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Constant circulation system, anyone tried?

Mark EathertonMark Eatherton Member Posts: 1,160
Ignore MC, maybe he'll grow up or go away.

SOme things you need to be cognizant of. THere is no communication between the thermostats and the pump, so you must incorporate a pressure activated bypass in the distribution piping circuit to avoid dead heading any pumps.

The final product will be extremely comfortable and as efficient as possible. This system goes against the typical AMerican way of Bang Bang control logic, but once you've started using them, they become easier to install then the typical American system.

Oventrop makes a 10 meter cap tube (33' in length) that can pretty much hit most exteriors, but if the length is a problem, you can always run the return line to wherever you need it to be to make contact with the control.

The system does require some education of the homeowner. The TRV's do not read in degrees F like the typical thermostat. It reads from 0 to 5. Each person has their own personal comfort setting depending upon the person as well as the setting. Generaly, once they find a good comfort zone, they leave the TRV's alone.

About the only disadvantage I can think of is the fact that you can't do a room by room programable set back. It has to be done system wide through the operating fluid temperatures.

Go for it, you will NOT regret it.



  • John McArthurJohn McArthur Member Posts: 157

    Anyone tried a constant circulation system? I'm installing DiaNorm panel rads in a couple of jobs and am thinking of a constant circ with TRV's. Seems like a good way to save energy, especially with a C/M boiler like the Buderus GB.

    Looking for suggestions also with mixing radiant in with the system.
  • R. KaliaR. Kalia Member Posts: 28

    No, you're the first.
  • ALHALH Member Posts: 1,790
    A great option

    Panel radiators, modulating condensing boiler, constant circulation and TRV 's are a great combination. It saves wiring, saves manifolds, and saves fuel. As far as using them with radiant, some manufacturers make remote trv's that can be used on a valve with a capillary tube between the two. The length of the capillary tube is a little limiting, and a TRV mounted on the wall is not as aesthetic as a digital thermostat. But if the house is all radiators with a couple radiant zones it might be a nice option to avoid the 24v wiring altogether.

  • J.C.A.J.C.A. Member Posts: 2,981
    I agree with,

    Moo Cow, you're the first...Unless you discount most of Europe. It's a FINE way to heat, just make sure you size the radiation so the boiler works in condensing mode 90+ % of the time. Chris
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Programmable Setback with TRVs

    Certainly possible room-by-room.

    Danfoss offers fully self-contained, battery operated, programmable operators for TRVs. Code # 013G2750 Nearly three times the cost of standard actuators however.

    They used to offer operators with a remote sensing bulb that sat against a little resistive heating pad. Center-tapped resistor offers three setback levels, but they do require an external 24VAC power source. Doorbell transformer can power hundreds. (I have these in my house, but they're no longer produced.) The little white square in the photo is the "heating pad". The three wiring connections are directly behind. The remote sensing element (looks like a little can) rests directly against the heater.
  • TedTed Member Posts: 1,718
    constant circ

    Typically the only type of "system" we install.

    You should consider the Buderus pump station with the GB142. It already has a press diff by pass in it.

    I have used the diaNorm panels too, and they're great. Just make sure you get them shipped to you in cardboard boxes, like the Buderus panel rads come. Otherwise, they will get damaged during delivery. Around here(Massachusetts) they just come delivered in their original plastic.

    If you know this, sorry, just had to say it.

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  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    For Radiant Panels

    Danfoss, Oventrop and perhaps others offer FHVs (Floor Heat Valves) that are very similar to TRVs with one big difference. They have an internal restriction adjustment (like a flow setter). The object is to set them such that head loss is nearly identical in each loop regardless of length. This prevents shorter loops from overheating the floor when the operator is turned up. Both air temp and panel temp modulating operators are available. Both have limits. 10k-12k btu/hr is about the max for the air temp modulating devices and the panel temp devices are best suited to small to medium bathrooms. I believe they would be especially well-suited for Warmboard systems offering a level of individual control/comfort that is truly unachievable by any other method.

