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Constant circ question??

Dave_22
Dave_22 Member Posts: 232
Anyone???

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  • Dave_22
    Dave_22 Member Posts: 232


    In a constant circ homerun system with panel rads and TRV's- what tells the boiler to fire? I have read that it comes on with a low outdoor reset temp,but does it always fire on low outdoor temp? If it does-isn't that kind of un-efficient? If all the TRV's are closed and the differential pressure valve at the manifold is open, what keeps the boiler from just hitting it's high temp limit and shutting down? What am I missing? This system would be a whole lower level with 2 bedrooms and a living room as one zone.Thanks in advance.
  • eleft_4
    eleft_4 Member Posts: 509
  • Constantin
    Constantin Member Posts: 3,782
    I hope this helps...

    ... a constant circulation system will have a dedicated circulator pumping water to and from closely-spaced T's the primary loop. Panel radiators and the like will "drink" from said secondary loop as much as their TRVs allow. When all the TRVs are closed, the water will bypass the radiators and exit via the differential bypass.

    In the meantime, the boiler sits on it's own secondary circuit that feeds into the primary loop, also on closely-spaced T's. A properly-adjusted OR controller will adjust the setpoint on the boiler based on the heating curve that characterizes the house. Here, the curve will match the water temperature of the primary loop to the temperature that is needed in the secondary system to keep the house comfortable.

    The boiler will fire whenever it doesn't meet that internal setpoint, and it'll only fire continously on design days, if it is sized properly. Depending on the water content of the boiler, it may or may not have long periods of inactivity between firing. Here, the thermal mass of all that water in the boiler acts as a buffer. The larger the buffer, the longer periods of inactivity as well as burn cycles.

    A boiler with modulating burner doesn't need as much of a buffer because it can modulate its firing rate down to meet demand. Currently, such boilers are only available in the US on the natural gas/propane side of the residential business.

    Conversely, a boiler with a on-off burner (i.e. residential-sized oil units) benefits from a large buffer in TRV systems because its continuous output only matches the heating need 1% of the year, if that. The buffer can be inside the boiler (i.e. Viessmann Vitola) or external (i.e. Superstor or similar insulated indirect tank).

    So, to answer your question, the system is efficient because the internal tank temperature of the boiler modulates up and down to meet expected heating demand. The lower the internal tank temperature, the lower the standby losses and the greater the savings. Should insolation or other heating mechanisms obviate the need for the boiler, the TRVs will ensure that they won't be a source of heat in the house. Then, the boiler just sits in the basement, maintains a even temperature and waits...
  • eleft_4
    eleft_4 Member Posts: 509
    Hey this answers it !

  • Dave_22
    Dave_22 Member Posts: 232


    I don't feel too bright right now, I know the secondary pump would run continuously, but what fires the burner? Would you still have 1 thermostat in the zone? What turns the boiler burner on and off? It sounds from the above posts that there is an aquastat measuring the secondary loop temp.Sorry- this ones just taking awhile to sink in!!
  • Constantin
    Constantin Member Posts: 3,782
    T-Stat?

    Theoretically, each TRV is a little thermostat that regulates the temperature in its area. So, if your home has a OR controller and all heat emitters are regulated by TRVs I don't think you need a thermostat on a secondary loop...

    That is, unless you're running a system with multiple tempeartures. For example, a radiant floor heating secondary loop may be fed by a 4-way valve to ensure that when the primary loop gets toasty due to a indirect water heater calling for heat that the floor doesn't get fried. On such a secondary loop, the thermostat would provide the feedback to ensure that the secondary loop doesn't get too hot.

    So while I see no reason that a indoor T-Stat is required it could give the boiler controller additional feedback, i.e. indoor and outdoor reset for even greater savings. Thus, in a home that benefits from significant winter-time insolation, the boiler will know that it can safely take the daytime hours off while the sun does its business of heating the home.
  • steve_89
    steve_89 Member Posts: 1
    high limit

    Here is one homeowner's take on your question:

    Couldn't you either set the thermostat very high so that there is always a call for heat or eliminate the thermostat and jumper the tt terminals of the controller so that in effect there is always a call for heat (either way you would let the trv's and reset controller maintain the desired room temp level).

    With a continuous call for heat the boiler firing would be regulated by the controller high limit. With my Erie outdoor reset controller, once the high limit is reached, the boiler won't fire again until the boiler water temperature has dropped by the differential that you choose (either 15 or 30 deg) The high limit is automatically reset according to the outdoor temperature.
  • Dave_22
    Dave_22 Member Posts: 232


    Yes- I can understand that, but on a warm day that sun has heated up the home, why would you want the boiler bouncing off it's high limit for most of the day?
  • Constantin
    Constantin Member Posts: 3,782
    Bouncing?

