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Outdoor Reset on a Buffer Tank?

Hi,

Apologies as this is my first post here, so please excuse any cultural snafu's.

I've recently had a Navien mod/con combi-boiler plumbed in series with a 30G buffer tank to smooth the hydronic load for our relatively small zones in a new renovation.
We've had the outdoor reset module added to the Navien which modulates the temp according to the weather, but the buffer tank is controlled is a fixed aquastat with a manual temp. setpoint.

1. Does this negate all the savings from using ODR as the the buffer tank is getting heated to the same temp regardless of what's going on outside?
2. Couldn't it also lead to a situation during warmer weather where the boiler can't bring the buffer tank up to temp because it's output temp is below the buffer tank setpoint?

I'm still learning about all this stuff, but I haven't seen this type of topic covered anywhere before.
If these are problems, how can I get around them to ensure I get the efficiencies of ODR, realizing that the buffer tank is not all that quick to change to changes in outdoor temp?

thanks
Matt

Comments

  • IronmanIronman Member Posts: 5,299
    I see no reason to have an aquastat on the buffer tank. It's just there to add mass to prevent the boiler from short cycling - wide spot in the hydronic road. It shouldn't need any kind of control on it.

    Can you post some pics that show the boiler, the piping and controls?
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rodhot_rod Member Posts: 12,014
    As Bob mentioned lets see the piping first. It may be as simple as moving the boiler sensor to the tank.

    There are a couple way to determine buffer control logic.

    One concept it to run the tank hot, say 180F, then draw it as low as possible based on ODR distribution. That will maximize the tank. This is most common with solar and RE boilers- pellet wood, etc.

    However the mod con suffers efficiency when running above temperatures that induce condensation.

    Or just treat the boiler and buffer as one vessel all controlled from the boilers ODR, basically turning the boiler into a high mass or high volume type heater.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mshackladymshacklady Member Posts: 7
    edited February 20
    Thanks for the input so far guys - making the buffer tank a large mass with no specific controls makes logical sense to me. I've attached a photo, and a rough schematic I drew up to try and explain the setup. Let me know if anything doesn't make sense. I'd be grateful for any feedback on the setup


  • hot_rodhot_rod Member Posts: 12,014
    Is that an indirect type tank, a coil in the bottom?

    Ideally to "use" the capacity of the tank, draw for the system supply from the top, return to the bottom.

    Looks like that piping would just use a small % of the tank?

    Some piping and control options in this journal.

    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • NY_RobNY_Rob Member Posts: 1,370
    You might want to consider replacing the old-school air scoop with a unit that has a particle magnet/strainer. Take a look at the Caleffi Discal Dirtmag. Better for your boiler and pumps.
  • IronmanIronman Member Posts: 5,299
    edited February 20
    As also previously mentioned, if the boiler has a "System Sensor" option, then place that in the sensor well of the buffer tank and let that control the temp. Then do away with the aquastat. The boiler will then modulate the temperature of the buffer tank based upon ODR.

    You'll probably also need to connect the boiler's "T T" terminals to zone panel's end switch ("TT" or "XX") to fire it during a call for heat.



    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • SuperTechSuperTech Member Posts: 949
    With the new ECM circulators I would highly recommend using a magnetic dirt separator to protect the circs and the boiler. The Caleffi Discal Dirtmag will do this for you and provide far better air removal than that old style air scoop and vent.

    What type of heat emitters do you have? I prefer to zone my boiler using zones valves and a single ECM circulator. A pump for every zone seems like overkill. Nice looking setup though. Looks like someone takes pride in their work.
  • mshackladymshacklady Member Posts: 7
    The tank is a Bock ST 30G tank. As far as I can tell it looks to be just a tank, without internal tubing. The piping also looks suspect to my (untrained) eye, I can't figure any reason for it, other than the manual shows it plumbed that way, but for domestic hot water not for radiant floors - I'm raising it with the installers.

    Bob B and Bob R - regarding the aquastat, based on the photos and tank you'd recommend disconnecting the aquastat once the piping is sorted out and piping the relay to the boiler?

