Chilled Water Buffer Tank Sizing

A Chilled Water Buffer Tank Sizing Guide for Optimal System Performance

If you’re looking for the best approach to sizing a chilled water buffer tank or vessel, you’ve most likely seen chillers cycle because the buffer vessel or tank is too small.

Have you ever been annoyed by temperatures that fluctuate as your load changes? As the load decreases, the room’s temperature drops abruptly.

The system fails due to a low temperature fault, necessitating a service call to the location to complete the reset. In addition, there is a possibility of flooding back and oil buildup in the compressor, among other potential issues.

Perhaps your problem was the opposite: going much over budget for an 8,000-liter buffer tank when you only needed half that volume.

Calculating the buffer tank size required for your chilled water system may appear to be a daunting undertaking, but getting chilled water buffer tank sizing right is essential for your chillers and other equipment requirements for your hvac system and customer satisfaction.

The chiller manufacturer should be your first stop, because all chillers can have nuances. Nobody knows their chillers better than they do, so our advice is to contact them first for a recommendation.

When is it appropriate to use a chilled water buffer tank?

A chiller requires a particular system volume to function in low-load mode without cycling.

Cycling can occur in low-load situations, such as when everyone who works leaves the premises at the end of the day.

The building load significantly decreases, necessitating an increase in the system’s volume to avoid cycling and subsequent damage to the chiller.

Identifying scenarios that require a buffer vessel or tank for chilled water

  • Water chillers require a certain system volume to operate in low-load conditions without cycling.
  • If the total system volume (TSV) is less than the chiller manufacturer’s requested capacity, a buffer tank is required.

The benefits of using a buffer vessel or tank in chilled water systems

  • Prevents temperature fluctuations and system cycling.
  • It ensures optimal chiller performance, improves energy efficiency and prevents potential damage.

Buffer Tank Sizing Fundamentals

Understanding the variables that affect the size of the buffer tank is crucial.

  • Determine the kW rating of the chiller and the necessary system capacity.
  • The total system volume (TSV) encompasses the water capacity volume of the chiller, pipe system, and buffer tank.
  • Flow rates and desired temperature fluctuations.

The formula: calculating buffer tank size accurately

First,the chilled water buffer tank sizing rule of thumb

  • Chiller manufacturers can differ in their system volume needs; however, four litres per kW seems to be the normal ratio for air conditioning system comfort control applications. Thus, chiller kW x 4 litres = required system capacity (litres).
  • However, multifunction chillers can be up to 11 liters/kW. This is due to the need to balance the heating and cooling within specific limits, which complicates the system and necessitates the additional volume.

Then the accurate calcualtion is:

  • Required System Capacity (L) – Total System Volume (L) = Required Buffer Tank Volume (L).

V=N x 60 x Z / 4.18dt

V = total system water content (litres)
N = the chiller’s first capacity step (kW).
Z = the minimum allowable running time (minimum 5 minutes).
dt represents the temperature difference at the minimum part load condition, which is, for example, 2 °C.

This allows the chiller to operate smoothly at low load capacities, and it should provide sufficient thermal storage to offer at least 5 minutes of operation after the unit has stopped running.

The Role of Bypass Valves in Buffer Tank sizing

The positioning of the bypass valve is important; it can’t be too close to the chiller.

  • To avoid damage from cold water flowing back into the chiller, position the bypass valve far enough away from the chiller.
  • The valve should be positioned to ensure the volume of water enclosed in that section of pipe meets the specification or minimum volume of water required by the chiller.

How do bypass valves affect the total system volume?

  • The TSV used in the buffer tank volume calculation should be adjusted based on the length of pipe required for the water to come out of the chiller and return through the bypass.

Avoiding Common Sizing Mistakes

The consequences of undersizing a chilled water buffer tank are significant.

  • There are temperature fluctuations, system cycling, and potential damage to the chiller.
  • Adjustments to the BMS and installing additional valves may help temporarily, but they cannot solve these problems.

Strategies for preventing undersizing

  • Use the correct formula to calculate the required buffer vessel or tank volume.
  • Consider variables such as flow rates, desired temperature fluctuation, and chiller kW rating.

Choosing the Right Buffer Tank

The process of determining the buffer tank capacity involves taking into account the stabilisation of the system and the actual water level.

