pex size to re-pipe radiators

Tom Giedraitis

I have discussed re-piping my existing steam radiators to convert them to hot water with two contractors. One insists that 3/8 pex is perfectly adequate while the other insists that it should be done using 1/2 tubing.

Both intend to set each rad up on a separate loop off of a manifold.

None of the rooms requires a single radiator output greater than 3500 Btuh.

What does the wall recommend?

Alan(CaliforniaRadiant)Forbes

No problem

using either one; 1/2" PEX will work up to 15,000 BTU's and 3/8" PEX will work up to about 6,000 BTU's.

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Geo_2

pex

Alan, we all know the safe amount of GPM's threw copper tubing 1/2 -15k, 3/4 40k, 1 80k, 1 1/4 140k and so on and I have been meaning to ask this to the wall, the adp't for pex are much smaller than the id of copper, do you guy's/gals still feel safe using those numbers w/ pex seeing the reduction in the adp's?. Thanks, Geo

hr

3/8\" pex

3500 BTU/hr, 20° delta T, .4gpm, 1.1fps, 2.3'head per 100 feet of tube. piece of cake!

A small restriction at the fitting should not bother you much. Unless you start putting a dozen or so in the circuit

3/8" pex will move 12,000 BTU/hr, 20° deltaT, 1.2gpm, 3.8 fps, 19.8' head per 100 feet of tube if you want to push the limit and stay under the 4 fps rule. Although head goes up fairly high.

Not an uncommon head number for the small diameter over the top type of products in use these days, however.

hot rod

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Alan(CaliforniaRadiant)Forbes

Geo

You're absolutely right; I spaced out.

But what are we talking about here; there's not going to be great distances between the manifold and the radiator; 20-30 feet of 3/8" PEX at 160° will only give you a pressure drop of 1-2 feet of head at .6 gpm.

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hr

3/8\" pex

3500 BTU/hr, 20° delta T, .4gpm, 1.1fps, 2.3'head per 100 feet of tube. piece of cake!

A small restriction at the fitting should not bother you much. Unless you start putting a dozen or so in the circuit

3/8" pex will move 12,000 BTU/hr, 20° deltaT, 1.2gpm, 3.8 fps, 19.8' head per 100 feet of tube if you want to push the limit and stay under the 4 fps rule. Although head goes up fairly high.

Not an uncommon head number for the small diameter over the top type of products in use these days, however.

hot rod

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