    I'd prefer not to keep the tank heated to a set temperature because the whole point of this combi setup was to do away with a tank water heater to save the costs of keeping hot water on standby.
  • hot_rodhot_rod Member Posts: 12,014
    The connections on that tank are usually for DHW applications where cold supply comes in the bottom, hot out the top.

    In your case with a closed loop circulation, I don't think you are getting a lot of buffering piped like that.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • SuperJSuperJ Member Posts: 556
    You want the boiler pulling off the lower port and the system return going to othe other low port. Boiler supply in the top side port, and system supply out the top.

    Or do the two pipe buffer with the secondary loop going out of the top side port, and returning to the bottom. And then put the boiler supply tee’d into the system supply and boiler return from the system return. This will reduce flow in the tank but is tougher to get all the details right.
  • mshackladymshacklady Member Posts: 7
    Great, so I'm not a ignorant newb then...just a newb :)

    Regarding the plumbing of the buffer tank, this may be a little philosophical, but does a 2 pipe configuration not give you more of a guarantee that the buffer will only be used when needed (i.e. the circuits come first, and the buffer second) vs. a 4 pipe configuration where the buffer comes first and the circuits second (because all water has to go through the buffer regardless)? Also, a 2 pipe config would allow you to provide water to the circuits at roughly the same temp. as the boiler supplies it, vs. a 4 pipe where, because of the buffer tank, you can't guarantee what temp will be supplied to the circuits?

    I'm just trying to figure out the advantages to a 4 pipe config, and therefore the best piping for my setup?
  • SuperJSuperJ Member Posts: 556
    edited February 20
    Sounds like you know more than your letting on ;)
    It's not hard to get it right, but there seems to be infinite ways to get it wrong too. And not every system actually stands to benefit from every possible optimization.

    Here's a good primer on buffer tanks. And 2 pipe versus 4 (@ about 9min15sec in).
  • hot_rodhot_rod Member Posts: 12,014

    Great, so I'm not a ignorant newb then...just a newb :)

    Regarding the plumbing of the buffer tank, this may be a little philosophical, but does a 2 pipe configuration not give you more of a guarantee that the buffer will only be used when needed (i.e. the circuits come first, and the buffer second) vs. a 4 pipe configuration where the buffer comes first and the circuits second (because all water has to go through the buffer regardless)? Also, a 2 pipe config would allow you to provide water to the circuits at roughly the same temp. as the boiler supplies it, vs. a 4 pipe where, because of the buffer tank, you can't guarantee what temp will be supplied to the circuits?

    I'm just trying to figure out the advantages to a 4 pipe config, and therefore the best piping for my setup?

    Yes you have it correct. With 2 pipe the boiler goes directly to the load when output and load match. Really no good reason to involve the buffer when load and boiler match exactly. So you always have instant boiler output to the load, no need to warm a cold tank first.

    Or a portion of the boiler output can go into the tank, or when loads shut down the entire boiler output goes into the tank. Then on the next heat call, the boiler only fires after the buffer is pulled down, so you really get several advantages with 2 pipe.

    There are a few critical piping details with two pipe, the connection at the tank needs to be large enough to handle the multiple flow rates, it becomes a hydraulic separator.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mshackladymshacklady Member Posts: 7
    edited February 20
    Thanks for the presentation - very useful.

    Back to the aquastat - I've read Idronics 17 a little closer and in the examples shown they do include an aquastat, but connected to an outdoor reset - See page 15. It reads as if the buffer tank is kept at in a heated state. See also page 29, example 1 on page 45, and others throughout the examples.

    So this reads to me that the tank should be maintained at the supply temprature, and modulated through an ODR controller - although there would be some lag in that due to the mass of the tank.
    Also, in that presentation that was just posted prior to this comment, at the 1:01 mark, a question is asked if tanks should be kept at supply temp, and after some humming and hawing the answer is no to avoid standby heat loss - so I'm really confused :)
  • IronmanIronman Member Posts: 5,299
    I wouldn't worry about standby loss. At 130*, the tank only loses 1/2* per hour. You can't compare a space heating system that averages 50 - 70% runtime with a domestic system that may see useage 2-3 times per day.

