Flow rates in PEX / Air Elimination / Heat Transfer

Discussion in 'Boiler Forum' started by CO-Chris, May 7, 2013.

  1. CO-Chris

    CO-Chris New Member

    Messages:
    3
    Location:
    Boulder, CO
    View attachment Boiler_Design_r0.pdf I'm new to the forum. I'm installing a Lochinvar WHN-085 with a SIT-040 indirect water heater in a home with 2,000 sf across 4 floors (500 sf / floor). I'm planning to use (400') loops on each floor.

    I'm going for maximum efficiency and I want to use a single variable speed (variable voltage input by boiler) pump for the entire system.

    So, I'm looking at the following (2) set-up options:

    Common to both:
    Calculated heat loss: 40 - 45,000 BTUH. Water heater first hour demand of 135 GPH drove boiler size.
    Max loop demand of 1.25 gpm assuming a 20 deg Temp Drop
    Assume about 4' of loss in central components
    Indirect water heater will demand 7.5 gpm at negligible head loss (1.25').


    First:

    Use 5/8" PEX-AL-PEX, which gives me about 6' of head loss
    Use a Grundfos UP15-42F/VS

    Second:
    Max loop demand of 1.25 gpm
    Use 1/2" PEX-AL-PEX, which gives me about 22' of head loss
    Use Grundfos UP26-64F/VS


    My thought is to use the smaller pump and the bigger low loss PEX (first choice). However, I'm worried that the flow rate will be below 2 fps always. Is that a problem? I've seen some posts mention that this will lead to 100% laminar flow and stratification of the flow stream?

    The next issue is air elimination. I'm thinking of using a 1" copper manifold and centralized boiler tubing. Most air eliminators need a flow of 4 fps through them. My indirect water heater demand will get me to 3.5 fps at the flow rate through 1". My thought is to reduce to 3/4" for the air eliminator to increase the flow rate and then increase 6" or so after that fitting? Any thoughts? I'm I getting too detailed? My other thought would be to use a Taco 4900 series, which doesn't seem to have a minimum flow requirement.

    My final issue are the transfer plates. I'll be running the PEX in the joist space and I need a transfer flux of about 20 - 25 BTUH/ SF. If I insulate under the PEX, should I still use transfer plates? I've looked at the Ultra-fins, thoughts? I'm avoiding the transfer plates because I want to install hardwood floors at some point and I really don't want to be concerned with punctures to my PEX from errant nails. If I go with the Ultra-fins, it looks like I'll have to manually adapt them for the larger PEX (if I go 5/8"). Thoughts?

    In summary:
    1) Issues with sub 2 fps in a loop?
    2) Go 1/2" or 5/8" PEX?
    3) Taper down to a 3/4" air eliminator or the Taco 4900 will work?
    4) Transfer plates?



    Thanks in advance.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    400' in a single loop is probably too long - better to limit it to around 200' or the water at the end of the loop will have cooled off too much and the head is higher making even heating more problematic. You can drive both loops on a floor with the same thermostat. You should never (well, almost never) size a boiler for your potable hot water needs...adjust the size of the tank. Running a bigger boiler for that purpose means most of the time, the boiler is way oversized which affects efficiency. To avoid hassles with nailing into the pex, I'd seriously consider an engineered hardwood floating floor that snaps together. www.kahrs.com has a huge selection and is top quality stuff. There are others that make similar stuff as well, but Kahrs is really good stuff.
  3. CO-Chris

    CO-Chris New Member

    Messages:
    3
    Location:
    Boulder, CO
    Jim--thanks for your response. I understand the concerns about the long loops but I guess I'm adverse to running a lot of loops in the joist space. That would be double the supply and return and lines and space is a premium. The structure is actually 4 stories with 500 sf per floor. As long as the loops are designed for the circulator, it seems I would be good. In fact, as I stated, using 5/8" with a 20 deg delta-T will put the flow below 2 fps. The low flow is my concern. The heat calculations show that it would work fine unless there was an issue with laminar versus turbulent flow.

    On the indirect water heater, it's not added on top of the space heating requirement. I'll have the indirect on a priority loop. Basically, I wanted 135 gallons first hour capacity and 75,000 BTUH is the required boiler size. My space heating only requires 45,000 BTUH. So, my boiler is sized at about 76,000 BTUH net of all applicable deratings.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    If you use a bigger indirect, you can achieve the first hour results with a smaller boiler which will save you money every time the boiler fires over its life. If your usage is mostly showers, then a waste water heat recovery system will let you keep the smaller tank AND a smaller boiler and maybe gain you some benefits on rebates. It will not help with filling a bathtub. I'm by no means an expert on this, but 200' is often the max suggested on a loop. Now, using 5/8" may make a difference in that.
  5. CO-Chris

    CO-Chris New Member

    Messages:
    3
    Location:
    Boulder, CO
    Jim: thanks for the idea on the shower waste water heat recovery units. I'm going to look into those.
  6. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    Drainwater heat recovery units are a bit hard to find in the US- they can be ordered (at full retail) through Home Depot, and most of the vendors have buy-direct web pages but you have to pay shipping from Canada (where most of them are located. ) EFI is the US distributor for one of them, and it's pretty easy to open an account and get a much reduced wholesale price on the select assortment they carry. They're a pretty good vendor, they ship orders promptly and don't tack on handling charges, you pay what they paid for the freight.

