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zone valves vs constant circulation

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Ken_40
Ken_40 Member Posts: 1,320
constant circulation is "superior" in any way to other methods, e.g., zone valves or circulator zoning and/or mixing valves is an apple and banana issue, not a "do you want whipped cream too"? issue.

The concept of constant circulation and how inherently efficient it is is for an extremely common system design; that being, the single zone home. ASHRAE/I=B=R discovered very high efficiencies when the circulator was on 24/7 and the 'stat worked the flame only - not the whole boiler. Other "issues" were also known like condensing, by-passes and the myraid of ways to overcome constant circulation shortcomings - in any adaptational form. The increase in fuel efficiency was substantial, on the order of 10% or more. With all the zoning and bypass contortions we have to take to achieve similar designs for MULTIPLE zones, the constant circ. simple and effective purpose and design has been become a nightmare of valves and controls at a higher expense than simply zoning and simple boiler controls, like O.Reset.

The only "sanctioned," single zone, constant circ. designs in operation have morphed into bastardized combination variations of the basic theme, but IMO are never justified with regard to the expense of added controls to achieve similar dynamics, and reduce overall expenses over long periods of time. The fuel savings disappear the more zones are added.
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Comments

  • Tim Gardner
    Tim Gardner Member Posts: 183
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    Ideal radiant setup.

    I'm trying to learn about radiant heating so that I can help advise my friend who is planning to put it into a large house. I have tried to understand various threads about constant circulation, and my understanding is that with constant circulation you can have more comfortable and more efficient heat.

    If I understand correctly, each radiant loop goes to a manifold, and better manifolds allow you to balance the loop by restricting flow in or out. You can also put one or more loops on a zone valve which will turn the loops on or off depending on the thermostat.

    But if constant circulation is good, wouldn't it be better for zone valves to adjust flow based on temperature rather then just turn the zone on or off? Do TRV's do this? If they do, are there TRV's that can be controlled electrically so that they can be scheduled and controlled remotely?

    Thanks,
    Tim
  • Dave_4
    Dave_4 Member Posts: 1,405
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    Radiant emitter type

    It also depends on the radiant emitter type - high mass vs medium mass vs low mass. A high mass radiant system (slab floor, gypcrete, etc.) benefits the most from a constant circulation/variable temperature control set-up. The feedback from the room thermostat to a control valve, even a modulating valve, on the particular high mass/slow response radiant loop is impossible to "tune" to react to room heating/cooling load changes and activate the very slow acting high mass radiant emitter. Concrete, even a thin 1.5" light slab radiant floor, with insulating materials on top of it (hardwood floors, rugs, furniture, etc.), takes over 2-3 hours to respond to a change in temperature in the radiant tubing.

    A light mass radiant system, like staple-up, or thin capillary tubes embedded in thin-set tile mortar beds, can be operated well from a room thermostat and a control valve since it has a faster response time from the radiant tube temperature change to the floor surface temperature response time.

    That being said, you will get much more uniform room temperature control with almost all types of radiant systems using a constant flow, variable temperature system control. The trick is designing the right local zone/manifold source flow temperatures from a single primary heating source. This is where you need to set up separately temperature controlled manifold feeds from primary/secondary take-offs and separately pumped and controlled zones, or even small plate HEX's for each separate temperature controlled zone, fed from the constant, or modulating, hot water primary heating source water(boiler).
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    low or high mass -- I still want more than on/off.

    For high mass, I understand that it takes a while for response. But I'm not sure why you would want to only control it with on/off.

    For low mass, you can get good response to a thermostat using on/off, but wouldn't it be even better if you could just adjust flow instead of on/off so that you could get the same control with lower temperature water?

    What I want is to be able to control rooms precisely and efficiently and interactivley to the extent that it is possible to do so. Since low temperature is inherently more efficient, I don't want to varry temperature more than I have to due to constraints of the material and weather, ie outdoor reset only.

