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Constant circ for dummies
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steven E.HEAT
Member Posts: 47
can you please direct me to a good and resonably priced thermal imaging camera. thank you
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Please explain constant circ w/ rads and TRVs
I'm a newbie at this and able and willing to learn. Is it explained as simply as, when the boiler's on, the circulator is moving HW through the loops that feed radiators?
Or is it more complex. Please explain.
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Depends on the system...
With a single temp zone it's a very simple solution. With multiple temp circuits and or zoning, the required the amount of complexity, controls, pumps and costs jumps greatly.0 -
Theory
I guess I would explain it as this:
Sizing the radiators(or radiant, or what ever) and piping to use the smallest possible circulator(to save electricity) and using the lowest possible water temps(to save on fuel) to heat a given space while circulating this warm water to match the btu loss.
I think that was a run-on sentence.
Massachusetts
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Constant Circulation
In it's simplest form and as I have applied it, it goes like this:
1) The circulator runs all the time when there is a call or potential for heat. This can be either by a manual switch or by an outdoor thermostat set to turn on the pump when it is below say 65 degrees.
2) The space thermostat fires the boiler to satisfy the space.
The principle is that the constant flow of water tends to even out and homogenize the space temperatures as opposed to cycling the circulator which will give perceptable warm and cold periods depending on your system mass.
You may find that even if you do not have outdoor reset, you get this by default because in colder weather the boiler will fire longer giving hotter water temperatures. Less firing in mild weather means cooler water temperatures. This is not to say this is perfect but is a side benefit. Proper OD reset is still preferable but if you are looking to cut corners (no, not you, Gene!
, it becomes an option.
As a further refinement I would add:
3) Put TRV's on all radiators to even out space temperatures and give individual control and to compensate for insolation (sun gains).
That's it!"If you do not know the answer, say, "I do not know the answer", and you will be correct!"
-Ernie White, my Dad0 -
It has to be designed into the system from the git go
It's not just a simple matter of having to wire a pump to run all the time. It is a European design which incorporates some non electric technology with some elctrocic technology, and throw in some hydraulic control devices (either pressure activated bypass or variable speed circulators) and you have a "system".
There should be a primary room master control sensor watching worse case scenario (north facing room with lots of glass) to dictate supply temprature and flow requirements (this zone is called a reference zone and is the only truly "free flowing" zone.)
All other zones are limited by a non electric thermostatic operator, either at the point of use (panel rads) or with a remote cap tube controller (large radiant surfaces).
A basic outdoor reset program is established, and either over ridden or under ridden depending upon what the indoor feed back loop is sending back to the master PID controller. Not all control logics are conducive to constant circ operation. Some people view it as being an easier, cheaper way to control their system, versus zone valve operation, and this is not the intent of this design. THe primary intent of this design is to provide the highest degree of comfort for the smallest amount of parasitic power consumption, all while maintaining the lowest temperature of operation possible.
Two possible logics that are compatible with this theory of opearation are the two big German boiler manufacturers, Buderus and Viessmann. Tekmar controls can be applied by knowledgeable persons, and I'm certain there are others out there that can be set up to work as well.
One noteable disadvantage to this system is that due to its simplicity, it is not as conducive to the new internet based control systems whereby a person can turn individual rooms up and down from a remote location. Generally speaking, you have the opportunity to shift the reset curve backwards by a predetermiend amount (say 20 degrees F) which would cause all TRVs to go wide open and undershoot most of their targets. A few days before you are ready to re-occupy the space, you'd send a signal to the logic pushing its reset curve back to an occuped postion.
Continuous circ with ODR is not for everyone, just those that understand its operation and are willing to live with the nuances associated with its operation (slow reset response, lack of electronic interface etc..)
Unless you are using a WILO Stratos pump, you will need to incorporate some means of dead headed pump protection (read pressure activated bypass) because other than the master controller, there is no communication between the zones and the pump.
