This article is part of the Radiant Heating Skills Workbook.

While a lot of attention is paid to efficient boilers and innovative radiators, the design of the piping system is often what makes or breaks a hydronic heat system. A good piping system can be the difference between a noisy, uncomfortable, energy-wasteful system, and one that brings even comfort to all the rooms in a house.

To design an efficient system, you must match the heat source to the “heat emitters” — that is, radiators and convectors. Some types of heat emitters are best suited for relatively high-temperature heat sources. For example, the familiar finned-tube baseboard convectors, used in many residential and light-commercial buildings, work well with water temperatures above 150°F, but not with low-temperature systems such as ground-source heat pumps (see table “Matching the Components” ).

Once you’ve chosen a boiler and some heat emitters, you need a piping system designed to get the most out of that heating equipment, both in comfort and efficiency. This article weighs the pros and cons of four piping methods that are suitable for use with the equipment often used in residential and light-commercial buildings.

The Series Circuit

In a series circuit, the simplest hydronic piping system, the radiators and boiler are on one common loop. Radiators near the end of the loop are often larger to compensate for the lower water temperature.
In a series circuit, the simplest hydronic piping system, the radiators and boiler are on one common loop. Radiators near the end of the loop are often larger to compensate for the lower water temperature.

In the simplest hydronic distribution system, all heat emitters are connected into a common loop, or “circuit,” with the heat source. In this arrangement, the water temperature gets progressively lower as it moves from one heat emitter to the next. This decreasing temperature must be accounted for when the heat emitters are selected and sized.

A common error is to size the heat emitters based on the average water temperature in the system. With a series circuit, you must size the heat emitters based on water temperature at their specific locations within the piping circuit. If you don’t, you’ll hear complaints about overheated rooms near the beginning of a piping circuit (nearest the heat source) and uncomfortably cool rooms near the end.

The main advantage of series circuits is the simple and low-cost installation. However, since water flows through all heat emitters whenever the circulator is operating, you can’t use a valve to regulate the heat output of a given heat emitter. If you did, you would restrict flow through the entire system. In other words, series circuits have the disadvantage of not allowing independent control of individual heat emitters to match comfort needs.

As a rule, series circuits are best suited for high-temperature heat emitters, such as finned-tube baseboard, in small buildings that are controlled as a single zone. They should not be used with heat emitters that have high pressure-drop characteristics, such as radiant floors and some fan-coil convectors.

One-Pipe Systems

A one-pipe system isolates the boiler from the main loop of pipe when the boiler isn’t firing. Tee fittings and thermostatically controlled valves draw water off of the main loop, send it through the radiators, then return it to the main line
A one-pipe system isolates the boiler from the main loop of pipe when the boiler isn’t firing. Tee fittings and thermostatically controlled valves draw water off of the main loop, send it through the radiators, then return it to the main line

A “one-pipe system,” or “Monoflo system,” as it is sometimes called, is a distribution system that uses specialized tee fittings to divert a portion of the hot water through a branch piping path. If a manual or automatic control valve is placed in the branch piping path, the water flow through a given heat emitter can be fully controlled. This allows you to control the rate of heat output from each heat emitter without affecting the whole system. One-pipe systems thus offer the potential for room-by-room zone control — a feature not offered by series circuits. In most cases, extensive zoning can be accomplished less expensively with a one-pipe system than with any other type of distribution system.

Because the heat output from each heat emitter can be independently controlled, one-pipe systems also allow you to oversize individual heat emitters. This feature can be nicely applied in a bathroom where an oversized heat emitter can be set to rapidly heat up the room prior to a shower or bath, and then reset to maintain normal comfort temperatures. If you did this with a series circuit, you would constantly overheat the room.

Finned-tube baseboard, panel radiators, and fan-coil convectors can be mixed and matched as desired, all connected as separate branches off the main distribution circuit. Each unit still needs to be sized according to the temperature of the water it receives from the main circuit. This main circuit generally follows the perimeter of a building, and runs beneath the heat emitters located on exterior walls. This layout saves money by minimizing the amount of pipe used between the main circuit and the heat emitters.

