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Residential steam boiler maintenance, operation, and chemicals

by Dr. David Zatz

If you have a steam boiler in your house, chances are it’s a great big mystery to you. Most homeowners don’t seem to keep the owner’s manual, which can be quite vague anyway. “Civilians” are not supposed to service the boiler, plumbers are. Unfortunately, many plumbers also have no idea what they are doing when it comes to steam heat.

adding chemicals to steam boiler

Much of information on this page came from the HeatingHelp web site and books written by Dan Holahan; and the advice and practice of master plumbers Jim Devor and George Hull. Jim and George had immense expertise, and could also pick up full-sized radiators and carry them down a flight of steps without any apparent effort. Any errors are likely my own; I do not claim expertise but am sharing experiences, and cannot be liable for any errors, bad advice, or other problems you may encounter after reading this.

I am not a plumber, but I’ve done a lot of reading, had a master plumbers patiently teaching me some of the basics, and have lived with steam heat for over nearly 20 years, including a boiler replacement and quite a bit of troubleshooting. I’ve done smart and not-so-smart things, and gotten a lot of help along the way.

Without further ado, here are the lessons I've learned, and some advice for you.

How steam heat works

It is possible to have a nearly silent steam heat system, which keeps the house heat in a reasonable range and doesn’t consume fuel too quickly; and it’s possible for a boiler to last quite a long time.

The most common type of steam heat system for houses, and the kind this page is about, is called “one pipe” steam: a single pipe goes to each radiator. If your radiators have a pipe on both sides, you have “two pipe” steam. In practice, the main difference is that two-pipe steam has different problems and steam traps, not used in one-pipe, are more likely to be an issue.

“One pipe” systems send steam into the radiator; water condenses out of it when it hits the cold steel of the radiator walls, and the water drips down through the same pipe the steam is coming up. Sound crazy? No question. Still, it’s cheaper to build and troubleshoot.

There could be two or three main steam pipes coming out of your boiler, or a single steam pipe that splits into two or more branches, but it’s still one pipe steam if the steam goes into the radiator and the water drains out through the same pipe.

The main steam pipe(s) in the basement (I usually see two in other houses) should run along the length of the house; at the end, it sort of branches into a much thinner pipe which runs back to the boiler. That’s the pipe that drains the water back into the boiler. Sometimes, at that point, there’s also an air valve. Sometimes the air valve is back at the boiler, just before the pipe goes down to the base of the boiler.

Steam pipes and insulation

Asbestos pipe insulation cost us $1,200 to remove; adding fiberglass insulation to replace it cost another $1,200, a price inflated by my request to use the thickest insulation possible, and our complex near-boiler pipe arrangement. Thick insulation pays off, especially on the near-boiler piping.
Make sure they cover all the fiberglass — there should be no exposed fiberglass, it’s not safe to breathe. Only use licensed, qualified companies; they should have the “space suits,” cordon off the area while working, and get a state inspection when they’re done. You won’t save money by hiring fools. Get quotes from a few local companies, I've been told that the main cost difference is how far you are from their home office.

Go down to your basement and look at your boiler — in particular, at the big, thick pipes coming out of the top of it. Are they covered in insulation? They should be, though not asbestos. (If the insulation is asbestos, and you bought your house in the last 30 years, the inspector probably told you about it.)

The pipes nearest the boiler are the most important to insulate, but all the steam pipes should be well wrapped. Part of the goal is to keep the steam “dry” — that is, to reduce the amount of water droplets being flug along with the steam up into your radiators. The pipes will get cold when the steam is off; when the steam hits the pipes, it condenses into water, and the water is then flung (by the steam) up into the radiators, hitting pipe bends at high speed along the way. That causes noise and inefficiency, not to mention heating up the basement unnecessarily (rather than the rooms). Insulating the pipes can help to increase the amount of heat getting into the furthest rooms.

You can save money by doing this yourself, but it’s better to have a professional who has the correct knowledge, gear, and materials.

Steam pipes are surprisingly small, once the insulation has been removed.

Radiator valves and “main line” valves

If you are going through a lot of water, you may simply have a leaking emergency valve. These cost around $20-30 (don’t cheap out) but make sure you get the exact model your boiler came with, if you can, or at least one with the exact specifications. There are many other possible causes, the worst being a rust hole in the boiler.

