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Radiator re-pipe. Copper or Pex?

JerseyDIYguyJerseyDIYguy Member Posts: 24
I live in a 90 year old house in the Philadelphia suburbs that has hot water heat and nice old radiators in every room. The house is about 1400 square feet and has 9 radiators of various sizes. The system was originally gravity fed so it has 3" black iron pipes suspended from the basement ceiling. Now I need to move all of the suspended radiator piping on the west half of the basement ceiling because I'm finishing the basement.

About 9 years ago the heating system was converted to gas (from oil) and changed from gravity fed to pumped. The boiler is a Weil Mclain CGa 5, 117,000 BTU unit. The installer ran 1-1/4" copper piping to and from the boiler to the original 3" black iron pipes.

I'm very much a do it yourself plumbing person. I've completely removed all of the original black iron water piping in the house and replaced it with copper. I've also done all of the plumbing (and electrical, structural, tile, finish, etc.) on 9 bathrooms and 3 kitchens over the years so I'm very comfortable sweating copper pipe. However, I have never worked with Pex.

I was discussing the need to remove and replace the black iron piping with a contractor friend and he suggested using Pex as far easier and less costly than copper pipe. A bit of digging here and on the web seems to confirm that it's doable, but my lack of experience with Pex concerns me.

I would start by demolishing the 3" black iron, leaving the smaller iron pipes (a mix of 3/4' and 1") that run directly to each radiator un-touched. Then I have a few options:

1) Cut the existing 1-1/4" copper, sweat new 1-1/4" copper to it, route the new 1-1/4" copper where needed to feed the radiators, and use fittings to reduce to 3/4" copper pipe running to and from each radiator. I'd use threaded brass fittings to join the new copper pipe to the existing iron radiator pipes.
2) Cut the existing copper, connect 1-1/4" oxygen barrier Pex, route the 1-1/4" Pex where needed and use fittings to reduce to 3/4" Pex running to and from each radiator.
3) Cut the existing copper, sweat manifolds to the supply and the return pipes, and run 3/4" oxygen barrier Pex from the manifolds to and from each radiator.

I'm comfortable with option 1 because I know I can do the job, but it appears to be the most work and the most $$$ by quite a bit. Option 2 appears OK, but option 3 seems a bit more elegant. With both 2 and 3 I'm concerned about using a product where I have no experience in a demanding application and pretty big consequences if it fails.

I'd looking for advice on the right way to proceed. Thanks!


  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    The way I would do it is to find a place for a manifold and run 1/2" pex-aluminum-pex home runs to each rad. I would not use a straight pex product in this scenario - it expands alarmingly when hot and will create noise and chafing issues. On a 1400 Sq ft home, 1" supply and return to your manifold should suffice. I'd use copper there.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    That's my option #3 which I like a lot (other than having no experience with Pex!). I'd definitely use Pex-Al-Pex for the job.

    Are there any Pex-Al-Pex products specifically for radiator heating systems? I've seen lots of Pex-Al-Pex advertised for underfloor radiant heat systems but those generally run at lower temperatures than a radiator system.

    What about manifolds? Any differences there?
  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    They pretty much all are rated for radiant or high temp duty. Same with manifolds. They can get blinged up ie stainless, flowmeters, etc etc for those special applications but for what you're doing, there's no need to get crazy.

    You're probably looking to get those rad sup plies down to 1/2" via new valves and return L's, or reduce somewhere to the pexalpex.

    Not sure what's available to you regionally, but my go to PAP has been Viega Fostapex and manifolds. I like Uponors EP manifolds but I find their PAP fitting system too exotic for my taste.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The Fostapex looks interesting. I'd need to buy a manual crimper tool and the pex prep tool but that should not be a real problem.

    Is 1/2" PAP large enough to supply the radiators that have 1" iron supply pipes? The largest radiator in the house is about 5' long (23 sections IIRC), almost 8" deep, and about 30" high.
  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    1/2" will flow about 15,000 BTU. That's a huge rad. It's good to get a room by room heat loss done and compare whatcha got vs. What you need for heat.

    I will have to look at my EDR book to get an approximate value on that rad for you. 15k needed in a room in a 1400 Sq ft house sounds excessive...but a heat loss will say.

    get back to you on that output.
  • bob eckbob eck Member Posts: 915
    Yes Viega Fostapex is a great pipe. Also take a look at using Watts RadiantPEX-AL it is great for running from a minafold to cast iron radiators and back to the manifold.
    Should he run reverse return. First supply off the supply manifold out to radiator would be the last on the return manifold. When using the Watts tubing you do not need to purchase a prep tool or special crimping tool.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The Watts PEX-AL looks good too, although I've had some bad experiences with compression fittings before (copper tube with ferrule and nut - not PEX). Any issues with the compression style fittings with Watts versus the crimp style fittings with Fostapex? I'm OK with needing to buy a few specialized tools for Fostapex if it makes for a better and more reliable outcome.

