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Replacing large steel pipes going to/from hot water boiler to cast iron radiators with PEX

mplsjot Member Posts: 4
I am finishing the basement in our 100yo 2-story bungalow in St. Paul, MN. The basement's ceilings (cement floor to bottom of exposed ceiling joists) are 7ft high or less so every inch of headroom that can be gained is a big deal. I have cleaned, primed & painted the exposed bottoms & both sides of the basement ceiling/first story 2" x 10" floor joists, rim joist end cavities & bottoms of the 1" thick rough-sawn sheathing boards under the hardwood flooring on the first story, as well as all the electrical wires & natural gas lines running thru them. Everything looks great, but things would be much better if I could get rid of the big old steel supply & return pipes that run from our hot water boiler (less than 5 years old) to the home's original cast iron radiators in each of the rooms on its first & second story. A set of large (2-3") pipes run just under the bottom of the basement ceiling joists directly above the landing at the bottom of the stairway that goes to the exterior door on the side of our house & then into the kitchen on the first story. I'm 6ft tall & need to duck quite a bit to avoid hitting my head on these pipes whenever I pass under them. In addition, they are preventing us from having a code-conforming bedroom in our finished basement because MN's 2020 Residential Code for Existing Basements states "the finished ceiling height shall be a minimum 6'4", including beams, girders, ducts, or other obstructions" & "a minimum 6'4" headroom is required in all parts of the stairway." My father was an electrical & refrigeration contractor & I spent 6 summers working for his company while attending high school & college, so I'm pretty knowledgeable when it comes to construction & mechanical "stuff." Unfortunately my dad's business was located in a small farm community where most homes didn't have natural gas, so I've got little to no experience with hot water & steam boilers/heating systems. I would like to replace the steel pipes going into the 1-1/4" inlets & outlets in each cast iron radiator with much smaller oxygen-barrier PEX pipes that can run through holes drilled in the ceiling joists. I've been reading every article/blog post I can find for this subject but unfortunately haven't found any that can provide me with the guidance I need to do so. I'd like to install two manifolds in the mechanical room where our boiler is located & then run two (supply & return) PEX pipes to each radiator in the house. From what I've read, it sounds like 1/2" PEX should be sufficient. So, do I need to run the PEX pipes all the way to inlet & outlet hubs in each radiator so I can connect them with 1-1/4" to 1/2" reducing bushings/valves or can I connect the PEX pipes to the existing steel pipes between the open ceiling joists in the basement so I don't need to try "fishing" the PEX thru the exterior & interior walls where the original steel pipes were installed when the home was built? The house's footprint is less than 700 sq ft so the pipes running to each radiator are not very long. There are only four radiators on the first story (front entrance, living & dining rooms & kitchen) & four more on the second story (3 bedrooms & one bathroom). One more thing - the hot water boiler was the house's only source of heat when we bought it, but we installed a Hartman pellet stove in the living room on the first story last winter that can heat the entire house on all but the coldest MN winter days & will be adding a 2.5 to 3 ton mini-split heat pump system with 3 high-wall units located in two of the upstairs bedrooms & the living or dining room this summer. So we'll soon have three different heating systems & will use whichever is the most efficient to operate at any given time. I expect we'll use the mini-split wall units to take the chill off in the early fall & late spring so we won't need to start the boiler or the pellet stove until the temps get & stay much colder. I removed all three of the cast iron radiators in the basement & expect the combination of the heat from the natural gas boiler, hot water heater & clothes dryer to keep it plenty warm down there, as I've had the exterior walls & rim joist cavities sprayed with 2-3" of high-density expanding foam insulation & all 3 of the windows are new high-efficiency ones. If not I can always add another min-split high wall unit down there. Any advice you have to share would be greatly appreciated! Thanks in advance for any support you can provide me with that will allow me to wrap this project up before my fall deer hunting season starts in September!


  • Ironman
    Ironman Member Posts: 6,579
    The old system was gravity flow. Has it been converted to forced flow with a circulating pump?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jamie Hall
    Jamie Hall Member Posts: 17,111
    Is the circulation of hot water pumped? Or is it still by gravity? That's a very important consideration! If it is already pumped, there really isn't that much hassle in replacing the pipes (probably iron, not steel) with PEX, and you can make the switch in the basement.

    If it's still by gravity, you can still do it -- but you'll have to add a pump.

    And either way, the trick will be to get the system properly balanced. For which reason... I'd very much suggest using a home run system to a manifold with balancing valves.

    How recent is the boiler? If it's more than 20 or so years old, it may be time for it to retire...

    A word on holes in joists: there are very definite restrictions on where, and how big, they can be. Never more than a quarter of the height of the joist, and less if possible; always in the middle third of the height of the joist; at least a foot between holes horizontally, and never closer than two to three feet from the ends or any support col
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • DavidinKenai
    DavidinKenai Member Posts: 10
    Brainstorming here, but could you tear out all of the boiler, its piping and the radiators? Does the mini-split heat-pump system switch to electric resistance heating (which I know really spins the meter), but if it's enough kW to meet code, usually you could operate the pellet stove.

    Reclaiming your head room is worth a lot and so is the space reclaimed from the boilers and radiators. Houses cost $100-200 per square foot and with all that space freed up, you might gain tens of square feet.