Somewhere in the basement, along the steam pipes, either right by the boiler or at the ends of the big pipes, are big air valves (described earlier). These die eventually and need to get replaced, preferably by the next higher capacity size. The main symptom that these valves are shot, is that the rooms farthest from the boiler don’t get much heat. Why not?

When your boiler starts generating steam, it flies through the pipes, but the pipes are already filled with ordinary air. The air has to escape through valves; so each radiator has an air valve, but they’re pretty small. The main steam pipes, you’re probably noticed, are very large. The air in the main pipes is supposed to escape via those valves in the basement (only one if you just have one main steam pipe). If the valves are stopped up, the air all has to leave through the tiny valves in the radiators, and the rooms at the end won’t get much steam because the air that’s supposed to escape through the main valves ends up escaping ever so slowly through the radiator valves in those rooms. (The valves automatically close when they sense the presence of steam, so that your radiators aren’t blowing scalding steam into the room — which would also mean that your radiator would go through a great deal of water.)

pressuretrolSo replacing those valves may be needed. They’re rather pricey. I have found the best place to replace them is through a dedicated plumbing supply store; their prices and selection tend to be better than the Internet, Home Depot, Lowe’s, and local hardware stores. A plumbing company like State Supply may be handy if you don’t have access to a local “contractor oriented” plumbing supply store. (If they have a fancy showroom, their prices are probably going to be high.)

If particular rooms’ radiators don’t warm up, the issue might be the valves in those rooms. There are multiple choices for valves, including a “Vari-Valve” which I purchased for every radiator in my house (paying literally half the hardware-store price at Universal Supply Group). These are made in New Jersey, and are somewhat adjustable, so they make just one type. The more common type (usually Gorton or Hoffman) is sold by size, the idea being that larger radiators need larger valves; to a degree, a larger valve can also make up for the failure of the “main line” valves, though it’s not really the right way to do it. Since our radiators are all pretty much the same size, I have the VariValves on most of them set to the full rate; the ones on smaller radiators are half open. Valves are moderately pricey (e.g. $12) but you only need to buy them once every now and then.

varivalveAs I said earlier, I chose VariValves partly because they’re made in New Jersey, still; but mostly, because they claim to have the highest flow rate of any radiator valve you’re likely to get. The company actually claims that their radiator valves flow as well as the Gorton’s main-line valve (size #2), at 0.8 CFM. Another benefit is that they are dead silent; the Gorton’s and Hoffman valves I've used sometimes whistled or bubbled or made other noises, but the VariValves are utterly quiet. (I have no affiliation with the company, and you'll notice there are no affiliate links here. If you want their valves, you'll have to get them on-line or from a plumbing supply store. They’re made by a company called “Heat-Timer.”)

Chances are, if you have proper valves fitted, your steam heat system will never build up enough pressure to register on even an “ounces” gauge — yet, many boilers are fitted with pressure controllers (e.g. “PressureTrol,” see illustration above for a vintage model) and gauges that register in pounds. The fact is, steam systems don’t work on pressure; they work on the fact that steam expands, and expands very rapidly (as in “about as fast as you drive down a country road”). Not showing pressure is a good thing. A low-pressure system is usually quieter and more efficient, or so I've read.

Maintenance: the simple stuff

water valveNow and then, you’re supposed to drain some water from the system. How much, and how often?

On a boiler with a traditional low-water cutoff (not an electronic one), you should let out water from the low-water cutoff system once a week, according to most boiler manuals (check yours to make sure). If your boiler is very old, this is what is has; if it’s new, it probably doesn’t. A dead giveaway is a valve right by the sight glass (the glass through which you can see the boiler water, if it’s not too filthy). As to how much, “until it looks relatively clean” is a good guideline. It should not take much water, you’re just getting rid of the sludge that settled down. Don’t wait for it to look like tap water.

The illustration on the right shows a typical unit. All the ones I've seen look like this, even on much newer boilers. That doesn’t mean they all look this way. When you open the valve, do it while the boiler is running; it should shut itself off while the water is being drained into the bucket (if you open the valve all the way). If not, you need to get a plumber in to fix the shutoff valve.