    So it seems to me strange if the only tool at my disposal is on/off. Why can't I dynamically control flow? It seems to me that zone valves are the enemy of constant circulation and low temperatures.
  • Dave_4
    Dave_4 Member Posts: 1,405
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    Modulating valves

    What you want is what I described for a low mass emitter - a "modulating valve". Tekmar makes them, along with appropriate controls. A modulating valve in my world is one that incrementally varies the flow, and is not an "on/off" valve. A modulating valve with a high mass emitter cannot be tuned to suit the long response time of the emitter vs the short time span thermal transient loads in the room(s).
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    I need help finding them.

    Thanks for the clarification. It sounds like "modulating valves" are indeed exactly what I was asking about. I went to the Tekmar website but I could not find anything named modulating valve. Can you recall a particular product name or number to search for?

    Thanks,
    Tim
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Not from Tekmar (didn't even know they made/supplied them) but here's a source for true modulating FHVs (floor heat valves):

    Danfoss FHV

    Two types of Danfoss FHVs:

    One type modulates via floor temperature and is suitable for areas up to about 100 square feet. Very well suited for bathrooms where a minimum floor temperature can take preference over a minimum air temperature.

    The other type modulates via room air temperature and is suitable for spaces with up to about 10,000 btu/hr design heat loss. Multiple FHV'd loops in the same room are possible, but there would likely be problems with "sharing" the load--e.g. one part of the floor may do nearly all of the heating in much of the weather.

    Using good, modern construction techniques you should be able to keep heat loss in almost any reasonbly sized room below 10,000 btu/hr. If the problem is "not enough floor area) common in kitchens and "sun spaces", supplemental radiators with TRVs can be used to make up the deficit.

    If, like many new homes, there is a large group of rooms generally open to themselves in the "common space", I would suggest an accurate mixing valve operating under an independent reset curve.
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    how about maximizing circulation?

    If I understand correctly, you are saying that constant circulation only makes sense for a single zone system.

    Perhaps I don't understand. I thought the basic logic of constant circulation was that it let you run lower water temperature which is therefore more efficient. Wouldn't that apply (if true at all) to multi-zone systems too?

    If constant circulation is no good for multizone systems, isn't it still better to at least maximize circulation in each zone?

    A lot of what you say rings true, because I am scared of over-complicating things.

    THanks,
    Tim
  • Noel
    Noel Member Posts: 177
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    define zoning

    with constant circulation, please.

    Zoning infers stopping and starting flow in parts of the system.

    Constant circulation does not. It modifies the water temperature in the emmitter.

    Noel
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    definition of zoning

    I want to be able to control temperature in different floors independently. For me, zoning just means independence. I'm trying to understand which mechanism is best for which conditions.

    I would also like to keep water temperature low. If the temperature is low, I can keep the floors "hot" as close to all the time as possible, even the low mass ones. So it seemed to me that I would want constant circulation where the water temperature is related to the overall heat load, ie ODR. This seems like it would give me the best efficiency.

    But how do I control different areas independently? Using traditional zone valves I turn on and off the water to the zone. Is this the only/best way to do it?

  • Brad White_9
    Brad White_9 Member Posts: 2,440
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    To me, the best of both worlds is....

    Both!

    For best comfort, the principle of constant circulation is hard to top.

    To make it effective of course (independence as you said), the control valve comes into play by injecting hot water into the secondary (floor) loop. If this valve modulates such as a 3-way (on condensing systems) or 4-way valve (on non-condensing systems), the smoothness of control, the ability to change temperature very closely tracking the target setpoint is hard to beat.

    To illustrate the contrast, picture a direct-coupled 2-way control valve, open-closed. Your emitter temperature will have pronounced spikes as the temperature enters at say, 125 degrees and leaves at say, 110, then nothing as flow essentially stops and it cools...

    Now, with constant circulation, the water is moderating the temperature, the transfer of BTU's is at a rate more proportional to temperature, rather than that sharp drop off curve. Bleed in a fractional amount of primary hot water and you can maintain your loop temperature within a degree.
  • Noel
    Noel Member Posts: 177
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    Each panel or slab would be controlled individually.

    Either a TRV (panels) or a FHV (slabs), or a motorized mixing valve with control and sensor ($$$) could do as you ask.