    Besides that, your pipes are uninsulated and giving off heat whenever there's flow through them. If it's going into a conditioned space, then it's irrelevant.

    Domestic standby loss is different because the heat loss may or may not benefit the space, particularly in cooling season.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • IronmanIronman Member Posts: 5,299
    And yes, let the boiler control the tank using a system sensor. In effect, the tank becomes an ancillary part of the boiler. The sensor would tell the boiler what the actual temp in the tank is since this can vary from what the boiler is seeing through its supply and return sensors depending upon how many zones are pumping through the tank.

    Since your boiler has an internal circulator that must be used to pump the boiler, I think you'll be better off using a 4 pipe configuration. Not every piping scenario works for every situation.

    The 4 pipe buffer tank that I posted has no internal coil. Using an indirect tank for a buffer has its limitations because the internal coil limits the amount of heat transfer when there's a low temperature difference (delta T) between the circuits. Hot_rod may have a better work around drawing for that scenario.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rodhot_rod Member Posts: 12,014
    It kinda depends on what you want to accomplish with a buffer. The tank can extend the run cycle of the boiler, basically turning a 3 gallon boiler into a 33 gallon boiler. You turn a low mass, low water content boiler into a high volume.

    And it can also provide a long drawdown before the boiler has to fire again, but only if the tank is maintained at or above a useable temperature.
    So you look at boiler run time and boiler off time.

    To get the most drawdown time you would run the tank to a high temperature, say 180F and draw the loads off the tank with outdoor reset control. In that case the boiler sensor goes into the tank and has a setpoint temperature control, tank always maintained at boiler operating temperature.

    Keeping in mind with a mod con boiler, maintaining a 180 SWT reduces the boiler efficiency when it runs above condensing temperatures. So pros and cons to both piping and control logic you chose with buffer tanks.

    Different between 2 and 4 pipe is with 4 pipe the water always flows across or through the tank. With 2 pipe flow can go directly to the loads, quicker start up, etc.

    Yet another method is 3 pipe where the system return flows directly to the boiler, across the bottom of the tank, without any blending in the tank, that would assure the lowest possible return temperature to the boiler, for best efficiency, most common with heat pumps to keep efficiency at max. Same 2 pipe function up top, flow from boiler able to go directly to the load.

    The pic shows a buffer tank simulator and how you can toggle zones, temperatures, tank capacity to model boiler on and off cycle intervals.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mshackladymshacklady Member Posts: 7
    Thanks for the info Bob - what software did you use to do the buffer tank simulator?
  • hot_rodhot_rod Member Posts: 12,014

    Thanks for the info Bob - what software did you use to do the buffer tank simulator?

    I use Siggys HDS program, a free trial version at this link


    https://www.hydronicpros.com/downloads/
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • styxplostyxplo Member Posts: 13
    How do you coordinate the outdoor reset with the buffer tank if the boiler has built in outdoor reset, but no external temperature sensor input?
  • Rich_49Rich_49 Member Posts: 2,540
    edited December 4
    styxplo said:

    How do you coordinate the outdoor reset with the buffer tank if the boiler has built in outdoor reset, but no external temperature sensor input?

    Taco PC 700-2 control offers ODR , a sensor that can be placed in the tank and an end switch which when the tank is pulled down will fire the boiler . We set these settings exactly as the boiler and it works very nicely . We use this on 2 pipe buffers with the load between the boiler and tank . This basically makes the only thing that turns on the boiler , the tank temp with the exception of Priority DHW of course .

    http://apps.taco-hvac.com/uploads/FileLibrary/PC700-2(102-097).pdf
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • styxplostyxplo Member Posts: 13
    Alright. I was thinking the tank would need its own ODR controller with a curve that matched the boilers ( assuming the temp sensor was mounted in the same location with similar characteristics) I’m trying to understand the idea behind the 2 pipe buffer with the heating load between the boiler and the buffer.
  • styxplostyxplo Member Posts: 13
    I found a good article explaining the 2 pipe buffer. It makes sense now. Just to make sure I understand the control side. Make both of the ODR curves the same. Use the Taco PC700 temp sensor on the buffer. When it closes its contacts use that to fire the boiler. Thanks Mark
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