I thought with dual-fuel the gas burner & heat pump were never running at the same time, since the incoming air from the heat pump coil into the gas burner's heat exchanger would be too high(?). The higher incoming air temp leads to lower heat exchange efficiency on the gas burner (especially if condensing) and going overtemp on the heat exchanger. This isn't a problem for resistance auxiliary heat strips (which can operate safely and at the same efficiency at any incoming air temperature and can easily be modulated), but a complicated control issue for combustion equipment.

Many 2- stage gas furnaces use a simple (sometimes programmable, sometimes not) timer to determine when to kick up to the high stage, with no direct thermostat control of the second stage.

Without so much as a model number it's impossible to take more than a WAG at the control scheme and thermostat compatibility. It's sort of like asking "What type of fuel injection controls do I need for my hybrid car?".

The heat pump is a Goodman GSZC180241 . The furnace is a Goodman GMVC960403BN
 

Stuff

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This is a new install so the installer normally provides a thermostat along with all of the other equipment. What's going on?

Also - they could install a dual fuel board that handles the switch-over between systems using an outside temperature sensor. Then you would only need a simple 2h/2c thermostat.
 

WorthFlorida

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I’m not familiar with a gas furnace and heat pump setup so I tried not to make a definitive answer. It’s become obvious that the furnace control board handles the heating control and not the thermostat.

However, I have a Honeywell rth9580wf and on page 10 of the installation manual you select type of heat. The after selecting heat pump further options will show but it is not in the manual. The thermostat may turn off the heat pump when the gas furnace is called on.
 
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Joel Schoenborn

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Goodman makes an all fuel control board (afe18-60a). If I buy a dual fuel capable thermostat, will I need that board or will the thermostat control the units.
 

Joel Schoenborn

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You have communicating units so should be able to use Goodman's communicating thermostat. What does the installer recommend?

https://www.goodmanmfg.com/products/controls/comfortnet-controls/ctk04-control

I am the installer. It has been awhile since I have been in the field I am not real current with communicating systems. Will this thermostat control both stages of the heat pump and both stages of the furnace. I did look into the ctk04. It looks identical to the Honeywell prestige 2.0. do you know if they use the same protocol, so I could use the Honeywell, which is about half the price.

Thank you
 

Dana

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If the heat pump is sized to handle the load at the 90th percentile temperature bin it's usually cheaper to just go with resistance auxiliary heat rather than dual fuel. The heat pump will still be carrying the majority of the load even at 0F. Have you done a Manual-J heat load analysis on the place? If you have a heating history with this house you can measure the heat load based on wintertime fuel use following these methods.

The outdoor temperature average in Fairfield IL in January is about 25F (the mean, not the high or low). At 25F the GSZC180241 is good for about 16,000 BTU/hr on it's own, no auxiliary heating.
 
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