As the North American hydronics industry has grown, so has the complexity of its systems. Many installations that are now featured as award-winning examples of hydronic heating contain hundreds — sometimes thousands — of parts and pieces. They can be so complex that only the person who designed and installed them understands how they operate, or can fix them when they don’t.

Although there are some buildings, perhaps even some single-family homes, that benefit from having a dozen or more heating zones as well as a wide mixture of heat emitters, they do not represent the mass market in which hydronics still only holds single-digit market share. Promoting the concept of hydronic heating with photos of elaborate systems containing lots of hardware can easily send the wrong marketing message. Instead of comfort combined with peace-of-mind reliability, the average consumer may be thinking — expensive, complex and what happens when it breaks?

Why build a space shuttle when a Cessna is what’s needed?

This month we’ll look at a relatively simple system that provides space heating and domestic hot water. It’s simple from the standpoint that only one type of heat emitter is used. Simple because the control logic is pre-engineered rather than customized. And simple from the standpoint of piping layout.

Perhaps you’re thinking that simple excludes being state-of-the-art. This is not the case. The system we’re about to discuss uses cutting-edge concepts such as outdoor reset control, boiler modulation and variable-speed pumping. It personifies the state-of-the-art in hydronics technology but with far fewer parts and pieces.


Combination system

A piping schematic of the system is shown in Figure 1. We’ll examine each subsystem independently and then look at how they all work together.

  • Heat source. The system’s only heat source is a mod/con boiler. A wide assortment of such boilers are available in North America, and most are capable of operating with either natural gas or propane. All have internal control logic that allows them to regulate the water temperature supplied to the heat emitters in response to outdoor temperature (e.g., outdoor reset control). The warmer it is outside, the lower the boiler temperature and the higher its efficiency. These “partial load” conditions represent the majority of the heating season. The system must be stable and efficient during these times, as well as under brief peak load conditions.

When it’s time for domestic water heating, a mod/con boiler temporarily ignores outdoor temperature and fires itself to an elevated setpoint temperature to drive all heat output through the heat exchanger in the indirect water heater. During this mode of operation circulator (P2) is turned off while circulator (P1) operates.

  • Buffer tank. This system provides room-by-room comfort control. Each panel radiator represents a separate zone. There will often be times when the heating load (perhaps a single room) is far lower than the minimum heat output rate of the boiler. Under such circumstances, any low-mass boiler with minimal water content will short-cycle. This can lead to premature failure of ignition and combustion components.

Short-cycling can be avoided by installing a well-insulated buffer tank in the system to provide additional thermal mass between heat production and heat release. The boiler maintains the temperature of the buffer tank based on outdoor reset control. This happens whenever the system is not in “warm weather shut-down” mode, regardless of what the space-heating demand happens to be.

Thus, a small heat emitter such as a bathroom towel warmer could operate at a “trickle” rate of heat transfer without forcing the boiler to operate until the buffer tank temperature drops below the required lower limit. Whenever space heating may be needed, there’s always warm water in the buffer tank ready to immediately meet the demand. A 25-gal. buffer tank would provide 44% more thermal mass than a 400–lb. cast-iron boiler with a 12–gal. water volume.

With a reasonable temperature cycling range of 10° F, it provides very adequate thermal mass, and can often be mounted directly below the wall-hung mod/con boiler as shown in Figure 1.

The buffer tank also hydraulically separates the pressure dynamics of the boiler circulator (P2) from those of the variable-speed distribution circulator (P3). It provides a low-flow velocity zone allowing air bubbles to rise to where they can be captured and expelled through an automatic float-type vent.

  • Panel rads. This system uses fluted-steel panel radiators for all heat emitters. These panels offer several benefits:
  1. They are very adaptable to different interiors. They come in an assortment of heights, widths and depths that allow them to fit into a wide variety of situations.
  2. They are one of the easiest heat emitters to install in new homes as well as retrofit applications. Being relatively light, they can be supported by almost any wall.
  3. They lend themselves to room-by-room zoning using nonelectric thermostatic radiator valves, as shown in Figure 2, fully modulating room-by-room comfort control without a mile of thermostat wires. This is a tremendous benefit relative to the complexities and installation time associated with wire-based thermostats in every room.
  4. Panel radiators have very low thermal mass and thus respond quickly to changing internal heat gain situations. In short, they warm up and cool down much faster than heated floors. This is especially desirable in buildings with significant internal heat gain potential.
  5. With proper sizing, panel radiators can operate at relatively low water temperatures and thus enable the mod/con boiler to operate in a condensing mode, even under design load conditions. I suggest sizing panel radiators for design load output with a supply water temperature of 120°. Operating panels at relatively low water temperatures also increase the ratio of radiant to convective heat output, which further enhances comfort.
  6. Each panel radiator is equipped with a dual isolating valve enabling it to be totally isolated from the remainder of the system if it ever has to be removed.
  • Distribution piping. Each panel radiator is supplied by a homerun distribution circuit connected to a common manifold station. These homer circuits can be PEX or PEX-AL-PEX tubing in either 3/8-in. or 1/2-in. nominal sizes (depending on flow requirements). Some of these piping segments could even come from “remnant” coils of tubing left over from radiant panel installations.

Because the panel radiators each have their own balancing valve, as well as dual isolating valves at the supply and return connections, it’s not necessary to use a manifold station equipped with such valves. A simple copper manifold such as the one shown in Figure 3 is fine.


The homerun distribution system provides several advantages:

  1. The small-diameter flexible tubing can be routed through the framing cavities of the building much like electrical cable. The ability to “fish” tubing through closed framing cavities is a tremendous advantage over rigid tubing, especially in retrofit situations.
  2. Homerun systems deliver the same water temperature to each heat emitter. This simplifies sizing since the temperature drop associated with series-type circuits is not present. It also allows each heat emitter to take equal advantage of outdoor reset control.
  3. In combination with thermostatic valves at each panel radiator, homerun systems allow the temperature of each room to be individually controlled. The master bedroom can be kept cool for sleeping while the master bathroom remains toasty warm for a morning shower. Unoccupied rooms can be set at lower temperatures to conserve fuel.
  • Cruise control. Another piece of the performance pie is a variable-speed, pressure-regulated circulator. These are now widely available in North America. An example is shown in Figure 4.

In this application, the circulator should be set to maintain a constant differential pressure. As the thermostatic valves on the panel rads open and close, the circulator senses the “attempt” to change differential pressure and responds with a motor speed adjustment. This keeps the flow through the active zone circuit stable, regardless of what other zones happen to be on, off or at some intermediate flow rate. Think of it as “cruise control” for differential pressure. It’s just the right amount of pumping effort under all conditions.

These circulators also operate on a fraction of the electrical energy required by a typical fixed-speed circulator with a permanent split-capacitor motor. Imagine an entire heating distribution system, in a typical North American house, operating on less than 45 watts of pumping power on the coldest day of the year! It’s possible with this technology.

Although simple in concept, this system can deliver excellent comfort on a room-by-room basis while operating at very high thermal efficiency and low electrical power consumption. It’s one of the easiest distribution systems to install in new and retrofit situations.

There are no controllers other than those already built into the mod/con boiler and the circulator. Wiring consists of supplying power to the boiler and three circulators. No room thermostats, transformers or zone relay panels are required. Domestic water heating is handled as a prioritized load allowing for the fastest possible recovery of tank temperature.

If you feel the systems you’re presently installing are getting a bit complex, consider giving this concept a try. It will quickly convince you that hydronic systems don’t have to be complex to be elegant.