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Initial target Flow Rate calculations

HydronicRookie Member Posts: 54
Hi all,

I'm slowly making my way through Siegenthaler's book to aid me in my 3-storey/3-zone home-run Pex cast-iron rad project. I'm just trying out some target flow-rate calculations to choose a pipe size for the respective rads and I'm getting numbers that don't feel right.

To get the overall flowrate of the system given a 62,000 btu/hr design load at 170˚ average water temp. and a nominal 20˚ drop - (62,000/(500 * 20)) - I get 6.2 gpm.

In looking at a particular zone, say with a load of 26,000 btu/hr - (26,000/(500 * 20)) - I get 2.6 gpm.

Now when I look at a single rad, my fear is that flow-rate is getting too low - Example: for the rad in the 1st floor washroom which has two outside walls and a load of 1000 btu/hr (*one of the smaller loads in the house) - (1000/(500*20)) - the flow rate is 0.1 gpm.

This now lies below even the 3/8" Pex-Al-Pex minimum of 0.6, that is, if I want to achieve a minimum flow velocity of 2 ft/second. Now I've read on this forum, that achieving that minimum isn't absolutely crucial, that there are ways of removing dissolved air in spite of the low velocity, and that laminar flow isn't that a big deal in pipes (if it even happens). But how low is too low?

Here are calculated rates for the other rads in that zone:

Living 0.7 gpm

Dining 0.4

Ft. Foyer 0.84

Kitchen 0.48

Bathroom 0.1

Bk. Foyer 0.08

My thinking is to use 1/2" Pex-Al-Pex for most of the rads in the house, but the minimum flow rate (* again, assuming I want to achieve at least 2 ft/sec velocity) is 1.2 and these numbers are way below that.

Have I got the right idea here or am I way out in the weeds? Thanks very much.


  • Rich_49
    Rich_49 Member Posts: 2,766
    AH hah

    The old Delta t / ASHRAE standards trap .   You have cast iron rads and they will do fine with even a wider than 20* Delta T , they work real good with a 30 - 40* Delta but you don't wanna do that unless you have unless your sure of your flow unless you have a condensing boiler and can go even a bit lower on the SWTs.  Here is the tricky part , ready ,  between 2 to 4 ft per second is real good for heat transfer . Question , do you want heat transfer in your supply and return piping where you receive no benefit form it ?   Those standards in my opinion belong with baseboard , HXs .    No air should be getting that far anyway with the caliber of eliminators that are readily available now .  A radiant system is really gonna mess with your head , wait till you see how laminar those numbers are .  Read further you'll get to the spot where Siggy tells you about short fat headers wuth laminar flow in them too . 
    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
  • Zman
    Zman Member Posts: 7,569
    Flow rate

    First off, you do not need 2fps with a continuos pex loop. It will easily purge air at 1/2 that.

    I do think you will have trouble balancing a system as out of balance as yours will flow rate alone.i also think you are likely to have air issues at .1 gpm.

    Is it possible to put trvs on in the areas that are over radiated.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Eastman
    Eastman Member Posts: 927
    rad average tempreture

    Are the rads really going to have a 170 degree average?  The supplies and returns are adjacent, how do the rads channel water through the sections?  Are you converting from steam?
  • Canucker
    Canucker Member Posts: 722

    I believe the rads are connected across the top and bottom, if I'm remembering the thread correctly. I don't believe it was a steam conversion
    You can have it good, fast or cheap. Pick two
  • bob_46
    bob_46 Member Posts: 813
    No diff

    Eastman, not to worry. You can read the full engineering report in the attached PDF

    The difference between the coefficients in the two equations is only one-half of one per cent. This deviation is easily within the limits of experimental error, and it may be concluded that the rate of heat emission is the same for water entering through the upper tapping as for water entering through the lower tapping. For this particular radiator the equation may therefore be written:
  • HydronicRookie
    HydronicRookie Member Posts: 54
    Just the sort of reassurance

    I was hoping for, thank you. This 3rd edition of the book hardly mentions CI rads, so I sometimes feel like I'm swimming in many unknown unknowns as a beginner.

    The boiler will be mod/con so I'm all for a greater deltaT, though what about the even smaller flow rates (i.e. half the size)? Surely, at these flow-rates, I'm looking at 3/8" Pex-Al-Pex for the lot of them?
  • HydronicRookie
    HydronicRookie Member Posts: 54
    Balancing act

    Thanks for posting your thoughts. I've been wrestling with the possibility of TRV's on each rad for a handful of days now - the issue being that all my rads are left-threaded, and the TRV tail-pieces seem to be all right-threaded. It amounted to a fairly lengthy forum-thread on here, where I received lots of good advice, but with no easy, clean, affordable solution. That being said, I'm still investigating ways to make it work.

    I'm curious to know what makes my system 'out of balance' in your view. Is it the fact that the flow rates are so low and so varied? Also, with regard to the trouble in balancing, is that because the balancing valves on the manifolds don't measure/adjust that low?

    Are there any remedies to air-issues I might encounter at these low flow-rates?
  • HydronicRookie
    HydronicRookie Member Posts: 54
    edited June 2014
    I simply chose

    170˚ average because it was the temperature that brought my calculated CI rad output closest to design load. And since outside temperatures won't match design-day temperatures but a few times a year, I imagine the water temperature will seldomly climb to 170˚ either. Or maybe I'm going about this all wrong, I don't know.

    The rads were always hot-water, and they're connectedat the top and bottom. I'm curious as well to find out how efficient or inefficient (in terms of distributing hot-water) the rads are made by having the inlet and outlet side-by-side. From what I've seen, its quite a common configuration here in Canada.
  • Eastman
    Eastman Member Posts: 927
    very useful information

    However, H.R.'s connection scheme isn't discussed, just top left to bottom right and bottom left to bottom right. Also, figures 15 and 16 reveal an impact on surface temperatures due to these flow distributions.
  • HydronicRookie
    HydronicRookie Member Posts: 54
    Just realized

    3/8" Pex-Al-Pex doesn't exist unfortunately, which means I'll have very low flow velocity in the pipes at 1/2" PAP, but I guess the upside is there will be a lower head-loss with the larger pipe-size.
  • NJ, Designer
    NJ, Designer Member Posts: 53
    edited June 2014
    Eliminate some rads

    IMO, I would look to reduce the amount of radiators, It is not necessary to have a small radiator in every "closet" small washroom, Foyer etc....... I don't see your layout but it sounds like removing the small radiators form the internal rooms and adding them strategically to the closest exterior rooms would save you some time money and headaches... 
  • HydronicRookie
    HydronicRookie Member Posts: 54
    Definitely fragmented

    Thats a fair observation and now that I think about it, there are a few instances where two smaller rads could be replaced by one bigger one. I guess I was just biased by the desire to use the one's i've got and placing them in locales where heatloss is worst (i.e. by outside doors and windows).