Cast Iron Radiator Re-Pipe to Pex Home Runs

cshorett

I am preparing to re-pipe my home's early 1900s cast iron radiator system from black iron branch arms to a 1/2" pex-al-pex home run system. The home has 12 radiators - 6 upstairs and 6 downstairs. The boiler is in the basement and the ceiling is now fully exposed, allowing access to riser ends which I plan to use as my tie-in connection for each rad. PAP will run back to a 12-port Caleffi inverted manifold with horizontal runs of varying length but all <65'. I do not plan to zone at this time. Basement heat will be provided via a new mini-split system.

A few questions:
- Planning PAP vs. hePEX given concerns over expansion noise and stretch w/ high temp distribution. Not a lot of providers out there these days w/ Uponor Multicore and Viega Fostapex seemingly off the market. Any product recommendations?
- Zoning upstairs vs. downstairs would be nice, but I've only seen individual loop valves which get spendy and complex w/ 12-ports. Is there a manifold zoning option I should be exploring with two 6-port manifolds instead?

Appreciate any guidance!

EBEBRATT-Ed

Why not install manifolds for upstairs and for down stairs and put a zone valve on supply feeding each manifold so you can zone up and down.

If you only need (2)6 port manifolds make a couple of them with tees and nipples for the supplies and make a 12 port for the return or make them from copper if you like to solder. You can also buy premade pipe manifolds.

hot_rod

The pex al pex tube is a bit less forgiving to work with, it kinks easily what you pinch it pulling through holed.

Use a soft flexible tube like any pex A
In some cases 3/8 may be adequate. Easier yet to work with.

cshorett

EBEBRATT-Ed said:

Why not install manifolds for upstairs and for down stairs and put a zone valve on supply feeding each manifold so you can zone up and down.

If you only need (2)6 port manifolds make a couple of them with tees and nipples for the supplies and make a 12 port for the return or make them from copper if you like to solder. You can also buy premade pipe manifolds.

Makes sense - didn't realize you could run zone valves w/ manifolds. Looks like Caleffi makes a 1" valve that would work well. I've heard better to run valves at return side to limit heat exposure, correct?

cshorett

hot_rod said:

The pex al pex tube is a bit less forgiving to work with, it kinks easily what you pinch it pulling through holed.

Use a soft flexible tube like any pex A
In some cases 3/8 may be adequate. Easier yet to work with.

Given distribution will be at ~180 degrees, I'd heard Pex A gets droopy and makes noise in the walls. Thoughts?

hot_rod

Properly supported pex at 180 is not an issue. Pex installation manuals give you the recommended spacing.
In the walls, might be wise to insulate it? They also make hangers and brackets for insulated pipe fastening. These plastic ones work well, plenty others to choose from.

EBEBRATT-Ed

Zone valves can be on the supply or return. Does not matter. In the "old days" they put pumps and valves on the return so they would see cooler water. Pumps should go on the supply or on the return but should pump away from the expansion tank valve can go in either location.

tropostudio

I'm curious if your black iron system is still gravity fed, or has it been modded with a circ pump? Do you plan on changing the boiler?  If not, why bother with switching to home-run piping for each radiator?

The old cast iron radiators are great emitters.  If you are intent on doing the home run plumbing, I'd consider a new boiler with outdoor reset, a constant pressure circulation pump, and TRV valves at each radiator.  Add another radiator or two to replace the basement heating that you lose by removing the black iron distribution pipes.

psb75

I too highly recommend thinking about an eventual boiler replacement with a modulating condensing boiler. You could then operate the system with a significantly lower supply water temperature. Its very good that you are keeping your high mass radiation in place. The home-run manifolds with pex A plumbing, outdoor reset and zoning potential all get you a very efficiently operating system. Lower water temperature and more constant circulation. I also recommend changing out the "black iron branch arms" if they are largely exposed in basement ceiling. Think of all of these upgrades as eventually paying for themselves!

tropostudio

If you are able to put a TRV on each emitter/radiator, you can get by with a single constant pressure or proportional pressure circ pump. The Grundfos Alpha is a good bet. You skip the zone valves or multiple circ pumps. Use manifolds with individual flow control at each homerun to set the baseline flow rates with TRV's wide open. The TRV's dynamically tune the flow to precise temp control at each emitter, allowing temperature variation per emitter (keep a bathroom warmer and a hallway a bit cooler, for example). You need to do a heating load calc and flow calc to optimize the set-up. Once done, it's set and forget with VERY even and consistent heat distribution, as the boiler just hums along with no short cycling.

JerseyDIYguy

I did a similar job a few years back:

https://forum.heatinghelp.com/discussion/154770/radiator-re-pipe-copper-or-pex

At the time I did the job I was told that you do not want to use conventional Pex for the job because it will allow oxygen into your system. That's why the layer of Aluminum is in Pex-Al-Pex, it blocks transmission of oxygen. It looks like the pex manufacturers are now making oxygen barrier pex of different construction and materials, so a little searching on the interwebs may be needed.