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Mono-flo or Parallel piping is small home?

I'm installing a radiant system in my 1,500 sq ft home in the SF Bay Area (moderate climate). The home is very well insulated. I've decided on Runtal radiators and in speaking with the Runtal rep he told me that a parallel piping system would be better than a monoflo system. His reasoning makes perfect sense: in a mono-flow system the water continues to lose heat as in passes through the radiators and that the last radiator will run less efficiently that the first. It also makes sense that in a parallel piping system the water temp is consistent through all radiators. Do you agree/disagree?

I'm attaching a zone diagram that I designed that has what I believe is a parallel piping system for one of the 2 zones planned for my home. Does it make any sense? Am I doing this correctly? I want to have flow control valves at each radiator to allow heat adjustment for the individual rooms. Comments appreciated. I'm a remodeling contractor but have limited knowledge of hydronics. Looking for help with the design and install, was told this was the right place for both. thcx


  • RJRJ Member Posts: 484

     Direct return      Make sure you install positive shut off valves on supply and return on each radiator as well as your balancing valve and drain and purge valves or air vents at high points,  these valves will help you when filling and draining system.  some prefer balancing valves on the return of the radiator,  ball valves not recommended for balancing, use a good quality balance valve that has gauge ports for installing a balance meter or gauges.   Bell&Gosset, Taco or Victaulic make good valves
  • SWEISWEI Member Posts: 7,356
    Reverse Return

    would be best if the layout permits.

    Panel radiators are usually sold with fittings that include multi-turn stops  (for balancing) and unions.

    TRVs would add another level of both comfort and control.
  • why 2 shutoff valves at each radiator?

    Thanks for the reply. I'm a neophyte trying to learn on the fly....not an ideal situation but I can't afford to hire an hydronics pro so I'm committed to learning.

    I understand about the air gap valves at the high points, makes sense to clear the line of air when filling/refilling, but I don't understand why I'd want shut off valves at both supply and return on individual radiators. I thought the purpose of a shutoff valve on individual radiators was to control the amount of heat output for each radiator. Is this correct? Why would 2 shutoffs be required on each radiator, one each at supply and return?
  • What is a "reverse return"?

    1). What is a "reverse return"?

    2). I'm using Runtal radiators which do not have built in valves on wall panels. My supply will be coming in through an exterior wall so I had planned to use an angle valve to control water flow to each radiator.

    Comments welcomed...thanks! I'm an idiot but getting slowly smarter with help from you guys :)
  • RJRJ Member Posts: 484

    shut off valves will allow isolation of individual radiators for any future repairs,  proper piping practice used by steamfitters for decades,  some balancing valves,circut setters are not 100% shutoff.   I worked in the Bay Area for 20 yrs,  If you dont already have a contractor call RMA   Redwood Mechanical or ABCO Mechanical
  • valves

    Thanks for the reply. I can't afford to hire a contractor, although I do know a heating contractor willing to consult for a fee....I'm setting that up now. I've been in the trades for years, I've sweated miles of copper and am confident I can assemble the system. It's the piping layout, valve positioning, radiator positioning I need help with. I had one guy tell me it was a waste to put radiators on interior walls and that it's better to put a single large radiator on one outside wall to heat a great room rather than use several radiators positioned around interior walls. It's that type of counter-intuitive info I'm having trouble understanding. I've purchased the Dan Holohan book on radiant heat but found it useless as it's geared toward in-floor heating. I haven't yet found a good basic primer on wall/baseboard radiant heating.
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    Reverse Return

    OK, I found an example diagram of "reverse return". Is this diagram correct?

    What is the advantage of a reverse return vs what I show in my original diagram? Not sure reverse return is practical since the last radiator in the loop is farthest away from the boiler and I'd need a lengthy run to return to the radiator. My original diagram looks to be more efficient from a total pipe required perspective.
  • SWEISWEI Member Posts: 7,356
    Reverse Return

    Will push roughly the same amount of water through each branch without a lot of throttling required, and tends to maintain the system balance even when zones are turned on and off.  It's not required, but it makes balancing the system much easier.