The best way to control one-pipe systems is to provide constant circulation of heated water around the main circuit during the heating season. Thermostats open and close as necessary to meet the heating demand of the individual rooms. Because constant circulation is used, it is best to connect the boiler to the system as shown above. The boiler circulator operates only when the boiler fires. At other times, water flow in the main circuit bypasses the boiler, reducing off-cycle heat losses.

Multi-Zone and Multi-Circuit Systems

A multi-zone system uses a separate main loop for each zone, providing water to each zone at about the same temperature. The preferred method is to use a small circulator and a check valve on each loop.
A multi-zone system uses a separate main loop for each zone, providing water to each zone at about the same temperature. The preferred method is to use a small circulator and a check valve on each loop.

Another method of zoning a hydronic system uses a separate piping circuit for each zoned area. There are two ways to set this up; using a separate circulator for each zone, or one larger circulator and several electric zone valves. I prefer the former method for the following reasons:

• The small zone circulators consume less electricity and operate only when their associated zone requires heat. By comparison, the single larger circulator in a zone-valve system must operate whenever one or more of the zones requires heat.

• When a single larger circulator operates with only one active zone, the flow rate may be high enough to create annoying flow noises in the pipes.

• If a circulator fails, heat is interrupted only to one zone. The other zones function as normal. Failure of the circulator in a zone-valve system will prevent heat delivery to the entire system.

It is important to note that a springloaded flow-check valve must be installed in each zone of a multi-circulator system. If there are no check valves, and just one zone is calling for heat, warm water will flow backward through circuits that are supposed to be off. This will limit the heat output of the active circuit. It can also cause unwanted heat to flow into heat emitters during warm weather when the boiler operates only for domestic water heating.

Multi-zone systems using separate circuits have another advantage: Each zone receives water at about the same temperature. This may allow the heat emitters to be sized somewhat smaller in comparison to a series circuit. If the heat emitters are appropriately sized, you can also operate the system at a slightly lower temperature, thus improving its overall efficiency.

Two-Pipe Systems

A two-pipe system supplies water to each radiator throughout the system at nearly the same temperature. All radiators are connected between a common supply main and a common return main. Two-pipe systems are more common in commercial buildings, and are a good match for condensing boilers.
A two-pipe system supplies water to each radiator throughout the system at nearly the same temperature. All radiators are connected between a common supply main and a common return main. Two-pipe systems are more common in commercial buildings, and are a good match for condensing boilers.

The most common type of hydronic distribution system in commercial buildings is known as a two-pipe, or parallel, system. In this design, which can also be used in residential systems, each heat emitter is located within a separate branch circuit that connects to a common supply main and common return main. Each branch circuit runs “parallel” with the others, allowing each heat emitter to receive water at about the same temperature. In theory, this lets you use smaller heat emitters in each room.

The preferred method for connecting the branch circuits to the mains is shown above. This design, called a “reverse return system,” results in balanced flows through the branch circuits.

This chart shows the typical operating ranges of various hydronic heat sources, heat emitters, and piping systems, although unusual circumstances may occasionally require designs outside of these ranges.
This chart shows the typical operating ranges of various hydronic heat sources, heat emitters, and piping systems, although unusual circumstances may occasionally require designs outside of these ranges.

Because each heat emitter receives water at about the same temperature, the temperature drop between the boiler supply and return will be less than that associated with a series piping system. A typical parallel system, for example, may have a temperature drop of only about 10°F between the boiler supply and return. By contrast, a typical series system could have a temperature drop of 20°F or more. The smaller temperature drop of the two-pipe system helps keep the temperature of the water returning to the boiler above the dewpoint of the exhaust gases, thus preventing flue-gas condensation.

Two-pipe systems are the best choice for use with low-temperature heat sources such as heat pumps or condensing boilers. Radiant floor systems can be considered two-pipe systems since each floor circuit is connected in parallel with the other circuits at the manifold stations. Twopipe systems also allow easy zoning by using valves to regulate flow through any given heat emitter.

Be sure to check out the rest of the Radiant Heating Skills Workbook.