    Would also be extremely well-suited to systems with TRVd rads and a "couple of radiant zones". No 24V wiring, etc.
  • John McArthurJohn McArthur Member Posts: 157

    Thanks for the replies. Our company was one of the first in our area to break the mold and do P/S heating systems several years ago. Now it's time to do it again :)

    I've been thinking about this for several years now and now the time seems ripe.

    One job is a warehouse/shop/office for our company with radiant concrete lower level and panel radiators on top floor. We like the Buderus GB-142 whenever we can use them. The second job is a NY loft style residence with slab on the bottom, staple-up on mid and panel rads on top. All with the GB-142 and a indirect WH.

    If my boss is willing to go for the loft I would love to go for it. He is definately willing to try it in his shop.

    The tips posted here will make the design much easier.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    So Nice to Hear...

    ...more and more wonderful heating contractors trumpeting the benefits of constant circulation, TRVs, no or few digital thermostats and boiler modulation!!!!

    As I've said and said, and done my best to give both objective and subjective data to prove:

    1) System design is simpler and requires fewer materials.

    2) System operation is easier to understand and troubleshoot.

    3) System comfort is exceptional.

    4) System efficiency is maximized.

    All it takes is the willingness to study a bit and the guts to try something new (to you).

    My most sincere belief is that these systems are both the future and salvation of American hydronics.
  • RoosterBoyRoosterBoy Member Posts: 459

    wont Constant circulation fire the boiler more with outdoor reset because the boiler water will cool down faster with water being circulated through the pipes all the time.

    i think if you leave the water in a well insulated boiler it may save more.

    yes the water in the pipes will be cold and the boiler temp in the boiler will drop fast on a call for heat.

    id think with Constant circulation on out door reset the boiler will fire on every 10 to 15 min to keep the temp in the boiler at the right heating curve even though the trv
    wont allow boiler water to enter the zone it still circulating though the loop and wasting energy

    i never seen a Constant circulation so please correct me
    if im wrong

  • ALHALH Member Posts: 1,790

    If all the zones are closed the boiler fluid just circulates in the mechanical room through the differential pressure bypass valve. Also the heat loss is less with a modulating condensing boiler because of the reduced fluid temps.

  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    The loft job sounds rough for what I consider "real" constant circulation.

    To me real constant circulation is continuous circulation of heated water to meet the heat loss demand with (perhaps) some headroom for individual room temperature adjustment.

    That loft job certainly has two grossly different supply temperature (tube-in-slab & staple up) requirements and two grossly different heat transfer methods (staple-up and panels).

    Is this bare-tube staple-up or tubing in good, heavy conduction plates?

  • RoosterBoyRoosterBoy Member Posts: 459

    is a buderus g115 a modulating condensing boiler?
    it says it can handle low water temps
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    I have a very hard time considering constant circulation without reset and without proportional flow control.

    In all honestly, this product, The Smoother, should prove superior to simply constantly running the circulator in a system without both reset and proportional flow control.

    wont Constant circulation fire the boiler more with outdoor reset because the boiler water will cool down faster with water being circulated through the pipes all the time

    In my limited experience, constant circulation combined with reset and proportional flow control will make a traditional boiler fire more freqently and for shorter periods. How much more frequently and how much shorter depends mainly on how much the boiler is oversized. In a system without reset and proportional flow control, constant circulation merely tends to average the temp of the boiler and the emitters between calls from the thermostat. Both good and bad parts to that. Again, that "Smoother" is doing its' best to find the middle-ground in a less-than-ideal (my opinion) system.

    i think if you leave the water in a well insulated boiler it may save more.

    Only if that boiler does not have a direct, straight-up path through the flue.

    id think with Constant circulation on out door reset the boiler will fire on every 10 to 15 min to keep the temp in the boiler at the right heating curve even though the trv wont allow boiler water to enter the zone it still circulating though the loop and wasting energy

    Yes, provided that the TRV settings have not changed it WILL fire every so many minutes for every so many minutes of time. The firing time will not change significantly--only the time between the firings. But no, the water circulating through the loop is not wasting significant energy unless the piping passes through something like an unheated attic where it's wasted anyway. The exception to this [may] be old gravity system with HUGE piping as the basement may wind up a bit warmer under constant circulation. The simple solution in any case is pipe insulation.

