Domestic Hot Water (DHW) Calculator

FlexEJ Domestic Hot Water Calculator

Find the power and storage required for your desired DHW flow.

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DHW Calculator

The FlexEJ Domestic Hot Water Calculator allows you to find the power required for instantaneous DHW or power plus storage volume required for semi-instantaneous DHW. The calculator will provide packaged plate heat exchanger and buffer vessel selections.

The FlexEJ LLH Calculator Alert

Please rotate your screen. Our LLH calculator requires just a little more horizontal space than we can squeeze into your current rotation.

Input Project Data

Please select your system’s primary inlet temperature, required DHW flow and available power. You may also choose which pumps to include with the packaged plate heat exchanger. For any input values not shown below please contact sales.

Assumed Values for both Primary and Secondary: Specific Heat = 4.185 kJ/kgK Density = 983 kg/m3

Primary Inlet Temperature °C Primary Inlet Temperature °C Input the temperature available from your system’s heat source in degrees C. This should be the highest normally available temperature. Required value.
Required DHW Flow Unit Required DHW Flow Unit Choose the unit of measure that you want to use for the Required DHW flow input.
Required DHW Flow^* Required DHW Flow Input either the instantaneous DHW flow rate or 1 hour peak flow rate for semi-instantaneous systems. Units as selected above.
Available Power kW^* Available Power kW Input the available power from your system’s heat source in kW. This should be the highest normally available.

Optional Storage L Optional Storage L Optional storage is in addition to any storage the calculator identifies as required to achieve the Required DHW Flow input. Optional storage does not alter the calculation results but does display available products.
Primary Pump Required Primary Pump Required The primary pump in the heat exchanger package is optional. The head required by the primary side of the packaged plate heat exchanger is shown in the results, if your system can provide this then select No. Otherwise, please select Yes and include the pump.
Secondary Pump Required Secondary Pump Required The secondary pump in the heat exchanger package is optional and the head required by the secondary side of the packaged plate heat exchanger is shown in the results. If the system is instantaneous you will require a recirculation pump, this can be included in the package – if desired choose Yes. If the system is calculated as semi-instantaneous (available power is less than power required for instantaneous) a transfer pump is required and will be included in the package.

Calculated Results

Based on your inputs the following has been calculated. Please call the FlexEJ team for assistance with the calculation, to request detailed calculations or to talk about delivery or customisation.

DHW Flow l/s DHW Flow l/s This is the input value expressed in l/s.
Available Power Flow l/s Available Power Flow l/s The DHW instantaneous flow rate based on the Available Power input value. If this is lower than the input Required DHW Flow then the storage is specified to achieve the required DHW flow as a 1 hour peak value.
Instantaneous Power kW Instantaneous Power kW The power required to provide the Required DHW Flow instantaneously with no storage.

Required Storage Volume L Required Storage for Available Power L The storage volume required to meet the input Required DHW flow on a 1 hour peak (semi instantaneous ) basis with the input Available Power. This value will be zero if the input Available Power is greater than or equal to the calculated Instantaneous Power.
Recovery Time min Recovery Time min The time taken to reheat the Required Storage Volume from cold with the input Available Power.
Phex Nominal Power kW Phex Nominal Power kW This calculator shows the available Packaged Plate Heat Exchanger products in 50kW steps. For more precise selections please contact our sales team and request a specific, detailed calculation.

PRODUCT RESULTS & CALCULATIONS

The following FlexEJ PhexPak Packaged Plate Heat Exchangers and Buffer Vessels have been selected based on your inputs. Details of the performance is shown for both primary and secondary circuits. This calculator shows the available Packaged Plate Heat Exchanger products in 50kW steps. For more precise selections please contact our sales team and request a specific, detailed calculation.

