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Risk vs. Reward

May 24, 2021
With enough money and effort, just about any steam system can be changed to hot water.

Have you ever considered converting a steam heating system to a hot water heating system? The rewards for a successful conversion include lower energy bills, more consistent temperature control, and quieter operation. What homeowner wouldn’t want all of these improvements?

Before you go around promising every customer or prospect all of the above benefits, you need to temper your enthusiasm with some sobering realities. The first and foremost of these realities is the fact that not all systems are a good candidate to convert. You probably noticed that I didn’t say that it couldn’t be done. With enough money and effort, just about any steam system can be changed to hot water.

There are two types of steam systems where the amount of money and effort become too large, even for really deep pockets. The first type has cast iron radiators that were made specifically for steam. Most modern radiators can be used as steam or hot water depending on how they are piped, but not the really old ones.

In the very early days of central heating systems, steam was the dominant system used. The cast iron radiators were constructed for the flow of steam, not water. There are radiators in your town that are not suitable for conversion to hot water. This is the first thing to look for. When I say not suitable, I don’t mean impossible.

I know this because I saw the conversion done on a house in Newport (Kentucky, not Rhode Island). The individual sections of the earliest cast iron sectional steam radiators were only connected at the bottom. Why does that matter? Since we are now going to try to fill the radiators with water instead of steam, the air has to be vented from the top of the radiator. With the type radiator that is only connected at the bottom, each of the sections will need to have a bleeder.

This was a large house for the small town of Newport with plenty of radiators. Plenty of radiators escalates to a whole lot of sections. It must have taken the contractor a whole lot of drilling and tapping to install the hundreds of manual air vents necessary to get the all the air out and the radiators full of water. This expensive extra week or so of labor usually prevents a conversion like that from happening.

The next thing I check is how many connections are on the radiation. With steam, it can work with just one pipe connected at the bottom. The steam comes into the radiator, gives up its heat as it condenses, and flows back out as water. With hot water, there has to be a way in and a separate way out. So we need two pipes per radiator to get the water to flow.

If the existing system only has one pipe connection to the radiator, a second line will have to be added. Then it has to be connected to a new return main and run back to the boiler room. If the house is in the middle of a major renovation, and any demo of the walls, ceilings or floors is acceptable, then this might be possible. However, most homeowners are not going to spend the amount of money required to add these extra pipes. I haven’t seen any houses with this conversion. The energy savings of a high efficiency boiler would never recoup that expense.

The best candidates in our area for a conversion are two pipe systems that were installed in the 1920’s. The system designers of that era were using pipe sizes that work well for circulated systems. With steam, the supply pipe size is always large enough. The return pipe is the one to check. Sometimes it can appear to be too small. But if you run the numbers for an oversized pipe to the radiator with an undersized pipe back to the boiler, the total resistance to flow is not out of the range for a normal circulating pump.

These 1920’s jobs have the supply pipe connected to the radiator at the top, with a steam trap or vapor device connected at the bottom, usually on the opposite side. The radiator valve is easy to replace because it isn’t at the floor. I recommend replacing all of them for two reasons. First, they are the most likely place to develop a leak. Second, with fresh valves you and the homeowner have control of the flow of water to the radiator, which means they can control room temperatures.

Now let’s talk about the other end of the radiator. The advice of my elders when I was just learning about this was to take the cap off the trap and remove the bellows assembly. If you were real ambitious, you then drilled out the seat inside the trap. The thinking at that time was to open up the path for the water to flow through. A bigger hole creates less restriction to flow. I see it differently now.

It can be very difficult to take that cap off, so simplify the operation. Leave the bellows in and the seat size intact. Now, when the water starts flowing, it hits the same size hole at each radiator, which creates pressure drop, but not that much and it is consistent to the point of self-balancing. The only problem we’ve encountered since adopting this method has been noise

A rattle can be created if that bellows inside the trap had previously broken off its anchor point. As water flows through and around it, it can keep a guy up at night. Of course, the only one reported so far was in the master bedroom of the guy that was Hall of Fame quarterback and Cincinnati native Roger Staubach’s center in high school. The contractor was able to get the cap off without too much trouble, remove the broken bellows, and give him a well-deserved night’s sleep.

If the thing on the other side of the radiator is a vapor device, you’re golden. They usually don’t have moving parts, so you don’t have to disturb them. “Bob’s your uncle” as they say in Canada.

This is but a brief discussion of all the risks that go along with a steam to hot water conversion, concentrating on the radiation. There are plenty of things with the piping in the basement that I don’t have enough space in this column to go over. I’ll save that for another time when the Kid asks me to explain this again. He’s picking up hot water basics pretty quickly, but as with most, the concepts and nuances of steam are harder to wrap your brain around.

Patrick Linhardt is a thirty-seven-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.

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