Water Hammer Mystery

valeeproperties

Hello,

I'm a property manager of a building that's probably 90 years old, and I've got a second floor unit which has been dealing with very noisy water hammer whenever our boiler kicks on, to the point that the tenant can't sleep through it.

Some background: the boiler is two years old, and it's a two-line steam heat system. The knocking only seems to occur in the piping below this unit, probably about 15-18 feet of horizontal run, and there is no knocking in the radiator itself. Nowhere else in the building is this an issue. We've had a contractor confirm that this length of pipe is pitched back to the boiler as I assume it should be. The steam vent valve on this radiator is new and adjustable, and no matter the setting, the problem remains. There is only one radiator in this unit (a studio apartment), and we attempted to raise the radiator itself to make up for any settling of the building which may have occurred and may have caused the pitch of the pipe to go in the direction of the radiator instead of the boiler. There was almost no play though, as the steam supply and return run through holes in a healthy looking floor joist right near the radiator that leaves maybe an 1/8th of an inch in any direction for the pipe to move. Additionally I shut the radiator itself off to make sure that the knocking was occurring solely in the supply line and not the return, which it was.

Going back further, last year we discovered a leak beneath the first floor (directly below the area in question by one floor) in the steam header which we promptly repaired. The knocking hadn't been an issue until this repair was made, nor was it an issue before the boiler was replaced two years ago. So it would seem that the new boiler caused a leak to be exacerbated, which, when repaired, then caused this water hammer issue. I'm curious if the leak beneath the floor was causing some relief in the line which we unwittingly got rid of. And if this is the case, would a vent or vents somewhere in the steam supply lines possibly offer the same kind of relief?

Any suggestions are welcome. Thank you!

KC_Jones

You say it's a 2 line system, do you mean 2 pipe as in 2 pipes hooked to each radiator? If it is indeed true 2 pipe system there shouldn't be vents on any of the radiators. There are 2 pipe air vent systems, but by the age you state I would doubt you have one of those

Some pictures of the new boiler piping and the repaired pipe would help as well as pictures of the radiators to verify what you have.

Hammer is water laying and being picked up by steam, but there can be many causes. If the boiler piping isn't proper you can have wet steam which can amplify small problems into bigger ones.

Jamie Hall

I'm confused. Is this two pipe steam? That is, there is a supply line which comes on at the top of the radiator on one end, and another pipe -- the return -- which leaves the radiator at the bottom, either at the same end or the other end?

Or are there two lines which come into the radiator at opposite ends at the bottom, with valves on them?

Or is it single pipe -- only one line, coming into the radiator at the bottom of one end?

If it is the first option, there should not be a vent on the radiator. The inlet valve can be used to control the heat.

Either of the other two options there must be a vent on the radiator. The valve(s) must be fully open at all times, and the vent controls the heat.

Now...

It would be very worthwhile to verify exactly when the knocking occurs. If it occurs after the boiler turns on, but shortly before the radiator starts to get hot at the steam inlet end, then the problem is water in the line feeding steam to the radiator. That line needs at least an inch of pitch, and it must be straight, no sags. If the interval from the noise to the steam arriving at the radiator is rather longer, the problem may be in some other horizontal bit farther back towards the boiler. It's surprising how far the knock from a water hammer can travel through the pipe.

All that said, you note that this started happening after the new boiler was installed. I almost hate to ask, but I have to: first, was the new boiler installed with at least the manufacturer's minimum piping for the near boiler pipes? That is set out in the boiler manual, and it is the bare minimum required to get reasonably dry steam. If the near boiler piping is not up to snuff, you could be getting wet steam, and it just happens that the resulting water hammer is in that particular location. Second, is the new boiler properly sized to the load? And third, is the new boiler operating at no more than 1.5 to 1.7 psi cutout? Not that higher pressure will cause water hammer -- it won't -- but it will annoy any leaks which might be present.

Fred

If this is a two pipe, why are there vents on the radiators? Most two pipe systems have traps instead of vents. Are there traps on the return side of the radiator? A picture of the return pipe at the radiator would help. It is possible that a trap has failed open and is allowing steam into the return line, causing the hammer. It may be the trap on that radiator return or it could be a trap elsewhere in the system. It takes steam and water to cause hammer. Also, make sure the return pipe doesn't have a sag in it. It may be pitched but if it has a sag, that will hold water (but if the trap is working, it shouldn't see steam).

