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HVAC Unit 33 Hydronic Heat Study Guide
Terms in this set (64)
What is hydronic heating?
It is using a system of pipes to carry hot water or steam into areas that are to be heated.
What is a terminal unit and give an example?
It is the component of the hydronic system that receives the heat and gives it off to the room. An example would be a radiator or finned-tube baseboard unit.
T or F: Air conditioning can be easily added to homes with hydronic heating.
False- it is very difficult to add air conditioning since there are no duct systems with hydronic.
What is a zone?
A zone is a separate area that is to be heated, usually a room. It usually has its own thermostat.
What is the unit called that provides the heat to the water?
It is called a boiler.
What is the definition of a boiler?
It is an appliance that heats water using gas, oil or electricity.
What is the most common temperature to which water is heated to?
What can happen if temperature of the flue gases drops below 130°?
The water vapor in the flue gases can condense back into a liquid. These condensing flue gases are very corrosive and can damage the chimney and the heating equipment.
How is a condensing boiler different from a conventional boiler?
A condensing boiler is designed to condense the flue gases in order to increase efficiencies by lowering the temperature of the flue gas below the dew point.
What efficiency are condensing boilers rated at?
What is the most common type of boiler found in residential and light commercial applications?
The cast-iron boiler.
What is meant by dry-base and wet-base boilers?
A wet-base boiler has the boiler water surrounding the burner flame. The cast iron is absorbing the heat directly and being kept from burning by the cooling effect of the boiler water.
A dry base boiler has a fire box under the boiler and will need refractory material oposite the burner (target wall) to absorb and help distribute the heat back up into the bottom of the boiler.
How does a steel boiler heat the water?
Steel boilers have a bundle of steel tubes through which the combustion gases pass on their way to the chimney. These tubes have baffles to increase the heat transfer surface area.
How does a copper water-tube boiler heat the water?
The heat exchanger is constructed as a series of finned copper tubes that are heated by the flue gases in the combustion chamber of the boiler. Since copper is such a good conductor of heat, they do not require as much surface area to heat the water. They usually do not contain as much water as cast iron or steel.
What is another source of heat for hydronic heating?
Geothermal heat pump.
Why are expansion tanks necessary?
Hydronic systems are closed loop and hopefully air free. When water is heated, it must expand. Without an expansion tank, the pressure relief valve would be opening each time the water is heated.
How does a compression expansion tank work?
It is a tank located above the boiler. It is filled with air at 0 psig. As water is added to the boiler, the air that is being displaced is pushed into the tank, causing the pressure in the tank to increase.
What is the formula for calculating the pressure of an expansion tank?
P = (H ÷ 2.31) + 5
H is the vertical distance between highest pipe in the system and the inlet of the expansion tank.
What are the two names for the type of pump used in hydronics?
A centrifugal or circulator pump.
How does a circulator pump work?
The motor in a circulator pump drives a device called an impeller. The impeller is a wheel with angled blades. When water gets caught in it, it is driven by centrifugal force to the outside of the spinning wheel. The shape of the blades then compresses the water and shoots it out in a jet.
T or F: Centrifugal water pumps are not positive displacement pumps.
True- the centrifugal pump is responsible for creating a pressure difference between the water at its inlet and the water at the outlet of the pump. It is this pressure differential that makes the water flow through the piping circuit.
What do you call it when water is forced to pump upwards and it meets pumping resistance.
It is known as feet of head.
What is the term used when the height of a pipe is more than what the pump can force water through?
It s known as cut-off head.
How much psi equals 1 foot of head pressure?
As the discharge valve is closed, what happens to the power consumption of the pump?
The water begins to recirculate in the pump and the power consumption reduces.
What is the point of no pressure change in an hydronic system?
It is the point in the system where the pressure remains the same, no matter what the system is doing. The point of no pressure change is where the expansion tank connects to the system piping in a closed-loop hydronic system
1 foot of head equals how much psig?
.433 psig. So 10 feet of head equals 4.33 psig.
How do you determine psi difference for a pump head?
Place a gage at the inlet and the outlet of the pump and the difference between the two numbers is the psi difference.
What can air cause in cast-iron and steel boilers?
Excessive corrosion. As the temperature of the water increases, the rate of corrosion increases as well.
T or F- trapped air can lead to noisy pipes.
How does an air separator work?
The air separator does not remove air from the system, it only separates air from the water. It does this by having the water flow through a mesh screen. The air bubbles collide with the screen and adhere to it. As more bubbles adhere, they get larger and break loose and go up to the air vent.
How does an air scoop work?
They are installed on horizontal runs of straight pipe and cause the air in the pipe to separate from the water and rise to the top of the pipe. Baffles or vanes at the top of the scoop actually scoops the air out of the pipe.
What happens to the air after it is separated from the water?
The air vent is responsible for removing it from the system. It is located at the highest point in the hydronic system. They accumulate the air. They can be manually bled from the system by pushing the button on the vent. They can also be vented automatically by an automatic air vent or a float-type air vent.
What is another name for a temperature-limiting control and how does it work?
A aquastat is a temperature-controlled switch that is intended to maintain the temperature of the water in a hydronic heating system. It has a temperature differential and will cycle the system off when it reaches the high temperature and cycle back on when it reaches the low temperature differential reading.
How does a high-limit control differ from an aquastat?
They both are devices that open and closes its contacts in response to boiler water temperature. However, the aquastat is an operational control while the high-limit control is a safety device that will open and deenergize the heat source if the boiler water temp gets too high.
What is the highest temperature that the water temp should ever get?
A typical high-limit control will open at 200 degrees. So 200 degrees is the highest temp it should ever get.