    I'm good with setting up the radiators on a reverse return. Would it be wise to connect the one really big radiator (discussed above) to the first outlet on the supply manifold and to the first inlet on the return manifold (and then reverse return the remaining radiators)? I'm guessing (just a guess...) that this would tend to increase the flow to that one radiator.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    Another wrinkle... The wife is a bit concerned about having 18 lines of orange PEX tubing running around the basement ceiling (though about half of those will be hidden once the basement is finished). So I looked into using Onix tubing from Watts. She'd be much happier with black tubing.

    Is Onix still susceptible to issues with "black gunk" that I've read about here? It looks like there haven't been any recent reports of black gunk issues so maybe Watts improved the product?

    Onix is listed as PEX-AL-PEX but some people have reported that the AL layer is very thin (Mylar-like?). Any experiences using Onix with cast iron radiators?

    I'm still debating the 1/2" versus 3/4" tubing to run to each radiator. Anyone have a link to an estimator/sizing tool?

    BTW, we had a cool night the other day so I turned the heat on in the house and ran the boiler for an hour. The system is set up with a crossover pipe and valve between the boiler output and return pipes. This mixes some of the return water with the boiler output water before going to the radiators. The valve was set partially open by the installer and I've left it that way since the system was first activated on his recommendation. The water temperature heading to the radiators (after being mixed with some return water) stabilized at 140F. The pressure gauge on the output side of the boiler stabilized at about 15psi.

  • SWEISWEI Member Posts: 7,356
    Onix is an EPDM tube product.

    Given the track records of PEX and innumerable rubber products over the past three decades, my money goes on the PEX, or PEX-Al-PEX if you need the stiffness (think exposed radiator risers.)

    As far as sizing, 3/8" PEX is easy to fish through existing wall cavities and at 5 FPS (a reasonable limit for a radiator runout) will carry 15,000 BTU/hr at a 20°F ΔT or 22,500 BTU at a 30°F ΔT. 1/2" PEX carries 67% more than 3/8" will.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    I ran some calculations based on radiator sizes and sections based on the info here: Cast Iron Radiator Heating Capacity Guide.pdf

    The 9 radiators total about 39000 BTU/hour for 140 degree water (they'd be about 73,000 BTU/h for 180 degree water).

    The largest radiator is about 7,000 BTU/h at 140F (about 13,000 at 180F). The smallest radiator is about 2,700 BTU @140F.

    I've done more digging and the Onix is off the list. I'm just too worried about potential gunk and corrosion issues. However, I told the wife about using white Pex-Al-Pex and she's OK with it so it looks like I need to choose between Fostapex and Uponor.

    I'm thinking that I'll sweat 1-1/4" manifolds to the boiler supply and return tubing and then run 3/4" Pex to all the radiators. That way there should be plenty of heat capacity for all of them. All the valves on the radiators work so I can throttle each of them individually.

    Any differences worth noting between Fostapex and Uponor? How about their connectors?
  • Rich_49Rich_49 Member Posts: 2,540
    edited June 2015
    1/2" would be more than sufficient to these cast rads . 3/4 is more than you likely need at the outputs you are stating and is more difficult to work with .
    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
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The 1/2" does have a lot of advantages (price, ease of install, better manifold availability...).

    Would using 1/2" versus 3/4" have any issues in terms of water temperature returning to the boiler or flow rate of the system? In the winter the system runs for at long time (an hour or two) when the heat first comes on in the morning (thermostat is set back about 10 degrees F overnight).
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    Interesting stuff about the flow rates. I double checked and the big radiator is currently fed by 1.5" black iron pipes. The smallest radiator feed pipes are 3/4" black iron. Everything is driven by a Taco pump on the 1-1/4" copper return tube so the flow rates through the radiators must be pretty high with the current setup.

    It looks like I'll sweat 1-1/4" copper manifolds to the supply and return lines on the boiler, then run 1/2" Fostapex to/from each radiator using a reverse-return arrangement.

    I'll use black iron fittings on the larger radiator pipes to adapt all of them down to 3/4" threaded NPT and then connect with threaded press pex connectors like this one:

    Any holes in this plan?

    BTW, thanks again for all the great insight.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    edited June 2015
    And now back to our regularly scheduled thread.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    It looks like I'll be starting the demolition sometime next month and will probably do the job over several weekends. Are there any issues with leaving the boiler and cast iron radiators with (presumably) a small amount of water in them standing open for a few weeks? Corrosion?
  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    edited June 2015
    Not for a few weeks. You'll be flushing things out at purge anyway. Going with the 1/2" PAP, off a manifold, reverse piped S/R? Alpha or Taco Veridian/2218 smart pump?
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    edited June 2015
    It'll be 1/2" Fostapex. Everything run off of manifolds. Reverse piped. The existing pump is a Taco 007-F5 (the system was converted from gravity fed to pumped about 9 years ago).

