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  1. #1
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    How to Use Outdoor Temperature Reset and Maintain Boiler Minimum Return Water Temp?

    Hello,

    I recently posted about improving the efficiency of an MAU at the condo building where I live. The building also has a hot water heating system that we want to improve the efficiency of as well. It has the following boiler and controller:

    Boiler:
    Make/Model: Raytherm H3-624-N
    Type: Non-Condensing
    Heat Exchanger: Cast-iron headers with copper finned tubes
    Minimum Inlet Temperature: 105℉

    Controller:
    Make/Model: Tekmar 261
    Key Features: Outdoor temperature reset; minimum temperature setpoint; PID control of supply water temperature; automatic and fixed target temperature differential modes

    I want to optimize the settings the controller. But I have some questions about what these should be:

    1) The Tekmar 261 only senses the supply water temperature coming out of the boiler. But I need to ensure that the return water temperature does not drop below the minimum inlet temperature for the boiler (105℉). So, how do I determine the minimum supply water temperature setting (BOIL MIN) such that the return water temperature is above 105℉ at all points on the “Characterized Heating Curve”?

    2) I understand that the top of the Characterized Heating Curve is the design point for the building heating system - the coldest outdoor air temperature (OUTDR DSGN) and the design supply water temperature (BOIL DSGN). The bottom of the curve is the design room temperature (BOIL INDR).
    All of these values are unknown because there is no documentation about the original design of the hot water heating system. So how should I determine these settings for the Tekmar controller?

    3) Does anyone know if there is a way to use the Tekmar 261 to sense the return water temperature and control on this temperature rather than the supply water temperature?

    4) How is the “FIRE DELAY” setting determined for the Tekmar 261?

  2. #2
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    Oct 2009
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    God's country - Shenandoah Valley, VA
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    You can consult Tekmar about the particulars of the control, but how the ODR curve is setup will be determined by two things:
    1. Flush gas condensation will occur if the curve is set below 140*. This must be avoided at all costs.
    2. What type of heat emitters do you have? That will also dictate the parameters of the curve.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  3. #3
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    Oct 2009
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    God's country - Shenandoah Valley, VA
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    You can consult Tekmar about the particulars of the control, but how the ODR curve is setup will be determined by two things:
    1. Flush gas condensation will occur if the curve is set below 140*. This must be avoided at all costs.
    2. What type of heat emitters do you have? That will also dictate the parameters of the curve.

    Here's a link to Tekmar's instructions:

    https://s3.amazonaws.com/s3.supplyho...61-install.pdf
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  4. #4
    Join Date
    Sep 2002
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    You could measure the temperature drop across the system and then set the minimum boiler set point at 105 + (system ∆ T) = min. boiler set-point. Wouldn't that about do it?

    Coldest Outdoor Temperature would be published information for your area, no?

    Lacking the original design data I would use 180 (which is pretty standard for rating radiation) as the number for the building equipment and 70 as the indoor as 68 seems to annoy most people.

    PHM
    ---------


    Quote Originally Posted by nielkfj View Post
    Hello,

    I recently posted about improving the efficiency of an MAU at the condo building where I live. The building also has a hot water heating system that we want to improve the efficiency of as well. It has the following boiler and controller:

    Boiler:
    Make/Model: Raytherm H3-624-N
    Type: Non-Condensing
    Heat Exchanger: Cast-iron headers with copper finned tubes
    Minimum Inlet Temperature: 105℉

    Controller:
    Make/Model: Tekmar 261
    Key Features: Outdoor temperature reset; minimum temperature setpoint; PID control of supply water temperature; automatic and fixed target temperature differential modes

    I want to optimize the settings the controller. But I have some questions about what these should be:

    1) The Tekmar 261 only senses the supply water temperature coming out of the boiler. But I need to ensure that the return water temperature does not drop below the minimum inlet temperature for the boiler (105℉). So, how do I determine the minimum supply water temperature setting (BOIL MIN) such that the return water temperature is above 105℉ at all points on the “Characterized Heating Curve”?

    2) I understand that the top of the Characterized Heating Curve is the design point for the building heating system - the coldest outdoor air temperature (OUTDR DSGN) and the design supply water temperature (BOIL DSGN). The bottom of the curve is the design room temperature (BOIL INDR).
    All of these values are unknown because there is no documentation about the original design of the hot water heating system. So how should I determine these settings for the Tekmar controller?

    3) Does anyone know if there is a way to use the Tekmar 261 to sense the return water temperature and control on this temperature rather than the supply water temperature?

