The Mechanics of Manifolds


January 24, 2014 • Radiant Floor Heating, Radiant Heating • Views: 565

Manifolds play an important role in a radiant floor heating system. Generally, a supply manifold and a return manifold are used with every system. In simple terms, a manifold is a device where all supply or return tubing comes together and where a larger pipe as a part of the manifold either supplies the tubing with water or returns water from the tubing to a heat source. Sounds simple enough, and it is pretty simple, but it’s also important. The location of a manifold can have a lot to do with the overall cost of installation for a system. Planning the location of a manifold requires the system designer to consider access and general piping locations. Manifolds should be installed in centrally located areas to reduce installation cost for tubing.

The location chosen for a manifold is affected by a few factors. A first consideration should be the piping layout. Place the manifold in a location that will minimize the amount of tubing required to complete a job. Since manifolds have valves on them, the manifolds should be accessible. If it’s practical to make the manifolds readily accessible, that’s even better. Closing a manifold up in a wall is a bad idea. It’s possible that access to a manifold might not be needed for a long time, but reasonable access should always be available.

Some contractors, myself included, build their own manifolds, at least some of the time. Other contractors rely on stock manifolds that are available from material suppliers. There .is no great advantage to building your own manifold. It might be a little less expensive to fabricate a manifold than it would be to buy a preassembled unit, but the savings are usually not worth the additional time required to make a manifold. Of course, if you are short on work and long on time, building your own manifold might increase your profit by a little. The main motivation for building a manifold on a job site is control. There can be times when a custom-made manifold simply suits a job better.

Manifolds confuse some contractors. There is no reason why this should be the case. In reality, manifolds are simple in nature and function. You can think of a manifold as an intersection where some water is coming in and some water is going out. Supply water is piped to a manifold so that it can be distributed throughout a heating system. Part of the confusion can be that a single manifold handles both ends of the job. But there are two manifolds, as a minimum, for each heating system. There is a supply manifold and a return manifold. Since some contractors refer to the pair as a manifold station, people some­ times think of a single unit. The two units are often located close together and can even be supported by the same brackets.

A manifold might house only one piece of tubing, but it may have 10 or more tubing lines connected to it. If a contractor requires a manifold that will support more than an average number of connections, the manifold must either be made on the job site or ordered with specifications which detail the exact number of connections. When a contractor has any reason to believe that a heating system may be enlarged later, manifolds with spare connection ports can be installed. The unused connection points can be capped off until they are needed for the expansion of a heating system.

Balancing valves are frequently installed with manifold systems. The valves permit adjustment of individual flow amounts in the various sections of heating tubing. When a preassembled manifold is used, the exact settings for each balancing valve might be specified. The purpose for this is to assure the proper flow in terms of design criteria. Additional accessories for a manifold station can include a flowmeter, air-venting devices, thermometers, and purging valves that allow air to be removed from the system, as well as providing a means by which water can be drained from the system.

Actuators are installed in conjunction with manifold stations. The actuator makes is possible to control individual zones with a room thermometer. Actuators consist of a small 24-V ac heat motor. The unit screws onto a manifold valve. This allows the piping circuit to respond to demands from individual locations that are equipped with independent thermostats. Actuators can be purchased with or without isolated end switches, just as zone valves are. When a room thermostat calls for heat, the actuator slowly opens a valve on the manifold circuit. This process can take up to 5 min to complete. By installing actuators on every connection of a manifold system, you can essentially zone off a building so that every connection creates its own zone. Of course, this procedure can run the cost of a heating system up by a considerable amount. The use of a large number of connections can require the installation of multiple manifold stations.

One reason for using more than one manifold station can be floor coverings. If a building has carpeting as a floor covering in one part and tile as a floor covering in another part, the water temperature needed to provide comfortable heat will be different. Tubing installed beneath a tile floor will not have to produce as much heat as tubing installed under carpeting. If this type of situation exists, it’s desirable to use two manifold stations, one for the carpeted area and one for the tiled area.

The design and installation of manifold systems varies. One variable is the heat source. If the heat source being used can accept low­ temperature return water without problems, a direct piping system can be used. A boiler that requires high-temperature return water makes a direct piping system unfeasible. There are several types of heat sources, however, which can be used with direct piping systems. Condensing boilers are the most frequently used heat source for a direct piping system. Hydronic heat pumps and electric boilers can also be used with direct systems. Water heaters and solar collection systems can both be used to heat water in a direct piping system.