    How much will your boiler "bounce" when the outdoor temps are high, the indoor loop temps don't drop because the TRVs are closed, and the boiler just sits there with nothing to do? If your boiler and the piping is well-insulated, it'll be hours before it'll fire whenever there is no heat transfer via the emitters in the home.
  • Dave_22
    Dave_22 Member Posts: 232


    OK- I can kinda see what your getting at here. So they events would kind of be 1)TRV's close 2)diff bypass opens 3)boiler circulates hot water in a circle and shuts down on high limit 4) boiler loops cools and refires boiler??? Is that what your saying for the most part? Sorry to be a PITA!
  • hydronicsmike
    hydronicsmike Member Posts: 855
    DAVE

    Short Answer: If all TRVs are closed, the Water Temperature is too high!

    Better answer with more detail: Call tekmar at 250-545-7749.

    Hope to hear from you.

    Mike
  • Constantin
    Constantin Member Posts: 3,782
    No worries, you got it.

    The thing to keep in mind is that the high limit doesn't have to be 180°F. It can be much lower than that, particularly if you have a boiler that doesn't have to worry about flue gas condensation.
  • Constantin
    Constantin Member Posts: 3,782
    good point!

    If all the TRVs are closed on a cool day with no insolation, then the OR curve isn't set right. If your boiler cannot modulate down that low w/o condensing, then a mixing system is in order. Thanks Mike!
  • Dave_22
    Dave_22 Member Posts: 232


    Hey-thanks all- I'm getting it more and more. I have an idea? How about a flow switch in the diff. bypass leg that "IF" there is full flow across the bypass valve, then interlock the burner!!!! I guess in the dead of winter, with the proper reset temp setpoint it will always run. I was kind of stuck on when the load is less than the low fire of the boiler.
  • Weezbo
    Weezbo Member Posts: 6,232
    it is possible to do constant recirc other than mentioned...

    comfort levels both physical and finacial to be determined by you the habby home owner.if you are having a bit of a chore visualizing what might be the best way to roll...there are articles in many books with pictures that show various system isolation techniques..these help make it easy to see and replace clustered messages with straightforward pictures...these pictures explain the whys and wherefores and the math behind them which allows you to pipe or control with the stradgey of your choise.............trv's have lets say a choise of temp.differential,the pipe feeding and returning from them have another,the pipe supplying that system can have another,this may be further mixed down to provide even more differential temp ....now all those options can be connected to a variety of tanks plumbing arrangements or appliances ...these also can have very specific piping arrangements and controls, and stradgies of operation....one of the better outlines of these variations is a book that is called Modern Hydronic Heating. the gentelman who wrote this book i have never met however from this corner of the world i would say it should be at least perused by a home owner...Mechanical contractors ,engineers,designers,architects,General Contractors and students of M.E ,E.E and P.E.'s would also find it a very useful book . check on goggle search for Hydronic designe studios. experience is recognizeable ,so, you will probably see somethings that you have seen or done and be able to recognize it immediately...what i mean by that too is that the book isnt a bunch of computer wannabe engineering that falls down about ones ears.. for the most part it is very apt application of a wrench twister with a mind like a steel trap. *~/:)i type slow...3 other posts were made since i started to reply...Mike is correct. the temp may need to be mixed down available to the constant circulation.and depending upon the temp of the boiler it may need some means of either steady state partial or full reset ...
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    I was kind of stuck on when the load is less than the low fire of the boiler.

    I can describe what a Vitodens does on a TRVd system (original gravity so LOTS of water volume) with constant circulation, no low-loss header, a single circulator, full outdoor reset and no wall thermostats or other digital control devices.

    As long as loss is greater than lowest fire, it's pure, smooth modulation. As long as TRVs are not changed, the supply and return temperatures float smoothly with changing outdoor temperature and your reset ratio.

    Once loss is less than the lowest firing rate, the burner can no longer fire continually.

    Exactly how and why it does what it does is unknown to me, but here are my guesses. Do not forget that it is a "learning" boiler, so the operation I describe may not hold true for other types of systems.

    Again, loss is less than lowest possible modulation.

    When the boiler temperature drops to some point below the current target, the burner fires. It begins at full output. If the boiler temperature rises to some point above the current target during this brief period of full fire, the burner shuts down. Amazingly (to me) these very short firings are extremely energy efficient. A surpring amount of condensate is given off and the flue temperature remains remarkably low.