Please also visit our main PhexPak product page for full information, technical guides and more. Both ranges of PhexPak and our Commercial Buffer Vessels are also available to buy at factory-direct prices and typically ship within 7 working days from our Domestic Hot Water shop in the main store. Do please call us direct for sales/technical support. We have a full technical team here who can help you with everything from the DHW calculations to delivery and installation.

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Frequently Asked Questions

A. A plate heat exchanger is a series of corrugated plates (usually stainless) placed in sequence to form a series of primary and secondary circuit channels that alternate throughout the plate pack.

A. A packaged plate heat exchanger starts life as a basic plate heat exchanger, it is then combined with control components such as a primary pump, control valve, temp sensor and various other optional extras. Do see FlexEJ’s range of fully-customisable PhexPak packaged plate heat exchangers with a wide range of power and flexibility options.

A. A gasketed plate heat exchanger uses soft gaskets to seal between the plates in the stack. The plates are compressed together between the end plates by long tie bolts. This arrangement is very flexible and allows the heat exchanger to be very closely optimised for the application. The stack can be dissembled for cleaning and may be reconfigured if conditions change. Do see FlexEJ’s range of fully-customisable PhexPak packaged gasketed plate heat exchangers with a wide range of power and flexibility options.

A. A brazed plate heat exchanger is constructed form individual plates which are brazed together in an oven. Brazed plate heat exchangers have a fixed performance range and are available in standard sizes. No gasketing is used, the brazing is the seal between the plates. This construction tends to be smaller and lighter than an equivalent gasketed type but does not offer the flexibility of disassembly / reconfiguration. Do see FlexEJ’s PhexPak Lite packaged brazed plate heat exchangers for FlexEJ’s off-the-shelf/stock options.

A. Plate heat exchangers work by forcing a medium through the small channels between the plates, causing the heat to be extracted from one circuit and transferred to the other.

A. Packaged plate heat exchangers manage and optimize the transfer of flow and heat by monitoring and reacting to a temperature sensor, usually placed on the secondary flow.

A. Phexpak is a fully modular and customizable product. It can be specified and designed to meet your exact requirements.

A. Hot water requirements for commercial projects are typically calculated using CIBSE guidance to meet the required standards. We utilise BS EN806 and BS 8558 to help our customers understand their requirements and can provide suitable product options to meet those requirements.

A. In a word, yes. A packaged plate heat exchanger is a great replacement for a calorifier for the vast majority of applications.

A. Not entirely, but it will significantly reduce the risk if installed to supply DHW instantaneously due to the vastly reduced volume of water in the system when the storage element is removed.

A. In a word, yes! Like our other products, the PhexPak is available to order directly from us. Just get in touch with our sales team to start the process.

A. Plate heat exchangers are one of the most efficient ways to transfer heat in the HVAC industry, in fact they are 99% efficient with only a 1% heat loss in most scenarios. Couple this with the most efficient pumps and control components available in today’s market and you have an unbeatable solution.

A. Packaged plate heat exchangers are typically installed freestanding in a plant room, and are connected via the 4 following connection points: Primary flow, Primary return, Secondary flow, and secondary return. Power is then supplied to the control panel and the unit is ready for commissioning and setup.

A. Packaged plate heat exchangers tend to cost more than standalone plate heat exchangers, but offer a wide range of benefits that a standalone plate heat exchanger simply can’t offer. They are also supplied with all the required pumps, valves and control components fitted to ensure optimum performance of the heat exchanger.

A. A calorifier is a storage buffer vessel that holds a body of water (most commonly DHW), and has an internal coil or similar heating element which circulates a primary heating circuit, designed to indirectly heat the body of water in the vessel.

Yes, the PhexPak is a great direct alternative to the Stokvis Econoplate packaged plate heat exchanger, which is no longer available. Please get in touch for help with your requirements or to request more info on the PhexPak.

A. Yes, please contact us with your product and project requirements and we will quote the PhexPak packaged plate heat exchanger.

A. Yes, please contact us with your product and project requirements and we will quote the PhexStore. PhexStore is a direct alternative to the Econostore, which is no longer available.