    The FHV would sense either air or slab temperature (you decide which type to buy) and modulate the flow of water into the slab. You can either modulate the whole flow IN the floor, or modulate the flow INTO a constant circ loop in the floor by injection mixing. The FHV could control just the injection amount, leaving the flow rate constant in the floor.

    You could use constant circ on your biggest zone, and disconnect the boiler-controlling end switches on all of the other zones, and use outdoor reset to control the water temperature. Crude, but effective.

    Noel
  • hr
    hr Member Posts: 6,106
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    First off

    do a room by room heat load calc. Then decide on zones. And then you'll need to know the supply temperatures required at the various zones.

    On a large home with 6 or more zones, and possibly several different supply temperatures, constant circ, in the true sense gets involved and expensive.

    Outdoor reset controls with indoor feedback and an accurate design will darn near get constant circ anyways. Think tekmar!

    A very simple constant circ can be had with 3 way zone valves. In position A the flow is through the zones, in position B it allows heat from the boiler or source.

    Are old friend Dan Peel, may he rest in peace, has dozens of 3 way Taco ZV constant circ systems spread throughtout eastern Canada.

    The very best reading on constant circ can be found in the December 2003 issue of PMmag.com Siggys "Zoning with Constant Circ" article.

    If you can figure out how to get registered it is in the archives over there. Three nice drawings including several with a mod con source.

    hot rod



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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Of course you're 100% correct about beginning with a room-by-room load calculation.

    When it comes to "deciding" zones and required supply temperatures however it's easy to get into the, "Which came first? Chicken or egg?" quandry. Do I (if necessary) "adjust" the zones (via tube spacing/floor covering, etc.) to suit the available temperature(s) or do I "adjust" the zones (via arrangement) to suit the available temperature(s)?

    Disclaimer: I have never used FHVs. Nor have I heard from anyone who has used them.

    If true constant circulation is the desire, I believe that FHVs can be used quite effectively without incurring extreme expense. Why?

    It's that "extra" adjustment that FHVs have over TRVs. Some (certainly Danfoss) have an accurate, metering-type "flowsetter" built in.

    Not only can this adjustment be used to compensate for different lengths of tubes served by a single FHV, I see no reason why it cannot (within reason) be used to adjust for different btu/sq.ft. load requirements. If loss per sq.ft. of heated area is relatively high reduce head and increase flow; if loss per sq.ft. of heated area is relatively low increase head and reduce flow.

    I won't claim this could compensate for gross difference in supply temperature requirement but as long as the heating contractor is actually involved with the design from step one, I can see two temperatures of true constant circulation serving most any home.

  • Tim Gardner
    Tim Gardner Member Posts: 183
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    great article

    That's a great article. Thanks for pointing it out to me.

    This article suggest three-way mixing valves with a conventional boiler where the loops are short circuited back into themselves when there is no call for heat, as you suggested in your post, with 3-way mixing valves for each zone.

    It alternativley suggests a condensing boiler with constant circ through a big supply circle. Each emitter loop is independent and has its own circulator which runs all the time. There are "bridges" between the supply circle and each emitter loop in which the flow is controlled by an additional circulator or a zone valve. In this case there is constant circulation in the supply loop and independently constant circ in each of the emitter loops, but heated water passes through the bridges only on a call for heat.

    Both of these ideas are direct answers to my question. I was actually thinking in terms of regulating flow volume, whereas these are ways of leaving flow constant and adjusting the temperature of the circulating water. So it looks like I need to adjust my thinking.

    Thanks again.
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    control of FHV's

    Unless I am not understanding, it looks to me like the Danfoss FHV's are entirely manual. If a user turns it from setting 3 to setting 5, there is no way to turn it back down other than going into that room and physically turning it back to 3.

    If this is the case, this limits its usefulness in terms of whole-house control and overall efficiency.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Global vs. Local

    The setback for the system is global at the boiler. At pre-programmed times of the day, the outdoor reset curve will shift to reduce the temperature at night.

