Some Tips On Converting From Steam To Hot Water by Dan Holohan


Before the 1920s, most steam contractors used cast-iron, column-type radiators. Those are the free-standing ones with the real wide sections. Column radiators were perfect for steam heating because they had a lot of internal space and they allowed the steam to rise up and displace the heavier air. In the case of one-pipe steam, the air worked its way out of the radiator air vent. With two-pipe, the air escaped through the steam trap (or that old Vapour gizmo) and left the system through a vent somewhere down the main. Having a radiator with lots of internal space also allowed room for the condensate to get out of the way of the steam.

The trouble with those old column radiators, though, is that their makers never intended for them to be used with hot water. Those individual radiator sections aren't connected across the top, and that presents a challenge with water. You have to look real close to notice this on those old radiators, and it's the very first thing you should look for when you're considering a conversion from steam to hot water. If you don't see those top push nipples, the conversion isn't going to be very easy - or practical. Water won't flow well through a sectional radiator that's not connected across the top. To make it work, you'd have to drill and tap each section and install air vents all the way across the top. Or, if you have a LOT of patience, you can wait for the system water to absorb the air and move it to the air separator. This could take months.

After the 1920s, steamfitters began using cast-iron, tube-type radiators (the ones with the thin sections). These were actually hot water radiators, but they also worked well on steam systems. Building owners preferred them because they were nicer to look at. Tube-type radiators do have push nipples across the top of the sections, so if that's what you have, you can move on to the next step.

And that next step is to check to see if your system is one-pipe steam or two-pipe steam. This may seem like an easy thing to do, but don't go by just one or two radiators. Many systems have a combination of both one- and two-pipe radiators, and that's perfectly normal - for steam. Look it over. While it's possible to convert a one-pipe radiator to hot water, it's sometimes not practical. You have to run a second pipe from the top of each radiator back to the boiler. Some folks use radiant-floor tubing to do this. They drill and tap the top of the radiator section that's opposite the supply valve and then run either PEX tubing or rubber hose back to the boiler, connecting it to the mains with cable ties as they go. Once back in the boiler room, they use manifolds with individual circuit-balancing valves to make their return connection to the boiler. Most plumbing & heating supply houses sell this gear nowadays.

A conversion from two-pipe steam to hot water is usually easier to do, but examine the system carefully before you make a decision. Many of the older two-pipe systems were Vapour systems. There may be orifices in either the supply valves or the return elbows. Those radiator return elbows may also have tiny check valves or stainless-steel balls that can mess up the flow if you don't know they're there. Look the job over carefully and if you have any doubts, go to the "Wall" at and ask us. We do our best to help you.

Before you break out the tools, do a complete and accurate heat loss calculation on the entire building. Don't settle for one of those cockamamie rules of thumb that has you measuring the cube of the building and multiplying by an arbitrary number. Rule-of-thumb methods will oversize your system by a ridiculous amount. Take the time to do it right. If you don't know how to do a heat-loss calculation, check out the Quick Sizing Forms available from The Hydronics Institute (

Once you get an accurate heat loss calculation, measure the existing radiation for square footage of Equivalent Direct Radiation (EDR). You're trying to find out if the radiation that's already in the building will be able to heat the place once the relatively cooler hot water is flowing through it.

Keep in mind that one-psi steam in a radiator will bring the surface of that radiator up to 215 degrees. Hot water, by comparison, is usually only about 180 degrees, tops. A square foot of steam EDR will put out 240 Btuh (British Thermal Units of Heat). Convert that radiator to hot water and you're only going to get about 150 Btuh out of that same square foot of radiation. So, for instance, if you have a five-tube, 26" high, 10-section, cast-iron radiator running on steam, it will emit 8,400 Btuh. Convert the system to 180- degree hot water and you'll get only 5,250 Btuh out of that radiator. If you're looking for a fast way to figure out how much EDR is in a radiator, get a copy of my book, The Golden Rules of Hydronic Heating, which you'll find in the Books & More section at

Once you've gone through that exercise, ask yourself if this lesser output from the radiators will be enough to heat each room on the coldest day of the year. This depends on the heat loss, and that's why you have to take the time to do both an accurate heat loss calculation AND a survey of the existing radiation.

If, after you've done your homework, you find the existing radiation can carry the heat loss on the coldest day of the year, you can move on to the next step.

Check the system for leaks. Keep in mind a steam system is used to seeing only about two-psi steam pressure. If there are leaks anywhere, you might not have noticed them at such a low pressure. This is especially true if the leaks are in the return lines. But when you fill the system with water the static pressure is going to be a lot higher. You'll notice those leaks for sure! It's so much better to find them before the conversion, if you can.


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