Connecting PEX to Hydronic Steel System

[gadolphus32]

I am finishing the attic in my 110-year-old house and want to add radiators by tying into the existing system. It's a hot-water system with old cast-iron radiators fed by 1-inch galvanized pipe. The boiler is in the basement and has a circulator. It's set to 180 degrees with a return temperature of 160 degrees, and 15 PSI.

There are already pipes up in the attic that used to connect to a leaky radiator, which I removed. I intend to tie into them with Pex and run the supply lines to the new radiators behind the knee wall that I'm building as I finish the attic.

The old radiator in the attic was apparently on its own loop -- or so I surmise because the pipes that fed it are currently capped, and all the other radiators in my house still heat up.

My plan is to do this:

1) Reduce the existing 1-inch pipes in the attic down to 1/2 inch using galvanized reduction couplings.

2) Use a 1/2 inch brass threaded-male adapter (i.e., an adapter that has 1/2 inch male threadings on one side, and 1/2 inch Pex crimp barb on the other) to transition from the steel to Pex.

3) Connect my Pex to the barbs and run it to the new radiators in a single loop that covers the whole attic. There will be three radiators total on this loop.

I'll use radiant heating Pex (the "orange" stuff) with an oxygen barrier, and I was planning to use the "plastic" Pex fittings rather than the brass ones, which a manufacturer says are rated for up to 200 degrees and 200 PSI.

I'll probably use the cheapo baseboard radiators, unless I have enough money left over to get something nicer. I assume the type of radiator I buy will not make much of a difference. I know I'll have to transition my 1/2 inch Pex lines back up to 3/4 to tie into the radiators (because as far as I'm aware, all modern radiators have 3/4 inch connections).

I am posting here to ask if I'm overlooking anything important, particularly because I know that there are nuances with a hydronic system related to pressure, corrosion and other issues that can be easy for a DIYer to miss. Thanks in advance for any advice.

 

[Jadnashua]

Not sure 1/2" pex will provide enough flow to heat things very well.

 

[Sponsor]

 

[gadolphus32]

Not sure 1/2" pex will provide enough flow to heat things very well.

Hmmm, thanks. I could go up to 3/4 easily enough. I was thinking 1/2 inch because I was just being a little cheap. 3/4 inch orange Pex is about 50 percent more expensive than 1/2 inch.

On the other hand, I guess with 3/4 inch I would not need to get expansion fittings to size back up going into each radiator. So the cost would probably about even out.

 

[Jadnashua]

The tubing manufacturer will list the maximum gpm flow for various sizes of tubing. Then, depending on the supply temperature, you can calculate how much heat you can provide. A manifold supplied with a large enough pipe then 1/2" or larger, depending on the heat required, might allow you to use smaller tubing, at least for part of the distribution system. Higher velocity (gpm) is inefficient, tends to be noisy, and not a great idea, in small pipes. Slower velocities and lower temperatures tend to be more comfortable and, if your boiler can handle it, the lower return temperatures can be lots more efficient because it allows the boiler to condense (not all boilers allow this and would be destroyed quickly if you let it happen).