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how does water get into radiators in a 1-pipe system?

weil_failweil_fail Member Posts: 69
while looking at different architectures of radiant/hydronic/hot water heating, it seems there are 3 main methods: series, 1-pipe, and 2-pipe. series and 2-pipe are easy to understand, but I'm a little bit uncertain about 1-pipe. the diagrams I see show a single loop of pipe, but then the radiators hang off in parallel to the main pipe. it seems like the radiator would be a higher resistance flow path, and most of the wanter would just circle back to the boiler without losing much heat. does it just rely on that slight pressure difference on each end to put water into the radiator, or is the main mechanism that he hotter water will rise up into the radiators, forcing out cooler water? (heat rising, cold sinking). I would assume that either way, it would be a bad architecture for a condensing boiler as the temperature drop seems like it would always be smaller than the other methods that require the return water to have gone through at least 1 radiator for the 2-pipe system and all radiators for the series system.

secondary thought/question: could one tap off of the return from a 2-pipe system, making a small segment of 1-pipe, and run a hydronic floor off of the cooler return water? obviously, you would need two thermostatic valves so that the water going to the floor is cut off if it exceeds the allowable temperature for your flooring, say 85°F/30°C. this seems like a way of extracting even more heat from return water, helping the efficiency of a condensing boiler, while also allowing slightly higher temperatures in the radiators than is presented to the floor loop (due to it passing through a radiator before getting to the floor loop). is this something that is done, or am I just being a mad-scientist with this idea?

Comments

  • Jamie HallJamie Hall Member Posts: 12,947
    I think that what you are referring to as a "one pipe system" is what would be called a "monoflo" system. If the Ts connecting the radiator runouts to the main were, in fact, just plain vanilla Ts, you are correct -- it wouldn't work. Water would just flow merrily along in the main. But they aren't. Or at least one of them isn't. Instead, the T has a clever arrangement inside which causes the flow resistance (or apparent pressure drop) along the main line to be effectively greater than or at least equal to the resistance through the radiator. This is usually some sort of venture arrangement affecting the main line, but not the lateral.

    So they do work, and work well -- although some folks have been known to use strong language when attempting to get all the air out of them.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,089
    @Jamie Hall

    I have a 1 pipe in my house (1955) so I know all about the strong language you are referring to. I put a bunch of valves in when I drained it so in the last 34 years it's only been drained a few times
  • weil_failweil_fail Member Posts: 69
    thanks, I didn't know quite what to search for, but when I added "monoflow" to my search I was able to find explanations of the venturi Ts.

    now that I know how they work, does anyone know if folks use monoflow off of a return line to run a hydronic floor? does anyone make a thermostat valve that is based on water temp? it would be really neat to run a radian floor off of a return for more efficiency. maybe more work than it's worth, though.
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    edited May 14
    The Mono Flow system is a way of heating up each radiator on a single zone evenly . As mentioned the system uses a special Venturi tee to create an pressure drop to move water though the radiator loop ...

    Condensing boilers are design for low temp systems .A system with large mass radiation use lower temperature supply temperatures . Radiators and radiant

    A primary secondary system which uses spaced tees and circulator for each zone would pipe the lower temperature heating loops down the line .. You may find them piped this way on large systems ....

    I have enough experience to know , that I dont know it all
  • Jamie HallJamie Hall Member Posts: 12,947
    I doubt that you could use a monoflow system for a radiant floor off the return line. The problem is that the system only works if the pressure difference through the monoflow T -- or Ts -- is enough to compensate for the larger head loss through the radiator runouts. A radiant floor, with even a relatively modest amount of tubing, has more head loss -- by a lot -- than a simple runout to a radiator and back, so you wouldn't get significant flow.