It does require a completely different mind set than the BANG BANG North American control logic, but it ends up usings less energy and provides a higher degree of internal comfort.
As a side note, I once set up the reference zone with the PAB to avoid total free flowing that zone and shorting the other zones. It worked quite well. When all the other sub zones were calling, little to no flow went through the reference zone, thereby causing the logic to increase the supply temperature, therby shutting down the other sub zones quicker, and when they were satisfied, bypass started occuring in the reference zone and it too became satisfied. I'd originally though possibly the reference zone might cool down too much, but have had zero complaints form the persnickity occupants in 6 years of operation.
ME0 -
Great minds think alike...
Some just type faster than others and have less to say...:-)
ME0 -
Yea..
I like my simple version better. Boy you guys love to type.
Massachusetts
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four-way mixing valve
I have a hot water system with four (pumped) zones, one of which has 11 ci radiators on it (the original house, circa 1865). The other 3 zones, serving additions, have 2 ci rads each. I'm thinking about putting a four-way mixing valve on the big zone and running the circulator (Grundfos 20-42) 24/7. I would regulate the mixing valve with the thermostat that currently controls the circulator. My primary heat source is a wood-fired boiler, with a separate gas boiler for backup.
Is it that simple, or are there other considerations I need to take into account?
I'd like to do this for a couple of reasons. First, I've heard that it's a better way to heat a house, with less wear & tear on the circulator. Plus, I'd like to use the large amount of water in the system for heat storage from the wood-fired (gasification) boiler.
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U luv 2 type 2, Ted
What Do U mean? "If you do not know the answer, say, "I do not know the answer", and you will be correct!"
-Ernie White, my Dad0 -
Nothing bad
I just think we should keep it simple. Don't get me wrong, you guys did a great job describing it. But sometimes you can talk to people about this stuff and we tend to go on and on and meanwhile the customer lost you at pumping away.
Massachusetts
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well Ted...
It has been my experience that when people keep explanations "simple" important details get left out and people start doing stupid things, like dead heading pumps, or not having reference zones and they waste things like enegy, time and equipment.
If we didn't think it was worth saying, we wouldn't have said it. I know I don't need any typing practice.
You also have the option of not reading it...
Nuff said.
ME0 -
I liked all of them Ted, including yours, but
I too agree, that you must tailor explanations to suit the audience. I know in about 30 seconds if I am talking over their heads or they are hungry for deeper expanations. When we are trying to sell a job, we will start out with the simpler version, ramp it up for the inquistive and recap the closing with the simplified version just to make sure they "got it." If the people are not as interested or can't grasp the basic ideas, I keep it REAL simple. Thanks for the descriptions guys. Mad Dog
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Constant Circulation with TRVs
Is it explained as simply as, when the boiler's on, the circulator is moving HW through the loops that feed radiators?
Not quite, but if you say, "When the structure requires heat, the circulator is moving heated water", you're awfully close. Unfortunately though, the reality isn't quite that simple.
I'm referring ONLY to systems using TRVs or FHVs in the following.
This may seem elementary, but first you have to understand how TRVs operate and what they can and cannot do.
TRVs are two-way valves. Fluid goes in one side and it comes back out at any rate between "full on" and "full off"--in other words a TRV is very simply a proportional flow valve. It's the job of the TRV actuator to regulate the flow. The actuator senses temperature (usually room air) and has an adjustable scale--it regulates the degree of opening of the TRV valve body in an attempt to keep the actual room temperature just equal to the room temperature setting.
If the sensor detects that the actual temp is dropping below the setting, the valve is opened somewhat; if it detects a rise above setpoint, the valve is closed somewhat. Note that it typically does not fully open or close the valve--it moves the setting proportionately to the deviation between actual and desired. It generally takes a difference of about plus/minus 4F to cause the valve to fully close/open.
So, the first rule regarding TRVs is that they regulate flow. They are ONLY capable of regulating flow. While it may look and feel like they are regulating temperature, they can ONLY do this by regulating flow.
