An expansion tank allows for the expansion and contraction of a system when it heats up and cools off and provides head pressure for the circulation pump. A buffer tank adds volume to a heating or cooling system and helps prevent short cycling.
What is a Buffer Tank?
A buffer tank is an insulated water vessel; they do not usually contain coils or heat exchangers. Instead, most will have top and bottom connections, and some will have a baffle plate internally. Buffer tanks shouldn’t be confused with thermal stores, as they are not a substitute for a hot water cylinder.
A buffer tank’s primary role is to keep a minimum volume of water “in circuit” when the heating load is very low. This prevents the heat pump from short cycling and provides a bypass route to maintain the minimum flow rate through the heat pump if most of the heating zones have shut down. The minimum flow rate and the minimum volume of water in the circuit are necessary to keep the heat pump operating efficiently.
What is an Expansion Tank?
An expansion tank is installed directly above a water heater on the cold water inlet to help contain the water pressure from water expansion when heated. There are several different types of expansion tanks, including bladder tanks and diaphragm tanks.
Read more about the differences between bladder and diaphragm expansion tanks in our blog post Bladder Tanks vs. Diaphragm Tanks.
Read more about the differences between expansion and pressure tanks in our blog post Expansion Tanks vs. Pressure Tanks.
Shop Bell & Gossett Expansion Tanks
Buffer Tank FAQs
Where is a buffer tank located in a hot water system?
Between the boiler and the heating system (zones).
What is the purpose of a hot water buffer tank?
A buffer tank provides additional water storage in the heating system to help eliminate boiler short cycling.
Why would my boiler short cycle?
The boiler is sized to heat an entire home or building. Not all zones call simultaneously; when smaller zones call for heat, the boiler will reach its high limit quicker than it should, which results in short cycling. Short cycling will lower heating efficiency and increase fuel bills. It will also result in additional wear and tear to the boiler and heating system. A buffer tank can help save energy, reduce maintenance, and extend the system's life. Short cycling can cause several issues: loss of energy efficiency, reduced compressor life, power network disruption, and, very rarely, sudden compressor failure due to lubrication starvation.
How does a buffer tank help reduce short cycling?
When a small zone calls for heat, it signals the boiler to come on. The excess heated water is circulated through the buffer tank as the circulator pushes the boiler water through the zone. This added volume prevents the boiler from short cycling. When heating demand exceeds the boiler output, the added volume of the buffer tank will satisfy the additional demand. When heating demand equals boiler output, the boiler water will flow through the tank to meet the demand.
How do I size a hot water buffer tank?
There are several pieces of information you need to know to properly size a hot water buffer tank. They are:
- Minimum boiler turndown output in BTU/hr
- Smallest load in BTU/hr
- Temperature differential in degrees F
- Boiler cycle time (minimum) in minutes
The formula for sizing a hot water buffer tank is:
What do buffer tanks do in a chilled water system?
Buffer tanks provide extra water volume in a closed chilled water system. The added capacity provided by the tank reduces cycling of the compressor unit, improves temperature control, and provides more consistent system operation. Buffer tanks are typically designed with an internal baffle. As the system water enters the tank, it is forced up and over the baffle before dropping back down to exit the tank, ensuring proper water circulation and full use of the tank volume.
Are buffer tanks used in every chilled water system?
No. Some systems have an adequate volume of water, so the extra storage provided by the buffer tank is optional.
Where is a buffer tank installed in a chilled water loop?
It should be installed at the suction side of the pump, which is the point of no pressure change.
Why are the flanges at the bottom of a buffer tank and not the top?
The connections and the baffle are located at the bottom of the tank to maintain a lower center of gravity. The tank would be top-heavy and more likely to tip over if they were located at the top.
What are the connections at the top and bottom of the tank used for?
The top connection can be used to install an air vent, and the bottom can be used as a blow-down connection. These connections can also remain plugged.
Will a buffer tank work upside-down, suspended from piping?
Yes, although measures must be taken to ensure the wet weight of the vessel is appropriately supported.
How do I size a chilled water buffer tank?
There are several pieces of information you need to know to properly size a chilled water buffer tank. They are:
- Chiller cooling capacity in tons
- Chiller recommended system water in Gal/ton
- Actual system water volume in gallons
The formula for sizing a chilled water buffer tank is:
Buffer Tanks By Manufacturer
Taco Buffer Tanks
Buffer tanks are often employed within HVAC systems to provide additional fluid volume to prevent short cycling of heating or cooling apparatus. Taco’s chilled water and hot water buffer tank line are built to last. They offer tank sizes of 50 gallons up to 3000 gallons across 22 tank volumes to meet the scope of job site-specific connection sizes specified. In addition, each model is available with system connections on the side of the tank, located either high or low, to meet application requirements.
Taco chilled and hot water buffer tanks also offer many insulation options to suit your application needs. Taco buffer tank applications include hot water heating systems, closed cooling circuits, air conditioning systems, hydronic heating & cooling, water/Glycol concentrations up to 50%, and solar/geothermal.
Taco Buffer Tank Features
- Designed, constructed, and tested to ASME Section VIII, Div. 1 requirements
- All fabrication is done in house
- Computerized selections to ensure fast and easy design
- Carbon steel construction standard (stainless steel optional)
- Flanged, grooved, or threaded connections are available
- A wide selection of insulation packages is available
- Available in vertical or horizontal configurations with various nozzle location options
- Available as 180° or 90° nozzle configuration for mechanical room space saving
- Available with anchor clips for floor mounting
- Solid internal baffle
- National Board Registration
- CRN registration (optional)
Taco Buffer Tank Performance Data
- Capacity: 50-3000 gallon standard
- Flange sizes: 2-12" standard
- Diameters: 20-84"
- Height: 50-160"
- Working pressure: 125 psi standard (optional higher pressures)
- Maximum operating temperature: 375°F (optional higher temperatures)
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AMTROL Buffer Tanks
AMTROL ASME buffer tanks add capacity to non-potable, closed systems to help reduce cycling, improve temperature control, and provide more consistent system operation. They are available for chilled water and hot water applications. AMTROL buffer tanks meet all ASME Section VIII, Division 1 code standards.
AMTROL Chilled Water Buffer Tank Features
- The internal baffle helps properly circulate water
- Available up to 1,040 gallons
- Connections from 3" to 12"
- Maximum working pressure: 125 or 150 psig
- Maximum operating temperature: 450°F
- Seismic restraints, sensor ports, and grooved pipe connections are available
AMTROL Hot Water Buffer Tank Features
- 2-port tanks add system mass
- 4-port tanks add system mass and hydraulic separation
- Available up to 300 gallons
- Connections: 2", 3", and 4"
- Maximum working pressure: 125 or 150 psig
- Maximum operating temperature: 450°F
- Seismic restraints, sensor ports, and grooved pipe connections are available
AMTROL Chilled & Hot Water Buffer Tank Specifications
- Shell: ASME-approved steel construction
- Flanges: ASME 150 lb.
- CWBT Series connections: 3", 4", 6", 8", 10", and 12"
- HWBT Series connections: 2", 3", and 4"
- Finish: red oxide primer
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