JUGHNE

And the always asked question; what pressure is the system running at? Where is the settings on the pressure control and have the pigtails been cleaned?

valeeproperties

-Jamie: This is a two-pipe system with the supply entering the radiator at the top left-hand side, and the return exiting at the bottom left-hand side (i.e. the first option). I was not aware that with a two-pipe radiator one doesn't want to use a steam vent valve; all others in this building have them and seem to work fine. We've had a couple steam heat specialists come by to check on the problem but they've not mentioned this to me before. Plus there is the threaded hole on the side of each of these radiators for a steam vent valve.

The knocking seems to occur at the beginning of the cycle just as the radiator begins to heat. I feel I can rule out the notion that the knocking may be occurring elsewhere in the building since all neighboring units report no noise.

I wish I had any knowledge about the installation specifications. We had the boiler installed by a trusted local HVAC company who we use regularly. Aside from the knocking in this one length of pipe, no other issues have persisted. I saw two meters labeled "cut in" but none labeled "cut out." They are both set at 2 PSI.

-Fred: Our contractor told me that the pipe didn't appear to be sagging, though I suppose he could have just been eyeballing it. I had considered this as a cause and asked him to check it but took his word for it since it's not so easy for me to access myself.

As far as a trap goes, there isn't one above the floor, and in fact we actually took about a 1x1 ft. section of the floor up right where the pipes come out to see what we were working with, but again, no trap.

-JUGHNE: I'm not sure about the pigtails, but we do have all of our boilers serviced at the beginning of each heating season. I saw three pressure gauges on the side of the boiler: the two labeled "cut-in" which I describe above as being set to 2 PSI, and a Honeywell Pressuretrol which reads 5 PSI.


Thank you for your responses!

KC_Jones

It could be an orifice system which can mean no traps or vents, pictures of the radiators would help. If it is as you describe I would question how expert your "experts" are. This is pretty common knowledge for steam, it would also give me pause about everything else they did for you.

Pictures would be a big help here, the radiators and the piping including the boiler piping.

Jamie Hall

If it's an orifice system as @KC_Jones suggests, which is quite possible given the age, the pressure is way too high. Those systems depend on a carefully calibrated orifice, usually at the inlet to the radiator (sometimes the outlet) to control steam flow -- and are intended to run at no more than half pound per square inch cutout -- and more like 4 ounces cutin. You will need to add a vapourstat set to those values (it can go in series electrically with the pressuretrols) to control the beast.

It won't hurt to add that anyway; if it's not an orifice system just dial it up to about 12 ounces cutout and it will be fine.

Can you post a picture of the inlet valve on the misbehaving radiator and also one of the ones which isn't misbehaving? May figure out more from that. Like, for instance, if someone replaced the valve at some point and it is letting way too much steam in way too fast...

JUGHNE

Is it possible that the leak was losing water in a sagged pipe and after those repairs were made the water would remain in the sagged pipe and when the steam got to it the water hammer would happen?

It still hammers if the rad valve is off....yes?

How about a picture of where the repairs were made also.

valeeproperties

Ok, I've got some pics here of the radiator in question. It turns out that quite a few of the radiators in this system don't have steam valves. I'm surprised we had two different companies come in and not point this out. Anyhow, I've adjusted this particular steam valve to be as closed as possible, and we're already talking about removing the rest in the building and plugging the holes.

Is it possible that the pressure settings of the boiler will need to be changed if we do this? Jamie, I know you said they are high to begin with, so I'm curious what may happen if we plug these holes without adjusting the boiler.

JUGHNE, unfortunately the repair which was made is now under a hardwood floor, but yes it hammers when the radiator is off.

Jamie Hall

Plugging the vents probably won't make things much better -- or much worse -- without changing the pressure. You've got to get that cutout pressure down. Then let's carry on and see what happens next...

valeeproperties

Thank you all for your insight. I'll have to set up a service call to correct the pressure on the boiler, but I'll post again once we've addressed this.

JUGHNE

In and out on the same end of the rad.....no standard trap visible....air vent on the other end (usually lower on the rad).

So to keep steam out of the return usually one has a trap on the outlet, a return line that drops down into a water seal or an orifice in the supply valve.

Or there were some outlet 90 ells that were designed to keep the steam out of the return lines.

Could this rad been disconnected during the repair and the orifice was misplaced?
Putting one in would not cause a problem, simply slow down the steam travel thru the rad which could be allowing the steam into your return water. A simpler solution than trying to examine the outlet 90 ell.

You might call your service people and ask them about an inlet orifice. Lowering the pressure might solve the problem also.

Mike Cascio

Do all the radiators have the vent right at the top of the radiator? If so, they were probably added at a later date to hide a systemic problem. You likely have an early vapor system with orificed elbows. That should be running a pressure of ounces not pounds. In order for everyone to give the best advice, some pictures of the boiler and near boiler piping are in order. Also some pictures of the supply and return piping and any odd looking devices connected to the basement piping would be great.