The second rule of TRVs is that since they are only capable of regulating flow and strive to maintain the desired setting by regulating this flow proportionally, that they "want" to be supplied with constantly circulating water heated at least enough to meet the load any time that heat is required in the structure. Stop the flow of water or let it fall too low and the setting they're trying so hard to maintain can no longer be achieved. Room temp will fall and when sufficient heat is again available the operator will be forced to open the valve quite wide. Without this constant circulation of adequately heated water, TRVs and their operators begin to resemble typical on-off thermostats and have less and less of their inherently proportional nature.
The third rule of TRVs is that they are fully self-contained. They can neither receive information from nor provide information to the boiler. They cannot tell the boiler "I'm satisfied", nor can they directly tell the boiler "I need heat" or "I don't need heat". ALL they can do is regulate flow--again from "full on" through "full off". This distinguishes TRVs from more common forms of "zoning" like via on-off valves or on-off circulators which do provide a control link between the boiler and the zone controller (typically a wall thermostat).
Many would stop here and say, "That's all the rules--or at least everything you need to know." But it's not ALL of the story..
While TRVs cannot directly provide control between the boiler and the emitters, they CAN provide indirect control--not only to the boiler but to each other! How do they do this? By regulating flow through the system as a whole as well as regulating how each TRV shares the available flow. HOWEVER, this method only works well when true constant circulation of heated water is used! ANY time that the structure as a whole requires heat, the circulator must be running with each* emitter controlled by a TRV. (*Carefully engineered radiant panels like bathroom floors are a notable exception provided that the supply temperature uses reset.)
So, TRVs provide proportional flow control, want constant circulation of adequately heated water, provide no direct control link to the boiler yet provide an indirect link to the load on the system.
Now, you ask, "How do I use constant circulation with TRVs?"
MAJOR RULE!!! No matter what you do it is possible for all or nearly all of the TRVs to be fully closed while circulation is being provided. Resistance to flow (head) will rise greatly and the circulator won't be happy--at best its life will be shortened considerably. The solution is a simple device called a "differential pressure bypass valve". This is a three-way diverting-type installed after the circulator driving the TRVd circuit. The diversion path bypasses the emitters and heads straight back to the circulator. In normal operation there is no diversion. If flow drops enough and head rises enough it begins to open and shortcut the water back to the circulator--just like the TRVs themselves, this is a proportional flow operation. It will divert whatever amount of flow it requires to maintain the maximum head loss (differential pressure) setting. If you've used a TRV on every emitter in a circuit, it MUST have a differential pressure bypass valve!
How do you control the boiler and the circulator?
One fairly common method was mentioned by Mark Etherton. You leave one emitter without a TRV (preferrably in a space that's relatively under-radiated--often high-loss, lots of windows--etc. and not subject to much solar gain), install a wall thermostat in that room and connect that thermostat to the boiler or boiler controller. Personally, I find this method a bit crude. It doesn't allow for true constant circulation of heated water--e.g. whenever that room is satisfied the burner and/or circulator stop. It eliminates (or at least reduces) the "indirect" control of the TRVs--both to the boiler and among themselves and consequentially makes it difficult to achieve room temperature setback via "supply temperature starvation".
If a wall thermostat must be used, my preference is to use TRVs on ALL emitters and mount the thermostat in an internal hallway that does not have a radiator. If no such space is available, try for a relatively under-radiated room but with a TRV on its emitter. For "normal" operation, such thermostat will be set somewhat higher than the desired room temperature--this provides the constant call for heat that ensures constant circulation of heated water.
If no wall thermostat is used (and your boiler isn't a Vitodens), how do you tell the boiler to heat? Simple. Jump the T-T (thermostat) connections! Why isn't this required with a Vitodens? Because there are no thermostat connections! Hmmm???? And YES, you CAN do this with any boiler including mod-cons! You MUST however COMPLETELY disable any "boost" function--the LAST thing you want to happen is for the boiler to keep raising the supply temperature as the heat call continues!!! You are making a continuous heat call any time the structure requires heat--that's your objective!