    A modulating boiler is the logical extension of constant circulation and proportional flow. You are now putting just the required amount of heat into the system. Efficiency is utterly maximized by providing this energy at just the required temperature to exactly maintain space temperature. Note however that in this perfect balance it is impossible to raise space temperature so in reality supply temperature is increased somewhat to allow adjustment for individual preference. During sleeping/unoccupied periods however, you can achieve some energy savings giving no "headroom".
  • RoosterBoyRoosterBoy Member Posts: 459

    thanks mike man everytime you respond to me i learn more and more i love it thanks
  • TedTed Member Posts: 1,718

    > is a buderus g115 a modulating condensing

    > boiler? it says it can handle low water temps

    Dont you own a G115?
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  • John McArthurJohn McArthur Member Posts: 157

    Unfortunately, it's bare tubing. It's tough selling the transfer plates to guys who've been using bare tubing for years.
  • John McArthurJohn McArthur Member Posts: 157

    > Is

    > this bare-tube staple-up or tubing in good, heavy

    > conduction plates?

    Unfortunately, it's bare tubing. It's tough selling the transfer plates to guys who've been using bare tubing for years.

    The shop job will be a great place to use the system. I figured the loft install would be problematic.

    Great responses on this thread, I learn something every time I log on. Thanks.
  • RoosterBoyRoosterBoy Member Posts: 459

    yes ted i do own a g115 but not sure what condensing and modulating are this is why i ask
  • TedTed Member Posts: 1,718

    Yes I thought it was you.

    No your boiler is not condensing modulating.

    With the control like the R2107, it can handle lower temps.

    And even their cast iron is capable of handling some degree or low water return.

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  • DanDan Member Posts: 388

    I have constant circ and outdoor reset on my system and I love it. Burner fires less frequently and for less time as it "keeps up" with the heat demand versus the old stop and go method. Rooms that were previously on the cool side are now very comfortably warm with constant and perfect matched heat for the demand. I also have an indoor reset which does wonders for fine tuning system as efficiently as possible. Once you go there, you will never ever go back.
  • RoosterBoyRoosterBoy Member Posts: 459

    homeowner do you have any pictures of your piping setup
    id love to see how a constant circulation is piped up

    also are you heating with panel rads or baseboards?
  • DanDan Member Posts: 388

    I dont have pics but my piping is standard; the constat circ is done with electronic controls; like you I have Buderus R2107 with outdoor reset and indoor sensor; its a fantastic setup.
  • RoosterBoyRoosterBoy Member Posts: 459

    thanks homeowner do you use panel rads or radient or basebords? how many zones?

  • DanDan Member Posts: 388

    Ted you were right on the money; the Buderus controls are a great fit for my system. I'm a little bit concerned about how fast the hours run clock is ticking (14 hours total in the past 4 days since I got the clock to move), because my own calculations are a lot less; but I think in the long run its got to be working more efficiently than without the controls. I appreciated very much your coming to look at my system and give a quote. I had to stick with my installer for basically the same quote because I need their oil and their maintenance. No hard feelings I hope; you have been gracious in offering advice and helping me learn. Thanks.
  • DanDan Member Posts: 388

    None of those roosterboy (wish I had Buderus panels). Its an old converted gravity system with a lot of water mass. The heat emitters are fin/tube convectors. Constant circ works especially great in this setup because the big pipes and water mass take a long time to heat up and it works a lot better to keep the flow turned on all the time and modulate temp with outdoor/indoor reset.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Here's an Idea for the Loft Job

    TRVs can still prove VERY useful on that upper level and at least the bare-tube staple-up is in the middle...

    Temp curve for the staple up almost certain to be the highest. Similar slope to the tube-in-slab, but likely a bit shallower. Ideal slope for the panels will be significantly steeper than either.

    Boiler will have to produce the curve required for the staple-up. I'd size the upper panels such that the design supply temp is very similar to the staple-up. Use TRVs on the panels and they'll deal with the flatter than ideal curve by throttling a higher portion of the flow as load decreases. You'll get exceptional recovery response from the panels in nearly any weather. This will save mixing a third temp for the panels while still giving them fully independent control.

