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Thermostatic Mixing Valves & Outdoor Reset

ewang
ewang Member Posts: 74
edited November 2020 in Radiant Heating
How does your typical thermostatic mixing valve work with Outdoor Reset on modulating condensing boilers?

For example, in a system where the OA reset sets SWT to 140F during a design day, if the mixing valve is set at 120F and there is adequate lower temp return available, everything is great.

But what happens during shoulder season when the TMV is still set at 120F?
Does the mixing valve send through all of the hotter supply water, or does it mix?





Comments

  • STEVEusaPA
    STEVEusaPA Member Posts: 5,313
    Motorized mixing valves would be better with ODR.
    steve
  • ewang
    ewang Member Posts: 74
    @STEVEusaPA I’m sure but I’d like to understand what happens with a non motorized mixing valve. 
  • hot_rod
    hot_rod Member Posts: 15,031
    a manual mix valve can work well also, it basically  tracks with the ODR. Boiler Ramos up, hot port sees that increase also. Most types can have an actuator added if you want tighter control or a second ODR function
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,387
    edited November 2020
    Where is the boiler water or hottest water coming from? Where are the circulator pumps? The water will flow to the path of least resistance. If the valve is designed to the ASSE 1017 cert, I believe it fails in the cold or "SAFE" condition, so the hot inlet side will stop all flow and only recirculate the cold will go to the mix on failure.
    When the valve is operational, the circulating water will try to reach its 120° set point mix temperature and allow for full flow from the 100° Hot inlet to the Mix outlet with minimum flow from the Cold inlet to the Mix outlet

    this may cause the circulating pump on the radiant loop to dead head against a port that is almost fully closed.

    I would call this a poor design that may not yield the desired result.
    The outdoor temperature needs to be cold enough to allow for the boiler to be at least 130° so the flow can be established for the "Cold" inlet of the valve to have some flow.

    What are you attempting to accomplish? 2 temperature from the same boiler? you may need a re-do on your design.

    Here is an illustration showing 120° setpoint for mix and a pump that has a 10 Gallon Per Minute (GPM) flow rate




    zoom in to see the flow rate and temperature. You will see, as the temperature in the boiler drops, the flow rate in the radiant is reduced to nothing. All the water is returned to the boiler with no flow to the radiant floor. (if this is how your circulator is configured)
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • ewang
    ewang Member Posts: 74
    @hot_rod Would you go the manual route over TMV?

    I'm just awfully curious on which way a TMV biases itself when presented with two temps that are lower than it's setpoint.

    Does it favor the cold side or hot side?

    If it favors the cold side, eventually I'd expect the TMV to completely shut off the incoming hot.

    If it favors the hot side, the lower temp radiant floors would be protected, but may cause that zone to cycle due to higher SWT's than needed.
  • ewang
    ewang Member Posts: 74
    @EdTheHeaterMan Hi Ed, thanks for responding. Your point on ASSE 1017 is a good one and the exact case I would want to avoid.

    To answer your question on one boiler supplying two temps - yes! Below is an example layout from WM Evergreen Manual.

    Replace the "Hi Temp Zones" with a manifold of panel radiators with TRV's and you have the system I'm planning on installing shortly.



  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,306
    It just goes full hot. If that water is 100 degrees the supply will be very close to 100 degrees. Think of it like a temp cap. If it's set to 120, then the mix wont go over that, but if the hot port is less than 120 then it will track with the ODR. The Honeywell AM series works in this way. Only thermostatic valves I've used on heating. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    ewang
  • ewang
    ewang Member Posts: 74
    @Solid_Fuel_Man Thank you! I think that answers all the questions I have. Now the choice is run with a manual valve that floats with ODR, or install a Honeywell AM that acts as a SWT limiter.

    Love this forum and everyone's willingness to help. Thank you!
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,387
    edited November 2020
    @ewang look at my edit above. the GPM is 0 thru the radiant when you use a thermostatic mixing valve as a temperature limiter with hot full open... does that make cold full closed?

    I believe item 30 in the diagram is a motorized mixing valve... not a thermostatic mixing valve

    Further analysis of the diagram indicates the valve will work as indicated by @Solid_Fuel_Man. my diagram would not work at all below 130° That is why I needed to know where the pump is located in relation to the heat source and the emitter.
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • Zman
    Zman Member Posts: 6,986
    If the desired curves are parallel and you just want it to track the main systems ODR, the manual valve is the way to go.

