PEX for replumbing radiator supply lines?

For old hot-water radiators from the early 20th century, can the supply and return lines be re-plumbed with PEX? If so, how would those be sized and what sort of PEX would one use? (Preferably, what model of Uponor Wirsbo?)

I used 1/2" Uponor Heatpex which has an oxygen barrier

What size pipe is/was connected to the radiators previously? 1/2" pex is closer to 3/8" copper, while 3/4" pex is a closer approximation to 1/2" copper or iron. Depends on what flow you need.

You don't need a crap load of volume. My set up is all 1/2" pipes to 17 radiators and everything feeds back to manifolds on the boiler. It's basically a home run set up. Works super and if you put valves on the radiators you can balance the heat also

You have to know how big the radiators are before you can determine the pipe size to them. What size feeds them now. 1/2" PEX would be TOO small for most "early 20th century radiators", REGARDLESS of how it is installed.

Uh, no it isn't. I can show you over a dozen jobs that have ci radiators piped with 1/2". What are you basing your opinion on? Radiators typically have an 1 1/4 inlet and outlet which is plenty large enough to handle a 1/2" line and then some. Go here and talk with Dan or John, they will set you straight. www.heatinghelp.com

With hydronics we are not looking for high flow rates, in fact just the opposite most of the time. As long as we can supply water at the proper temperature and maintain a delta T of close to 20 degrees that is all we need regardless of pipe size.

quote; As long as we can supply water at the proper temperature and maintain a delta T of close to 20 degrees that is all we need regardless of pipe size.

That is the point, "cast iron radiators" have a large radiation area so they need a constant supply of adequate hot water. 1/2" PEX is like feeding them with 3/8" i.d. copper.

All I can say without having to type in four pages of engineering specifications is that I have been using 1/2" pex feeds for over 15 years now and never ever had a single flow or heat problem. And BTW, for many years we fed radiant heat with 3/8 copper.

It all comes down to how much heat needs to be supplied at each individual radiator...the larger the line going to it, the more heat because of the larger volume. Now, you may not need a lot of heat, but using too small of a supply line means on a very cold day, you may be incapable of keeping the space warm. Whether that would happen in your circumstance is impossible to say, since we don't know your operating temperatures, flow rates, heat loss, or radiator sizes.

For hot water hydronic or radiant heating applications, the following equation can be used:
GPM = 0.002*BTU/(Temperature Drop),
where Temperature Drop is the difference between supply and return temperatures in the system and GPM is the amount of flow the circulator must produce.
Since most of the radiant heating systems utilize a 20F temperature drop, the formula can be changed to:
1 GPM = 10,000 BTU/hr,
meaning that for every 10,000 BTU's of heat load the circulator must output a 1 gallon per minute flow.
Assuming that system calls for 100,000 BTU/hr, a circulator pump should have a minimum 10 Gallons Per Minute flow rate at a given pressure drop.
The next step is to calculate the head loss, or pressure drop in the system.
Head loss is associated with friction of the water against the internal surface of the pipes/tubing in the hydronic or radiant heating system and restricts flow rate a circulator can produce.
Although radiant heat manifold and PEX tubing sizing are a different topic, let's assume, for example, that a manifold has 8 outlets with 1/2" PEX tubing installed at 300ft length per loop and the system calls for 72,000 BTU's.
Using the formula above, we can determine the flow rate required for our given zone: 72,000 / 10,000 = 7.2 GPM.
Flow rate through every selected circuit of the manifold equals Flow Rate divided by number of Circuits:
7.2 GPM / 8 circuits = 0.9 GPM per circuit (assuming that the circuits are equally balanced).
Using a Pressre Drop Table or Pressure Drop Chart, supplied by the PEX tubing manufacturer, a pressure drop per ft of tubing can be calculated at a given GPM flow rate.
NOTE: Pressure drop data supplied by manufacturers may be available both in PSI (lbs per square inch) and in foot (ft) of head.
For conversion, use the following equation: 1 psi = 0.434 ft of head (for fresh water).
In this example, pressure drop per 1 ft of 1/2" PEX tubing at 0.75 GPM flow rate would be approximately 0.03 ft of head).
Considering that each individual PEX tubing circuit is 300 ft long, pressure drop per circuit would be 0.03 x 300 = 9.0 ft of head.
Since PEX tubing circuits are in parallel to each other, pressure drop per circuit is always the same as the total zone pressure drop. So, the total pressure drop is: 9.0 ft of head.