    I agree with Mike that TRV's and FHV's are theoretically the way to go with a constant circulation modulating condensing boiler. It's a very simple and synergistic way to control the temperature. FHV's are not inexpensive, and they tend to "decentralize" the manifolds. The piping is just not as simple as with TRV's where there is often a single circuit in a room. I investigated The Oventrop Uniboxes a while back, but FHV's are limited by the maximum allowable flow rate and the price is still higher than thermostats and zone valves.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Yes, FHVs require human adjustment for a gross change of operation. In a nicely constructed new home using radiant floors there's essentially no need for adjustment of a truly proportional system save for areas that are typically unoccupied. Constant changes when humans "play" with their toys will only introduce inefficiency.

    You are confusing "usefulness" with overall efficiency!
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    sounds great -- how do you do it?

    It seems to me that what I want is to regulate the amount of hot water into the loop based on room temperature, not just circulate the cooling water. What am I missing?

    When there is a call for heat in the zone, the 3-way or 4-way valves start adding hot water. This brings the room to temperature and then the call ends and the mixing valve prevents fresh hot water from entering. The room begins to cool down while the water circulates and cools down with it.

    But how is that different than just turning off the zone? Does the circulating water cool off faster because it is cirulating, allowing you to set a tighter delta?

  • ALH_4
    ALH_4 Member Posts: 1,790
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    Indoor and Outdoor Reset

    With the proportional valves you get indoor reset as well as outdoor reset. The indoor temperature controls the flow, while the outdoor temperature controls the fluid temp. With a single reset control and no flow control, you cannot account for solar gains, for example.
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    accomodation for efficiency's sake

    I have only one zone in my house (steam heat, no trv's). My programmable thermostat has an up arrow that my family uses when they are cold. The setting reverts to normal either when the temperature is reached or when the next time period arrives, whichever is first.

    If they raise the temperature and press "Hold Temperature", the setting does NOT revert. A manual TRV or FHV, you'd have to remember to set it back everytime you wanted short term adjustment. It would be like having an automatic "Hold Temperature" for every adjustment.

    An ideal system needs a way to revert temporary settings automatically or else they become permanent by default.

    For true effiency you have to accomodate humans or else they will beat the system every time.
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    sounds interesting

    That sounds like a promising idea. Please elaborate. What type of valves? Where do you put them: each zone? Main loop?

    Thanks,
    TIm
  • Brad White
    Brad White Member Posts: 2,398
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    Think of this

    Water has mass and when moving the mass flow is constant. Call it a thermal flywheel which can be adjusted in temperatures incrementally with the right valve and controls.

    Take the same mass of floor and tubing (however thin or massless or massive it may be, but think thin for our discussion). Flood the tubing with warm water and get it up to temperature. Then stop the flow. The temperature will decay fairly rapidly, more quickly at first because the water and air temperatures are further apart. As the temperatures become closer together, the rate of loss invariably decreases. Over time you will see peaks and valleys of temperature above and below setpoint if you were to graph this.

    That first drops after withholding flow are steepest as you can see.

    Now go back to continuously circulating water but bleed in minute amounts of water, pulsing in enough hotter primary water proportional to the rate of loss. Do this and the temperature will be much more consistent.

    Each zone or manifold for splitting has a circulator and the mixing valve by definition is on the inlet side of that circlulator. Does that help? Not meant to be complete but I do not want to get too vertical here!
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Unknown
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    What you describe is overshoot and undershoot

    The valves seek and find equilibrium.

    They MATCH the load exactly, with no delay, and no overshoot or undershoot.

    They adjust before you become uncomfortable, seamlessly.

    Think of it like this; you set the thermostat at 70.1°, and they throttle back to maintain 70.0, and never satisfy the thermostat that runs the circulator, until the weather satisfies it.

    No bang-bang.

    You also need to decide if you want a very fuel efficient system, or if you need the fuel guzzling options of having every thermostat connected to that (V-8 engine with a 4 speed) boiler for speed and convenience.

    Noel
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    very helpful

    That is great, and I think I am starting to see the light. I'm going to summarize/respond to the original thread below in hopes of eliciting further response without fear of falling.
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    summarizing and more questions

    If I understand correctly, Brad White proposes keeping the water flowing in the zone so that the water and the zone cool at the same rate. Then by adding hotter water to the zone's loop using the right controls, you can tune the temperature with high precision.