    Have you considered piping as a single zone, with TRVs on each radiator?
  • ZmanZman Member Posts: 3,517
    Home Run

    Panel radiators will work well in reverse return as suggested. The idea setup is to run a supply and return from each panel back to the mechanical area. These lines would likely be 1/2". This is true parallel piping. From there you can control and balance it and way you please. I like TRV's because it gives great local control.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • reverse return

    Frankly I don't see the advantage of what you refer to as "reverse return" over what I'm showing in my original red and blue diagram. I do see a big disadvantage in the reverse return scheme in my layout because it would result in an additional lengthy pipe from the last radiator back to the boiler. Could you explain how the "reverse return" is better than the scheme I diagramed, and why you believe it is worth the extra 75' of return piping?
  • SWEISWEI Member Posts: 7,356
    Direct Return

    Water is lazy, to quote our esteemed host.  Direct return piping creates a "shortest route" for the water to take through the first radiator in the string.  It's harder to push water through each successive radiator as you get further from the boiler.  This can be countered by throttling down the balancing valves on the first radiator, less on the next, and so on.  If the flow changes from a variable speed pump, or a zone valve closing, the system balance will change, with different ratios now flowing through each branch in the system. explains this in some detail.

    If you put TRVs on each radiator (in effect turning each into its own zone) the piping becomes less critical, since the TRVs will throttle up and down in to maintain temperature.  A manifold system is sometimes easier to install, but often a hybrid branch and loop system makes a better fit.
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    TRV's and reverse return questions

    Thanks to all for the informative responses. I want to make sure understand 2 of the concepts mentioned:

    Reverse Return: The head pressure created in the reverse return drain line actually strengthens the input flow of hot water to upstream radiators. Is this correct?

    Thermostatic Radiator Valves: automatically adjust the input flow to individual radiators to control the temp in each room. Is this correct?

    It was suggested that if TRV's are used then a reverse return isn't needed. Is this a misunderstanding? Or, is it better to use both a reverse return layout AND TRVs?

    One concern I have is that with a reverse return system, you are asking the pump to push a higher volume of water due to the added length of piping required. How does this effect efficiency?
  • Is this a correctly drawn Reverse Return diagram?

    I've revised my drawing to what I believe is a Reverse Return system. Does this look correct? Is anything missing/incorrectly drawn?
  • BUMP!

  • SWEISWEI Member Posts: 7,356
    Diagram looks good

    Though if the house is anything approaching rectangular, your original drawing offers a shorter route. 
  • diagram looks good

    "Though if the house is anything approaching rectangular, your original drawing offers a shorter route."

    Yes but didn't we agree that a reverse return solution was better for balancing than the direct return shown in the original drawing? I'm do I achieve a "short route" similar to that in the first drawing, but using a reverse return system? The 'longer route" in the reverse return drawing is due to the length return of the boiler return back to the boiler room. I'm open to suggestions on how to solve this...thanks.
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    Balancing a reverse return system

    I've read several web articles that say line balancing is critical to a successful reverse return system. If I understood correctly, they were saying that a balanced system is one in which the boiler input line is approximately the same length as the boiler return line. The problem I'm having with my design is that my last radiator ends some distance from the boiler, causing the boiler return line to extend much longer than the boiler input line. Seems to me in an ideal reverse return system, the First radiator and the Last radiator will be approximately the same distance from the boiler, thereby allowing both input and return lines to be equal in length. Am I thinking correctly? The only way I can achieve this on my house is a single zone that begins at a radiator located about 10 feet from the boiler and ends at a radiator also about 10 feet from the boiler. This would allow both input and returns of the same length, hence a balanced system. Is this correct thinking?
  • SWEISWEI Member Posts: 7,356
    Sorry, second drawing

    not original drawing.

    A difference in supply and return line lengths prior to the first branch on each is not important, as long as the trunk lines are the right size.
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    Need clarification

    "A difference in supply and return line lengths prior to the first branch on each is not important, as long as the trunk lines are the right size."

    That statement confused me. In a reverse return there is only a supply line prior to the first branch (radiator), correct? And, the return pipe begins at the outflow of the first radiator. In your statement you refer to a return line prior to the first branch. I don't understand.