    4) How is the “FIRE DELAY” setting determined for the Tekmar 261?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  5. #5
    Join Date
    Nov 2019
    Location
    Calgary, Alberta, Canada
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    Thread Starter
    Quote Originally Posted by bobboan View Post
    You can consult Tekmar about the particulars of the control, but how the ODR curve is setup will be determined by two things:
    1. Flush gas condensation will occur if the curve is set below 140*. This must be avoided at all costs.
    OK, 140 F noted. This is the minimum supply temp, not return temp, correct?

    However, Raytherm has confirmed for me that the minimum inlet water temperature to the boiler is 105 F. So I'm thinking it might be possible to set the minimum supply temperature lower than 140 F.

    I think the minimum supply temperature could be calculated like this:

    (105 F) + (expected temperature drop between supply and return)
    + (maximum error in expected temperature drop between supply and return due to error in outdoor air temp reading)
    + (maximum differential temperature the Tekmar allows around the target supply temperature)
    + (other error margin)
    = Minimum Supply Water Temperature

    I believe the lowest return water temperature would occur at the lowest ambient temperature where the Tekmar controller calculates the minimum supply water temp as its target. At lower ambient temperatures the Tekmar would calculate a higher supply water temperature along the ODR curve, so the return water temperature would also be higher.

    Right now our mechanical maintenance contractor has the minimum supply temperature set at 155 F, which I'm thinking is too conservative and we aren't getting the full benefit of ODR.

    2. What type of heat emitters do you have? That will also dictate the parameters of the curve.
    We have "fin-tube convertor" (according to Tekmar documentation) type of heat emitters in the apartments, and a couple of "Cabinet Unit Heaters" on the inside two exterior doors.

  6. #6
    Join Date
    Nov 2019
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    Calgary, Alberta, Canada
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    Thread Starter
    Quote Originally Posted by bobboan View Post
    You can consult Tekmar about the particulars of the control, but how the ODR curve is setup will be determined by two things:
    1. Flush gas condensation will occur if the curve is set below 140*. This must be avoided at all costs.
    2. What type of heat emitters do you have? That will also dictate the parameters of the curve.

    Here's a link to Tekmar's instructions:

    https://s3.amazonaws.com/s3.supplyho...61-install.pdf
    Actually I've already download this from the Tekmar web site and read through it, but thanks anyhow.

    It think I've got a handle on how ODR works. I just need to get a better understanding of the settings that should be entered into the controller. I also intend to discuss with our mechanical maintenance contractor before making any changes.

  7. #7
    Join Date
    Aug 2001
    Location
    Pavilion, NY
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    Install a lochinvar LTV-06 bypass valve. They are pre-set to 140deg but replacement thermostatic elements can be used in a few different temps I believe.

    Sent from my SM-N960U using Tapatalk
    ...

  8. #8
    Join Date
    Nov 2019
    Location
    Calgary, Alberta, Canada
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    Thread Starter
    Quote Originally Posted by Poodle Head Mikey View Post
    You could measure the temperature drop across the system and then set the minimum boiler set point at 105 + (system ∆ T) = min. boiler set-point. Wouldn't that about do it?
    I was thinking about doing that. But the ∆T changes with the outdoor air temperature, doesn't it?
    This is how I understand it:
    At "Coldest Outdoor Temperature" (design point), ∆T is at a maximum.
    At an outdoor temperature equal to the design room temp ∆T=0.

    So the ∆T should change along a straight line as the outdoor temperature changes. Correct?

    The critical point seems to be at the lowest outdoor temperature that the Tekmar calculates the target supply water temperature to be "BOIL MIN", the minimum allowable supply water temperature. So I would need to measure the ∆T when the outdoor temperature is at this point.

    At higher outdoor temperatures the Tekmar will calculate a higher target supply water temperature along the ODR curve. The ∆T will increase, but the supply water temperature should increase more, so there is less risk of going below the minimum return water temperature.

    Coldest Outdoor Temperature would be published information for your area, no?
    Yes, I should be able to find that.
    So when these heating systems are designed do HVAC engineers use the actual published data for the coldest outdoor temperature, for example -36.3 F? Or do they use an assumed value at a convenient number, like say -20 F, -30 F or -40 F?

    Lacking the original design data I would use 180 (which is pretty standard for rating radiation) as the number for the building equipment and 70 as the indoor as 68 seems to annoy most people.

    PHM
    ---------
    OK, noted.

    I've also seen 180 F mentioned in other things I've read. It's the default value used for "Fin-tube convectors" in the Tekmar manual. So, it would probably be a pretty good assumption that this is what the original designers used.