    I've seen setups where the supply and return manifolds are intentionally oriented in opposite directions (e.g. in the supply manifold the water moves to the east and in the return manifold the water moves to the west). Is that just a way of aligning the inputs and outputs in space with a reverse return system? The way my system is set up both of the manifolds would have the water moving to the east. Should I reverse one of the manifolds?
  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    As long as you do a "first out, last back" on the rad home runs, that's important. It'll send the system into a better balance. I wouldn't be concerned with supply/returns to the boiler except for orientation practicalities. I would rethink the venerable 007. Much more flexibility with an ECM pump.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The ECM pump sounds interesting but I likely won't own the house long enough to make the return on investment worth it. The good news is that the existing pump has shutoff valves on both sides so if I need to change it in the future it's an easy fix.
  • Bob Bona_4Bob Bona_4 Member Posts: 2,083
    You have energy rebates there in NJ? Here in CT, one of those pumps with rebate is costing the same as a conventional.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    We do have energy rebates in NJ, however they really only apply to full system installs, not just swapping pumps. I'm no where near needing to replace the 9 year old boiler.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    Things got delayed a bit (summer happened) but now I'm ready to start demolishing the old black iron pipes.

    What's the best tool for cutting black iron? I've got reciprocating saw blades for cast iron and I've also got an angle grinder with metal cutting wheels. Any other tools I should try?

    Also can someone identify the fittings in the photo?

    The pipe is 3/4" black iron and the fittings look like they might be some kind of union or check valve. I'm especially curious because the threaded pipe nipples to the left of the fittings seems to barely go into the fitting.


  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The demolition is done, all the new fittings and manifolds are in, and I have one more question:

    I'm installing 1/2" Fostapex and using their Pex Prep tool to shave off the outer pax and aluminum layer before inserting into the Pex Press fitting. When the shaved end of the tube is inserted all the way into the fitting there is still about 1/8" of the black core Pex tube sticking out of the fitting. Is this much of the core tube supposed to stick out? Could this create issues with oxygen getting into the system?

    BTW, I ended up using a recipro saw and metal cutting blades to demo the black iron pipe. It went well.
  • njtommynjtommy Member Posts: 1,105
    Sounds like a nice project a ECM pump would really be ideal for your system. Even a mod con boiler when the time is right.
  • hot_rodhot_rod Member Posts: 13,494
    Not an issue with exposed tube, it is stripped back under the crimp collar also and exposed to O2 I suppose
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    The install is finished and the system has been running for a few months now. I’ve bled all the radiators a few times plus there's an air extractor at the top of the piping above the pump. There’s an expansion tank and it sounds hollow when I tap on it.

    I’ve noticed that the system is running hotter and at higher pressures with the new PEX-AL tubing installed. With the old black iron pipes it would run at about 140 degrees F and 20 psi max. With the new configuration it heats up to 155 degrees and about 35 psi. Once it hits 155 the burner shuts off and the pump keeps running until the temp drops to about 130 and the burner re-lights. If the thermostat stops calling for heat during the cooldown the pump will shut off.

    Are these temps, pressures, and behaviors normal?
  • SmileyBluesSmileyBlues Member Posts: 1
    I am in a similar situation with my radiators and I was wondering if you had any pictures of the finish product. MY spouse is not yet convinced that Pex would be the way to go.
  • njtommynjtommy Member Posts: 1,105
    Yea please post pics and let us know how the project is going. Even though it hasn't been all that cold this year. I'm sure the house feels better.

    Yup would absolutely go with pex of the cost of copper. You can just stub the copper out and convert to pex under the floor for esthetics.

    My buddy is in the process of buying a house with all cast iron rads. Current boiler is 50 years old. Can't wait to see it.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    I'm traveling the world for the next few weeks (Florida for fun and India for work) so I won't be able to post pics for a bit. I'll just say that I'm really pleased with the way the FostaPex installed. The entire job went very smoothly and quickly, even though I had no prior experience with any kind of Pex.

    The 1/2" tube has proven to be more than adequate for my radiators. They heat up much more quickly now compared to the previous large diameter black iron pipe setup. Also the change in my basement has been dramatic. Getting rid of all of the hanging 3" iron pipe has really made things open up (plus I don't have to duck anymore down there! - I'm 6'5").

    In hindsight there are a few minor details that I'd handle differently but nothing major. I'd do this sort of Iron to Fostapex replacement job again any time.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    As promised here's some pics of the finished job. They're a bit busy but you get the gist.

