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P/S Pipe, Pump, and Flat Plate Sizing?

CDeckerCDecker Member Posts: 13
edited November 2019 in THE MAIN WALL
Hey guys,

I was wondering if someone could double-check my math in calculating my pipe sizes and flat plate heat exchanger sizing?

I have a tight house and garage that are heated with forced air as the primary heat source. During re-modeling, I have added staple-up radiant throughout most (but not all) of the house for floor warming. I also installed pex in my basement and detached garage slabs.

This past summer I installed 14 loops of 5/8” pex on 6” centers throughout my 1600 square foot driveway for snowmelt. I am in Buffalo, NY. All of this tubing has been under air pressure, and I am now ready to purchase a boiler and fire everything up.

Originally I was looking for a large (300K BTU) mod-con boiler to handle everything. I then was advised it might be better to install (2) smaller units and cascade them, so that they can modulate lower for the majority of the time when I’m not melting snow. I considered using tankless units, but received a lot of negative feedback here on that approach, so I’m going to stick with boilers. I would like to use a flat plate heat exchanger as my hydraulic separator, and keep the glycol out of the boilers and my indirect. The staple-up, slabs and snowmelt would all be on the opposite side of the Flat Plate, and would contain 50% prop. glycol.

I used RadiantWorks Pro to perform a full heat loss calc. Based on those inputs, the software has provided a Snowmelt Boiler Load of 216,000 BTU's and a Radiant Boiler Load of 67,400 BTU’s. The driveway is approx 1,600 Sq Ft and the house is approx 2,500 + 800 sq ft garage. This is what Radaintworks provided me:

Manifold 1 - Garage & Basement Slabs - 3.3 GPM, 8.6’ Head, 20° Delta-T, 90.5° Supply Temp
Manifold 2 - Staple-Up Radiant - 2.2 GPM, 5.8” Head, 10° Delta T, 103.5° Supply Temp
Manifold 3 - Staple-Up Radiant - 1.9 GPM, 6.1’ Head, 10° Delta-T, 107° Supply Temp

Snowmelt - 13.6 GPM, 2.5’ Head, 35° Delta-T, 124.5° Supply Temp

My thoughts were to put the radiant on one zone, and the snowmelt on another. That would leave my radiant zone pump and piping to handle 7.4 GPM and 8.6’ Head, so 1” pipe and a 007 or 0010 pump?

My snowmelt zone pump and piping would need to handle 13.6 GPM and 2.5’ Head, so 1.25” pipe and a 005, 007 or 0010 pump?

My secondary loop piping would need to handle all of that above, so 21 GPM and 1.5” pipe?

Now here is where I get confused…

If I go with (2) 200k BTU boilers and a flat plate heat exchanger, how do I size the GPM for the primary loop? Does the primary loop pump need to overcome the head loss of both boilers combined, or just 1 since they are plumbed in parallel? Same with the flow, do I need to double the flow requirements because I need to flow through both boilers, or is that not the case because they are in parallel?

With regards to the flat plate, is it as easy as just finding one with the proper BTU rating? I was looking online at 320,000 BTU 80-Plate Heat Exchangers, and they have 1.25” ports. What if my pipe is larger than that? I realize the primary loop pump only needs to overcome the loss of the boiler(s) and the flat plate, but how do I know how many GPM I need to provide on that primary loop to get enough heat to the flat plate? How do I know how big my primary loop piping needs to be? I have no idea how to properly size this pump without that data…

Thanks in advance for the help. I’m really trying to figure it out on my own without bothering everyone. For those of you that do know how to calculate this, I would really appreciate you teaching me how to do it? I enjoy understanding how all of this works.

Thanks again!


  • hot_rodhot_rod Member Posts: 13,313
    Maybe sketch a drawing of your proposed piping.

    A flat plate would not be a good hydraulic separator. By definition a separator is a very low pressure drop device. A big wide spot in the piping so to speak.

    Pipe connection size is not critical on a plate HX, the capacity and associated pressure drop is what you want to look at.

    Most of the quality brand HX manufacturers have online calculators to help you select. Usually with an example or tutorial. You can input flow rates if you know them or the program will calculate if you put in BTU transfer required. Some programs will spit out multiple choices and sizes. Be sure to add the glycol % when you size it.

    Here is an example of some of the math involved in long hand sizing to give you an idea why the software is a must quicker way to compare performance and sizing options.

    Many of the online sales of HX don't give you a lot of data on how they came up with that output or performance number, best to size exactly to your application.

    Idronics 15 has some examples of multiple boiler piping into a hydraulic separator and options for secondary piping.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mikeg2015mikeg2015 Member Posts: 1,178
    Sounds like the radiant floor is supplemental, not primary heat. Are you sure you want the total output ot be 67k? Might want ot run just 80F waternd make it feel a little arm but only output maybe 30k.

    If that single zone is only 30k and high mass, Could size the boiler primarily for snowmelt. If you end up with two boilers however, you might be better to just do offset sizes with 100k for radiant and 200k come on for snowmelt and down run run at less than 100% except on initial pickup.