In the three cases above, "control radiator", "master thermostat" or "jumped T-T", you are well advised to set up a system a warm-weather shutdown. Just use a simple remote-reading setpoint controller with the sensor OUTDOORS and use its dry contacts to shut down the entire system when the outside temperature rises above some point. Believe it or not, 55F is frequently a good choice for the shutdown point. In the "control radiator" or "master thermostat" scheme, simply put it in series with the wall thermostat. In the "T-T jumped" scheme it merely replaces the thermostat.
So how do you control the temperature of this heated water that's constantly circulating anytime the structure requires heat?
With a conventional boiler you can technically use only the aquastat. This is NOT however advisable. Why? Because flow through the system will vary in direct proportion to the load on the system. In mild weather, system flow will be VERY low. The TRVs will be barely open--velocity through their orifice will be high and you'll get excessive wear and noises.
MUCH better to use outdoor reset. As the outside temperature rises, the supply temperature setpoint drops. With a conventional boiler you MUST however be aware of the potential for damage via condensation of the flue gasses. While old original gravity systems are essentially immune, more modern systems are not! An ESBE type TV thermostatic bypass valve [likely] offers the best (inexpensive, simple, reliable) protection.
If gas is your fuel, hopefully you've used a condensing (the lower the temps the better) and modulating (vary the fire to the load) boiler!!!! This is where a fully TRVd, constantly circulating system will truly shine. It's also where you get to take best advantage of the indirect control ability of TRVs!!!!
With a mod-con, I'd really suggest TRVs on all emitters. Connect your warm-weather shutdown control to the T-T connections--perhaps in series with your "master" thermostat. The warm-weather shutdown control should also disable the secondary (emitter) circulator if primary/secondary piping is used. If you're using a Vitodens by Viessmann there are no T-T connections; if a GB by Buderus, use the RC-10 controller as this is the type of system it's designed for...
With a mod-con driving a fully TRVd system there is a term you should know. "Heat Authority"
"Heat Authority" has EVERYTHING to do with TRVs as it describes the amount of constantly circulating heat available. "Heat Authority" is also HIGHLY related to both efficiency and adjustability.
A heat authority value of 1.0 means that the temperature--thus energy--is exactly equal to the temperature required for all of the TRVs to maintain their setpoint. Higher than 1 means that the temp is higher than needed; less than 1 means that it's less than needed.
With a mod-con, a heat authority of 1.0 is by definition the most efficient possible. However it would make it impossible to raise space temperature--after all, it's JUST adequate! In reality, heat authority is typically above 1.0. How much higher is a matter of preference and lifestyle. To get greater heat authority with a mod-con all you need do is increase the reset curve. You'll be able to raise space temperature faster the greater the heat authority, but there will be some "hit" with regards to efficiency. Provided the occupant has access to the curve, he or she is in COMPLETE control of both efficiency and "speed" and able to balance the two for their lifestyle.0 -
Ted Gregoriadis
See what Mike T. typed then tell ME and yours truly that we are wordy...
Thanks Mike for the validation!
Cheers-
Brad"If you do not know the answer, say, "I do not know the answer", and you will be correct!"
-Ernie White, my Dad0 -
OK ME
Let's be clear. The person asking the original question is a builder/homeowner/not an installer. I happen to think a simple explanation is required in this case. My Opinion. That's all.
Like I said, you guys did a great job explaining.
Massachusetts
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Yea..
I don't have the time to read Mike T's post. But I'm sure it's right too.
Massachusetts
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Agreed...
but who else do you think was reading this???
As a practical instructor, it is important to cover ALL the bases, not just the easy ones.
We in the trade understand all this stuff, and we get bored with explanations over and over adnausem.
But the people who are NOT in this tade come here to learn, and people like myself, yourself and Brad come here to teach.
