Can radiators share risers in two-pipe steam?

rjobrien

Hi. Interested homeowner here. I've been studying the layout of our early 20th-century two-pipe steam system in our three-story house. One thing I don't get is why there are separate risers supplying radiators on upper floors when the radiators are situated only feet apart. Why would the installers add the expense, labor, and intrusion of individual supply (and return) lines -- that technically take more effort by the boiler to prime. In some cases a supply line was installed in a wall cavity and an adjacent supply line runs through the room inches away on the room-side of the wall. Why not just tee them off up by the radiators and supply both with one riser? I know that means the radiators would take twice as long to heat up, but I would think we're talking about a matter of minutes, and the savings in materials and boiler energy would be worth it. I'm not making any plans to rearrange my pipes -- I just want to understand the logic. Thanks!

Intplm.

Simply put, the system you have is heated with a gas called steam. This steam vapor/gas is a gas that always wants to be a liquid.
That liquid is water that has cooled and is no longer steam but has turned into condensation.
The condensation needs a place to go. The system is piped so the least amount of condensate is developed in the system and is also piped so that the condensate has a place to go and that place is back to the boiler.
A proper piping arrangement allows steam to heat the radiators and also allows for the proper elimination of the condensate to drain back to the boiler to be reheated.
If the system was piped the way you asked, it wouldn't take a matter of minutes to heat. in fact, it may not heat at all because the condensate has a lesser chance of draining back into the boiler. It can block the steam from the radiator allowing for little to no heat at said radiator. It would also make a lot of banging noise when the steam and condensate collide in the pipes. Enough noise to disrupt the entire household every time the heat turns on. And can even damage the system.

Steam is a gas that always wants to be a liquid.
@rjobrien I hope this helps.

Jamie Hall

Most likely convenience. Or change in plans. Or sometimes a really big radiator will want a separate riser, to avoid using too big a pipe.

However, there is one other consideration: it is much easier to maintain the correct pitch on horizontal runouts at the top of the riser that way. In fact, you simply can't put a simple T at the top of a riser and go off left and right and have the correct pitch on the horizontals.

On a couple of other points. Steam systems don't prime anything. The steam moves in a pipe in response to pressure differences (which are very small) and not what the pipe is connected to. Sp having two radiators off one riser will be neither faster nor slower than having just one. Nor will the radiators off a single riser take twice as long to heat up. They may be a little slower, if the riser is undersized, but not much.

EBEBRATT-Ed

The riser needs to be large enough for the connected radiation load. If you have 2 radiators on the same riser you need a larger riser.

Larger pipe is more difficult to work with inside a wall cavity so maybe that is why they opted for individual risers.

And as @Jamie Hall mentioned maintaining the correct pitch and having room for swing joints in the wall is problematic.

You could use a single riser to feed 1 ff rad, 1 2d floor rad and 1 3rd floor rad if it was the right size without any piping problems.

The other unknown issue is was the steam system installed when the building was built? Or was it added sometime after?

If the second and 3rd floor risers are exposed, it is likely it was an added system after the house was built.

rjobrien

Thanks, everyone. That's insightful. @Jamie Hall , by "primed" I just meant that I understand steam needs to bring a riser up to temperature before it can pass through as a gas. I understand this takes a lot of energy and assumed a shared riser would conserve that extra energy. I didn't consider the horizontal pitch off the tee in two different directions -- that might be something. Does two-pipe steam need a pitch? I thought the vacuum system in two-pipe allowed for level supply lines.

@EBEBRATT-Ed, the house is 1890 and I think there was some sort of hot-air grates setup from a boiler or furnace originally, but a later renovation led to a two-pipe steam system. They only sliced through floor joists in one room (that's always fun!).

Jamie Hall

Two pipe steam does need pitch -- just less of it than one pipe. Or a slightly smaller pipe. The condensate doesn't ever fill the pipe, so even if there were a vacuum in the returns it wouldn't matter -- it's still gravity flow. And, in most two pipe systems, there is no vacuum -- at least not most of the time (there is in some designs with some special main vents, but most, no). In fact, it many original steam systems (especially "vapour steam") the dry returns were open directly to the atmosphere -- no vents at all, just an open pipe! Later systems got fancier... but even so, if all is going well, the main vents (which are all on the dry returns) never close.

JUGHNE

You should check how the air is vented out of the system.
Slow venting leads to slow heating and other problems.

Do you have vents on the radiators?
Is there a pump near the boiler?

rjobrien

@JUGHNE, no pump near boiler, only a Hartford loop. No vents on radiators, only one master vent on return main down by boiler. The steam system works great, I don't have performance issues. (And since it ain't broke, I won't fix it.) I just look at these parallel risers going up to radiators that are feet apart and wonder why they didn't share a riser.

Jamie Hall

Sounds to me as though it was done right... the first time.