Hydronic Pipe Materials

[UDP10]

In hydronic piping systems, my company has always specified copper pipe for sizes 2-1/2" and smaller and black steel for 3" and larger. No one at the firm really knows where the separation came from. Is it just cost? Are there other notable differences?
If copper pipe is cheaper at larger sizes, or steel at smaller, is there any reason not to change the spec?

 

[btrueblood]

Most heating service can be done with pipe sizes 2.5" and less. Copper has good corrosion resistance in hot water/glycol service. Cooling water typically requires larger pipe sizes (less available delta-T), and black iron can stand up to chilled water/glycol with the right corrosion inhibitors.

 

[stanlsimon]

It is possible to use copper in larger dimensions however the problem comes when trying to braze or solder the larger diameter (3 or 4")fittings, it takes a special talent to make a leak proof joint. There is a special multi headed torch which keeps the whole fitting hot but not all pipe fitters know how to use it. Right now with the price of copper being high, probably not an issue. Roll grooved gasketed (victaulic)fittings would be another solution.

 

[Compositepro]

Scaling and corrosion is more of a problem in steel pipes and causes more problems in small diameter pipes.

 

[moltenmetal]

Closed loop hydronic systems don't suffer much from scaling. Open loop systems where oxygen is constantly re-introduced are another matter entirely.

The switch-over from copper to steel pipe is a matter of capital cost plus fabrication labour versus maintenance cost/risk in balance. Threaded carbon steel pipe could be used at the smaller sizes (1/2 through 2"), whereas Victaulic grooved end pipe/fittings are often used at the larger sizes. Threaded, done properly with the correct sealant system, will serve very well, but done poorly it may leak. Copper pipe will stay tight guaranteed, and is less labour to install than threaded steel- but the fittings and pipe cost significantly more, particularly at the large sizes. Now that copper has soared in price, you may start seeing more threaded used again. For one thing, there'd be much less risk of having the material stolen off your jobsite!