    If the boiler temperature has NOT risen to the shutdown level during that brief period, the boiler modulates all the way down to lowest output. It stays at lowest possible output until one of two things occurs:

    1) Heat loss increases to the point that the lowest modulation can no longer maintain the target temperature (e.g. smooth modulation) or;

    2) The boiler temperature exceeds the target temperature by some amount. When this happens, the burner shuts down and the process repeats itself.

    --------------------------------------------------------

    I believe that much of the learning ability of the boiler is there to idealize operation when loss is less than lowest modulation. The boiler doesn't seem to have to learn how to modulate--it seems only to have to learn how to operate efficiently--while maintaining comfort--when it's forced into digital operation.

    In all honesty, it seems to learn how to use the water in the system as a buffer. Over time, my Vitodens has learned to allow a greater variance away from the target temperature. This has increased the length of the periods of lowest possible output and decreased the frequency of the little injections at full output.

    When heat loss is VERY low there's another problem in a system without indoor thermostats and with constant circulation. The boiler doesn't know when to stop producing heat--after all, the differential pressure bypass just opens and the boiler is effectively heating only itself with the heat gradually escaping via the flue and the jacket.

    That's where you need some form of warm-weather shutdown. Some electronic controllers may have this function built-in. With the Vitodens, it's rather strange.

    You have to look at the "Sun dial" (the normal mode setting) VERY differently. It no longer has ANY relation to the inside temperature. Remember: the Vitodens has no built-in provision for measuring indoor temp.

    You set the "Sun dial" for the OUTDOOR temperature at which you want the boiler to shut down! Say 57° or so. Moving the sun dial down that low makes a corresponding downward parallel shift in your reset curve. To correct for this, you add the same temperature (don't forget that the shift is ALWAYS in degrees Celcius) back into your curve.

    Now, the Vitodens will shut down at whatever outdoor temp you want but the overall curve is not affected. I went a step further. I set my "Sun dial" for 60° and added in the necessary positive shift to the curve. I used my old warm-weather shutdown control to switch to the "Moon dial" setting whenever the outside temperature is at the point where the burner stops modulating (40° in my case). The circulator goes into fixed speed (VERY low in my system) and the boiler target is further depressed by the "Moon dial" setting of 57°. While I loose nearly all ability to increase the room temperature whenever it's warmer than 40° outside, the significantly depressed reset curve has resulted in phenomenally high efficiency in such conditions.
  • Dave_22
    Dave_22 Member Posts: 232


    Thank you very much- this post has taught me a great deal about constant circ, TRV's and outdoor reset!!!! Thanks to everyone!
  • hydronicsmike
    hydronicsmike Member Posts: 855
    One more Qustion I may have for you Dave...

    By the way, Thank you Constantin.

    Alright Dave, to my question, how is your boiler being controlled? I am wondering if it is set to modulate to maintain a specific setpoint water temperature as opposed to modulating to maintain an Outdoor Reset Water Temperature.

    In the first szenario, I could see how the boiler is constantly modulating, how the TRVs may be closed and whyyou asked this very good question about shutting the boiler off.

    In the second szenario, you would have an Outdoor Reset Control, which could be made by tekmar (If I may suggest so). This Control could modulate the boiler then to maintain an Outdoor Reset Water Temperature at any given time, resulting in additional Energy Savings by maintaining a lower water temperature when ever it is able to do so, resulting in less losses. This would also allow your TRVs to open up. The tekmar Reset Control would also disable (turn off) the boiler when ever the load in the system is less than the boiler puts out during minimum modulation.

    Ideally, the TRVs should be almost fully open at any given time. But this is only possible while maintaining the lowest possible water temperature while still satisfying the heat requirement. If this is possible to attain, this would be the point at which your system would be most efficient. Nothing is more efficient than constant circulation. But not if it is going through a bypass with all the TRVs closed.

    I hope this all makes sense. Sometimes I wonder if my writing is clear as English is not my first language. If you still have further questions or would like to discuss with someone who does nothing but deal with your kind of questions or situations, call tekmar Control Systems at 250-545-7749.

    Best Regards,

    Mike
  • Dave_22
    Dave_22 Member Posts: 232


    Mike, Clear as mud! Nah-only kidding. I totally understand what you are saying now. My biggest question was what happens when the load is lower than the minimum fire. Of course, if I would of just asked that first!!! But I got a lot of other good info out of it too. This is a system still in design to replace electric BB. It will be designed at a low water temp with a modulating, condensing boiler with outdoor reset. I like the Tekmar function of turning off the boiler under minimum fired loads- that is exactly what I was talking about. You've been a great help.
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