Cast iron radiator at high flow rate

hot_rod

I have never seen pressure drop listed for cast iron sectional boilers. All I have heard is it is near zero. Probably much like a sectional cast iron radiator, large header tubes connection the sections.

Cast boilers do like like they are shrinking in size and water content, however those older legacy designs are probably using the same castings for sections that they used 50- 60 or more years ago. Most had large tappings for supply and return, even in small btu sizes.

The key as I understand it to gravity hydronic systems is large sloped piping, very low pressure drop in the distribution piping and radiators themself. Also limited horizontal piping distances. Vertical risers with short branch distances horizontally.

Steamhead

Pressure drop will depend on the flow rate. For example, consider a gas meter. It is rated for a certain number of cubic feet per hour at a certain pressure drop, but can handle more gas if one can tolerate a higher drop.

Typically, in gravity systems the boiler supply and return tappings are the same size as the mains, which are at least 2-inch even on the smallest systems. Also, each main has its own tapping on the boiler. Show me a residential boiler with that many big tappings.

hot_rod

Lots of speculation, to get an answer what does or can work would take a deep dive into this gravity design guide. They talk a lot about pipe and fitting resistance, as well as velocity increases going from a 36∆ to a 20∆.

Anyone up for running a design on a hypothetical modern day gravity installation?

Typical residential boilers have 1-1/4 taps, so a single riser system would be limited to that pipe size. Although an increaser at the boiler to 2" or larger could be in those friction calculations somewhere?