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1" vs 3/4" baseboards

Stenty Member Posts: 74
Here's a question;

What effect would installing 1" fin-tube baseboard in lieu of 3/4" fin-tube have on the delta T across the length of the baseboard?

Would the btu/ft go down?

Would the supply water spend more time in the 1" tubing?

1" M copper has just under twice the volume of 3/4" M copper.


  • icesailor
    icesailor Member Posts: 7,265
    edited June 2014
    One Vs the Other:

    Some of us have pondered this question and gotten a serious migraine over it. Many others just say cluck it and have a salesperson from the supply house give a quote for the material.

    Me, I looked at it like this:

    If you're going to be considering using 1" copper baseboard, it will be commercial baseboard. 1" or 3/4" (plus 1 1/4") will fit in the cabinet. Look here at #95 baseboard. With all things considered equal for comparison, 1' of 3/4" fin tube baseboard will give you a grand total of 1159 BTU's per hour. The same 1' of baseboard in 1" baseboard will give you 1228 BTU's per hour of output. A grand total of 69 BTU's per hour for your exercise. That #95 Slant/Fin is some seriously ugly and expensive to put in a house compared to all the other options.

    But that ought to give you a start. I'm getting a headache just thinking about all those circulator curves.

  • Snowmelt
    Snowmelt Member Posts: 1,394
    edited June 2014
    Bigger trim baseboard

  • RobG
    RobG Member Posts: 1,850
    edited June 2014

    Your response makes no sense, and I quote:

     "Or I inch copper baseboard is more commercial use. Have to do the math.

    1 inch can handle more btu , can also handle more btu's


    What I think you want to do is lower the water temp which you can do, just make sure your using a condensing boiler or a water heater just dedicated for that room.

    Give more details, what's your room heat lose is and b.b. that you want to ad. "

    Use a condensing boiler for one room? A water heater? Please re-read your responses before posting them. (someone might actually take you seriously and blow themselves up).

  • Snowmelt
    Snowmelt Member Posts: 1,394
    edited June 2014
    My fault

    Thought they where doing a whole job.

    Scratch the water heater comment .

    Need more info

    1 inch will carry more btu then 3/4.

    Still need to do the math. How many btu to heat the room vs how many btu that baseboard actually carries.

    My fault didn't mean to mislead.

    How ever if your doing just that one room with oversized baseboard, you can lower the water temp and still stay comfortable. That is if that room was on its own zone.
  • Stenty
    Stenty Member Posts: 74
    The skinny

    Here's the skinny;

    The loads are small and only 1/2" is need to supply-return each zone. I'd like to keep the return temps down to keep the boiler in condensing mode.

    While researching baseboards, I saw the 1" fin-tubes and thought that the large diameter would slow down the flow of water in the fin-tube and therefore give up more heat as it traveled and therefore a higher delta T.

    I now see that the reduced friction can increase the gpm but 1/2" will rule the majority of the circuit.

    What if the 1" did increase gpm. Can it not be slowed by changing the pump or its speed?

    What's the best solution?

    I've seen bb ratings to be the same on some models whether its 1" or 3/4".
  • SWEI
    SWEI Member Posts: 7,356
    edited June 2014

    a larger diameter tube has greater surface area in contact with the fins, which might result in more heat transfer.  But then you have less fin area exposed to the air, which might result in less heat transfer.  Modeling this might make an interesting project for an engineering grad student.

    Speed of the water is completely up to you and your pump.  Slower water = greater ∆T = lower average temp = less heat output.  That assumes, of course, that the supply temp is held constant.
  • Snowmelt
    Snowmelt Member Posts: 1,394

    Is something wrong with the 1 inch baseboard, are you having problems, is that zone the only zone with 1 inch?

    If you look at the fins are they bigger, are the fins a heavier gauge.

    There's still a lot of unanswered questions.

    My post was right about lower water temp.

    I just hate it when I'm right and someone says I'm wrong.

    In general if you have a larger pipe you can carry more gpm there for you can go longer with the linear feet of heating or lower the water temp as I said in original post.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    I look at it from this view point.

