- CONTROL STRATEGY
Control systems can be divided into a couple of categories those that circulate water continuously, and those that circulate the fluid intermittently. Each of these control methods can be further divided into two further groups:
- systems in which the temperature of the water is fixed at a constant temperature.
- systems in which the fluid is varied in temperature depending on the outside air temperature and/or the inside air temperature.
All boilers have aquastats that control the output fluid temperature within a users specified range. The simplest type of system uses a boiler with controls set to deliver a fixed temperature output every time the heat is called for by the thermostat. As the heat loss increases from the building, the boiler will run more minutes of every hour to deliver the necessary BTUs.
Try to keep the temperature of the water supplied to the floor as low as possible. This minimizes the possibility of overshooting the desired interior temperature when the heating system comes on. In all cases, the system designer must first balance the cost, ease of operation, and sophistication of local installers to come up with the appropriate strategy.
- CONTINUOUS PUMP OPERATION
When pumps operate continuously, boiler or zone valves can be turned on and off by a thermostat sensing indoor air temperatures. In this type of situation the water is circulated continuously throughout the boiler and the heat is turned on and off, if it is single zone heating system. In a multiple zone environment the fluid is circulated throughout the boiler all the time but is intermittently circulated throughout the individual zone when the room sensing device (thermostat) tells the zone to open or to close. The temperature of the fluid going into the floor is governed by two things. One the boiler control setting or a mixing valve that mixes return water with the supply; two control the temperature of the actual fluids entering the floor heat system. - ADJUSTING WATER TEMPERATURE
You may choose to adjust the water temperature feeding the floor heat system in response to the changes in the outside air temperature. This can be done by manual methods or by an automatic device called an indoor/outdoor controller. In either of these situations the manufactures recommend minimum boiler water temperature. It is also just as important to make sure to not thermal, shock the boiler by introducing extremely cold water into the boiler, this is most important for cast iron boilers. - BOILER CONTROLS
Boilers operate in an on/off configuration, but some are equipped with a modulating gas valve that can control the flame in an infinite variable range from 100% to 20% output. - SYSTEM CONTROLS
One way of controlling the system is by means of a primary and secondary pumping off the boiler. This type of system maintains the proper flow required for the boiler and the proper flow for the floor heat system.A second way of controlling the flow to the zone and maintain proper fluid temperature is with the use of a three way mix valve. These valves mix a variable ratio of hot water coming out of the boiler and cooler returning water from the slab. This temperature controlled mixture is then returned to the floor. A 3-way mixing valve may either be manually adjusted in response to changes in the outdoor air temperature or it may be automatically controlled by an outdoor reset control. A three way valve is slightly simpler to install than a four way valve, and the return temperature to the boiler may be lower. An optional bypass line, required for some low mass boilers or where higher boiler water temperatures are needed.
A third way of changing the temperature of the water going into the floor heat system is through the use of a 4 way mixing valve. These valves have four ports that interface with the floor heating loop and boiler loop. A water control disc rotates around a shaft and sends more or less hot water, as required, from the boiler to the supply feeding the heating system. Similarly, the return water from the floor heating system enters the right side port and is either recirculated to the floor through the top port, or is guided back to the boiler in the bottom port, or most commonly the water is divided by the valve between the two ports.
- OUTDOOR RESET CONTROLS
Automatic controls, known as outdoor reset controls, monitor the outside air temperatures and adjust the temperature of the water going back to the slab in response to changes in the outside air temperature. They may adjust the temperature of the water by either controlling mixing valves or by direct control to the boiler(s). In most radiant slabs the water is increased by one degree for every two to three degrees that the outside air temperature falls. - INTERMITTENT FLUID CONTROL
The above reset control works great in a large common area, but what is needed for the multiple zone installation is the water temperature to be set for the worst zone in the project. Then a zone valve is used to shut the circulation of the heat off to the room that is easier to heat, this will prevent any over heating of the zone or room.
SECTION 4
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