DIY Heat Bank

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This article is about making a DHW Heat Bank using readily-available separate components.

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Example Construction

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DHW secondary components assembled ready to install (remotely from cylinder). A service isolation valve and in-line 'Y' strainer are visible in the mains water inlet (bottom right) which are not shown in the diagram. The electrical connections to the pump and flow switch, and the triac electronics are not shown in this photograph.


Normal Y-plan (or S-plan, W-plan etc) vented CH+DHW system with

  • Direct HW cylinder (a standard Indirect cylinder could be used, either with extra tappings via e.g. Essex flanges, or Tee-ing the connections from the boiler into the top and (existing) bottom connections of the cylinder.)
  • Pump (standard 15/50 CH circulator type)
  • Plate Heat Exchanger (PHE)
  • Thermostatic Mixing Valve (TMV) 22mm
  • Flow switch wired to switch on pump when flow is detected



 Unit 8, Waterside Business Park
 Redwall Close
 S25 3QA
 Tel.: +44-(0)1909 551 107
 Fax: +44-(0)1909 550 885

Copper brazed plate heat exchanger model:- BP12M-20-G1G1


Compact gasket-free Brazed Plate Heat Exchanger consisting of corrugated pressed stainles steel plates (1.4401), which are brazed together with copper in a special vacuum process. Designed, produced and tested in sites with quality systems in accordance with DIN ISO 9002 requirements. Pressure-Vessel Approval according TÜV and EC Pressure-Equipment-Directive (CE).

Specification for 1 x M12-20-GG
Operational data: Side A Side B
Media: Water Water
Condition: liquid liquid
Heat exchanged: 50 kW
Mass flow:
Volume flow: 0.64 0.24 l/s
Temperature inlet: 82 10 °C
Temperature outlet: 63 60 °C
Pressure drop: 29.65 3.91 kPa
Surface margin: 1.49 %
Content: 0.5 l
Heat transfer area: 0.22
Design pressure: 30 bar
Design temperature: 200 °C

Note this is probably under-sized for this application: 100kW would be preferable.

Flow Switch

Farnell order code 730890 Manufacturer Part Number FS22 FLOW SWITCH, 22MM 3.75 L/MIN; Switch Poi £11.60 + VAT & p&p

This is a reed-switch whose contacts are rated 15VA, max 1A AC, into resistive loads.

The pump is rated 82W, 0.36A @ 230V AC and is an inductive load so the circuit shown is used to drive it.


Suitable Components:

Part Suitable Component Suppliers order code (CPC)
Flow Switch Also available from CPC SN35601
Triac STMicro BTA16-600B SC07585
The Metal Oxide Varistor (MOV) Epcos B722 Series RE03667
Snubber network
contact suppressor
(shown on the motor)
can either be assembled from basic components, or a module such as the Ampohm FE-SP-HDR-23-100/100 could be used. FT00715

An alternative requiring no electronics would be to use a flow switch capable of switching the pump directly. These ara available from DPS.


This is a standard 22mm Thermostatic Mixing valve used for limiting the temperature of water to washbasins in hospitals, schools etc to prevent scalding. Available from Screwfix, Toolstation etc.

See Also