A. Yes, FlexEJ can quote and supply DHW buffer vessels from our standard range up to 1000l or bespoke design and manufacture anything larger.

A. Yes, FlexEJ can offer help and support either remotely or on site if required, utilising BSEN806 and BS8558 to determine DHW requirements.

A. Yes, FlexEJ can quote and supply a huge range of heat exchangers for DHW and LTHW applications.

A. FlexEJ can quote and supply a whole host of plantroom accessories, from product specific insulation jackets to full unvented system component kits. Please get in touch with your requirements.

A. Yes, in April 2024. Please contact FlexEJ for quotes and technical assistance on packaged plate heat exchangers, buffer vessels and hybrid buffers.

Yes, in April 2024. Please contact FlexEJ for quotes and technical assistance for all your packaged plate heat exchanger, buffer vessel and hybrid buffer requirements.

Please visit our full FAQ page to see all our frequently asked questions.

Inlet Temp °C Inlet Temp °C Primary Inlet Temperature selected for calculation.
DT °C DT °C “Delta T”, the difference between Primary Inlet and Primary Outlet temperatures.
Flow l/s Flow l/s Primary Flow Rate calculated at the Nominal Power and Primary DT of the package.
Power Rating kW Power Rating kW Nominal Power rating of the package.
DP kPa DP kPa The pressure drop of the heat exchanger package primary side at the Primary Flow Rate.
Outlet Temp °C Outlet Temp °C Primary Outlet Temperature at the Nominal Power and Flow of the package.

	Outlet Temp °C Outlet Temp °C Secondary Outlet Temperature – assumed value.
	Delta Temp °C Delta Temp °C The pressure drop of the heat exchanger package secondary side at the Secondary Flow Rate.
	Flow l/s Flow l/s Secondary Flow Rate calculated at the Nominal Power and Secondary DT of the package.
	DP kPa DP kPa The pressure drop of the heat exchanger package secondary side at the Secondary Flow Rate.
	Inlet Temp °C Inlet Temp °C Secondary Inlet Temperature – assumed value.

Inlet Temp °C Inlet Temp °C Primary Inlet Temperature selected for calculation.
DT °C DT °C “Delta T”, the difference between Primary Inlet and Primary Outlet temperatures.
Flow l/s Flow l/s Primary Flow Rate calculated at the Nominal Power and Primary DT of the package.
Power Rating kW Power Rating kW Nominal Power rating of the package.
DP kPa DP kPa The pressure drop of the heat exchanger package primary side at the Primary Flow Rate.
Outlet Temp °C Outlet Temp °C Primary Outlet Temperature at the Nominal Power and Flow of the package.

	Outlet Temp °C Outlet Temp °C Secondary Outlet Temperature – assumed value.
	Delta Temp °C Delta Temp °C “Delta T”, the difference between Secondary Inlet and Secondary Outlet temperatures.
	Flow l/s Flow l/s Secondary Flow Rate calculated at the Nominal Power and Secondary DT of the package.
	DP kPa DP kPa The pressure drop of the heat exchanger package secondary side at the Secondary Flow Rate.
	Inlet Temp °C Inlet Temp °C Secondary Inlet Temperature – assumed value.

Storage Required L Storage Required L The volume needed to meet the Required DHW Flow on a one hour peak semi-instantaneous basis given the input Available Power.
1000l Vessel
750l Vessel
500l Vessel
250l Vessel

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DHW Calculator

The FlexEJ LLH Calculator Alert

Input Project Data

Calculated Results

PRODUCT RESULTS & CALCULATIONS

No products have been selected – your requirements are outside the scope of this web calculator but well within the capability of our product range.

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phexpak solution

Performance

phexpak lite solution

Performance

REQUIRED AND OPTIONAL STORAGE SOLUTIONS

FlexEJ Commercial DHW Buffer Vessels

Storage Volumes

Frequently Asked Questions