    An indoor temperature of 70 F also seems to be pretty common from what I've read. So I guess it would also be a pretty good assumption that this is the value used in the original design.

    One thing I forgot to ask in my first post is what differential temperature (∆T) is typically assumed at the design point (coldest outdoor temperature) when these systems are designed?

  9. #9
    Join Date
    Jun 2012
    Location
    Northern NV
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    Are you making hot water with this boiler as well?
    “A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.”
    ― Robert A. Heinlein

  10. #10
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
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    It's time to hire a professional to assess the entire system and explain how it operates. The best anyone on this forum can do is offer educated guesses based n your descriptions. The downfall is that we have no idea exactly how each component the system was designed to interact with the others. There are several ways hat system could have been designed and each will operate differently. Incorrect setup could lead to increased energy usage and decreased comfort.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  11. #11
    Join Date
    Jan 2004
    Location
    PA
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    So when these heating systems are designed do HVAC engineers use the actual published data for the coldest outdoor temperature, for example -36.3 F? Or do they use an assumed value at a convenient number, like say -20 F, -30 F or -40 F?
    They would use -36 or -40 if the published temp was -36.3°F

    Has the HOA or condo association put you in charge o reducing cost? Remember, one slip up and you could cost them a lot more money replacing a broken/rotted out boiler, and no heat for a week or so.

    The boiler may have been set to 150 min to allow for heat loss in the piping, and or the GPM through the furtherest fan coil/heat emitter. Copper tube aluminum finned baseboard heat out put drops a lot if the average water temp drops below 140 in them. So lowering the water temp below 150 could cause some of the other units to not heat up as well or as warm as they should. Plus, when the boiler water temp is lowered, you lower the temp that they are allowed/able to keep their condo. So you would need to find out how warm all of the other condos keep their condo, before lowering the water temp.

    At what outdoor temp does the reset start increasing water temp?
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  12. #12
    Join Date
    Oct 2009
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    God's country - Shenandoah Valley, VA
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    Quote Originally Posted by nielkfj View Post
    OK, 140 F noted. This is the minimum supply temp, not return temp, correct?

    However, Raytherm has confirmed for me that the minimum inlet water temperature to the boiler is 105 F. So I'm thinking it might be possible to set the minimum supply temperature lower than 140 F.

    I think the minimum supply temperature could be calculated like this:

    (105 F) + (expected temperature drop between supply and return)
    + (maximum error in expected temperature drop between supply and return due to error in outdoor air temp reading)
    + (maximum differential temperature the Tekmar allows around the target supply temperature)
    + (other error margin)
    = Minimum Supply Water Temperature

    I believe the lowest return water temperature would occur at the lowest ambient temperature where the Tekmar controller calculates the minimum supply water temp as its target. At lower ambient temperatures the Tekmar would calculate a higher supply water temperature along the ODR curve, so the return water temperature would also be higher.

    Right now our mechanical maintenance contractor has the minimum supply temperature set at 155 F, which I'm thinking is too conservative and we aren't getting the full benefit of ODR.



    We have "fin-tube convertor" (according to Tekmar documentation) type of heat emitters in the apartments, and a couple of "Cabinet Unit Heaters" on the inside two exterior doors.
    As I mentioned, the boiler water temp must be kept at 140* or higher to prevent flue gas condensation which would rot the boiler and flue. The manufacturer's number of 105* has nothing to do with that; it's simply the lowest RWT that the boiler can withstand to prevent thermal shock. The 140* number would apply to any atmosepheric gas boiler.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  13. #13
    Join Date
    Nov 2019
    Location
    Calgary, Alberta, Canada
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    Thread Starter
    Quote Originally Posted by kangaroogod View Post
    Install a lochinvar LTV-06 bypass valve. They are pre-set to 140deg but replacement thermostatic elements can be used in a few different temps I believe.

    Sent from my SM-N960U using Tapatalk
    Right, I have seen bypass valves mentioned in some of the stuff I have come across on the internet. This would work for sure. But I'm trying to find out how to do this without modifying the system. I will likely need to have a higher return water temperature to be safe, and hence compromising efficiency somewhat.

    Those Lochinvar valves I think have a themoplastic inside which acts as a heat sensor. The thermo-plastic expands or contracts with the outlet temperature, and it is this expansion and contraction which actuates the valve open and closed.

    Years ago I used to use these types of valves in the design of jacket water cooling systems for industrial engines. They would maintain a minimum jacket water temperature for the engine.

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