Old Gravity system

Mike Reavis_2

I left a house this morning that dates back to WWI. The current 210k Amer. STd water boiler has a pump. The house is 3 floors, radiators on 2. 5" steel pipe main in a loop around the basement. I know that if you have enough distance from a tapping feeding a radiator off a main to the return to the main that you can induce flow in the rads. But I hadn't considered that this would be workable in the gravity days. I am still wondering what the guy looked like who fitted out this house with 5" mains!

Mike

Mike T., Swampeast MO

It's not distance that makes the flow--it is the difference in the density of water at different temperatures. The density difference is very slight so the pipes have to be huge and offer nearly zero resistance to flow. (In fact when converting to forced flow you assume that the resistance in the system IS zero! You reduce the main sizing by ½ minus one pipe size in order to actually introduce some resistance for the circulator!)

The distance from the tapping to the main does come into play but mainly in the vertical plane. The higher a radiator above the boiler the greater its ability to induce gravity flow. That's why you'll often find that rads of nearly the same EDR will be tapped smaller the more floors above the boiler. They layed out their mains to keep horizontal runouts as short and direct as possible as these tend to inhibit gravity flow.

5" mains ARE huge! Particularly with "only" a 210k btu/hr boiler. I'd say the installers bore resemblance to Popeye!

Mark J Strawcutter

regular fittings

If you ever get back there, check and see if they didn't take the supply off the top of the main and bring the return back into the side - I think that's the way it is for 1st floor rads. Uppers are the opposite.

The heating Q&A here, as well as Dan's "how come" book talk about how they did this.

Mark

Mike Reavis_2

there were tappings out of the side of the main

as you thought.
Mike

Mike Reavis_2

and you should see the size of this pump.

Apparently, someone thought the big pipes meant big pump. There are probably hundreds of systems like this where the pump has seized, and the homeowners only notice that the heat is not as even as it was, say 5 years ago!
I see lots of two pipe gravity systems throughout the area. They do a great job turning cast-boilers into piles of flakes, and plug up copper boilers. Some even have the install manuals, with the suggested piping instructions stuck in the rafters--still sealed from the factory! This is the first single pipe gravity system (converted) That I can remember.
mike

Mike T., Swampeast MO

Single-pipe gravity

No wonder the main is so monstrous! The velocity in that main was so low under gravity that they were actually using the different densities inside the main and enhancing such with the way the connections were made.

Really freakish systems when you start thinking about using different temperatures inside of a single pipe with everything moving ever so slowly forward...