    For the tube-in-slab a fixed (by proportion) mixing valve should work fine if the required reset slopes are similar. If significantly different, a proportional reset-controlled (indoor or outdoor) thermostatic mixing valve would be preferable (budget allowing).

    The Buderus' master indoor temperature controller should be located on the middle (staple-up) floor. It will strive to keep the temp as low as possible and circulation as continuous as possible in the least efficient [transfer-wise] area.

  • ConstantinConstantin Member Posts: 3,782
    Good idea...

    .... another one I saw was at a Viessmann training course. They like their 4-way and 3-way valves, but not every job needs or can afford multiple, independent motorized mixing valves.

    Jim explained that a workable alternative for RFH with multiple kinds of RFH emitters is to install a second, non-motorized 3-way mixing valve off the high-temp circuit and adjust it until the water temps from the two circuits go up and down in harmony with the heat loss.

    Also, depending on the layout of the space, the heating needs in the loft area may be very low.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Designing a Constant Circulation System w/Proportional Flow

    Honestly quite easy provided you're using a two-pipe system.

    Will assume that a condensing/modulating boiler is also used with no low temp limit.

    For panels, iron rads and even baseboard (cast or fin):

    1) Calculate heat loss; both room-by-room and total. If calculation is based on Manual J, you can (IMHO) safely subtract 10% - 15% from these losses.

    2) Make a full-load delta-t assumption. 20° is the standard, but other values can be used. Higher delta-t requires a higher supply temp but lower flow to achieve the same average emitter temperature. Lower delta-t requires a lower supply temp but higher flow to achieve the same average emitter temp. If you are planning to use a Vitodens, STOP and read the message titled "Proportional Flow the Vitodens Way" to avoid a, "So now you tell me!" situation.

    3) Determine your flow requirement (both structure wide and for individual areas) using YOUR ASSUMED delta-t and the simple formula: gpm = Btuh / (500 * delta-t)

    4) Decide upon your average emitter temperature. With condensing/modulating boilers lower is better for both comfort and efficiency but it comes at the expense of increased emitter size. Do not forget that your maximum supply temperature will initially be set as maximum average + (1/2 * delta-t!!!)

    5) Size your radiation such that it meets the design load at your desired average radiation temperature as closely as possible. Possible exceptions include slight oversizing in areas like baths and bedrooms. Again this comes at a price (both in space required and emitter cost), but it will allow these areas to be kept significantly warmer than "average" with faster response time. It will also enhance radiation and retard convection when these spaces are kept at "average" or below average temps. Multiple emitters will be preferable in very large spaces or where there is no physical room for a single emitter of sufficient size. EACH and every emitter should use its' own TRV! Multiple TRVd emitters in a single space "get along" just fine. It's extremely difficult to calculate for and predict the operation of multiple emitters in series on a single TRV!

    6) Determine your emitter placement. Contrary to popular belief and provided that insulation and "tightness" are both reasonable emitters need not be installed on outside walls underneath windows! Your goal is to place emitters such that they are extremely unlikely to be blocked by furniture, draperies, etc. You want to MAXIMIZE the radiant potential--anything that covers or blocks the emitters will GREATLY reduce the potential to spread radiation throughout the space! If you have designed for a fairly low (say 140° or less) average temp, give HIGH consideration to placing panels on interior walls in best view of exterior walls and windows. When placed this way tall, narrow emitters are preferable. 2 to 1 aspect ratio (twice as tall as wide) is good.

    7) Size your TRVs. All TRV manufacturers will have a table for doing this. Use your calculated design flow rates for each space (from step 3) and choose TRV valve bodies that provide this flow within the manufacturer suggested range of allowable pressure drop. Record the design condition TRV pressure drop for each space.

    8) Determine your general piping design. Basically this amounts to home-run (manifold) or traditional main-and-branch piping. In far-flung structures using manifolds, multiple manifolds will often result in material savings. Reverse-return is absolutely unneeded with main-and-branch systems. In larger systems you will achieve significant material savings by sizing the mains for constant velocity (e.g. after say 2 gpm of design flow has been consumed, drop the main size such that fps velocity is similar (but not greater) to before that amount of flow was used.)

  • Greg GibbsGreg Gibbs Member Posts: 75
    J. that you John?