    But I'd start with looking at your cost per BTU for pellets and for natural gas. I'd expect NG to be cheaper than pellets even if pellets are bought by the ton and for NG to be much cheaper if pellets are bought 50 to 100 pounds at a time, retail. Perhaps you should be maximizes use of your boiler and not your pellet stove?
  • mplsjot
    mplsjot Member Posts: 4
    Sorry for delayed response - got tangled up in a nasty 4-car accident. A new boiler was installed right before we bought the house. It’s a U.S. Boiler Co. Model X-205 that was manufactured in 8/16. It’s Max Input is 140 MBH, Htg Cap is 117 MBH & Net AHRI Rating is Water 102 MBH. It has a Grundfos Alpha 15-55 FC circulating pump. I also measured the iron pipes going to each of my 8 radiators. 3 of the runs have1-1/2, 4 have 1-1/4 & the small bathroom has 1”.

    And yes the boiler is more efficient to operate than the pellet stove, but getting the temp where you want it is challenging with both. I assume “balancing the system” will be much easier if I install an 8-port manifold & run 1/2” or 3/4” (if there’s any benefits to doing so) PEX with oxygen barrier to the supply & return iron pipes that go to each radiator. And we don’t mind the cast iron radiators & the amount of space they take up at all. Except in the kitchen where they consume what could be 2 large base cabinets. If/when we update out kitchen we’ll most likely get rid of them & put radiant heat in the floor of use a mini-split to take the chill off if it’s necessary to do so. Once I make some energy efficiency “updates” to the double casement & single pane picture & leaded stain glass windows it might not be. Thanks once again for all of your support. Looking forward to running my/our plans by my inspector!
  • hot_rod
    hot_rod Member Posts: 15,041
    On the right path. Make or calculate the upgrades and do a good heat load calculation. It will show you where the radiant floors are and option and where you may need supplemental heat, btu/ sq. ft. Sometimes kitchens are a tough heat for just radiant floors, not enough surface with cabinets and appliances.

    The pencil and paper work is the key to a successful design and install.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    JUGHNE Member Posts: 8,960
    I don't know of any residential minisplits that have back up heat in them. There are some metal cased commercial inside units that have heaters.
    In any event I would not rely on mini HP in MN to keep me comfortable when your real winter sets in.

    To avoid drilling floor joists, have you considered fastening the pex on the bottom of the joists near the outside walls. Then a 2" deep soffit could cover the tubing. A removable painted plywood cover would keep everything accessible. Or if you have a center beam supporting FJ's they could run along the side of it.
    You may have other plumbing runs to cover in this manner also.
  • Jamie Hall
    Jamie Hall Member Posts: 17,111
    On those windows. Make sure your updates don't damage the value of the windows -- particularly the stained glass ones. There are very very good solutions to making old windows have good insulation value and become essentially draught free which completely retain their value and, for a pleasant surprise, cost a good deal less than replacement with even mid-range modern windows.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mplsjot
    mplsjot Member Posts: 4
    My brothers own/operate the Western WI electrical & refrigeration company my dad started (the one I worked for during HS & college summers) & they say their Twin Cities-based suppliers/wholesalers don't even stock AC-only mini-splits anymore. So every mini-split sold/installed up here in the Frozen Tundra is a heat pump these days & there are several that will keep producing good heat when the temps drop as low as -20 F. My last house had 220V electric in-floor heat cables in the basement cement floor of the big addition we built & a high-efficiency natural gas forced air furnace with a heat pump coil & an electric heat coil in its plenum. Xcel has some really good off-peak rates up here so we heated the cement slab up & used the heat pump &/or electric heat coil when off-peak rates were low & let the heat from the cement slab & the natural gas furnace keep things warm when off-peak rates weren't available. That was about as efficient as you can get. I won't be knocking out the cement floor in the basement I'm remodeling to do that again, but am planning put in-floor heat cables in the bathroom & possibly the big rec room if I decide to go with tile or laminate. And I'm not a bit worried about drilling holes thru the 2"x10" ceiling joists in my basement as our building code gives me plenty of room to drill ones big enough for four 1/2" PEX runs & I hate "boxed in" pipes & ducts in basements because they often exist because the folks who installed them were too lazy to do things the right way & yes that was not acceptable when working for my dad!
  • mplsjot
    mplsjot Member Posts: 4
    Yeah there's no way those original windows will ever get replaced as long as I'm alive! The previous owners replaced all the double casements in the second story with vinyl "pocket" replacement windows that have double pane insulated glass - which is fine because somebody painted all the wood trim up there white before we bought it. But on the main floor all the amazing wood trim, doors, stairways, open doorways, columns & built-ins are what sold us on the house. My girlfriend & I both grew up in old houses & love the character & warmth that only natural woodwork can provide! Some of the old windows have combination storm windows outside & I'm planning to buy some of the new friction fit inside storm windows for the two big picture windows to make them a little more efficient. And the big oak front door is so awesome that I removed the old single pane of glass & replaced it with an energy efficient double pane with Argon gas between the panes that cost more than a new insulated steel or fiberglass door would have cost. As they continue to tear-down old houses in our neighborhood & build new ones & rip the tops off 1-1/2 story bungalows so they can build full second stories with modern building materials, houses like our will always attract buyers who are willing to pay a premium for old home character with updated, energy efficient mechanical systems!