Heck, most of the people in the trade do not understand the concept of continuous circ with outdoor reset... They're all from the school of Bang Bang U :-)
Bottom line Ted, were all just here trying to help out our fellow man, be he a practised tradesman or homeowner.
When I am face to face with a homeowner, if I see his eyes starting to glaze over, I shut up and ask him if he has any unanswered questions. With the internet, if they ask, I have to assume they want to know as much as I can divulge, not just the highlights...
Peace...
ME0 -
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ME
we have acouple of systems out there that do not have a reference zone. We used a Tekmar to run the circ., give the boiler a curve and WWSD. All panels have TRV's and we ( on one ) installed a PBV and the other has a free flowing by-pass on the pex manifold.
It seems to work fine. I like these systems so maybe you can give me some insite where this is not correct ? Do I HAVE to have a refernce zone ?
I have found that on some systems, its TOUGH to get that room sensor to be in just the right spot and have the customer LEAVE IT ALONE.
I am leaving for ISH in a few hours so I might not read your answer. Maybe answer here for the guys and to me e-mail.
Scott
PS: Could you keep it shorter that Mikes answer, I got a small bladder.
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This thread made my day!
Considering the fact that I'm flying blind with my Energy Mgt. System in one of my buildings, and hiccuped dramatically when I arrived here at 6 am, just because my workstation computer is down, and I cannot see what it is doing! Mike and Mark's explainations were both good. Being a former Grand Master of bang-bang theory and applications, and not having worked much with this new fangled technology, I printed this entire thread, as well as saving some other selected ones, and am saving it for when the time comes to install a baseboard/mod-con system in my wife's house. It's been many years since I've done a new hydronic system of any type, and am looking forward to the project (I must be nuts...it's a 100 yr. old house!)I hope that when it is done, my Dead Man Dad will be proud! Thanks to all!0 -
Learning
Thanks Mark.
To me learning is everything. The day I become "unteachable" is the day I'm in trouble.
Steve Minnich
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But how much do I save with CC?
Thank you all for the great explanation of constant circulation -- from a somewhat educated homeowner.
I am planning a new hot water system in a house near Chicago that I am renovating. I guess the system I have in mind will be "bang-bang" -- zone valves and thermostats all over the place. It appeals to me to be able to progam the time and temperature of each zone -- which will in some cases be individual rooms. Of course the water temp will be controlled by ODR.
I want warm radiators in the morning in the kitchen when I get up and in the family room in the evening, without every room in the house necessarily following this same heating pattern.
My question: is CC a money-saver and if so how much? Or is it just someone's idea of what's more comfortable? And with my "bang-bang", couldn't I just set all the thermostats to "hold 68" and run the circulator all the time to achieve the same result? I don't see this as a savings -- doesn't turning down the temp at night and/or in certain rooms always save money? And I don't see how I can do that with CC, without running around turning TRV's up and down. And if I want my kid's room 70 all day and night I can't achieve that will CC either can I? (unless I set the whole house warmer than I want and turn all the TRV's down).
Maybe it's my experience with living with and loving steam heat, but I guess I want to walk in a room and feel the warm radiator (or not), and I could see being disappointed with CC.0 -
I'll second that
Am wondering a similar thing and would be interested to hear ideas about the "reference zone." For example, consider a tekmar driving two circulator zones. For the tekmar (260), it looks like once you add the indoor sensor, it uses that as the point of comparison to the control's "occ" or "unocc" temp setting and adjusts the water temp accordingly. That should ideally achieve constant circulation in that zone. However, if the sensor is in the colder of the two zones, and the colder zone is appreciably colder than the other, then the stat or other form of control in the non-sensed zone will bounce off setpoint and you're back to intermittent circulation. Not the case with TRVs I assume. I may not be thinking this through?0 -
I would look at it more
as a comfort-enhancer rather than a money saver. Your pumping cost will go up of course so it pays to use the smallest circulator or lowest speed which will do the job.