    The amount of heat output from fin tube baseboard is largely dependent on it's ability to create a convection current of air moving across the fins. Their are several factors that determine the airflow, total static pressure drop through the air side of the baseboard, fin temp and the big one, fin surface area per foot of baseboard.

    The main advantage to using 1" vs 3/4" IMO would be the increase in possible loop length. GPM does not increase heat emission. Heat emission is a result of Temperature difference between the emitter and the space to be conditioned. If 3/4" and 1" are very close in the BTU/h per foot spec, The only reason I would ever choose 1" is if I was stuck with having to have a loop length that required more GPM than a 3/4" pipe could reasonably handle.

    If you want lower water temps with equal BTU/h per foot output, I would choose a product like this http://smithsenvironmental.com/heatingedgegreen-brochure.pdf

  • icesailor
    icesailor Member Posts: 7,265

    Harvey, I don't totally agree with you on a point. That the fin tube is the constant. The enclosure and design of it is the real decider of output. Take for example, Slant Fin Baseboard. They offer #15, #30 and #80 baseboard. All have different outputs using the same elements. It is the amount of air that can be heated passing thought the inside of the cabinet that determines the output. Measured by the water temperature entering and the temperature drop as it leaves. #80 may actually have a wider element. But when compared on an equal basis, the smaller cabinet of the #15 has a lower output. With the same GPM flowing through the pipe. The problem comes when someone feeds a loop with 3/4" pipe, some 3/4" fin tube, then sticks a piece of 1" fin tube in the middle. The flow will only be equal to the smaller 3/4" pipe, regardless of the 1" in the middle.

    A better example of what I saw a lot of is running one long series loop. Say, 100' long. What is the temperature entering the first emitter? 180 degrees? When it comes out 100' later, the return water is 140 degrees. Was the last room cold as opposed to the first one? If you split the 100' loop into two, 2-50' loops will work a lot better?

    You can also buy #15 Slant Fin with 1/2" copper elements. If you feed it with 3/4" copper and reduce it to 1/2" into the elements, you don't increase the output. Rated design of the emitter enclosure is as important as the temperature drop through the unit. I used to get "contractors" wanting to get away from baseboard and they wanted to practice their design/finish carpenter skills by making fin tube enclosures out of wood. They never worked worth a crap unless you doubled the length or doubled up the elements. I did do a couple that worked better. One was to use the standard baseboard. I split 3/4" ash boards to 5/16" and screwed them to the front covers, and one on the top. It looked OK. Just time consuming. Quality time on my band saw. and picking over the lumber yard stock.  End caps were a ****.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239

    I read your post then I reread my post then I read your post again.

    I think we are saying the same thing basically?

  • Stenty
    Stenty Member Posts: 74
    Ignore that Heating Edge .pdf

    The specs shown on that particular brochure are not the true specs.

    The true or latest specs are in the .pdf that I've attached. The other specs were just too high to be believable so I did some research and contacted the company. They sent me the .pdf enclosed.

    I'm hoping the generous specs of that other brochure were just typos.
  • Stenty
    Stenty Member Posts: 74
    Temp drop

    Just like a significant temp drop along a long run of connected baseboards results in lower output at the far units, a significant temp drop within one baseboard element would lower the output of that baseboard.

    The longer the water spends in the element, the more heat is lost by the water while running through a baseboard unit and therefore, a higher delta T but a lower output.

    increase the flow and the water is hotter all along its length and therefore a higher output.

    I started this post with the idea that the water in a  1" unit would move slower, despite the gpm remaining the same, and therefore increase delta T but lower btu output.

    I came up with the idea to reduce return temps to a mod/con. There is no existing system installed, just an idea for now.
  • Rich_49
    Rich_49 Member Posts: 2,738

    or turbulent flow is what you want for thermal transfer In a baseboard application .  The fluid moving slower will more often than not just aggravate an already poorly performing piece or pieces of baseboard . By increasing the pipe size and keeping flow the same you will create laminar flow thus impeding heat transfer .  Instead of entertaining the thought of changing baseboard type and size I suggest using a Delta T pump . In this way you can insure optimal heat transfer at all conditions , indoor and out . The very middle parts of your system will always have the average temp , beginning will be just above average and the farthest parts will be just below average . Now all that is left is room by room sizing of the baseboards at those temps and you're done . Math is awesome !
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Snowmelt
    Snowmelt Member Posts: 1,394
    Rich is right

    Do the math first , IMO your making a simple baseboard job way to complicated.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    Tying up loose ends.