    My balance questioned referred to the lengthy return line that exits the last radiator then makes a long solo run back to the radiator (re-attaching my drawing for clarify). Does this represent an imbalanced system?
  • SWEISWEI Member Posts: 7,356
    edited October 2013
    Supply lines

    meaning the fat red and blue lines in the above diagram.  The difference in length you describe is not an issue, as long as the lines are large enough to carry the volume you need without adding too much head to the system.  If they maintain the same diameter through their entire length (not reduced as they get to the end of the line) then it's really just length and diameter.

    As mentioned above and also in the other thread you started, TRVs will reduce the usefulness of reverse return piping.  If you use them, you also won't need two zones.
  • Reverse Return Balance

    Thank you for the response.

    I've read in several publications that in order to maintain system balance (without introducing balance valves) that the input and return lines must be the same length. In my drawing, the return line is substantially longer than the input line. This seems at odds with your comment. Could you clarify? Thank you.
  • GordyGordy Member Posts: 7,122

    How reverse return can benefit a TRVed system. Because I'm not seeing an advantage worth the labor, and materials.
  • SWEISWEI Member Posts: 7,356
    Input and return lines do NOT have to be the same length

    the goal is to have the total trip (supply -> emitter -> return) the same length for each emitter.  The difference between the lengths of the supply and return pipes between the heat source and the first branch is immaterial.
  • SWEISWEI Member Posts: 7,356
    I don't believe I said that

    or did I miss something?
  • clarification

    "The difference in length you describe is not an issue"

    I interpreted that as meaning a difference in lengths between supply and return pipes would not affect system balance. This is a key point I've been struggling to understand. I'm hoping you could provide some more explanation to fill in my knowledge gaps....and there are plenty :).

    I greatly appreciate your help.
  • GordyGordy Member Posts: 7,122

    Not saying you did trying to put two threads together here. But I see the OPs drawing is using TRVs, and he's trying to implement reverse return in the same drawing. Maybe I missed something Kurt?
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    time to reset the conversation

    First off, I'm experienced in the area of hydronic heating so any confusion falls at my feet.

    I'm attempting to design a simple system to heat my small home. Here's what I've concluded from various articles I've read the last several days:

    1) One pipe series system is not desirable due to the pressure and heat loss as the unput travels through the radiators.

    2) A 2 pipe direct return system is better, but is prone to system imbalance due to the "path of least resistance" effect of the return piping. This effect can be somewhat minimized with the use of TRV's.

    3) A 2 pipe reverse return system avoids the "path of least resistance" problem of a direct return system and thus is an inherently more balance system than either of the above choices. Optimal balance is achieved in a reverse return system when the input and return piping are about the same length. To some extent, pressure imbalance in a reverse return system can be offset with TRV's on each radiator in the system.

    Does this make sense so far?

    I've decided on designing a reverse return system to optimize pressure balance, and am adding TRV's not specifically to balance the system but to maintain a desired temperature in each room where a radiator is located.

    So now there is still an unanswered question with respect to the piping lengths in a reverse return system. As I mentioned, I've read in a few articles that optimal balance is achieved in a reverse return system when the input and return piping is the same length. In my latest design, the return pipe is nearly double the length of the input pipe. This is because my last radiator in the system is far away from the boiler, necessitating the lengthy return pipe length. So 2 questions here really: 1) does the long return piping diminish system pressure balance as I've read it does? and 2) does it really matter if I'm using TRV's on every radiator?
  • GordyGordy Member Posts: 7,122
    The idea is

    To have the same distance traveled from the supply of the boiler to the return of the boiler in each rung of the ladder.

    Soon the first closest radiators supply water will be the last to return to the boiler. That's where the reverse comes in. But this is only going to give balance of each circuit has close to the same resistance.

    I would use the TRVs and call it good. How often will all heating loops call for heat, and for the same length of time. TRVs will be throttling demand on each loop. How drastic are the heat loss differences in each zone?
  • hot rodhot rod Member Posts: 7,077
    Home run piping?

    Is that an option? Delta p circ
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Home Run Piping is not an option choice.
  • RobGRobG Member Posts: 1,850
    An easier way

    An easier way to do the reverse return piping would be to start at boilers supply and connect to BR1 then BR2 then MasterBR and end at Office. that way you avoid looping all the way back around the systerm. hope this helps.

