Question on boilers in Summertime
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  1. #1
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    Question on boilers in Summertime

    I am a Southerner and do not have a lot of experience with boilers and steam heat or hot water heat, but recently I went to inspect a building for general operating issues--not HVAC--up north in a "cool" climate. (Not cool now, hotter than down south, but summers are short up there with long cold winters.

    Even though it is summer, with temps in the 90's and even 100's, the boiler and its water loops are still on, and they are using reheat to temper the air.

    I asked the tech why did they not turn the boiler loops off and/or stop circulating hot water throughout the building, and he said that would be bad. He said without circulating the water, the chemical treatment might not be dispersed properly and cause a lot of problems.

    Since I am not a cold climate person and don't have any experience, I did not argue, but it goes against my energy efficiency gene to be reheating general office space. It is not like it was a Surgery Center, where reheating is common. Just general office space, with fairly low humidity, and inside temps also kept fairly low--between 69 and 72. But radiators on.

    Is this standard procedure for buildings with boilers and hot water loops?

    Or can we raise the chilled water setpoint a bit, turn off the boilers, and save some money?

  2. #2
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    You humidity is going to be determined largely by the chilled water setpoint. If you raise it, it will start getting muggy and/or hot depending on flow rates, coil size and airflow.

    Commerical systems, largely, are designed to operate year round at a certain supply temperature, usually around 55F, and airflow is adjust as needed to maintain building temperature. Chilled water temps are around 45F and that gives you a nice 50F dewpoint for comfort cooling.

    For efficency you get into using economizers, or instead of steam you have some chillers that will generate hot water or use seperate DX coil in a hot gas reheat system (forget hte correct term).

    But it's pretty normal to run the boilers in medium to large commercial buildings year round.

    Cna you shut it down in higher temps. Yes, a lot of facility managers do this. Can it screw up the system's balance? Yes, you can run into dehumidification and overcooling issues.

  3. #3
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    When I first arrived at the facility where I work now (an art museum in Texas) the boiler operated year round. I did careful study of how all of the mechanical systems in the building operated over a year's time, and concluded that operating the boilers in summer was not necessary, and a huge expense. We started a trial shutdown one August about three years ago and have since gone offline starting late April and refiring around Halloween.

    The museum has saved a lot of money by doing this, and our interior environmental temperature and humidity targets, which are more strict than typical Class A structures due to an extensive art colection, has not suffered at all.

    We do keep the hot water loop circulating all summer in order to keep it properly treated.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
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    Boilers are run year around in souther climate for hot water reheat.

    If thats the application the chill water set point should be left alone.

    The hot water reheat raises controls space temp as chill water coils control humidity.

    Shutting the boiler down could make for a nice cold and wet space.

  5. #5
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    Quote Originally Posted by Six View Post
    Boilers are run year around in souther climate for hot water reheat.

    If thats the application the chill water set point should be left alone.

    The hot water reheat raises controls space temp as chill water coils control humidity.

    Shutting the boiler down could make for a nice cold and wet space.
    We have not had that problem, and two of our air handlers are hot deck/cold deck systems.

    There are other sources of reheat besides forcing air through a hot water coil. And if the building uses VAV boxes, that's another option.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
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    Its a specific application.

    You wouldn't use electric with hot water coils typically. With VAVs electric pocket heaters are usually used on exterior zones only.

    Hot deck cold deck isn't the same as using thermostats to control 3 way valves to open and close zoned hot water coils.

    In this specific scenario 55 degree air is provided at constant volume. Hot water controls space temp.

  7. #7
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    Quote Originally Posted by Maikerum View Post
    I am a Southerner and do not have a lot of experience with boilers and steam heat or hot water heat, but recently I went to inspect a building for general operating issues--not HVAC--up north in a "cool" climate. (Not cool now, hotter than down south, but summers are short up there with long cold winters.

    Even though it is summer, with temps in the 90's and even 100's, the boiler and its water loops are still on, and they are using reheat to temper the air.

    I asked the tech why did they not turn the boiler loops off and/or stop circulating hot water throughout the building, and he said that would be bad. He said without circulating the water, the chemical treatment might not be dispersed properly and cause a lot of problems.

    Since I am not a cold climate person and don't have any experience, I did not argue, but it goes against my energy efficiency gene to be reheating general office space. It is not like it was a Surgery Center, where reheating is common. Just general office space, with fairly low humidity, and inside temps also kept fairly low--between 69 and 72. But radiators on.

    Is this standard procedure for buildings with boilers and hot water loops? Yes.