If you were to graph temperature over time, the sine waves would be shallower and longer than with bang-bang control where you may have shorter wave peaks and deeper crests and valleys. All of this depends on your system mass and other variables of course.0 -
agree
I agree with Brad, and can add a real-world data point. Since going with near-constant circulation (reset schedule and subsequent coasting interrupts, as does solar gain), I have been amazed at how much more comfortable the house is. Set the temp for 68F, and the house just [i]is[/i] that temperature. No discernible peaks and valleys, just comfort. Bang/bang was never like that.
As for savings, because the ODR is running the supply temps, I saw about 23% savings in half the fall shoulder season, and expect 23-30% in the spring.
The nice thing about hydronics is there's a bunch of options for getting what you want/need that end in the same place. The hard part is figuring out which path you want to take.0 -
TRVs, Setback, Cost and Efficiency
TRVs and daily setback are not mutually exclusive--by any means!
One quite effective way to use setback with TRVs is to merely reduce the supply temperature on a timed schedule. At least one boiler (Viessmann Vitodens) has this capability built-in. Not sure about other boilers--particularly mod-cons--however. When you reduce the supply curve such that heat authority is less than 1.0, the rads are no longer satisfied and the space cools. The TRVs will eventually open wide, but provided there's reasonable flow balance in the system all spaces will cool fairly consistently. This method can be called "setback via starvation". Provided the house needs some heat, water will still be circulating and the boiler will still be providing heat--just at a level insufficient to meet the load at "normal" room temperature. Because heat is still being provided, the rate of room temperature decline will be virtually weather-independent.
Using the "control radiator" or "master thermostat" control schemes you need only lower the thermostat setting. This will stop the boiler from heating (and probably the circulator as well) until the room temp falls below the reduced setpoint. The colder the weather, the faster the room temp will fall to the reduced setpoint.
TRVs operators are available with timed setback. Danfoss (for one) makes them. They do however cost considerably more than standard operators.
Of course you can always lower individual TRV operator settings as well. If you have any rarely used spaces you can use the most efficient form of setback--CONSTANT setback.
If you've used daily setback, the supply temperature required to recover from such setback in any reasonably fast manner will be considerably higher than had no daily setback been used. Unless you have unbelievably oversized cast iron radiators, they will be nice and hot during recovery in most weather.
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As to cost:
"Zone valves and thermostats all over the place" don't come for free! They also complicate things--often requiring significantly more piping, lots of wiring and often rather expensive and elaborate controllers. With a large number of zones you're likely to have more than one circulator for the emitters--and don't forget that many installers prefer to zone strictly via circulators.
TRVs don't come for free either, but they're not particularly expensive. TRVs often simplify the piping as it only takes a two-pipe, direct return loop as opposed to numerous zones all heading back to a central location. (TRVs can be used with one-pipe systems, but it requires some extra work and you don't always get their full benefit.) Overall, there's a good chance that the cost between TRVs and "lots of zones" will be very similar--perhaps even lower! TRVs are also known for extremely an extremely long life with essentially zero maintenance--such cannot be said for zone valves and zone circulators!
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As to the effect on system efficiency:
Yes, the circulator will be running anytime that heat is required. Note the operative word here, "the" circulator. In many cases it takes only one circulator to operate the entire system. Unless you have special circumstances like radiant floors requiring a different supply curve, there's no reason to ever have more than two circulators (one primary for the boiler; the other secondary for the emitter) in a TRVd system. Depending on the number (and type) of circulators used, a constantly circulating TRVd system may well wind up using less electricity! Regardless, I'll venture that it wouldn't add much in the way of electric costs to nearly any system.
Variable-speed, high efficiency circulators are on the horizon. They're currently available in Europe and [should] be here soon. Such is installed inside the two smaller Vitodens models. I'll venture to say that a Vitodens directly driving a fully TRVd system will use less electricity than nearly any reasonably comparable system you could find in the US. These "smart pump" aren't cheap however and would be a quite expensive way to zone compared to TRVs that will use only one for the emitter circuit itself!