Once a month or so, it also helps to let a bucket of water out of one of the lower valves; this will be much more mucky. This might be a gallon or two. I usually shut off the boiler for a while beforehand, let the water cool down, but more important, let the house cool down so that when I've refilled the boiler, it immediately goes on and heats the water.

sight glassOne thing most plumbers agree on is that fresh water kills a boiler, so whenever I put water in, I try to make sure the boiler will activate soon. If that means filling it at night, or waiting a day or two for colder weather, that’s what it means. (Of course if the water is really low, it’s another story.)

For this reason, I've also started letting out water every other week, rather than every week, on my boiler, which has a more modern sensor-type low water cutoff (note: since adjusting chemical dosage and using filtered water, this has only made sense on a monthly basis). You can’t release water from the cutoff valve because, well, there isn’t one. Instead, you release the sludge from the bottom. The plumber hooked up the fresh water supply to go right through the Hartford loop, which means that rather than letting the condensate sludge up the loop, every time you put in fresh water, it goes right through it and clears it out. (I do wish there was a cutoff between that and the boiler so I could clear out the loop without dirty water getting into the boiler.) Weekly sludge removal doesn’t seem to add much.

Following an alert from Dan Holahan, I also started using filtered water (reverse osmosis) to keep salt and chlorinates out of the boiler. Apparently both can rot out a boiler quickly, which explains why boilers in my town have started dying left and right — largely since the local water company build mini-mansions on the reservoirs’ “buffer zones.” The result is already visible: the water in the boiler seems to be much cleaner. Essentially, I now run just enough fresh water from the pipe through to clear out the Loop (the return), — and since most residential boilers are not plumbed this way, that’s often a pointless step — then pour in filtered or distilled water through the emergency valve hole. It is a nuisance but the boiler does not go through much water. (See below for the exact method.)

Maintenance and cleaning: dealing with surging and such

“Surging” is when the water in the boiler (as shown in the sight glass) bounces around quite a bit while the boiler is on. It’s usually caused by dirty water, and it’s “bad.” That brings up cleaning.

Skimming is a way to get oil and grease and such out; for this, you’d want the water level to stay roughly normal. In essence, since you can’t take the top of the boiler, the way to “skim” the top is to remove something that’s roughly at the waterline, like a pressure indicator or (a little higher) the safety valve or (a little lower) the lower sight glass support, and then gently run water out of the boiler while gently letting water run in. If it’s a spot lower than the water usually sits, you'll need to let out some water from the bottom first. Skimming requires time and patience; it’s better to do it very slowly and let the top layers of the water slowly drip into a bucket, according to the experts.

There are other ways to remove oil from the boiler, including using various chemicals. One I found amusing was made up mainly of baking soda; it bonds with oil and grease and can then be removed. It’s a pain to get in there, though. Our Peerless boiler’s manual recommended using a small amount of dishwasher detergent. You may need to do a few applications if your plumber did not clean the oil off the pipes before installing them (pipes come coated in oil to prevent rust or because it makes life easier for the factory or just to confound boiler owners, I haven’t decided which).

At the start of each year, I also empty the boiler out completely, then flush it out — the fresh water feed is on one side and there’s a low valve on the other, so I put a bucket on the one side and now and then put the fresh water on full force to push the sludge and such across into the bucket. Then I put in the boiler chemicals (more on these later), fill it up to the proper level — letting some water run through the sight glass bracket and into a bucket — and fire it up. Again, I only do this when I know it'll boil water as soon as I'm done. Fresh water = bad; boiled water = okay.

rust inhibitor

The preventive boiler chemicals I've been using are in this lovely retro “Utility Wonder Products” bottle, and I think the primary active ingredient is sodium sulfite. I can probably buy sodium sulfite much more cheaply without the cute bottle (around $12), but I have no idea how much to put in; sodium sulfite is often recommended but not often with specific dosages, so I use the bottles for convenience and because they also include some sort of indicator which turns red when the water’s too dirty. How does it know? Well, I’d assume it’s like indicator stopbath and relies on pH, which would seem goofy. In any case, the stuff has made a major huge impact in reducing corrosion, if the amount of gunk coming out and the cleanliness of the sight glass are any indications. My boiler has needs two bottles at the start of the year; your boiler may need just one. This year I put in a bottle and a half at the start of the season and added the rest in January and that seemed to work better, since new water is added now and then.