  • Calculate the water required for system stabilisation using the formula:M1 = QT / (C △ T), where Q is the unit’s heating capacity (in KW), T is the defrost time (in seconds), C is the specific heat capacity of water (4.187), and △ T is the maximum allowable drop in water temperature (in °C).
  • The actual water volume of the system is the sum of the water capacity per meter of the floor heating pipe and the total length of the system pipe, including the water capacity of radiators.

Then look at this webpage for our range of chilled water buffer vessels or tanks and select your appropriate buffer vessel for chilled water

Material considerations, installation guidelines, and maintenance tips

Consider material options such as carbon steel or stainless steel.

Ensure proper installation and maintenance to prevent corrosion and damage.

Regularly inspect and clean the buffer tank to maintain optimal performance.

Electric multipoint water heaters

Electric Multipoint Water Heaters for Instant Hot Water

Electric multipoint water heaters 9kW to 36kW from Flexiheat UK

 

Electric multipoint water heaters are an excellent solution for supplying hot water to multiple points with an instantaneous water heater.Thus the term “multipoint instantaneous water heater”

Our multi-point electric water heaters have state-of-the-art heating technology, allowing for precise temperature regulation to provide personalised comfort. Typically, our units with a rating output of up to 27 kW come with digital displays and adjustable temperature settings for easy water temperature monitoring and adjustment.

This multipoint instantaneous water heater range is typically used in domestic and commercial applications such as shops, factories, student accommodation, etc., supplying hot water for one outlet or multiple outlets, depending on the flow rate of the shower or taps (sink taps for dishwashing, food preparation, etc. or toilet-type taps) being used.

You will require a three phase power supply to this multipoint instantaneous water heater range – you needs the Amps to heat water on demand.

What is an Instantaneous Electric Water Heater?

  • An instantaneous electric water heater is a type of water heater that provides hot water on demand, without the need for a storage tank.
  • A multi point electric water heater heats water instantly, using electricity, and supplies it to various points of use, such as sinks, showers, and basins depending on the flow rates required.
  • Instantaneous electric water heaters are ideal for residential and commercial applications where water heating is required quickly and efficiently with energy savings compared to storage water heaters.

Benefits of Multipoint Water Heaters

  • An electric multi point water heater,which is a tankless heater provides hot water on demand, eliminating the need for storage water heaters.
  • Energy-efficient, as they only heat water when needed.This instantaneous water heater range all have a ErP “A” energy rating.
  • Lower utility bills because there is no standby heat loss that you have with a storage tank system.
  • Suitable for multiple points of use, such as sinks, showers, and basins or to provide instant hot water for an appliance such as a dishwasher or washing machine.
  • Compact design, ideal for compact spaces or remote areas for point of use such as a sink or shower that require instant hot water.
  • These wall mounted multipoint electric water heaters can be used in commercial water heating settings, such as offices, restaurants,student accommodation and hotels, as well as larger households.

Key features to consider for instantaneous electric water heating

  • Look for an instantaneous water heater with a suitable power rating (the kW rating) to meet your hot water needs.
  • Safety features: Ensure the multipoint hot water heater has features such as a high temperature cut out, which is a safety cut out for overheating protection.
  • Multiple points: Consider a water heater that can supply hot water to multiple hot water outlets / points of use.
  • Check the flow rate of the multipoint electric water heater to ensure it meets your hot water needs.

Choosing the Right Size and Type

Consider the number of points of use and the flow rate required.

Choose a water heater with a suitable power rating (in kW) to meet your instant hot water needs.

Overview of the product, including its features such as wall mounted etc and benefits.

Review of the product’s performance, including its flow rate at your required water temperature increases , the maximum water temperature it can produce, and energy efficiency.

Flow rates for hot water fixtures

Standard shower head: 7 to 8 litres per minute flow rate

Standard shower head 7 to 8 litres a minute flow rate

Standard shower head 7 to 8 litres a minute of flow @ 40C

A standard shower head requires 7 or 8 litres of hot water per minute, whereas European Water Label “green band” water-saving shower heads require 6 litres per minute. Think of this standard as an “ErP” rating for bathroom products, taps, etc.

Kitchen sink taps: 4 to 6 litres per minute

The “spray taps” for basins and sinks range in capacity from 2 to 4 litres per minute.

Be careful to check the flow rates on your spray taps, as all our instantaneous water heater units require a minimum flow of 2.5 litres per minute to operate.

Modern bath mixer taps have an approximate demand rate of 12 litres per minute.

