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CI Radiators, converting iron pipe to pex in the basement

I'm doing a basement remodel and am looking to sanity check some work I am going to perform. In short, I want to remove all the iron pipe from the basement and run pex from a manifold to the different branches, connecting back to the old pipes where they leave the basement.

Boiler is an older Weil-McClain, 137K BTU. There is a single Taco circulator on the return side. When the boiler was installed (80s?), 1.5" copper was run from the boiler. On the supply side it connects to an ancient Thrush 120 flow control valve and then off to the iron. On the return side there are a couple of old Walworth thingies - not sure if they are to balance the flow? Pictures of the setup are attached.

The house has 13 radiators with a sum of 703 sq ft of radiation. Boiler seems to run around 170 degrees, which gives a radiation output of 105K BTUs (I realize the water temp can be adjusted up or down). I haven't done a heat loss on the house, which I realize should be done to get an accurate picture.

To the plumbing proposal. The 13 radiators are on 10 loops from the basement. Here is the load per loop:

Loop BTUs
1 7800
2 12000
3 12750
4 8550
5 9750
6 12600
7 22500
8 9450
9 6000
10 4050

I am planning to use a RiFeng manifold and 1/2" pex-al-pex. I like the manifold approach because I can then balance the loops centrally instead of dealing with frozen radiator valves all over the house. I'm looking at the RiFeng over alternatives such as a Watts mostly due to cost.

The 1/2 Pex-Al-Pex and the manifold ports should be provide enough flow for all loops except #7 (22.5K BTUs). This loop is a 1.5" iron pipe in the basement that rises to feed two radiators on the 2nd floor and one on the third. I can't split these to their own loop without tearing the house apart. To provide additional flow, I am considering creating loop 7a and 7b, with two 1/2" runs into a 1.5" tee at the pipe. (meaning 11 loops on the manifold). 5/8 pex would probably flow enough on a single loop but I'm not confident that manifold can flow enough on a single port.

My next concern is that the aggregate flow through the manifold would nee to be > 10 GPM. The manifold has 1" supply and return connections. Can 10 GPM get through that? Seems like it would be really pushing capacity. Therefore I am thinking of going with two 6-port manifolds instead of a single 12-port. I would tee off of the 1.5" copper on the boiler to a pair of 1" lines feeding the manifolds.

Finally, would an air separator, such as a taco vortech, make sense here? If I install this on the supply from the boiler to the manifold(s) would it eliminate the need to go around the house bleeding individual radiators?

Any thoughts, comments, suggestions?



  • Ironman
    Ironman Member Posts: 6,560
    Can you post some more pics? Particularly of the boiler with its near piping, you expansion tank and it's connection and some of the old ping in the basement?

    A couple of first observations:
    1. I would not use any Chinese manifolds, particularly Rifeng. I've had a couple of jobs where customers supplied them and they leaked like sieves. Now, they get my manifolds or they get someone else. You get what you pay for. Look at Rehau, Caleffi, Roth, Uponor.
    2. With 1/2" pex run from manifolds, most of the time you're not gonna get more than .75 gpm per loop, if that much. That translates to 7500 btu's at a 20* delta T. You'll need 5/8" or 3/4" for those rads requiring more.
    3. Anytime you modify an old system, there are consequences that may not have been foreseen. If you're gonna err, do it by using larger runs. Maybe 5/8" minimum and 3/4" on the larger rads.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    MNOldHouseSteve MinnichRobGicesailor
  • MNOldHouse
    MNOldHouse Member Posts: 4
    Thanks for responding. I've attached a few more pictures as requested.

    You can see where I already ran some 1" pex to replace one of the loops (actually feeding a few off of that). I'm having balancing problems hence my desire to change the manifold.

    I'll look into those other manifold brands.
  • SWEI
    SWEI Member Posts: 7,356
    If you're looking for an inexpensive manifold, check out the Sioux Chief models with balancing valves. They make about a hundred different non-valved configs as well.
  • 10kREDLINE
    10kREDLINE Member Posts: 15
    I had a similar situation to you. I had large pipes feeding cast iron radiators in a reverse return config. I ended up eliminating all of the large piping, and installing 1/2 inch manifolds on both the supply and return ( one manifold set for each zone using a zone valve on the return ), with home runs to and from each radiator. Some radiators larger or smaller than others. The house now heats up in half the time, and I've noticed fuel savings. Straight forward near boiler piping, pumping away. No primary/secondary etc..
  • MNOldHouse
    MNOldHouse Member Posts: 4
    I'm a little confused...did you run 1/2 pex to each radiator? How big (BTUs) were they?

    Also, how many zones do you have? I'm assuming you have a single zone valve to the return on each manifold, rather than a value on each loop of the manifold. If that is the case, how are you balancing difference size radiators on the same manifold?

  • 10kREDLINE
    10kREDLINE Member Posts: 15
    Yes 1/2" to each radiator, largest rad approximately 15-18k btu . 3 zones, with a pressure differential bypass. Yes, at each return manifold is the zone valve. There is no specific "balancing" done. Not that i just left it to chance, but rads were fairly similarly sized, although there is one fairly small radiator on the first floor zone, everything heats fairly evenly. Also I believe the the differential bypass helps everything to run that much smoother, keeping the pump close to its curve. Obviously if all 3 zones are calling, everything heats a little slower due to the amount of heat required. But it takes no where near the time it used to.
  • 10kREDLINE
    10kREDLINE Member Posts: 15
    Just to be clear, in one zone I have 3 "large" radiators, 2 "medium" and 1 "small". I guess you could call it my most diverse in terms of size. With that zone specifically or any other zone calling, I have not found a balancing issue and i had been looking for any issues after i completed the retrofit.. They all heat evenly.
  • MNOldHouse
    MNOldHouse Member Posts: 4
    Thanks! Any chance you can post a picture of your setup?
  • 10kREDLINE
    10kREDLINE Member Posts: 15
    absolutely. give me a little while i'll post it up
  • 10kREDLINE
    10kREDLINE Member Posts: 15
    i dont know why theyre so big, sorry
  • vincyking11212
    vincyking11212 Member Posts: 7
    Hi, I’m trying to do a similar setup and need some advice prior to installation next month. I bought a 7 section manifold to feed 6 radiators and 1 hot water heat blower, a navien NHB-55 (Mod con boiler), and running Homeruns 1/2 inch pex al pex. However I am concerned about my design. I am converting the entire house from a 140k BTU steam heat system with DHW to 2-Navien mod/con with single zone use. What are your thoughts?
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,309
    This post is almost 5 years old. Best to ask the moderator to start a new thread and you can put in a link to reference this post.

  • Steamhead
    Steamhead Member Posts: 14,838
    I just replied to the new thread. I think you're wasting your money.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service