    John? Steamboat John? I just set up a training on this very subject w/ Barry Engleman. It will be in Silverthorne
    Tue. December 13th @ 4:00 PM...I hope to see you there, if you need help sooner please give me a call. -Greg
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Give Me Head [loss]!

    Depending on your perspective, this is either the most enjoyable or the most distasteful part of the design process.

    Fortunately however it is much easier than the process that the dead men had to deal with for gravity systems. It's also much more forgiving than traditional manifold or main-and-branch systems that do not use proportional flow control.

    The head loss through an individual emitter is the sum of:

    1) Head loss of the emitter itself (zero for iron rads, very low or zero for panels, low to significant for baseboards). (Manufacturers supply head loss data for panels and baseboards.)

    2) Head loss through the branch (or home-run) piping.

    3) Head loss through the manifold (home-run system) or that emitters' share of the main piping (main-and-branch).

    4) Head loss of the proportional flow device (TRV) itself.

    With head loss size does matter! If you want to shortcut, just oversize all of the piping and TRV bodies. If you need 5 gpm use 2" or so manifold/near boiler or main piping and use ¾" branchs (or home runs) and TRV bodies--jump to 1" if more than 50' or so of combined supply/return in the branch or home run.

    Done this way the system will work. Your head will be exceptionally low, and the TRVs will provide their own "balancing" head loss. Two problems with this:

    1) Your material cost will be outlandish!

    2) Open too many TRVs too widely and you may not get flow through the entire system. Of course you could "solve" this problem by using a Blood & Guts 100 for your secondary circulator...


    The REAL GOAL is to find the highest head loss total to, from and through any given emitter in the system.

    You already know two important things: design flow rates (system and individual emitter) and head loss of the TRV body @ this flow.

    The next step is to plan your branch or home-run routes to each emitter. Make your best estimate of length (both return and supply) and don't forget to include "equivalent length" of any fittings in your planned route. Do this for each emitter.

    Now compute the branch or home-run head loss to each emitter using readily available tables/software/slides, etc. Your pipe size will be the same as the TRV body size. No need to split hairs--10th of a foot or inch is more than adequate. Make note.

    Now add any head loss in the emitter itself (again zero for iron rads and most likely zero for panels).

    For manifolds use your total system flow rate and size such that you are at/below the maximum suggested flow rate for a given size of pipe. If close to the max suggested (and particularly if the manifold piping is anything but "short") I'd suggest bumping up one size to keep head loss in this part of the system low.

    For main-and-branch and multiple manifold systems, it's a bit more difficult. With main-and-branch systems the system flow is reduced after each branch takeoff. (That's why you'll find it economical to size your mains for constant velocity in larger systems.)

    The main head loss to an individual emitter is defined by the flow and restriction in the mains up to and including the point of takeoff.

    If you are using good design software, the job is easy provided you remember the GIGO rule... If not, it does get a touch hairy.

    You will find it VERY useful to make a good-sized drawing of the main system. (Provided you've kept your supply/return lines close and parallel (VERY good idea) you need only make a drawing for one.)

    Label with lengths (and pipe size if you're using diminishing mains) before the 1st takeff and between each subsequent takeoff.

    Label with the flow rate just before EACH takeoff. If total system flow is 5 gpm, write 5 gpm just before that takeoff. Say that first emitter needs 0.5 gpm. Write 4.5 gpm just before the next takeoff. Continue until you reach the end.

    Now compute the head loss for each segment of the design using your flow rates and pipe size(s).

    Once this is done, merely add up the segments before each takeoff, DOUBLE (for the return side) and add to the rest of the values for each emitter.

    For a multiple manifold system, the process is similar to the main-and-branch. You'll likely find it easiest to make a big manifold to supply all of the sub-manifolds. If at all possible, try to balance your sub-manifolds by keeping total flow rate for the required emitters in each similar. Also do your best (via pipe/sub-manifold sizing) to keep head loss through each sub-manifold similar. e.g. if you need 3 gpm at each of two sub-manifolds and one is significantly further from the main mainfold, you should increase size of piping (and manifold) to keep head loss in both similar. While balancing the manifolds in this way is not absolutely necessary, it will help prevent one manifold from "hogging" the flow from another if a number of TRVs in one are opened wide. It will also aid when using "setback via supply curve starvation".