    I think this is close to or similar to the system proposed by John Siegenthaler in the Article mentioned by Hot Rod above.

    I still have a couple questions: This could work if each zone has its own mixing valve and its own circulator. Is there any way to do this with fewer circulators?

    Also, if the mixing valve is wired directly to the call for heat, does too much hot water get put into the loop before the thermostat is satisfied? If so, is there a good way to control the mixing valve so that it shoots in just enough hot?

    Thanks,
    Tim
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Proportional Flow

    TRV's and FHV's control the flow proportional to the room temperature. At first, it seems like you give up control by using non-electric, non-programmable controls, but they work so well that you set them and forget them.
  • Brad White
    Brad White Member Posts: 2,398
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    The way to use fewer circulators

    is to have each emitter set use exactly the same temperature of water under all conditions. If you can do that, theoretically you can use one circulator for all common temperature users.

    In practice, however, sunlight (insolation), internal gains and mass differences will thwart your best efforts. Yes, what I am describing is "Circulator Intensive".

    As for the rate of hot water flow into the secondary (loop) zones, you need what is known as PID control, Proportional Integral and Derivative. Essentially your sensing means and control "self educate", learning that reacting too quickly to a perceived drop in floor temperature results in too rapid a rise and overshoot. Next time this happens, it learns to not move so rapidly.

    I use the analogy of a young child learing to ride a bicycle. Start wobbly, over-compensate, lean left then right, gain balance, then ride smoothly. The adjustments become very fine, precise and it looks easy!
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • hr
    hr Member Posts: 6,106
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    We told you it would get

    component extensive to accomplish this ;)

    Roth and a few other manufactures build multi zone constant flow, mixing devices. I'm not sure they sold all that well?

    I'm not a fan of excessive circs. Really most residential should be "hydronic-able0 with one or two circs. The new smart pumps (delta P)will make this even more do-able.

    If you can get over the look, and design within their flow rates, that Oventrop Uni-box TRV is a sweet setup. With a mod con on reset you would have a really comfortable, constant circ system. A smart circ would do away with energy shedding PAB valves.

    hot rod

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  • Tim Gardner
    Tim Gardner Member Posts: 183
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    which smart circs do you like?

    I'm leaning toward a simpler system in terms of components, though I did find the discussion helpful in understanding the concepts.

    I like the idea of a smart (variable) circ. Have you installed any? Are you able to recommend any particular models?
  • hr
    hr Member Posts: 6,106
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    Wilo

    may be the first to market a small smart circ over here. I believe the Stratos is due in June or July? Steve?

    Rumor has it Grundfos may skip the expensive and time consuming UL listing to bring the Alpha to this market?

    I recently compared a 15-58 to a Alpha Pro. 60W vs 17W and compared the heat output difference.

    You will need to attend the Foothills Conference this month to "see" the results.

    http://www.hydronicsalberta.com/foothills/index.htm

    hot rod

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  • scott markle_2
    scott markle_2 Member Posts: 611
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    Tn4

    Have you looked at tekmar Tn4. This control will create essentially constant circulation in the zone of highest demand. Other zones will pulse width modulate (calculated on/off cycle) based on room sensor feedback. A one temp system can effectively serve multiple radiant zones if their actual temperature requirements are within 20-30 deg. (check this with tekmar)This means that different floor surfaces or rooms with differing heat loss can often operate very stably without the need for additional mixing. Additionally the on/off cycling for multiple zones is sycronized so as to maximize the amount time that circulation is occurring.
  • Chris_82
    Chris_82 Member Posts: 321
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    We need to be a little more careful about "efficiency" with constant circulation, it is expensive. No one ever said it was going to cost less! From Taco using their old (5 years old) numbers it costs about $23.00 per each 007 and many articles in the 1980 issues of P& M confirm this, in additional electric costs and at a minimum most constant circulation systems have three circulators running mostly full time. In addition to the additional costs for fuel! They are not economical. They are purely a comfort and noise issue product. Remember all of the original Wirsbro handbooks and fliers? "Sell Comfort" not price?
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    breakeven point

    As I understand it, using lower temperature water to produce the same amount of heat takes less fuel for the boiler. Additionally, providing more even heat decreases the likelihood of the user raising the thermostat.