    There are many temperature controlled mixing valves, which is what I think you are referring to. In principle you could set up such a valve and some other plumbing and a bypass to feed the floor -- but you'd have no effective control of flow rate. Much much better to run the floor off your primary/secondary manifold, if you have one, as a separate zone with a mixing valve for temperature control and its own circulating pump and thermostat or temperature sensor.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
    mattmia2
  • weil_failweil_fail Member Posts: 69
    edited May 14
    yeah, I suppose it could be done with a thermostatic mixing valve that puts the floor in series when it's below 85F and cuts it out when it's hot. I'm not sure if control of flow rate would be a huge deal, so long as you're just using the floor as a comfort item and not the primary temperature control of the room. worst case, you end up with a system that gets hot too quickly during really cold times and you don't get much heat to the floor, but it saves you the manifold, pump, thermostat, and the complexity that comes with creating a whole separate loop. you could do this with one of those fully integrated wall-mount mod-con boilers. obviously, this would be something one would experiment on their own house, as a customer may be upset if their radiant floor isn't warm on the coldest days
  • PerryHolzmanPerryHolzman Member Posts: 127
    I believe it would be difficult to add a radiant floor to an existing monoflow T system (I have such a system with cast iron baseboard in my 1954 house - works very well).

    I also retrofitted a mod/con boiler, with outdoor reset, which varies the system supply temperature between about 80 and 155F depending on the outside temperature, with a return temperature 5 to 10F lower than the supply temperature (my pump is moving water faster than the original one - as designed temperature drop was 20F with 140F supply. Very efficient.

    For most of the heating season the loop temperature is higher than what you would want for a radiant floor; and I don't see how you could effectively mix down.

    Hope that helps,

    Perry
  • weil_failweil_fail Member Posts: 69
    Perry, my thought was that as the whole system heats up, it will ramp from ambient up to 140F, or whatever, and water flows through the floor while it's ramping up, but when the return water it hits ~90F (output water ~110F), it bypasses the floor, leaving that 90F water in the loop to heat the floor up.

    if your boiler is lit 24/7, then that would definitely not work. it also wouldn't work well if your boiler does not modulate but just jumps to 100% right away. but if you have a decently designed mod-con boiler, you should get roughly 3 times per hour gradual ramp of water up to ~90F into the floor.
  • PerryHolzmanPerryHolzman Member Posts: 127
    @ weil_fail

    I believe you have a basic misunderstanding about how hydronic heating systems work, and mod/cons as well.

    The system does not heat up from cold every day. It heats up at the start of the heating system and stays hot until the heating system is shut down in the late spring/early summer. Conventional boilers will normally heat up to 140 F (or whatever their temperature setting), and a thermostat would turn on and off a circulating pump or open/close a zone valve.

    A mod/con system with a temperature "reset" curve works a bit different. It's designed to fire 24/7 the vast majority of the heating system and normally constantly circulates warmed water through the system 24/7 (zone valves may control individual circuits). It varies the temperature of the water needed to appropriately heat the house based on the outside temperature.

    The only time that the system sees 80-90F water is at the start and end of the heating system. My system (in Wisconsin) runs and constantly circulates though my monoflow T cast iron baseboard system typically from November into April above 105F supply temperature, and on the coldest days at 155F ( we can see -30F temperatures, not windchill) at 155F. It often starts in mid September and the system supply temperatures are in the 80F to 100F through the end of October, and also these same lower temperatures are typically seen starting in April and extending into May (the Circulating pump runs constantly from typically prior to October 1 and through at least April 15).

    You cannot effectively mix radiant floor/wall/ceiling needing 80-90F water with a house that the rest of the system needs a higher temperature to heat the house if you are using the same boiler circulating water.

    It is theoretically possible to add a separate heating water circuit with a heat exchanger (hot side supplied from the boiler, cold side your new heating circuit) with appropriate controls and its own circulating pump. That would likely be very costly and take of a fair amount of space as the heat exchanger would have to be large enough to function with say 90F inlet water.

    Without the heat exchanger/separate heating circuit you cannot just add a radiant slab to one room if the rest of the house is heated with radiators that operate at normal radiator temperatures. You will need to either convert the entire house to radiant heating (slab/walls/ceilings) where the Boiler outlet temperature will never likely exceed 95F or add/replace some kind of radiators that are designed to heat the room at nominal 140F water on the coldest day of the year.

    What is your existing heating system? Monoflow T or not, Radiators or not, radiant wall/ceiling or not, etc. What is your current normal hot water supply temperature the coldest day of the year?

    If you are designing from new... Why are you wanting to mix systems?

