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Is there a thermostat that can control 2 heating systems?

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Some of you were kind enough to help me out last year with choosing a new heating system. I ended up installing a Burnham Alta 120 boiler and it ran great all winter. We switched from oil to propane and used less propane than I thought we would while keeping the house warmer than we did the past couple years. This boiler is for a 12 zone ultra-fin radiant system with 12 thermostats.

Our second floor air conditioner now needs replacement and we decided to go with a heat pump that we plan on using to heat the 2nd floor in the fall and spring only. We will use the radiant system during winter.

The thermostat that currently controls our second floor air conditioner also controls the heat for our bedroom zone. Is there a thermostat available that we could use for the heat pump and also for the boiler for our bedroom zone and switch between the two heat sources when the seasons change? I'm trying to avoid having two thermostats on the same wall. I'd like the thermostat to also have wifi. Thanks!

Joe

Comments

  • ratio
    ratio Member Posts: 3,654
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    Any thermostat that offers "dual fuel" heat pump operation should work for you. A Honeywell TH7321WF will give you everything you're asking for, but you might need an outdoor sensor to make the changeover happen automatically.
  • DCContrarian
    DCContrarian Member Posts: 122
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    Is your second floor thermostat a "regular" thermostat, in that it's just an on-off switch? A lot of heat pumps use "smart" thermostats that can modulate the indoor fan speed and the outdoor compressor speed to match the heating and cooling load. If that's the case then you have to use the manufacturer's thermostat. A quick way to tell is to pop the cover off and look at the wiring, a "regular" thermostat will generally use regular thermostat wire with a brown sheath and several colored wires inside that attach with screws, a "smart" thermostat will generally have a special connector.

    The question of when to switch from heat pump to the boiler is a tricky one, it depends on your cost of electricity, your cost of oil, the capacity of the heat pump and the efficiency of the heat pump as the temperature drops. Be leery of anyone who gives a blanket rule of thumb like "always switch at 32F," the correct answer is going to be different for every house.
  • Ironman
    Ironman Member Posts: 7,379
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    We run across this scenario often. You need a dual fuel stat and outdoor sensor for changeover. You’may also need an isolation relay in the AHU to bring on the radiant.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
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    I have made several wiring diagrams for some of the folks on this site that have unusual systems. try this link.
    How do I Connect this Wire?. Look at the post about 8 down from the top for heat pump with hydronic

    I believe that you want a single control that will, at some predetermined temperature event, automatically change to operate a radiant floor heating zone (or zones). This is tricky, because the radiant system is a very slow system to heat up and to cool down. If your floor is cold and you switch the heat pump off in favor of the floor heat, you may be waiting for several hours for the floor to give you the heat you need. I’m not sure that you will be happy with that.

    Most thermostats use an ON/OFF function based on the room temperature. Consider that the outdoor sensor has made the determination that the heat pump is no longer the better option. That outdoor temperature control will operate some type of ON/OFF relay or switch that will change the room temperature control from operating the heat pump in favor of the LP Gas boiler. During that time the heat pump is off, and the floor heat system is taking its good ole’ time making the floors warm with its low temperature water, the room temperature drops from 68°to 60° or lower.

    I’m pretty sure that you will not have the Happy Wife Happy Life going on in your home. You are going to need the radiant floor heat to start several hours before the heat pump is allowed to shut down. I learned this the hard way in my home in brigantine NJ. My radiant floor heat was unable to keep up with the demand whenever the outdoor temperature was below 14°F outside. That was rare but when it happened the multi stage thermostat would not kick in the gas furnace without overheating the home and shutting off the floor heat completely. I tried to design a system with one thermostat for two years.

    I finally gave up and placed a simple analogue thermostat on the wall next to my WiFi thermostat. The Analogue thermostat controlled the furnace at 68°F and I would just keep the floor heat thermostat at 72°F when the furnace turned on. That way the floor heat would stay on even if the furnace made the room go over 68° by a degree or two. Although my issue is the reverse of your issue, the principle is the same. Radiant floor and hot air heat are better off controlled with separate thermostats.

    If you still want to try it, I can give you a logical layout for the wiring system. I can try to adapt it to compensate for the different heating times of the two different systems, but I feel that you will ultimately end up with two thermostat in the end. Let me know if you still think it is the way you want to go.