    The next steps depend on what type of boiler is being used. If using a Vitodens you will find the best material economy by designing for a SINGLE variable speed circulator and omitting the low-loss header! IMHO, this is the "real" Viessmann way.

    As a general rule, other condensing/modulating boilers require primary/secondary piping. Some might be suited to a single circulator driving a system with proportional flow but such must be approved by the manufacturer!

    When using primary/secondary, I believe you will be best off designing for a secondary (emission system) flow (at full design) very similar to primary (boiler) flow.

  • hydronicsmikehydronicsmike Member Posts: 855
    Greg and Barry

    What an event. Wish I could be there with you guys!! Shamrock and George T.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Proportional Flow the Vitodens Way

    The Viessmann Vitodens is a bit different from other condensing/modulating boilers. It is designed with proportional flow control (TRVs) in mind. When you've used TRVs on ALL of your emitters, you can take advantage of this design and use ONLY the built-in (in the two smaller models) or one external (the larger models) variable speed circulator that is under the control of the boiler. No low-loss header + no additional circulators = lower cost + greater simplicity + highest optimization.

    Vitodens boilers allow a wide range of acceptable flow through the heat exchanger. While wide, it is certainly not without limit.

    The 6-24 model, with a gross output of 81mbh has a maximum flow rate of 6.2 GPM--with exceptionally low head loss. With 6.6 feet of head loss, the flow rate is reduced to 4.6 gpm.

    The graph (supplied with boilers with built-in circulators or with circulator for others) that matters most is the "residual head pressure".

    The "residual head pressure" is the amount of head pressure available to the emission system after having passed through the heat exchanger.

    When selecting your design situation delta-t you are well advised to check your required system flow against the residual head pressure chart. If it appears that you have very little head to work with, increase your design flow delta-t! Do not forget that increasing the delta-t decreases your average emitter temperature! In some cases, you may need to increase your emitter size--particularly if you are trying to reduce cost by minimizing the size of the emitters! Vitodens boilers have lower max supply temperature limits than "normal" boilers. Do not forget this!


    Once you have computed your head losses for each emitter, find the highest.

    Use this head loss in the "Residual Head Pressure" chart and read the maximum allowable flow at that head loss.

    As long as your total system design flow requirement is below this level, you're good to go!

    If the Vitodens will not be able to produce your flow design requirement at your highest emitter head loss you have a number of options:

    1) Increase your design delta-t. This will lower your flow requirement at the expense of higher supply temp. RECOMPUTE head loss at the new level! Beware of the change in average emitter temperature!

    2) Decrease head loss to some emitters. If the 2nd or 3rd highest head loss for an emitter is within parameters, increase the pipe and TRV size to the high-loss emitter(s). RECOMPUTE head loss for these newly sized TRVs and pipes and verify that you are now OK.

    3) If you are using a main-and-branch or remote manifold distribution system and if you have significant head loss in the mains/manifold system, increase the size of the mains/manifold system to reduce their head loss to "very low", or even "insignificant".

    If the residual head loss table allows significantly more flow that your design system requirement:

    1) You can use a lower design delta-t. If material cost is paramount you can, perhaps, reduce emitter size. If emitters are not downsized, your design supply temp will be lowered and you'll achieve greater fuel economy. You'll also be able to lower the maximum speed of the circulator and use less electricity.

    2) You can use smaller piping in your mains (or manifolds), branches (or home runs) and perhaps TRVs (when reducing the size of TRVs make CERTAIN you are still within the acceptable flow range for that size).
  • John McArthurJohn McArthur Member Posts: 157

    Yes it is. Hope to see you at the training. Came to The Wall to learn some more cool stuff. I haven't been on here in quite a while. Always great info.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Proportional Flow Control with Other Condensing Boilers

    With other condensing or condensing/modulating boiler you will use primary/secondary piping and pumping.

    In every manufacturer example I find, a fixed flow primary circulator is used. This ensures adequate flow through the HX at all times.