    When in doubt, go with the tallest and fattest that fits, since the heat transfer efficiency increases with increased surface area. Natural Resources Canada maintains a list of models sorted by third-party tested efficiency at 2.5gpm shower flows.

    They're basically a section of copper drain pipe with a tightly wound slinky of squared-off potable copper around it. To work it has to be dead-vertical so that the surface tension of the drain-side water can spread it into a thin film over a majority of the drain's surface. The output of the heat exchanger needs to feed both the water heater's cold input as well as the cold side of the shower to achieve the tested efficiency.

    [​IMG]

    [​IMG]

    [​IMG]

    This turned out to be THE solution to showering capacity for my low-BTU output combi heating system. I can shower all day if I wanted to, and that's even while serving calls for heat from zones.
  7. LamdaPro200

    LamdaPro200 In the Trades

    Messages:
    10
    Location:
    New York
    Recovery is going to be dependent on your temp rise which is the difference between the incoming cold and the DHW set point.

    75,000/500/ (120-40) = 1.875 gpm x 60 Minutes = 112.5gph continuous rating. Just add the usable storage for 1st hour recovery. No matter the size it can only produce what you feed it. A larger indirect just increases your usable storage for that initial dump.

    Have been doing radiant for more then 20 years and we never, ever design for a 20 degree delta in a joist heating application. If using real good alum extruded plates such as Uponor Joist Trak or Viega Climate Trak you could use a 15 anything less of a quality you are going to want to shoot for a 10. You want the floors to be evenly heating across the mass. I don't know of anyone that makes a 5/8" plate nor would I want to pull it. Typical application is 3/8". You can't get any more btu/hr out of 1/2" then you can with 3/8", just get a 50' longer loop length. Max loop length on 3/8" is 250'. 500 Sqft feet would require 3 Loops of 250' 8" cc. Flow rate on 10,000 btu/hr would be .4gpm per loop @8' of head.

    As for air elimination a simple Spirovent will do the trick and once you purge and get rid of air, you shouldn't be get air back. I believe on the Loch you can cap off the high end of the modulation rate for the heating circuit independent of DHW. You could conceivably have one boiler with two different top ends. As for my pumps I'd be using the variable speed circ for my boiler pump and let the boiler logic control it. Pipe pri/sec or LLH and use Delta-T pumps for the radiant. Delta-P type pumps work well but they still operate on a fixed curve. Might be a funny inverted curve but it's still fixed and it runs that same curve whether it's October or January. A Delta-T pump will vary it's speed based on what the emitter, in your case radiant is pulling out thus increasing and decreasing its speed to maintain a delta-t.

    Best of luck on your project.
  8. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    ...which is why the indirect needs to be sized for the biggest tub you have to fill, but beyond that it doesn't much matter.

    Even if run to depletion with a gusher-sidespray shower in conjunction with drainwater heat recovery, the delta-T is much lower, since the incoming water is and has been already much-improved.

    It's actually impossible to deplete the tank to below 105F with a legal single-head shower, and priority zoning on the indirect when used with drainwater heat recovery. Even with 35F incoming water, 105F shower head temp, and 2.5gpm shower head flow that's only about 87,500BTU/hr, and you can get more than 75,000BTU/hr out of the Lochinvar, Without DWHR you can eventually deplete the indirect with a shower, but even with a 30% DWHR unit it would be clawing back ~26K of what was going down the drain, which more than covers the shortfall, which means it can recover to the full setpoint temp even DURING an endless 2.5gpm shower.
    Last edited: May 9, 2013
  9. LamdaPro200

    LamdaPro200 In the Trades

    Messages:
    10
    Location:
    New York
    A 60-inch Power-Pipe System, for example, can raise the cold water temperature from 10°C (50°F) to as much as 24°C (75°F), under equal flow conditions.
    (From www.renewability.com). So we'll use the 45 Degree Rise..

    79,000/ (45 x 500) = 3.5gpm x 60 = 210gph continuous.. So yes because you reduced Temp Rise you get better recovery. Can't argue that, the math never lies. You increased recovery by roughly 45%. I corrected myself, the Heating Capacity of a WHN085 is 79,000 Btu/hr.

    Indirect doesn't necessarily need to be sized for the biggest tub dump. What if set point was 140 and I mixed down?
  10. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    "What if set point was 140 and I mixed down?"

    Do the math- it's 7th grade algebra.

    You'll get some tub-fill capacity boost, but to get very much you'll need more of a bump than moving from 130F to 140F. (Figure on 108-110F for tub fills, not the ~105-106F for showers.)
  11. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Sizing the tank to the largest tub is first grade math, though it has been a while...

    100% laminar flow and stratification of the flow stream, not likely.

    Extruded plates under a finished floor, no nails after. Insulation below.

    200' for 1/2" O.D. PEX for residential heating, 200' of 1/2" ID PEX for snow/ice melting.

    Proper heat loads and professional design help instead of online granny design.

    You could get a fancy heat recovery "Power Pipe" or leave the water in the tub until cool, saving over a grand, investment zero.
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