    Other things to consider
    1. The floor heat may be more comfortable than the Heat pump
    2. You may be comfortable with the floor heat at 64°, but you will need the heat pump to be at 68° for comfort.
    3. Having one thermostat that will prioritize the radiant floor heat over the heat pump heat will need to be set so the radiant heat is set higher than the heat pump for some time, during the hours of transition from HP to Boiler.
    That is the opposite of what you really want the thermostat to do

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • DCContrarian
    DCContrarian Member Posts: 122
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    OP said he had an "ultra-fin radiant" system. I took that to mean hydronic baseboards.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
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    OP said he had an "ultra-fin radiant" system. I took that to mean hydronic baseboards.

    I believe that UltraFin os a brand name of this add on for PEX or copper tubing
    It will increase the surface area of the tube to transfer more heat to the floor above the tubing. The tubing is not actually touching the sub floor in this design. When using low temperature water, you create a 2" plenum between the insulation and the sub floor.
    It must be completely sealed from any convection currents between the floor, the floor joists and the insulation, otherwise the convection currents may move the heated air from the plenum away from the place where the heat is needed.

    If you are using high temperature water in the tubing, you may not need the insulation to allow more convection currents to make the heat under the floor above transfer as needed.
    The downside is that the room below the floor is also heated. If that space is an unoccupied crawl space, then that heat is wasted. If it is a basement that is used for a children's playroom, or other occupied space, then the heat may not be wasted.

    You design as needed to get the result you want.

    Still going the have lag time for the changeover from HP to LP in either case.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Joe107
    Joe107 Member Posts: 27
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    Thanks everyone for the replies.

    Our heating system is the UltraFin system that Ed included pictures of. It is a high temp system because that's what they were recommending at the time (15 years ago).

    The heat pump that's going to be installed will be a single stage unit with no heating unit in the air handler.

    I am not looking for automatic switching between the heat pump and the boiler or to use the boiler as back up heat. (We actually can't - the other rooms on the 2nd floor each have their own tstat for heat.) I want to turn off the heat pump and turn on the radiant heat when temperatures start getting colder, say early December, and then switch back to the heat pump in probably late March. We don't want, what I consider to be, cool air from the heat pump blowing around during winter.

    I did look up the Honeywell TH7321WF but nothing is coming up. Was this replaced by the Honeywell TH8321WF?

    I'm not familiar with dual fuel thermostats but I'm thinking they're meant for automatic changeover to backup heat. I'm looking for one that can handle two heating systems and allow me to choose which one to run.

    Thanks again,
    Joe
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
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    Then I would just have a manual switch that will disengage the heat pump while engaging the LP Boiler zone. This will be a manual switch that can be near the boiler or near the thermostat.

    That switch will operate a relay that uses the heat pump thermostat that will normally operate the compressor to operate the LP boiler zone.

    To be clear about the manual change over
    During the preseason and postseason mild temperatures the heat pump will be operating all the rooms on the second floor. All the heat zone thermostats on the second floor will be set to OFF or at a low temperature and the heat pump thermostat in the master bedroom will control all the heat for the second floor via ductwork. When you decide to make the changeover you will set the change over switch on the master bedroom from HP to LP boiler zone, and also turn on the other LP boiler zones on that floor. In the spring you will reverse the process.

    Since you are manually making the change 2 times a year, then it is much easier to make that happen.

    Is the air handler for the HP in the same area as the LP boiler zone control?
    What zone controls are you using for the LP Boiler? Zone Valves, Circulator pumps, multi zone relay control?
    Knowing the zone device and the heat pump brand name & model number will help in selecting the thermostat and the additional relay needed to make the manual switch-over

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • ratio
    ratio Member Posts: 3,654
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    Joe107 said:

    …I did look up the Honeywell TH7321WF but nothing is coming up. Was this replaced by the Honeywell TH8321WF?…

    Oops, sorry. The part number is indeed TH8321WF.

    If you're interested in getting all the thermostats on the internet, you might look into the TH8110R thermostat. It's the single stage RedLINK wireless model, but it looks identical to the TH8321WF. It will require one RedLINK gateway (THM6000R) per 4 thermostats, but it may be cheaper overall.

    We use these stats a lot. I just came from a job yesterday where I installed 6 gateways to get them remote access to their 21 units.

  • Joe107
    Joe107 Member Posts: 27
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    @ratio Thanks for clearing up the model number. I will look into it and the RedLINK

    @EdTheHeaterMan What you wrote about the change over is all correct. The air handler is in the attic and the boiler is in the basement but I'm told new thermostat wire is needed from the air handler down to the basement and then outside to the condenser, so a switch could be placed in the basement.