pressure valve

So how do I add the chemicals in the first place? In my boiler, the easiest way is through the emergency pressure release valve. First, I use the pipe to get some leverage and loosen it; then I remove the pipe (if it sticks, use a plumber’s wrench to get it off) from the valve. This is all a lot easier if someone has done it recently or if you have a new boiler, so you may want to have a plumber do the chemicals and such, then as soon as he’s left, if he hasn’t used thread sealant, take it apart again and put the sealant on... the sealant makes it much easier to take things apart later.

thread sealant goop

There are a couple of advantages of using this valve. It’s all by itself on my boiler, and easy to reach; the steel pipe (placed there by state law because if there is an emergency steam release, it’s less likely to scald people after coming through the pipe) gives me leverage to start out without needing a wrench — I obviously wait until the boiler is cool before touching it; and it’s above the water line so I can add chemicals or whatever without losing water.

adding chemicals

It’s also a convenient but not ideal skimming point, because it’s too high up; for skimming, which thankfully I only had to do in the first year, I took out the pressure valve, which is closer to the water line (and on the other side of the boiler). I need to use the plumber’s wrench to get the valve all the way back on; it’s too loose if I finger-tighten, and needs to get an extra three-quarter-turn that the wrench makes easy.


As with everything you do to the boiler, try to do this sort of thing on a moderately pleasant day, not when the temperature is below freezing.

Not yet written: the sight glass

Coming someday.

Keeping the heat on when the power is not

Proceed at your own risk

You can prepare for power outages with the help of a plumber, electrician, or just someone clever with wires, but this is a process that requires caution and can result in your destroying your boiler’s sensitive electronics, if it has those, (and it probably does).

I do know several people who have done this, both during an outage and in preparation. It may or may not be legal in your area.

First, a preface: steam boilers use very, very little electricity. Hot water systems have pumps; steam systems do not. The electrical parts of a steam boiler are:

All together, these devices use very little power indeed. You can run a boiler for many hours off a car battery, through a high quality, previously tested inverter; and of course through a good quality generator — in both cases, they must be capable of clean, electronics-friendly power. We are not talking about truck-stop, Porter Cable, or Wal-Mart supplies.

Generally, a boiler has a cutoff switch right on it — it’s part of the code in many places. (I had another cutoff switch installed on the top of the stairs. If there’s some boiler emergency, I don’t want to be right next to it when I shut it off.) That is where the power goes in. Taking note of polarity, one can take the wires and hook them up to a standard three-pronged outlet, then alter the boiler power input so it is connected to a three-pronged plug. At that point, if the power goes out, you can take the plug out of the outlet on the boiler’s side, and plug it into an extension cord leading to the generator or power inverter. (It makes more sense if you can see it.) Just make sure you do not screw up the polarity, if you do this — and realize it is a risk. (If you try this out, do a dry run on a warm day.)


bad installI have had many plumbers in my house to troubleshoot our steam heat system, thanks largely to a boiler replacement done by completely unqualified people who, in the end, were kind enough to refund my money, though not my time. A large part of the reason for the refund was, they turned out not to be licensed plumbers and were actually not supposed to do the replacement. They also lied about not needing a permit. If nothing else, that taught me not to be desperate for heat when choosing a contractor; they happened to be in the house when the old boiler had just died (they were there to give quotes on air conditioning, which I suspect they also were not qualified to install).

In any case, they chose a boiler which was, as I'd requested, quite efficient — but also about twice as powerful as needed. Bringing in “real plumbers” to diagnose this problem resulted in a lot of very bad advice. (What happens when your boiler is “overfired,” or more powerful than it should be? Well, it shoots up much more steam than your house can handle, more quickly than it can handle, and the water condensing from the boilers falls back and is shot back up by more steam, so that every radiator sounds like a coffee percolator.)

In the end, there were two plumbers who seemed to know what they were doing and, this is important, were willing to follow the instructions that came with the boiler. There were five more plumbers who either had no clue what a steam system was supposed to look like, or refused to even think about following the instructions. Given that the pipes are now considered a part of the “boiler system,” that’s bad. The instructions must be followed or you'll have “wet steam” — steam with lots of water droplets being flung up into the radiators. One of those plumbers refused to make the pipes all steel; it’s much easier for them to use copper. It can also reduce the boiler’s lifespan, especially if it’s used in the wrong places.

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