    Or can we raise the chilled water setpoint a bit, turn off the boilers, and save some money? Maybe.....but there is more to it then that.
    What kinda of air system are we talking?

    General office spaces can have very diverse loads across a common AHU.

    What kinda control system is in place?

    By resetting CHW and DA temps based on zone temps, you might be able to shut down the reheat loop. But do you have the controls in place, somebody with the knowledge to program and tweak it, and the building and system that can handle it?

  8. #8
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    Quote Originally Posted by motoguy128 View Post
    You humidity is going to be determined largely by the chilled water setpoint. If you raise it, it will start getting muggy and/or hot depending on flow rates, coil size and airflow.
    Just a question....you state humidity is based off CHW setpoint. Let's say it's a standard VAV system with single duct cooling only boxes with reheat. Your
    AHU's are running modulating valves. Your primary air temp is set to 55 degrees. Will lowering your CHW setpoint lower your overall humidity?

  9. #9
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    Quote Originally Posted by ascj View Post
    Just a question....you state humidity is based off CHW setpoint. Let's say it's a standard VAV system with single duct cooling only boxes with reheat. Your
    AHU's are running modulating valves. Your primary air temp is set to 55 degrees. Will lowering your CHW setpoint lower your overall humidity?
    Can I answer that ? I see a problem in his logic.

  10. #10
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    Quote Originally Posted by Six View Post
    Can I answer that ? I see a problem in his logic.
    Go for it. My question is open to all.

    The reason I pointed this out......is I have had heated discussions with other techs about this.

  11. #11
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    Quote Originally Posted by ascj View Post
    Just a question....you state humidity is based off CHW setpoint. Let's say it's a standard VAV system with single duct cooling only boxes with reheat. Your
    AHU's are running modulating valves. Your primary air temp is set to 55 degrees. Will lowering your CHW setpoint lower your overall humidity?
    At my building, RH is measured in the return air duct. Not ideal, for various reasons, but for the most part it works. The control protocol is set up so if return air RH exceeds setpoint, the signal from the Rh sensor overrides the discharge air temp setpoint, and drives the modulating chilled water valve open. This makes the coil colder and thereby it condenses more moisture. At no point is the chiller chilled water supply temp accessed and changed.

    Latent heat removal capacity is driven by coil surface temp. That said, if the discharge air temp is set at 55 degrees and there is no RH override protocol, making the chilled water colder to the coil may not increase moisture removal capacity. In fact the valve may throttle down more to prevent overshooting the DAT set point. This would mean less water, albeit colder, moving through the coil. Faster rate of sensible heat transfer, yes, which could lead to warmer water leaving the coil than with warmer entering chilled water (which would translate to more flow through the coil).

    Thoughts?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
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    Quote Originally Posted by Shophound View Post
    At my building, RH is measured in the return air duct. Not ideal, for various reasons, but for the most part it works. The control protocol is set up so if return air RH exceeds setpoint, the signal from the Rh sensor overrides the discharge air temp setpoint, and drives the modulating chilled water valve open. This makes the coil colder and thereby it condenses more moisture. At no point is the chiller chilled water supply temp accessed and changed.

    Latent heat removal capacity is driven by coil surface temp. That said, if the discharge air temp is set at 55 degrees and there is no RH override protocol, making the chilled water colder to the coil may not increase moisture removal capacity. In fact the valve may throttle down more to prevent overshooting the DAT set point. This would mean less water, albeit colder, moving through the coil. Faster rate of sensible heat transfer, yes, which could lead to warmer water leaving the coil than with warmer entering chilled water (which would translate to more flow through the coil).

    Thoughts?
    "If" the space humidity is low and the spaces are satisfied/nearly satisfied why would one continue to pump in 55*? Why run the boiler? why not reset the chiller?Why provide a higher static when the VAV's are all but closed? why do anything when it is not needed?

    We do this because that is the way it has been done forever and ever.....

    The controls and the tools available now provide us the opportunity to do all this and then some, while saving the customer a boat load of cash in the process.

  13. #13
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    Quote Originally Posted by ascj View Post
    Just a question....you state humidity is based off CHW setpoint. Let's say it's a standard VAV system with single duct cooling only boxes with reheat. Your
    AHU's are running modulating valves. Your primary air temp is set to 55 degrees. Will lowering your CHW setpoint lower your overall humidity?
    Its a unnecesary waste of energy and you'll be running with the three way valves in by-pass which increases coil temp which increases humidity.

    Lowering Chill water temp is usually something that I have to tell my ignorant "engineer". to stop doing.

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