    In a previous post I mentioned that increasing GPM to an emitter does not necessarily constitute a higher heat output. I would now like to explain just what I meant by that.

    First off, the BTU output of an emitter is largely dependent on the temp difference (TD) between the space to be conditioned and the average temp of the emitter. While increasing the GPM at a constant water temp does increase the average surface temp of the emitter, everything does not always work the way you might think. You see, the relationship between GPM and BTU emission is not proportional as one might assume. It actually follows a non-linear curve. What I mean by that is, at the bottom end of the curve (low GPM) a numerical increase in GPM yields a large increase in BTU output. At the top end of the curve (high GPM) that same numerical increase in GPM yields a negligible increase in BTU output. This is of course at a constant supply water temp.

    So GPM is what it is. It is a sweet and simple measurement of the amount of water that is flowing through a pipe. 

    Moral of the story, You cannot easily predict the output of a heat emitter by increasing the GPM unless you know for sure what the delta-t is going to be at that higher flow rate. Defer to the manufacturers ratings on the emitters.  

    Addressed to no one in particular. Just cleaning up.

  • icesailor
    icesailor Member Posts: 7,265

    Harvey, I agree with what you say.

    No one ever taught me what I know. I figured it out with the available information I was able to find.

    The Slant Fin table for #2000 baseboard (once #15). It comes as either 3/4" or 1/2" Baseboard. Both with the exact same ratings.


    With either 3/4" or 1/2", at 1 GPM or 4 GPM, the rated flow is 580 BTU's/Hr at 1 GPM and 610 BTU's/Hr at 4 GPM. Per Foot for both of them. I understand that the rated flow at 1 GPM equals 500 GPH with a 20 degree drop. but the determining factor is the temperature going in to the baseboard or loop and the drop coming out/leaving. The old IBR tables I had for designing emitter systems listed maximum lengths for pipe sizes so that you could keep the Delta T within the parameters of the circuit lengths. Why you could feed 14' of cast iron baseboard with 1/2" tube but 15' needed 3/4". Restriction and heat loss through the emitters. Because a series looped circuit considers each baseboard as part of the circuit length, there is a limit to how long you can make a 3/4" series loop and still have a 20 degree drop, as opposed to a 1/2" loop. Or, why you do split loops with a 1" supply and two returning 3/4" circuits. The only real reason you would really use 1" elements or 1 1/4" elements is to use longer length circuits in commercial applications.

    Then, there's individual home runs with 1/2" PEX which is less than 1/2" nominal size inside. I see some proposed PEX Home Run systems with manifolds that can not possibly meet the maximum lengths listed in flow tables. But I'm not doing them.

    Once I learned how this was all supposed to work, I used to put fittings on the supply and return at the boiler and run the system so I could measure the pressure differential and the temperature differential. I almost always did some form of reverse return with zone valves. No matter what I did, the leaving supply and the returning water temperature was never more than 20 degrees. Except during a period when thermal lag was occurring when the controls were starting and stopping the burner. As a zone dropped out, the Delta T became closer.

    Some seem to think that installing higher output larger tube elements will increase the output of the emitter. But if the circuit is 3/4" and actually flowing at 1 GPM, if there is a section of 1" element in the middle of the circuit, the GPH should drop.

    And according to the tables of the 3/4" and 1/2" #2000 baseboard, 1- GPM with 180 degree water gives you 580 BTU's per hour but increasing it to 4 GMP only gives you a 30 degree increase(610) in output. Hardly worth the effort. For myself, after experience and to save time, I had no way of accurately knowing the actual GPM's, just picked 600 BTU's per hour per foot and knew that the true flow rate was between those numbers and also because I/we always "rounded-up" (Rounded off to the next highest standard length baseboard), we never had a problem. Or at least I never did.

    I hope I make sense.