Water in radiator

abellia

Hi,

I've got a one-pipe steam system hooked to a Nesbitt radiator (convector, so they call it). When the system fires, the air comes out of the vent and then I get water out of the vent and lots of sloshing noise from the radiator. It's tilted to the riser, so I would think water should be draining back to the boiler.

The riser pipe is on a cold outside wall. The pipe is certainly not insulated. The wall may or may not be. Could the steam at first be cooling so fast as to leave a bunch of water in the radiator before all the air leaves the radiator? Could there be crud in the radiator that's keeping water from draining back? There is no valve between the steam pipe and the radiator.

Thanks for any ideas,

Jamie Hall

It is possible that the problem is in the riser -- but it could also be that the vent you have on that convector is too fast, and is letting steam in faster than the condensate can get out. Have you tried a slower vent?

abellia

Thanks.

I have a variable vent turned very low.

How would the process work where the condensate wouldn't drain back? Doesn't the boiler cycle for just this reason -- to allow time for the condensate to drain back?

Hap_Hazzard

The reason the boiler cycles is that it sends enough steam to the radiators to warm the living space up to the temperature called for by the thermostat. The condensate should drain continuously. If your near boiler piping is bad or your water is very dirty or too basic, your boiler might be throwing excessive amounts of water into the system, in which case you'll see the level in the gauge glass dip very low during the heating cycle. You may also see the level fluctuate a lot while it's boiling.

Is the radiator you're having trouble with the closest to the boiler?

Can you show us pictures of your boiler and the pipework around and above it?

Where are you located?

abellia

I'm in New Hampshire. The boiler piping is kind of strange. I could take a picture if you want a laugh or shake your head, but I don't think this is the issue. I'm really only having trouble with water in one radiator and it's quite far away from the boiler.

If the condensate is draining continuously, why does a steam system bang if there is water in the pipes? I thought that the banging happened when steam pushed standing water against some pipe corner or whatever? Is this only an issue when there is standing water when the boiler first starts steaming?

EBEBRATT-Ed

@abellia

It doesn't matter how far the boiler is from the problem. Bad near boiler piping will cause your issue. So will running to high of a steam pressure or dirty water in the boiler and a 100 other things including pipe pitch and uninsulated pipe

Hap_Hazzard

Differential shock can happen whenever you have a closed system with high velocity steam moving over water that's deep enough for wave propagation, like a steam pipe where steam is flowing out while the condensate is trying to flow in the opposite direction. If the wave peaks reach the top of the pipe, they form a slug that blocks the steam, and the steam pushes the slug to the end of the pipe where its energy is dissipated as noise and possible damage to the pipes, fittings and supporting hardware. Differential shock is the most dangerous and destructive form of steam hammer.

A milder, but still undesirable form of steam hammer is thermal shock, which happens when a cloud of steam encounters cold standing water in a closed system. The cold water causes the steam cloud to collapse into condensate, which takes up 1,600 times less space, resulting in a large, almost instantaneous drop in pressure. This also makes noise and throws water around inside the pipe, but it's less energetic.

Both of these are more likely to occur at startup, because the steam velocity is highest when the pressure is lowest, and any standing water left in the pipes will be cold enough to cause thermal shock. (Hot water has a high enough vapor pressure to allow the steam to contract more gradually.)

This YouTube video demonstrates both kinds of steam hammer.

Jamie Hall

Water hammer in steam systems occurs when some water -- usually better to thought of as a puddle than as a slug -- gets picked up by the steam velocity and hits an obstruction, such as an elbow. There are a number of ways a puddle or something like a puddle can form in a near horizontal pipe, and it can be a little hard to visualise. Basically, though, suppose that you have a thin flow of water on the bottom of the pipe -- the returning condensate -- and you start to move steam over it (either in the same direction it is flowing or in the opposite -- opposite is much more sensitive). At low velocity nothing much happens, but as the velocity increases, waves will start to form in that smooth flow of water, and eventually -- depending on the amount of water -- the waves will become big enough to be pushed along and bang.