    If in doubt on the plate and frame, go up one size to drop the approach temp for economy and to be “safe”. Too small and it won’t work. IF you don’t get the ground warm enough, the system becomes a really good at making ice.

    How do you prevent the rooms without radiant from being cold?

    What are the calculated supply and return water temps... especially return water temps? They determine the boiler output due to impact on efficiency. At 100F you’d be around 93% I’d estimate. 95% @ 90F.
  • CDeckerCDecker Member Posts: 13
    edited December 2019
    @hot_rod Thanks for the reply, greatly appreciate your advice!

    I downloaded idronics 15, and I am still reading...

    Thanks for the screenshot of the long hand Flat Plate sizing also. That's what I was looking for, but I'm going to try to find an online calc as you mentioned, to check my math. Where did you pull those page(s) from? I would like to read more, but I did not see those pages in Idronics 15?

    As requested, I have attached a drawing of what I'm thinking for a pipe layout. This is the drawing I made when I was considering 2 Tankless units, so some things might look funny, but the concept is the same..

    I think I'm on the right track with the secondary piping? I think my pipe sizes and everything are good there? Just trying to figure out the primary. Is my thinking correct in that with the Flat Plate, the primary pump only needs to move flow through the boilers, flat plate and primary loop piping, and be able to move the full capacity of both boilers, so 400k BTU's to the flat plate?

    So when using these online Flat Plate calcs, it appears I'm missing information to perform a calc? I do know how many BTU's I want to move (400k). I know that my secondary loop is 50% prop glycol, has a GPM requirement of 21, and a supply temp requirement of 124.5. Now it wants my primary loop input temp, output temp and flow? How do I know that? I thought the calculator would tell me what my input temp should be on the hot side if I want 124.5 coming out on the cold side?

    I'm trying here, just getting confused on the math...

  • CDeckerCDecker Member Posts: 13
    edited December 2019
    @mikeg2015 Thanks for the reply!

    You are absolutely correct, the radiant floor is supplemental. I also believe that you are correct in that the actual load will be much much less than Radaintworks has stated. This is why I am hesitant to install a large 300-400k boiler that will only modulate down to 65,000. It will be WAY overkill on a majority of the winter days when its not snowing.

    I agree on upsizing the flat plate, however I wanted to check with the pros here first, because it has been my understanding that going to large can cause the glycol not to move through it fast enough, and could cause blockage issues?

    You are correct in that it either works, or it doesn't. There is no middle road. I'm confident I put enough tubing in the slab. 14 loops of 5/8" at 6" on-center, 220' lengths. I probably could've gotten away with half of that, but I wanted to be confident it would work, and pex is cheap.

    I also forgot to attach the image for @hot_rod above, so here it is...
  • hot_rodhot_rod Member Posts: 13,313
    Idronics 23 is an excellent issue on heat transfer in general. Towards the back we show more on HX sizing and a snowmelt example on GEA software.

    Approach temperatures "thermal penalties" can be played with, it will cost you more HX size to get closer approaches

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • CDeckerCDecker Member Posts: 13
    @hot_rod Not sure where my first reply to you went? I tried going back to add the diagram and poof, it was gone.

    Anyways I registered and opened the GEA (now Kelvion) calc. See the attached screenshot. Am I correct in that I want the output temp on the "cold" side to be the highest required temp by my zones (in this case, 124° for the snowmelt?). It won't do any calculation without an output temp on the primary side. I have no idea what this would be, and I flat out guessed at 120..

    Screenshot attached.

    This still doesn't seem to provide me my GPM on the primary side. How do I know what size pipe and pump I need?

    Thanks again,

  • hot_rodhot_rod Member Posts: 13,313
    you need to put more temperatures in and the A side needs to be warmer than the B side.

    Use the example I attached above, is shows 180 supply 30∆ delta on the A boiler side. Adjust the temperature inputs for what you plan on supplying to the A side.

    It shows flowrate and pressure drop.

    For pump sizing you need to add the pressure drop of the rest of the piping in the circuit.

    idronics 16 takes you step by step through sizing a circulator.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • CDeckerCDecker Member Posts: 13
    Ok, Starting to get the hang of this. Well, maybe..

    In order to move the proper amount of BTU's across the Flat Plate, I'm starting to discover that it's going to take a higher boiler temperature and a MUCH higher ∆T than the 30° I was planning on. More like 55°+.

    Question regarding GPM's on the hot side. If I have a boiler where say my target GPM is 7, and I install 2 of them in parallel (cascaded), do I need to double my GPM on the primary loop to get enough flow across both of them, or will the pump see them as 1 because they are identical units and in parallel?

  • hot_rodhot_rod Member Posts: 13,313
    Would it make more sense to dedicate the correct size boiler to the snowmelt? Just glycol that system? Not a lot of value in modulating a snowmelt boiler, usually you want all the output the boiler can muster until the snow is gone. Copper tube boilers work well, although not as efficient they are simple, a lot less maintenance, and affordable. How often will the snowmelt operate?

    If you still want two parallel boilers , use a hydraulic separator, and just glycol the snowmelt via the plate HX.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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