    But running the circulator more obviously takes more electricity.

    Are you saying that the circulator eats up more than can be saved by the lower temperature and evener heat?
  • Tim Gardner
    Tim Gardner Member Posts: 183
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    ideal

    I looked at their literature and spoke with their rep today. If the product works as claimed, this looks like a good solution. Thanks for pointing it out.
  • scott markle_2
    scott markle_2 Member Posts: 611
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    efficiency

    I worked recently for a very established radiant only contractor who uses tn4 almost exclusively. The tolerance of this system is great enough to allow quik trac wood floors and in slab pipe on the same (temp.) buss. My only compliant with tn4 is that the room sensors are a bit cumbersome for the average person, although rumor has it that a 2 button version is in the works. however in reality radiant heat lends it self to a set it (68) and forget it approach.

    If you are really motivated to create something highly efficient and you are planing on using a mod-con boiler. Then I would consider some effort be made to ensure that a high delta t on the boiler is maintained throughout a range of operating conditions. One problem with primary secondary piping (as I see it) is that changing flow rates on the heating side of the hydraulic decoupler complicate balancing of primary secondary flows. If flow on the boiler side exceeds secondary flow then the desirable high delta across the boiler will not be maintained.

    While I have some ideas on how this could be accomplished I have as of yet not seen any real world setups that address this issue. So far most of my work has been with smaller essentially single zone or trv rad. situations were direct connection to boiler was viable.
  • rb_6
    rb_6 Member Posts: 222
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    Nirvana

    It helps to understand that zone control valves do not open to heat they close to prevent over heating.

    Why you may ask?

    Well, the heat loss calculation will be used to establish flow.

    Flow is achieved by creating a sufficient change in pressure to overcome the system resistances. This is the job of the circulator (aka pump).

    The resistances in the installation are to be verified on site by balancing to a worse case scenario – i.e. all valves open and each zone getting its design flow. (see footnote 1.)

    To prevent disturbances in the hydraulics which happens when valves close, the fluid temperature is adjusted down when the outdoor temperature is warm and up when its cold. This keeps the valves open.

    This is what “constant flow- variable temperature” is all about. It’s about keeping the hydraulics constant and using temperature to regulate heat transfer.

    Except in the real world the sun comes up, the kids come home with their soccer team, someone turns on the oven etc. As the temperature rises the valves are told to close to prevent overheating…this closing causes a pressure disturbance.

    If the valves are a true modulating type the disturbance is spread over the actuators closing time (measured in minutes) and stroke distance (measured in a few millimeters). On/off valves, typically close in 30 to 45 seconds. Think about drinking through a straw - slowing the flow by squeezing (modulate) or clasping quickly (on/off). On/off or modulating? Can you imagine it?

    So what to do with the disturbance created by a valve closing? It’s not a problem because the smart contractor will have installed some method to regulate the disturbance. A pressure activated bypass valve is a common solution. The bypass valve is the air traffic controller of hot water heating systems. When flow is not required, it puts it in a holding pattern until it is needed.

    Now, the folks who make control valves are in the match making business. They look at the personality of the heating system and the way they’re regulated. They have a menu of valve types to best suit the system. Of the three more common types, the linear valve works best with many weather compensated systems where 50% flow transfer 50% of the systems output. That is why 99% of all TRV have linear characteristics - they were designed for European panel radiators controlled by weather compensation (aka indoor/outdoor control, aka reset control).

    If the radiant system can be designed around a single low temperature by manipulating spacing or surface area, then what you have is the makings of a perfect system.

    A single low amp circulator, condensing boiler controlled by a weather compensator, fully modulating TRV’s and a spring loaded bypass. It’s a bullet proof system and as close to nirvana as one can get.

    It is a bit deeper than this but hopefully you get the essence of it.