  • response to "an easier way"

    If I did as you suggest yes it would shorten the lengthy return pipe, but it would also nearly double the input pipe length so no improvement in balance really....unless i missed something?
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    I've concluded that....

    ...the only way to attain line balance via equal length of both input and return pipes in a reverse return system is to have the first and last radiators in the system approximately equal distance from the boiler. Does anyone disagree with this? If so, what am I not understanding? Input pipe exits boiler and continues run until it reaches last radiator; return begins at last radiator and continues back to boiler. If first and last radiators are same distance from boiler, then input and return pipes will be of approximate equal length. I do not see any way around that rule.

    The counter argument I've heard here is that if you are using TRV's, then it really doesn't matter if the the system is imbalanced.

    ...And still another argument I've read here but don't fully understand is that a lengthy input run back to the boiler doesn't necessarily create system imbalance, and that balance is measured in the region between first and last radiators only. This may be correct but I have no clue on the physics involved with this theory. I'm hoping someone a lot smarter than I will come along and provide a thorough explanation of system balance in a reverse return system: how it's measured/ where it's measured/why a lengthy return pipe doesn't affect it.
  • GordyGordy Member Posts: 7,122
    edited October 2013
    Head and flow rates in parallel

    With out any control valving head in a parallel piped system will balance equally among circuits. Flow rates will vary though depending on the head loss of each circuit. By adding the extra pipe for reverse return you are adding more head loss to the system piping.


    How long is the main supply / return piping? With out the added piping for reverse return.

    How much difference is there in each circuits length?
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    Question for Gordy

    Not sure what you mean by "How much difference is there in each circuits length?" I'm a hydronics noob and not completely familiar with the jargon. Are you asking for the distance from where the radiator input leaves the main supply pipe and to where it enters the return pipe, for each radiator shown?

    For your first question, I'll take precise measurements and respond shortly.

  • GordyGordy Member Posts: 7,122
    edited October 2013
    If you think about it

    Parallel piped is really like a longer version of a hydronic manifold. What it does do is decrease head loss in a system verses a home run set up do to larger dia supply/return piping. How much really depends on the area you are trying to heat. There is also remote manifold stations
  • GordyGordy Member Posts: 7,122

    That is a circuit or loop
  • Scott_Mountain_View_CAScott_Mountain_View_CA Member Posts: 203
    edited October 2013
    Answer to Gordy's distance questions

    "How long is the main supply / return piping? With out the added piping for reverse return."

    Total input piping = 123 feet (as measured from the boiler to the last radiator)

    Total return piping = 83 feet (as measured from the first radiator to the last radiator; i.e., excluding the long return from the last radiator back to boiler)

    Return Pipe from last radiator back to the boiler = 123 feet

    "How much difference is there in each circuits length?"

    Each radiator will be located 5' from the input/output pipes. Therefore a total of 10 circuit feet excluding the radiator (5' input to radiator; 5' retrun to return pipe). All radiators are approximately the same size.
  • GordyGordy Member Posts: 7,122
    edited October 2013
    I think

    Your well balanced already if your radiators, and circuits are as you state. Your main supply / return should be close to identical lengths with out the addition of a reverse return pipe length added in,

    At 123' your supply temp may lose a couple of degrees by the time it gets to the last radiator, depending on if the pipe is insulated, and or if it's in a conditioned space.

    I think I would opt for TRVs skip the reverse return, and go. The TRVs will give you zoning.

    If your not happy with the result you could always reverse return later, but I personally am not seeing a gain here with reverse return being your circuits are pretty much going to have the same head loss.and if you are going to use TRVs. If your not going to use TRVs then reverse it.

    Remember pipe sizing plays a crucial role. In both the ability to deliver the required flow rate, and btus needed. With in the acceptable velocities for a quiet comfort system.
  • Response to Gordy

    Gordy, thanks for the response.

    When you say to abandon the reverse return would you go with a direct return as an alternative? That was my original design (see the diagram in the first post on this thread).

    I was planning on 3/4" supply and return piping, and 1/2" to/from each individual radiator. Is this sufficient? In either scenario (reverse or direct return), should I be concerned about shrinking the pipe diameter in the supply and/or return as I move further from the boiler?

    I had a friend suggest a 1" input/return pipe size. This seems excessive in this small system.
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