What is a water-regulating valve?
Water coming in from the main water supply line may be too high to work safely in a hydronic system. A water-regulating valve is installed in this main water supply line and it reduces the pressure to a safe working pressure.
What is the highest pressure a hydronic system can get and what devices make it that pressure?
30 psig. Both the water-regulating valve reduces the outside water source to 30 psig. In addition, the pressure relief valve is set to relieve if the pressure is greater than 30 psig.
What is the device that deenergizes a system if the level of water drops below the desired safety point.
A low-water cutoff valve.
Zone valves, what do they do and how do they work?
They are thermostatically controlled valves that control water flow to the various zones in a hydronic system. When a zone calls for heat, the zone valve is energized and slowly opens. When the zone call for heat ends, it slowly closes.
There are two types of zone valves: gear motor actuators and heat-motor actuators. Gear motor uses a motor to open while heat motor uses a bimetal strip to warp and open the valve.
What are balancing valves?
They are devices that make the resistance to the water flow the same for all flow paths in a system.
Why are pressure differential bypass valves necessary?
If a system is using two port zone valves, there can be excessive pressure if multiple zones are closed. This can result in noisy and banging pipes. The pressure differential bypass valve is located between the supply and return headers. It opens as zone valves close. The circulator can keep pumping the same amount of water and any excess hot water is recirculated back to the boiler.
What are flow control valves?
They are actually check valves. They prevent water from back flowing to a terminal unit when the circulating pump is not operating.
What is the outdoor reset control?
It measures the outside temperature and adjusts the water temperature to higher or lower depending on the outside temperature.
What type of heating is used with finned-tube baseboard units?
Air enters at the bottom of the heating units and passes over the hot fins of the finned tube unit. Heat is given off to the cooler air and causes it to rise by convection.
How many lb/hr equals 1 gal of water flow per minute?
1 gallon of water weighs 8.33 pounds.
1 gal/min = 8.33 lb/min
8.33 lb/min x 60 min/hr = 499.8 lb/hr rounded up to 500 lb/hr
T or F - in a series loop, the temperature of the first terminal unit can be higher or lower than the last terminal unit with the use of a thermostat.
False - the temperature of the first terminal unit will always be higher than the last terminal unit. As the water moves down the piping, it gives up heat.
In a series loop, what is the formula for calculating water flow and the amount of temperature drop across each terminal unit.
terminal 1 20000, terminal 2 30000, terminal 3 50000
Water supply temp 180 degrees
Water return temp 160 degrees
GPM = Q(total) / (500 x Delta T)
GPM = (20000 Btu/h + 30000 Btu/h + 50000 Btu/h) / (500 x (180-160))
GPM = 100000 / 10000
GPM = 10 gpm
Delta T = Q1 / (500 x gpm)
Delta T = 20000 / (500 x 10)
Delta T = 4 degrees
Delta T = Q2 / (500 x gpm) = 30000 / 500 x 10 = 6 degrees
Delta T = Q3 / (500 x gpm) = 50000 / 500 x 10 = 10 degrees
Describe a one pipe system.
One main piping loop extends around the occupied space and connects the outlet of the boiler back to the return of the boiler. Each terminal unit is connected to the main loop with two tees. The gpm flow off the tee into the terminal unit is less than the gpm flow on the main pipe. The less flow is due to higher resistance.
What are diverter tees?
They are special tees designed to increase resistance in the main loop pipe section between two tees.
How does a two pipe direct return system work?
This system uses two pipes, one to carry water to the terminal units, and another tor carry the water back to the boiler. The terminal units are arranged in a parallel circuit. The terminal unit closest to the boiler will have the shortest piping run with the least amount of resistance and will have a higher water flow compared to the unit that is furthest away.
What can be used to keep the resistance the same for all the terminal units in a two pipe direct return system?
Balancing valves are used to increase resistance of the shorter piping loops. The closer to the boiler, the more resistance is added to the valve.
How does a two pipe reverse return system differ from a direct return system.
They both use parallel piping circuits but the return pipe is opposite. The closest terminal pipe is the first to receive water but the last to get back to the boiler. Thus each terminal pipe has the same amount of water flow back to the boiler. They do not require balancing valves.
When is a mixing valve used?
They are used to combine two water streams of different temperatures to produce water flow that is at a temperature between the two entering temperatures. Common types are three way valves and four way valves.
What is radiant heating systems?
They rely on circulation of hot water through circuits that heat the shell of the structure instead of the air. They are concealed in the floors, walls and sometimes ceilings. They use polyethylene tubing (PEX) instead of copper pipes. The average water temperature is much lower than regular hydronic- 110 degrees
What are the three most common types of radiant heating system applications?
Slab on grade - PEX is placed right in the concrete slab before it is poured.
Thin slab - when there is an existing floor in place, PEX is placed on top of the frame floor, stapled into place and concrete is poured over it the existing floor.
Dry- the PEX is stapled below an existing floor Insulation is positioned below the PEX.
How do tankless domestic water heaters work?
A coil is inserted into the boiler. The domestic hot water is quickly heated by the boiler which eliminates the need for a hot water tank.
What would happen if a hydronic system was installed without an expansion tank?
The relief valve would constantly go off.
Where are terminal units located?
In the heated spaces.
What are impellers usually made of?
What are the three main parts of a circulator pump?
Motor, linkage and impeller.
Where are pressure reducing valves located?
They are located on the main water supply line.
What temperatures do radiant systems usually operate at?
105 - 120 degrees.
Describe a forced air discharge heater.
They are normally used for commercial or industrial. They have a heavy copper tubing for the heating element with aluminum fins. Fans circulate are across the heating element.
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