The primary application of a buffer tank is to reduce heat pump, chiller, or boiler short cycling. Hydronic buffer tanks are used in systems operating below the design load condition, which is most of the time, or in systems having several low BTU cooling or heating loads calling at different times. This can cause the heat pump or the boiler to short cycle, resulting in reduced operating efficiency and shorter equipment life.
The primary application for the HF-22 BT is to reduce low-mass modulating, condensing boiler short cycling. The high and low left side connections are used for boiler supply and return. The top connection is used to supply the distribution system and the low right connection is the return from the distribution system. If the top connection is piped to the line supplying the air purger and vent, the tank will be self venting.
Buffer Tank Sizing - Calculating Capacity
The Heat-Flo buffer tanks are a simple, cost effective way to improve overall system operating efficiency by reducing
unnecessary equipment short cycling. The recommended capacity or volume of a buffer tank is based on four variables.
1. The duration of the heating or cooling source “on time”. (minutes)
The desired length of “on time” for each run cycle depends on the type of equipment used. Heat pump and
chiller manufacturers typically recommend a minimum of 5 to 10 minutes on time, and boiler manufacturers may
recommend a minimum of 10 minutes “on time”. Check with your equipment manufacturer. Generally, the longer
the on time, the higher the overall operating efficiency.
2. The minimum rate of heat input. (BTU/HR)
This is based on the heat pump or chiller output, or the boiler output at the minimum firing rate if the boiler has a
variable input system that ramps input down as the demand decreases.
3. The minimum system load (BTU/HR)
This is the demand placed on the system with the smallest zone calling for heat.
4. The allowable tank temperature rise. (deg. F)
This varies depending on the type of heating or cooling system used, and on the design of the distribution system.
Chillers may require a tight, (6 deg. F), differential to assure good dehumidification and prevent freezing, heat
pumps may require a 10 deg. F differential to maintain a high COP, and boilers with hydronic heating distribution
systems may require a differential anywhere between 10 to 40 deg. F depending on the application.
The following formula determines the tank volume:
V=T X (Q heat input - Q min. heat load)
Tank temp. rise X 500
V = Buffer tank volume (gallons)
T = desired heat source “on cycle” (min.)
Q heat source = heat source output (BTU/HR)
Q min. heat load = heat output to minimum load
Tank temp rise (deg.F)
Water to Water Heat Pump Example: Town and Country Mechanical wants a minimum heat pump on time of 10 minutes.
The heat pump output is 46,500 BTU/HR. The smallest zone is a 7,000 BTU/HR bathroom. The allowable temperature
differential is 90 to 100 deg. F for the radiant heating zones.
V=10 X (46,500 - 7,000) = 79.0 Gallons minimum volume. Choose the HF-80BT buffer tank.
(100-90) x 500
Click here to see a drawing of a buffer tank application.