    Perry
  • weil_failweil_fail Member Posts: 69
    @PerryHolzman , thanks for the explanation. I appreciate you taking the time; that definitely explains why it wouldn't really work.

    this isn't really a plan to modify or add a boiler, it was just a thought I had.
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    Keep the Mono on it's own zone , The radiant on its's own zone , You're choice Auto or manual mixing . Always use an floor sensor for radiant .Control works for comfort..
    Food for your thought .....You don't want to mix two different animals with heating .
    I have enough experience to know , that I dont know it all
  • jumperjumper Member Posts: 1,464
    Is monoflow really cheaper? Single pipe is bigger.
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    Comfort is not cheaper you have to pay for it ...
    I have enough experience to know , that I dont know it all
  • SuperTechSuperTech Member Posts: 1,344
    Is there any advantage to having a monoflo system nowadays? The same comfort could be accompanied by piping the emitters home run style back to manifolds at the boiler with temperate gauges and balancing valves. The only advantage I can see is the additional water volume is good for avoiding short cycling
  • Big Ed_4Big Ed_4 Member Posts: 1,357
    edited May 20
    It's been done before .A spaghetti system were installed in the 50's they used 3/8" copper or steel tubing . One would cringe replacing the boiler on the steel tubing system .. The steel I believe call modo tubing ( I fogot ) were used during the Korean War .

    I would think back before Pex a Monoflow would be easier to install then bending tubing , around the house . They used tubing benders it seems , Nice even bends ..

    A Monoflow would be a better choice with a large baseboard loop . They were installed when baseboard first started to get installed .The tees were properly spaced . The drawback was the bleeding .

    The problem with a Monoflow system is the main line tends to hang lower in the basement to get that uphill riser . Back then a finished basement was painted walls and floor .
    I have enough experience to know , that I dont know it all
    SuperTech
  • SuperTechSuperTech Member Posts: 1,344
    I believe my house originally used a monoflo split loop. 1-1/4" to two 1". Actually I think the 1" was originally the supply, because the diverter tees would be on the inlet side of the cast iron baseboards.
    Somewhere along the way the piping was was changed. The 1" pipes were made the return, another two 1" pipes were run along with them so the baseboards could be piped in a two pipe reverse return setup. Unfortunately two cast iron radiators were removed from the kitchen in favor of fin tube baseboards. I'm planning on changing them out to panel radiators.
    Surprisingly I don't have any air problems after refilling the system. A good purge takes care of it, along with hy-vents at the boiler. Of course now I have a micro bubble resorber and I'm pumping away from the expansion tank.
  • PerryHolzmanPerryHolzman Member Posts: 127
    edited May 20
    Big Ed_4 said:


    A Monoflow would be a better choice with a large baseboard loop . They were installed when baseboard first started to get installed .The tees were properly spaced . The drawback was the bleeding .

    The problem with a Monoflow system is the main line tends to hang lower in the basement to get that uphill riser . Back then a finished basement was painted walls and floor .

    It is true that you really need a basement with some vertical space for a monoflow loop. In my house (built 1954) the monoflow loop is some inches below the bottom of the rafters.

    Not only that, the main pipe is slightly uphill and each corner is progressively higher than the next (by exact increments - the contractor was a perfectionist - and it was his house as this house was built for the local plumber heating contractor in the area and he did the plumbing and heating), and the return end is clearly higher than the start of the loop. That prevents bleeding and venting issues.

    The presence of a this monflow loop rules out ever installing a finished ceiling in this basement. Now admittedly, you could build a basement with extended height so you could install a drop ceiling below a monoflow loop.

    On the other hand, in Wisconsin a basement does not add to the "taxable sq ft" of the house if it is not finished; and it's not finished without a finished ceiling. I've seen many people shocked at the increase in taxes when they "finish" the basement and make it into formal living space (usually a family room).

    However; a reality is that my monoflow system with cast iron baseboard really works well, and is mechanically very simple without anything extra that could need maintenance. The base loop has 1 pump, no valves of any kind (no TRVs, balance valves, solenoid valves, etc), and will likely last the entire life of the building with no other required maintenance (other than changing the boiler and occasionally replacing the loop circulating pump). If building new construction with a basement, I'd very seriously consider doing it again (yes, its initially more costly than other options - but totally comfortable to the point that people have mentioned that they like how uniform the heat is in my house in the winter, unlike their house).

    Perry

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