In counterflow -- the steam going one way and the water the other -- the thinner you can make that stream of condensate the better, which is why pipe slope is important. Further, the less condensate the better, obviously -- which is why slowing the vent on a banging radiator can help in one pipe steam. Keeping the velocity down with bigger pipe helps too, but clearly there is an overkill happening there!

In two pipe steam, counterflow should only happen in the runout feed. Hammering in that runout can occur -- and does -- from the usual suspects, but is usually only there at the start of a cycle, since the only condensate in the runout should be from condensing right in that pipe -- which will stop when the pipe is fully heated. Obviously, insulation helps if you can do it. However, you can get some hammering if the steam is wet -- full of water droplets -- throughout the cycle, which is why near boiler piping can be a problem.

In one pipe steam, you can get hammer throughout the cycle, if the pipe is too small for the radiator or the pitch isn't adequate.

In either, you can get bad hammer if there is a sag where a puddle can collect -- or even a relatively flat section of pipe.

abellia

This is a really great response that I'm going to have to read a few times. Thank you.

On my original question...I realized that I have a more general problem. My system is kind of nutty in that it has 4 cast iron radiators that are no problem and 4 convectors that I turned off by closing the air vents because of noise/water out of the vent. I also have a loop that goes to a heat exchanger to run radiant heat for a floor. It's a mess not of my construction.

Anyway, it seems like all of the convectors make water noise and leak water out of the air vents, but I have no issues with the cast radiators. Is it possible that the properties of the different types of radiators themselves would end up causing a problem that would lead to water in the convectors? I don't seem to be able to turn down the vents sufficiently on the convectors to get heat but also prevent water from building up in the unit such that it exits the air vent and bangs around.

I insulated the piping in the basement of the home a few years ago, but the runs to individual radiators (a mix of copper and black pipe) are uninsulated.

Jamie Hall

Convectors on one pipe steam are almost always a problem. They are great at condensing steam, but not so hot on letting the inevitable condensate back out.

There are approaches ranging from subtle to sledge hammer... slow the venting as much as you can. Give them lots of pitch (yes it may look crazy -- that's what enclosures are for) (and if they are multi-pass back and forth, that's almost impossible to do). Worst case add a false return from the opposite end and tie that back into the steam main with some height and through a trap. Sometimes works...

Hap_Hazzard

Piling on to what Jamie said about convectors, even if you get the darned things working right, which usually requires divine intervention, there's no getting around the fact that these units are substantially less massive than cast iron radiators. This comes in handy when you're hauling them to the scrap yard to recycle them, but you don't get that much for them because of the low mass and because they're mixed metals. Where it doesn't come in handy is: keeping you warm.

Convectors do a great job of consuming lots of steam and giving off heat in a hurry, but if they're anywhere near your thermostat, they'll shut off your boiler before the radiators really get going, and once the boiler shuts off, they cool right down as quickly as they heated up. They don't retain that warm glow between heating cycles like the radiators.

The end result is that, as you walk around your house, you'll keep going from warmer to cooler, but not always in the same places.

If I were you, I'd keep an eye out for used cast iron radiators.

abellia

The way that the radiating unit is fastened to the enclosure makes it nigh impossible to tilt them anymore unless I tilt the enclosure. The convectors have three pipes that run from side to side. I didn't realize that they may be looping back and forth. I figured that the steam just entered all three pipes simultaneously. Sigh. What a hassle.

Thanks for the advice.

abellia

This may be a crazy idea, but would it be terrible to convert from a one-pipe steam system to a two-pipe system by adding and steam traps to the radiators/convectors and running PEX from the steam traps back to the boiler?

Jamie Hall

I wouldn't bother with the radiators. Fussing with vents usually gets them pretty well balanced, and unless they are banging...

Converting the convectors to two pipe certainly would help -- but it's a hassle, to put it mildly. You really shouldn't use PEX above the water line (and it's a little dubious below), however. Yes, if the traps are working perfectly, you might be able to get away with it -- but only because the pipe isn't under pressure. The condensate will, if the system has run for any length of time, be good and hot -- 200 or even more -- and PEX simply can't take that. And, of course, if a steam trap fails...