    In general they seem to move LOTS of water through the HX--most likely more than you will EVER need to move through your emission system.

    This makes your job a bit easier, as you don't have to ensure that flow through the emission system is within an allowable range. The primary/secondary has "decoupled" your emission flow from the boiler flow.

    So, you need only size your emission circulator to provide your design flow requirement at the highest head loss you find to, through and from any emitter. You'll likely find that only a very small circulator is needed for the emission system. One of the newer multi-speed circulators on "low" will probably suffice for small to medium-large systems. More pumping power won't really hurt because the TRVs provide their own resistance to keep the system in balance. You will however use more electricity than necessary and the normally silent TRVs may produce velocity and/or throttling noise in certain conditions (e.g. wide open TRV and when the TRV begins throttling down to setpoint after having been turned up considerably).

  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Bypass Valves

    Differential pressure bypass valves are a MUST for systems with proportional flow control. In conditions of very low demand due to mild weather or high setback, all of the TRVs will be nearly or fully closed. This will result in "dead heading" of the emitter circulator. The TRVs have no way of communicating back to the circulator to tell it, "HEY--I don't need any heat--shut down!"

    To eliminate this problem a differential pressure bypass valve is installed between the supply and return of the emission loop. The only real placement requirement for the valve is that it MUST be installed AFTER the secondary (emitter) circulator. Set the differential pressure adjustment a touch above your design flow head loss and you're good to go. In systems with extremely low head loss, this may have to adjust by setting all of the TRVs to minimum, pressing your ear against the differential pressure bypass valve body and adjusting until you hear the hiss of bypass, the continuing until the hiss becomes more like a "shhhhh".

  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Controling a System with Constant, Proportional Flow

    With a Vitodens, the job is easy as the boiler is not intended to be controlled by a traditional thermostat. With a simple, one-circuit TRVd system your only control wiring will be for the outdoor temperature sensor.

    With other boilers that are intended to be controlled by a normal wall thermostat there are a number of different options. Most involve "tricking" the boiler a bit by providing a constant call for heat. In periods of low demand, the burner will stop firing when it exceeds the reset setpoint even if the thermostat is still calling. Once demand increases, the burner will cycle until demand increases above the lowest modulation level. While this shouldn't cause a problem with the control system of condensing/modulating boilers, I would verify through the manufacturer. Just verified via W-M and HTP that the Ultra and the Munchkin can be controlled in any of these three ways. First method uses the least materials, 2nd method allows whole-house setback, 3rd method is inferior to either of these in my mind.

    1) Instead of an indoor thermostat, install a warm-weather shutdown control with the sensing bulb outdoors. If the control has two independent contacts, wire your emitter circuit through one set and the thermostat connection through the other. If control has one set of contacts, you'll use it to control a double-pole relay. Once the outdoor temperature falls below your setpoint, the emitter circulator will run and the boiler will receive a call for heat. When outdoor temp rises above your setpoint, the emitter circulator will stop and the boiler will no longer receive a call for heat.

    2) You can mount a traditional thermostat in a central location. (You have TRVs on ALL rads.) For "normal" operation the thermostat is set higher than the desired temp(TRV setting) in the space. This ensures a continual heat call. In this case, you'll still be best off installing a warm-weather shutdown control. You will want to wire such that it both controls the emitter circulator and opens the t-stat circuit (thus overriding the wall thermostat in mild weather). Setting the thermostat down will become a "whole house" setback and the system will operate as if the TRVs do not exist. This is where your decent initial balance will help as the TRVs will be "starved" and nearly wide open.

    3) You can omit the TRV in one space and install a thermostat in that room. A warm-weather shudown control used as above is still a very good addition. This is likely the worst option as this creates a "master and slave" situation. When TRVs are installed on all emitters there are no masters and no slaves.

    With a boiler like the new Buderus wall-hung that uses an indoor reset temperature controller, follow the manufacturer recommendations. This may involve placing the sensor in a space (like an interior hall) with no emitter. It may involve placing the sensor in a space where the TRV has been left off. Either should work, but do realize that this is inherently a "master-slave" control system. If placed in a space with an emitter, slight undersizing of that emitter will ensure some headroom for the remaining TRVs.
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