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Advice needed for converting black iron to Pex Al PEx

basementremodel3 Member Posts: 2
edited August 2020 in Radiant Heating
My goal is to gain head room in a basement by replacing the supply / return lines for a boiler that feeds 11 radiators. I would prefer that the lines run in the joist bays or at minimum just below the joist.
There are currently NO zones, it is just two main lines located in the basement that have lines that branch off to each radiator.

I’ve looked into Pex Al Pex and that seems to be the route I should go. I only found one video that relates to exactly what I want to do, EXCEPT I don’t want a manifold with 22 lines (there’s a total of 11 radiators in the home). What I would like to do is convert just the main supply/returns that are located in the basement.

Is it possible to replace just the main supply / return in the basement with the Pex Al Pex? Or do I have to do a manifold with connections to all the radiators? I attached a link to the “This Old House” video.

Pictures - https://imgur.com/gallery/LFSUwGE
Radiators to Pex Al Pex: https://www.youtube.com/watch?v=5jfzmIOUSzA&feature=youtu.be


  • fenkel
    fenkel Member Posts: 162
    How is the system currently working/setup?
    One supplyline? does it have zones?
    Pictures would help alot..
  • kcopp
    kcopp Member Posts: 4,393
    Need to be careful w/ this set up.
    If there are diverter tees involved and you take them out you could seriously effect the operation of the system.
    We need more info.
  • basementremodel3
    basementremodel3 Member Posts: 2
    Here are pictures. It is NOT zoned and I don't feel there is a need to zone given that the house is not that big. In the basement is just one supply and one return, feeding each radiator individually.

  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,937
    edited August 2020
    Does your system have a pump?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Ironman
    Ironman Member Posts: 7,350
    That’s an old gravity system that’s been converted to forced flow. You need to be very careful about altering it as you can easily throw it out of balance.
    If proper piping practices are followed, it can be done, but it ought to be under the supervision of a hydronic a pro who understands that system.
    There may also be orifice plates in the rads that need to be relocated. And you may end up needing to replace radiator valves which involves removing radiator spuds that have been in them for 100 years.
    Every DIY video makes things look easier than what they really are.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,700
    I would go the home run and manifold , no need to go 3/4" run , 1/2" would be plenty . On the video they used manifolds with flow regulators . It would be much better if they spun one of the manifolds around to help balance pressure and flow of the circuits ......
    I have enough experience to know , that I dont know it all
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,567
    Pipe sizing between the boiler and the emitters (radiators) is very important. If you change the 2" and 1-1/2" pipes to something smaller, the balance designed into the larger pipes will suffer.

    The new PEX Al Pex pipe would need to be large enough to handle all the capacity of all the radiators. This may require 1" pipe inside diameter if your boiler is over 40,000 BTU capacity and if it is larger than 100,000 BTU Capacity, you may need 1-1/4" from the boiler to the first and second radiator branches.

    I'm not sure that size is easy to come by. Here is some info you can use to familiarize yourself with how water is used to heat your home. It is all about temperature difference and gallons per minute flow rate.


    Page 4 and 5 of this text will get you on the road to understanding the basics of your heating system.
    The last page has pipe sizing guides for iron and copper pipe. Pex is closer to copper pipe size.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,567
    edited August 2020
    One option is to use home runs from a new manifold near the boiler to the 3/4 iron pipe where it goes into the basement ceiling. My guess is that you want to get rid of the low hanging large pipes. In this picture, it would be the 2" pipe and the 1-1/2" pipe and the 1" verticle pipe.

    This is easily accomplished by cutting the 1" pipe with a saw. then the hard part. See if you can use two 18" or 24" pipe wrenches to remove the 90° elbow from the pipe you want to keep. (The one that is above your new ceiling hight). If this does not work, then apply some heat with an acetylene torch with a large orifice tip (needs a lot of heat) and try again. If that fails you may need to cut partially thru the fitting being careful not to disturb the threads on the pipe nipple you are going to keep.

    After you have the pipe threads exposed, you can add a 1" x 3/4" reducing elbow or coupling. On the female threads, you can use a PEX x Iron Pipe adaptor and run Pex back to the boiler room manifold.

    I might have the pipe sizes wrong, It could be a 1-1/2" x 1-1/4" x 3/4" Tee. But the concept is the same.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • fenkel
    fenkel Member Posts: 162
    It looks like an older converted steam to hot water system..
    The problem you will run into is balancing the heat between your heat
    emitters. If you do it wrong, only a few emitters will provide heat, the others might get warm and some could be cold..
    With this system, you want the last emitter to be the first back to the boiler...that will maintain even heat distribution in this system...
    Get a copy of a navien install manual, they have a working diagram  using the same system as you have.. ( I've enclosed a copy of the Navien diagram)
    I might be concerned with junk in this system...if it was a converted steam system, you'll have a lot of sediment in it and changing pressures might loosen it up and possibly plug up your Bosch boiler..for good practice, a good strainer in the return line to your boiler would be recommended..
    next, pipe size could also cause flow issues, I'd look into Pex that's at least 1 inch in diameter.. could be expensive... 
    you might be better off running your system in series,  from one radiator to the next,  then to the next,etc and back to boiler....  the problem with this is: the last radiator will be colder and that area might not heat as well.... to cure that you 'd want to have zones... like bedrooms and bathrooms on one zone, other rooms on the other zone....  but this could/will cause your boiler to short cycle, which will cut the life of you boiler down...
    the nice thing about your current setup is it holds a lot of water in those 2 inch pipes, the more water in your system the longer the boiler runs and the more efficient your boiler gets.. 
    question is at this point.. how much do you want to spend to do it correctly?
    if you're a DYIer , you might be opening a can of worms... you'll only catch your mistakes when its below zero outside and you have poor heating in the home...