A constantly circulating, fully TRVd system has an advantage not found in traditional zoning (unless you zone each room individually). This is there ability to indirectly communicate between each other and the boiler via flow. The best way to understand this is in a south-facing room with significant solar gain.
As the sun heats the room, the room temp will try to go above setpoint--as it does, the TRV operator will close the TRV valve more and more. There will be less and less flow (thus less and less energy consumed) through the radiator. This causes an excess amount of available energy in the system as a whole. The boiler will fire less frequently (or if a modulating boiler will reduce its firing level). The remaining radiators will STILL modulate to produce their setpoint. In effect, the energy provided the sun will appear to have "moved" into the system!!! The only other control system capable of doing this is pneumatics--WAY too complicated and expensive for a normal residence.
A room (like a kitchen) with high occupancy heat gain will have a similar effect. The better your insulation/weatherization the more you will notice the effect--both on your comfort and in your fuel bills.0 -
Student here
Just letting everyone know I for one enjoy,hunger, thirst,Yurn, for the descriptive explanations you hydronic wisemen dish up here.
This is like watching the history, science, and modern marvels of hydronics, and steam.
Your words are not wasted on blind eyes. Thanks to all of you wisemen/teachers for your time, and effort!!!
Gordy0 -
ir cams
can you please direct me to a good and resonably priced thermal imaging camera. thank you0 -
Wow! Thanks for the great explanation. A couple questions re the system.
Is supply to the radiator emitters done by a single run to each off a header near the boiler? Or is a single "loop" header, such as 3/4" PEX, running supply to each rad with a 1/2" short drop?
And one more biggie: Can we have a "two temperature" house, the lower level heated with in-slab radiant, and the floor above that one all heated with the rads and constant circ?0 -
Numerous ways to pipe with TRVs.
You can use individual "home runs" from each emitter back to a central header. If the structure is spread out, it's often better to use one or more "remote" headers in addition to by the boiler.
A traditional two-pipe system is fine as well. Here you have a pair of mains (supply and return) that generally loop around near the perimeter of the structure with branch connections to each radiator. Branches are sized by the load of the radiator; the mains are sized by the entire load. Ideally the mains will size down as more and more rads are served--this reduces material cost and aids with balance if "setback via temperature starvation" is used. Unless the system is very small (output wise) and very compact (length of mains wise), 3/4" is probably too small--at least for the beginning of the mains.
Unless a branch run is extremely long a 1/2" branch piping and 1/2" TRV body will service most reasonably sized radiators. TRV bodies larger than 3/4" usually have the same flow characteristics as 3/4", so they're typically only used in retrofit situation--in other words, 3/4" TRV bodies are the largest you'd ever be likely to use in a new system.
There's utterly no need to use reverse return with TRVs. The TRVs themselves will balance the flow in a simple direct return system. TRVs will certainly compensate for some piping sins that would really screw up the system balance were TRVs not used, but the real goal is to design a nice, two-pipe direct return system essentially as if the TRVs weren't used.
One-pipe (series and diverter tee) systems can work with TRVs, but it requires extra piping (bypass lines) in a series system and extremely careful engineering/valve/tee selection in a diverter tee system. With any one-pipe system the TRVs will not have the same level of independence as they have with two-pipe systems. For that reason, any new system really should be two-pipe.
You can CERTAINLY Have a two-temperature system. The lower temperature circuit is produced via a mixing valve and drives its own circuit (almost always with its own circulator). The boiler fires to a level sufficient for the high-temperature circuit and the mixing valve does its job to produce the lower temperature circuit. More elaborate systems will find the mixing valve operated such that it mixes down to reset curve completely independently of the main reset curve. Fixed temperature or fixed proportion mixing can usually be used without any serious problems however--just use a normal wall thermostat to control. If you go "whole hog" and use FHVs (similar to TRVs but designed for floors), I'd suggest mixing via an independent reset curve.0 -
Here's a link to an older thread with much more detail regarding how to pipe for constant circulation with TRVs.0
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