    _________

    1. Almost all properly designed systems have flow rate calculations done, but very few have the design loop completed by verifying the flow in the field. So why do the calculation?
  • What

    And just think not too long ago almost everyone, even some big name people said "the more pumps the merrier". Wow nothing like doing a 180 degree turn on design. The next new thing will be one adequate sized circ. and balance valves. What a novel idea.
  • scott markle_2
    scott markle_2 Member Posts: 611
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    re:nirvana

    Very helpful, my problem with differential bypass and primary secondary is the loss of highest possible delt T. Ideally as you said trv's are open all time because of full outdoor reset. However when they close some of the flow bypasses the emitters diluting (warming) the flow and lowering the potential condensation (latent heat) "bonus". I know it's a bit obsessive but I really love the idea of plus 100% efficiency (net caloric value).

    My nirvana is a direct conect mod-con, Constant circ. full outdoor reset, Generously sized radiation with mico- zoning (ie. trv,s) with some core rads. left wild ( ie. no trv's) to prevent dead heading. no bypass valve. and a variable speed circulator that adjusts to maintain head in response to changing system conditions (trv's) Also it seems that the trv's and reset curve should be adjusted in such a way that at target room temp. there is some flow restriction from the trv's. This way if a upward user adjustment is made there is some capacity for this without readjustment of the curve. This would also make the rads. more responsive to recovery from restricted flow states. Slow recovery from setback is a real problem for systems were the curve is set so close to actual load. This is part of the allure of the Tn4 system for those high end people for whom comfort is everything.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Nirvana Realized

    Very nice Mr. Bean!

    Very nicely stated Mr. Bean, but you left out one thing: System load.

    To put the "bit deeper" in the simplest terms I can muster while assuming a modulating source of heat:

    A true constant circulation system begins with, "constant flow--variable temperature" just as you said.

    In a reasonably perfect world such "constant flow" could also occur at a constant rate with the "variable temperature" controlled by nothing more than the outside temperature because outside temperature gives a reasonable estimate of system load.

    The KEY is actually maintaining that "variable temperature" to all of the emitters regardless of the conditions experienced by the emitters.

    Excepting a warehouse: Because this IS the "real world" there is NO WAY you will maintain that "variable temperature" without also varying flow.

    The flow--somewhere--must be varied to maintain the "variable temperature" to just meet the requirements of the emitters with the emitters themselves varying their average temperature to meet the load.

    Mark my words. Within two years we will have mod-cons or aftermarket controllers other than the Vitodens that can control a variable-speed primary (or ONLY) circulator!

  • rb_6
    rb_6 Member Posts: 222
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    Good stuff

    I hear you TAW. We did a webinar on this a few months back. You’ll appreciate the message. Valves vs Circulators

    As the cliché goes …there really isn’t anything (well almost anything) new under the sun. Remember when wall hung boilers back in the 80’s were chastised by those who swore boilers were supposed to be big, heavy and floor mounted. I suppose today’s burner and casting technology and application awareness makes the timing better this go around.

    Yes Scott and Mike, the direct connect mod con with big delta t’s = big gains all around.

    In addition to both your valid words, big delta’s = lower flows = smaller pipes = smaller fittings, hangers, insulation = less solder, flux and labor plus with smaller pipes = smaller volumes = less cost for chemicals and glycol ( if req) and since there is less volume the expansion tank can be smaller and oh yes, smaller pipes and lower temperature means less transmission losses…throw in the long term trend to reduce building loads to zero and you can see how briefcase sized mod con micro plants could evolve over the next few decades...and yes having restriction in the TRV is a key element of control. Contrary to common practice bigger is not better when it comes to valve Cv.

    It’s cool to see people focusing on lowering return temperatures in heating which can be done as Mike pointed out by adjusting flow (slowing it down)see the infuence on heat transfer at low velocity in the slide presentation. Many of these concepts we find in district energy applications where sub stations are not permitted to return warm fluid back to the plant. Some systems actually penalize users if this happens.

    If you haven’t already found this more recent TA material…it’s a nice addition to ones library. Be sure to download the rest of their stuff.
    Why and when variable flow HVAC systems require differential pressure control.
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