Please note:

Drench showers with hot water flow rates from 15 to 30 litres a minute

Please be aware that drench showers can have massive flow requirements, ranging from 15 litres to pretty much any flow rate; 30 litres per minute for a drench type shower is not uncommon. For these water heating applications,a large instantaneous water heater will be required or even a direct storage unvented DHW cylinder due to the high volume of water required to feed this type of shower.

Product range

These instant water heaters feature four power ratings or output ranges in a single unit. The 27kW and 36kW electric multipoint water heater, on the other hand, only offers a single power setting.

For instance, you can choose between the 17kW, 18kW, 21kW, or 24kW power outputs on our 17kW to 24kW instantaneous water heater.

This affects the draw-off point flow rate; therefore, at 17 kW, it will produce 8.1 litres; at 24 kW, it will produce 11.6 litres a minute, at the standard temperature rise of 30 °C, for daily use in the winter period, as that’s when you want hot water the most, as you can see in the following data table.

Power output of the instantaneous water heater9kW to 15kW power rating17kW to 24kW power rating27kW36kW
Energy efficiency classAAAA
Heat capacity ( power setting options)9/11/12/15 kW17/18/21/24 kW27 kW36kW
Electrical supply – All require a 3 Phase power supply 400V 3N~400V 3N~400V 3N~400V 3N~
Mains water supply pressure (Bar) min – maximum1 to 101 to 101 to 101 to 6
Hot water flow rate at  Δt=30 in litres per minute4.3/5.2/5.8/7.28.1/8.7/10.1/11.61317.3
Rated current (Amps)3×13/15.9/17.3/21.73×24.6/26/30.3/34.63 x 393 x  39.9
Dimensions – W x D x H (in mm)245 x 126 x 440245 x 126 x 440245 x 126 x 440315 x 140 x 492

Please note that our instantaneous water heater range requires a minimum flow rate of 2.5 l/min to start heating the water. They will not operate if the flow is lower than this point.

For a given kW output from the hot water heater – as the water temperature increases, the flow rate decreases is the basic rule of physics.

The 9kW to 27kW have a digital display for easy monitoring and precise temperature control.

Digital display on our instantaneous electric water heaters from 9kw to 27kW from Flexiheat UK

The digital display on our 9kW to 27kW instantaneous water heater range

Download the 9kW to 27kW multipoint instantaneous water heater range manual

and our 36kW instantaneous water heater has a tradtional temperature dial and is really suited to commercial settings.

36kW electric instant water heater from Flexiheat UK

36 kW Electric instant water heater – with a manual temperature control dial

 

Download the 36kW Electric multipoint instantaneous water heater manual 

Installation and safety considerations

  1. Make sure a qualified electrician installs the water heater.
  2. Follow the manufacturer’s installation guides and instructions to ensure the warranty.
  3. Ensure the instantaneous water heater is located (wall mounted) in an area with adequate space to service or inspect the water heater and as close as possible to the water outlets being served.
  4. The water heater must not be installed in a site that is exposed to the danger of an explosion and where the temperature might drop below zero C, as the water in the heater will freeze and damage the water heater, which will invalidate the warranty.
  5. Check the water heater’s safety features, such as the high temperature cut out i.e., a safety cut out to prevent overheating.
  6. No pressure relief valve is generally required on an instantaneous water heater unless the water pressure can get above the 10 bar the 9kW to 27kW electric multipoint instantaneous water heater models are rated for.The 36kW electric multipoint instantaneous water heater is rated to a maximum water pressure of 6 bar.
  7. No Expansion vessel is required for an instantaneous water heater, as they supply instant hot water there is no hot water storage to account for unlike tank-type water heaters.

Energy Efficiency and Running Costs

Instantaneous electric water heaters are energy-efficient, as they only heat water when needed they reduce your water heating energy consumption.

Look for a water heater with a high energy efficiency rating- Our electric multipoint instantaneous water heaters have an “A” energy rating.

Consider the running costs of the water heater, including electricity consumption and maintenance costs.

Contact us

Should you require further assistance, check stock (we normally have the units on the shelf for the next working day delivery or select express delivery times i.e. timed, weekend saturday delivery etc), or get prices on our energy-efficient and cost-effective electric multipoint water heaters with accurate factory fitted temperature control and supplying hot water with no standby heat loss, then please call our sales team on 01202 822221 or click here for our email contact form, as we deal with both the public and trade customers.