    The new condenser is a Bryant 235SAN 2-ton model. I don't have the model number of the air handler. The radiant zones use Taco zone valves with Taco zone controllers, #ZVC406.

    Thanks,
    Joe
    EdTheHeaterMan
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
    edited April 19
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    It appears that this model heat pump is using a new Puron refrigerant R454B and this is a fairly new product. The wiring diagrams for this equipment are not publicly available yet . However it appears that the standard thermostat wiring is going to be used on your equipment. In order to develop a wiring diagram for you to accomplish what you want done I will use an older air handler model that may be similar.

    This will take me some time to develop the logic and relays necessary for your changeover switch. I will PM you and also post the diagram here for you when it is ready

    Once you have the actual equipment in your possession, If the wiring diagram is different, then I can make the needed adjustments
    Edit:
    What zone is the master bedroom on the ZVC406?
    What other zones are cooled by the heat pump that are connected to the ZVC406? To further explain, if the heat pump is operating and you are using the zone thermostat in the master bedroom for the HP, what other zones on the ZVC 406 are not going to need LP boiler heat? what thermostats are in those rooms? what ZVC zones are connected to those thermostats? I am guessing here but with 12 zones are all the HP zones on one ZVC and all the other zones on a different ZVC. that would make it easy to use only one of the ZVC controls that needs to be disabled when the HP is heating.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,128
    edited April 20
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    @Joe107, Here is what I have come up with:

    I find that if you look at the attached file, it is easier to zoom in on the diagram to see the details. Mobile devices may be better with the screenshot above this paragraph.

    If you can get the Bryant Dealer to make all the wire connections in the basement, in a junction box, you will be able to add a DPDT toggle switch and make that switch the "Changeover" switch. If the Bryant Dealer is not comfortable with connecting the Hydronic Zone to the thermostat, then you may need to do this ADD ON yourself.

    There is a note on the diagram:
    After Your Bryant dealer installs your Heat Pump, you will have the R from the thermostat connected directed to the R in the air handler.(See Cut Wire) Hopefully, those connections will be in the basement as shown in the Junction Box. You can cut the R from the thermostat and place it on the Com of the DPDT switch.
    Then place the NO term. from the DPDT switch to the R wire from the air handler.
    Then take R from Taco ZVC thermostat zone terminal and place it on NC on the DPDT switch.
    Use the other pole from the DPDT switch to connect the W wire from the thermostat to the to Com on the DPDT switch and connect W from the ZVC zone thermostat connection to NC on the DPDT switch.
    Follow these instructions to get the Changeover Switch to work.
    You will need a Double Pole, Double Throw toggle switch to get the switch to work with the 2 separate transformer systems (air Handler has a 24 VAC transformer and the Taco ZVC has 2 24 VAC transformers). You do not want to cross connect those two transformers when making this install. You need to connect the Common or C wire for the thermostat to both the ZVC406 and Air Handler transformers ( the infamous C wire).

    Since your thermostat will be using the W wire from the thermostat to operate the Taco ZVC zone, there will be three things that you need to do each changeover.

    1. Flip the changeover switch from Heat Pump to Radiant
    2. Set the thermostat in the master bedroom to Emergency heat. This will not allow the compressor or reversing valve to get power from the Taco ZVC transformer
    3. Set the thermostats set point on the other zones thermostats to operate their respective zones.

    When is is time to change back in the spring,
    1. You set the Changeover switch from Radiant to Heat Pump, this is the summer setting and yoy use the master BR thermostat for changing from Normal HP Heating to Cooling.
    2. Then you will set the Master bedroom thermostat to normal (not emergency heat) or heat pump heat
    3. Then set all of the other zone thermostats on that floor to off, or set the temperature to the lowest setting in those rooms.

    This is the simplest changeover wiring I can come up with, as long as you are OK with the manual nature of the changeover.

    This is the DPDT switch you will need for the Changeover. It is the one that is ON/ON for Under $12.00. It is available at www.granger.com or perhaps your local hardware store.
    https://www.grainger.com/category/electrical/industrial-controls-automation-and-machine-safety/manual-and-foot-switches/toggle-switches?attrs=Contact+Form|DPDT~ATTRS&filters=attrs,webParentSkuKey&searchQuery=dpdt+switch&webParentSkuKey=WP5932491&sst=4&tv_optin=true

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Joe107
    Joe107 Member Posts: 27
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    @EdTheHeaterMan

    Ed, thanks so much for taking the time to put that diagram together. I will talk to my installer about it and hopefully we can make this work.

    Joe

    EdTheHeaterMan