    1st photo shows the boiler in beige and green expansion tank (gray water heater in backgound). The copper pipe with two yellow handled valves surrounding Taco pump is the output from the boiler running up to the air separator in red. From there the output turns to the right and the output manifold. The return manifold is in the background and is a bit lower then the output manifold.

    Second photo shows the manifolds viewed from the side. The output manifold is on the left. I wove 5 of the output and return pex lines into pairs and ran them along the ceiling joists in the center background of the picture. The remaining four outpu lines run up and to the right. The unconnected pex tube at the lower right is for a future hydronic towel warmer that I'll install when I renovate the basement bathroom (you can just barely see the valve for this line installed on the manifold to the right of the pex tube).

    Third photo is a close up of the return manifold. showing the Fostapex press connections.

    Fourth photo shows two of the output/return pairs near the connections to the radiators. The black iron pipe at the lower right is an example of how I transitioned from pex to iron using a brass press fitting.
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    If I had it to do over again I would have made a few changes:

    The output and return manifolds were located based on the existing 1-1/4" copper pipes that connected to the old large diameter black iron pipes. In hindsight I wish I would have done a bit more work to locate the manifolds a bit higher and I might have oriented them north-south instead of east-west. This would have made running the pex a bit easier and would have saved some space to the right of the boiler.

    I also would have used a manifold with 1/2" outlets rather than 3/4" and I would have used copper sweat pex press fittings rather than threaded brass. I used sweat fittings to connect to two of the radiators that already had copper pipe connected and sweating was simpler and the fittings were less $.

    I'm really happy with the way things turned out overall. The heating system performs better than it used to (it heats the house faster), I eliminated a bunch of obstructions to renovating the basement bathroom, and I don't hit my head on black iron pipes anymore!!!
  • BurbmanBurbman Member Posts: 7
    Just found this thread, and my set up is almost identical to yours. We just started some remodeling and have run into floor height issues tiling the bathroom etc, so this may be the perfect time to do this re-pipe.
  • BostonVictorianBostonVictorian Member Posts: 1
    I have the exact same setup as the OP in my 1900 home in MA. I am interested in having more specific control over certain rooms and want to zone the Rads. This re-pipe is what I am interested in doing, both to decrease the thermal capacity of the system with lots of 3" black runs, but also to zone the house.

    Could you have gone a step further, and simply put either zone valves or individual circulators onto the supply or return manifold runs, assuming that you have thermostats in each of the corresponding rooms? Would this allow the ability to individually heat each rad? My home is piped with 1/2 Honeywell Unique valves I don't have an option to instal TRVs at the rad unless I go crazy.
    Fred Rappuhn
  • Fred RappuhnFred Rappuhn Member Posts: 104
    I will like to read a reply to the above question...

    "simply put either zone valves or individual circulators onto the supply or return manifold runs"
  • BurbmanBurbman Member Posts: 7
    I'm getting ready to tackle this in the spring. My house is almost identical to OP's and am debating whether it makes sense to split the house into 2 zones, 4 rads upstairs and 5 downstairs. I already have a t-stat on the 2nd floor for the central air, and it would be easy to run a wire to it from the basement. Of course this means a second circulator and new boiler controls so not sure if I would gain anything. The house heats pretty evenly now.
  • JerseyHvacGuyJerseyHvacGuy Member Posts: 1
    I also have a question on all the work that you did on the system. Did it make a difference in your hearing bill?
  • pecmsgpecmsg Member Posts: 1,450

    I also have a question on all the work that you did on the system. Did it make a difference in your hearing bill?

    If you want to lower the heating bill Tighten the envelope. High efficiency, new piping, new emitters generally have NO return on investment. The house looses X BTU's per hour and you have to produce that. Slow down the looses!
  • JerseyDIYguyJerseyDIYguy Member Posts: 24
    There has been a small decrease in heating costs, though it can be difficult to judge given year-over-year temperature differences.

    The biggest difference is that removing hundreds of feet of black iron pipe in the basement and replacing with PEX means the system is delivering much less heat to the basement. All that black iron pipe effectively served as a spread out radiator. The PEX gives off much less heat. So while I haven't reduced the heat demand or loss in the upper floors of the house I've reduced the loss through the basement.
  • BillyOBillyO Member Posts: 199
    looks great, nice job. I love viega products
  • gt0081agt0081a Member Posts: 1
    @JerseyDIYguy , love the conversion! Like others I'm in a 1920 Foursquare with a near identical heating setup as you. I'm heating 7 rads on two floors from the basement and would like to achieve the same results (less knots on my head and more livable space in the basement. Two questions:
    1. Are you willing to share the approx. cost of the conversion?
    2. Green plumber question here: is brass to cast iron OK as far as dissimilar metals go?
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