Modern hydronic heating systems have evolved in complexity over the last few decades; no doubt we’ll continue to refine and improve upon them. With this evolution, the need for proper piping design and technique has become more important.

Simple systems, like a boiler and one or two baseboard zones, are pretty forgiving to piping error. However, systems that include radiant heat or fan coils aren’t so tolerant. A single piping error in a radiant system, for example, could lead to a system that refuses to deliver the expected comfort and efficiency - or it just won’t work at all.

It’s hardly surprising to learn that most contractors who successfully install radiant heating systems have (a) learned from a lot of expensive, time-consuming trial and error, or (b) have received the training to make sure the system goes in right and delivers the comfort and efficiency the customer expects.

I know from personal experience there’s usually some of both in all of us.


As boiler technology has pushed operating efficiencies to new heights, the need for correct near-boiler piping has become more critical. “Near-boiler piping” generally refers to the piping and system components that are installed directly to the boiler and connect it to the distribution piping.

These components typically include the system expansion tank, fill (pressure-reducing) valve, air separator, and, in some cases, depending on system design, a circulator and system purge.

The arrangement of these components in the near-boiler piping is a critical point of consideration when installing the system. Not only must these components be sized properly, but they also must be installed in specific relation to one another to ensure proper system function and performance.


Working well is only one part of the picture, however. Future servicing of the system should be considered, too. A well-designed and installed hot-water heating system will not only provide efficiency and comfort, but will also be highly serviceable.

Fortunately, most boiler manufacturers now offer their preference for specific near-boiler piping in boiler installation manuals. These piping and component details are adopted from the Hydronics Institute; the recommended piping schematics help installers avoid myriad problems.

Most boiler manufacturers have adopted this piping arrangement, and it’s no coincidence. The piping detail covers all of the concerns in a very efficient manner. Some of the key issues addressed are:

• Placement of the circulating pump, with its suction port immediately downstream of the expansion tank, ensures that the pump’s developed head pressure is added to system fill pressure. This helps control air problems.

• The fill valve is piped to the system with the expansion tank. This safeguards the valve from fluctuating pressure caused by the pump. Without this step, unwanted fill water and excessive system pressures are sure to happen.

• Optimal placement of the system air separator - there’s no better place for this component than at the point of highest temperature and lowest pressure. This is where dissolved gases are most likely to come out of solution and are most easily arrested and cast out of the system.

• Inclusion of a system purge - after all, if a need arises, we want quick and efficient purging of an entire multizone system, including the boiler.

Taking this a bit further, some boiler manufacturers show an isolation valve between the system and the expansion tank. In a modern system with the pressure relief valve installed directly on the boiler, the 100-year-old engineering directive to not place a valve there - from the days when the relief valve was installed on the expansion tank - can now be ignored.

The benefit of this valve is to enable easier servicing of a diaphragm-type expansion tank. A little-known fact about these tanks is that the air pre-charge must be checked annually. The only way to accurately do this is to isolate the tank from system fill pressure; the air charge can be accurately measured only when there’s no system pressure on the wet side of the diaphragm.

Just like a tire on a car or truck driven each day, a diaphragm tank, over time, can (and most likely will) lose some of its pre-charge. Just like riding on a soft tire, underinflation can lead to premature tank failure. With an isolation valve, matching the air pre-charge to the system fill pressure becomes much easier to do.


In my years of attending and conducting contractor training seminars on hydronic design and installation, I’ve noticed that the old excuse of “This is the way I’ve always piped a boiler,” is slowly giving way to the realization that there is indeed a better way to do it.

The near-boiler piping - like the arteries and valves nearest the heart - play an important role in seeing to it that the rest of the system gets what it needs. If it doesn’t, you’ll hear from your customers, all too often when it’s late. And cold.

A quick scan of boiler manufacturer piping diagrams reveals that a specific, smart, well-thought-out arrangement of components in the near-boiler piping will ensure highest operating efficiency and greatest ease of commissioning and service.

It is our collective hope at this company that installation contractors will take these recommendations to heart. After all, we want your installed systems to perform as expected, and so do your customers. If they’re happy, you’ll be happy too. They’ll sleep better. You will too.

Meanwhile, down in the mechanical room that new mod-con is humming along beautifully; pumps, valves, and other components are working in concert; and warmth flows like hydronic heaven.

Publication date:06/13/2011