    If you want a more dynamic curve, these are really simple to set up. https://www.supplyhouse.com/Taco-I075C3R-1-3-4-3-Way-Outdoor-Reset-I-Series-Mixing-Valve-w-Sensor-5203000-p

    I would not even consider a TMV in your application unless ORD is undesirable for some reason.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ewang
    ewang Member Posts: 74
    @EdTheHeaterMan - Hi Ed, I see you updated your post. The schematic you drew is definitely different than the way WM lays it out in their manual. And it appears that item 30 is just listed as, "Mixing Valve, if any".

    @Zman - The manual valve seems like the best option to obtain constant circulation throughout the whole design. The only benefits of the TMV is that the floors would "feel" warmer during the shoulder times, and it would act as a true protection for the flooring material.
  • Zman
    Zman Member Posts: 6,986
    The "feeling warmer" design is tricky. If it is warm out and you send hot water to the slab, the space will overheat and then not turn on again until it cools off making the floor feel not so warm.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Solid_Fuel_Man
  • Dave H_2
    Dave H_2 Member Posts: 478
    As your "close approach" happens (boiler water temp gets close to TMV setpoint), the water temp doesn't stay fixed, some TMV valves need at least 5 degree diff others up to 27 degree.

    Remember your customers have never asked for radiant floor heating......what they wanted is a warm floor. Modulate the water temp being sent to the floor based upon the heatloss. if you send a fixed temp to radiant all heating season, 120 when its a design temp and 120 when its 50 degrees out, you get a much shorter run time when its 50. Floors cool down, air temp is satisfied, but the floors aren't as warm as they were when it was 0 outside.

    A motorized valve will track not just the desired water temp to the room but if you think about it, it is performing load reset!

    That is different than what the boiler is doing, the boiler is modulating but all it knows is that you may have one big honkin zone. It really doesn't know what is going on inside when there are multiple zones in a house.

    When you use a modulating mix valve, you now add load reset.
    Boiler reset + Load reset = system reset!

    "Modulate the world!"

    Dave H.
    Dave H
    mikeapolis
  • hot_rod
    hot_rod Member Posts: 15,031
    Yet another control option, building on what @daveH_2 mentioned is ODR with indoor feedback. Really what you want is for the boiler SWT to respond to is the inside temperature condition.
    Most homes have sizable internal gains when occupied, cooking, laundry, showers, hh tanks, refrigerator, lighting, computer, people. So just because outdoor has dropped 10° doesn't necessarily mean the boiler SWT should drop.

    You can find thermostats with logic to do this, the tekmar components are one option, they have promoted ideal control with indoor and outdoor since they started.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 15,031
    ewang said:

    @hot_rod Would you go the manual route over TMV?

    I'm just awfully curious on which way a TMV biases itself when presented with two temps that are lower than it's setpoint.

    Does it favor the cold side or hot side?

    If it favors the cold side, eventually I'd expect the TMV to completely shut off the incoming hot.

    If it favors the hot side, the lower temp radiant floors would be protected, but may cause that zone to cycle due to higher SWT's than needed.

    Yes the valve will go full hot unless it is designed to have 100% close off. Some brands and models of TRVs do, some do not.
    It is not part of the ASSE 1017 listing, that they shut off completely. Nor are check valves.

    ASSE 1070 is a point of use valve and it requires checks, to meet the listing, has to be 120° max. temperature setable.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mikeapolis
    mikeapolis Member Posts: 43
    How about a Taco I Series?
  • hot_rod
    hot_rod Member Posts: 15,031
    Here is the valve I have on my system. It can be manual, or add an actuator.
    When you size a control valve, size by flow rate not by pipe size, Here is an example of a 1" performance curve. Keep the valve within the operating range for best control.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    ewang
  • ewang
    ewang Member Posts: 74
    Thank you. I think this will be a great thread for others that have similar questions.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,387
    hot_rod said:

    Here is the valve I have on my system. It can be manual, or add an actuator.
    When you size a control valve, size by flow rate not by pipe size, Here is an example of a 1" performance curve. Keep the valve within the operating range for best control.

    Damn, Hot rod, You have access to all the good toys!
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,306
    Did you catch that Bob has a non-caleffi valve?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    EdTheHeaterMan