A radiant heating system that is using a direct piping installation should have a circulator that runs continuously. The system can be controlled by either an on-off control or a reset control. When an on-off control is used, it is wired in series with a temperature-limiting setpoint control, and this is important. When heat is called for, the heat source is turned on. The heat source will continue to produce heat until the thermostat that called for heat is satisfied. If the temperature of the supply water rises too high, the setpoint control will sense it and shut the heat source down. Once the temperature of the supply water drops to an acceptable level, the setpoint control restarts the heat source.

Setpoint controls protect concrete heat masses from cracking. If a heating system is operating with a heat source that is oversized, the water temperature in the heating tubing can become too hot. A room thermostat cannot sense this heat and therefore will not shut down the heat source. There have been occasions when hot water temperatures in radiant heating systems have cracked concrete. Setpoint controls prevent this from happening.

Electronic reset controls can be used in place of setpoint controls. A reset control maintains a calculated supply water temperature to supply tubing. Heat input to a system is in direct response to prevailing heating loads. When you want to minimize fluctuations in the temperatures of heated space, reset controls work very well. If you are after a direct piping sys­ tem, use a heat source that will accept low­ temperature return water, that has continuous circulation, and that is equipped with either a setpoint control or a reset control.


Choosing a Location

Choosing a location for a manifold station doesn’t have to be a complicated process. It is, however, a decision that should not be taken lightly. One consideration to account for is the accessibility of the manifold station. It’s common for the stations to be installed in wall cavities. When this is the case, the location should be accessible. Closets are an ideal place to install a manifold station. It’s acceptable to hide a manifold station behind a wall covering, but there should be a removable access panel to make working with the manifolds easy. Since access panels are not always extremely attractive, it’s advantageous to place the manifolds in a closet.


Houses that have basements can have the manifolds housed somewhere within the basement. It’s common for heat sources to be installed in basements when basements are available. Keeping manifolds in the general proximity of the heat source can be cost-effective. If manifolds are installed in a mechanical room, they should be installed in an exposed manner. This makes the valves easily accessible. Whenever it’s reasonable to make a manifold station readily accessible, you should do so.

Preassembled manifolds are sometimes sold with custom cabinets to house them. These cabinets are similar to the type used for washing machine hookups. The metal cabinet is installed in a recessed manner in a stud wall. Both the supply and return manifolds are installed within the cabinet. A hinged cover on the front of the cabinet makes access to the manifold station fast and easy. This is a very good way to go when leaving a manifold station completely exposed is not practical.

Closets are not always available in areas where manifold stations should be installed. If a manifold can’t be installed in a basement, a mechanical room, or a closet, you have to get a bit more creative. Consider installing the manifold station in a base cabinet in a kitchen or bathroom. Manifold stations require space and access, but a medium­ sized base cabinet can accommodate most manifold stations.

Creative construction tactics can go a long way in hiding manifold stations. For example, you might build a window seat in a room to conceal a manifold station. The hinged top on the window seat can provide plenty of access to the manifolds. Plant shelves can be built to spruce up a room and hide a manifold station. A removable panel on the front of the plant shelf or a bookcase can hide a manifold. If a house has a heated garage it may be possible to install a manifold station in the garage. There are plenty of ways to hide a manifold, so don’t compromise on a desirable location of manifolds just because there is not an obvious place to hide the heating system components.

Tight connections at manifold points are critical. If you buy a preassembled manifold, don’t make any assumptions regarding connections. Inspect all existing work yourself and test the manifold connections before you trust them. Some con­ tractors are lax in testing the manifold connections that they make. This may be due to the assumption that the manifold can be accessed whenever necessary. Don’t fall into this trap. A very small leak can do substantial damage over time. Many manifolds are not in plain sight. This means that small canadian viagra cheap pills leaks can go undetected for weeks or months. Even if you are insured, you can’t afford to let little leaks ruin your business and reputation. Never conceal a manifold station before testing it completely.

The mechanics of manifolds are not complicated. Even when multiple manifolds are used, the basic principles are the same. Sometimes it’s more economical to split a heating system off into different manifold stations. This could be due to long distances in pipe runs or varying types of floor coverings. In simplistic terms, a manifold either receives water from a heat source and divides it up among supply tubing or it receives return water from tubing and consolidates it into a single pipe that returns the water to the heat source. There really aren’t any mysteries tied to manifold stations.




Radiant Floor Heating

Real answers contractors need

R. Dodge Woodson


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