Amazon HXIN Unvented DHW Cylinder

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Labelled Cylinder Drawing

Amazon HXIN labelled cylinder drawing.jpg

System Overview

The HXINUV plate heat exchanger assembly as fitted to unvented hot water cylinders (Telford Tempest) enables faster heat up and recovery times than possible with most conventional indirect coil type cylinders, as the full output of the boiler or primary heat source can be utilised as an instantaneous water heater. Heat up times are further reduced by feeding the Domestic Hot Water (DHW), as it is created, directly into the top part of the cylinder. This enables greater performance to be achieved than standard from a given cylinder capacity; alternatively, it allows a smaller than standard cylinder to be used where space is restricted.

The HXINUV will be supplied pre-plumbed and wired to the Tempest unvented cylinder in either single or twin cylinder variants.

Principle of Operation

The HXINUV assembly uses two cylinder thermostats linked together on a latch to provide a buffered mode of operation for the heating of the cylinder. The ‘Buffer On’ thermostat is the standard dual control/safety thermostat fitted to the cylinder itself (which includes a ‘high limit’ overheat cut-out facility). The ‘Buffer Off’ thermostat is tee’d into the plate heat exchanger assembly. This dual thermostat arrangement allows a minimum set volume of water to be heated which helps reduce cycling of the boiler - this can be an issue with conventional single thermostat systems - and thus the dual thermostat aids boiler efficiency.

When the cylinder temperature at the 'Buffer Off' position drops below its set point, it energises the 'Buffer On' thermostat; If this is simultaneously calling for heat it turns the plate heat exchanger pump on. In parallel, this calls for heat from the boiler or heat source via the two port energy cut out motorised valve. Primary heat from the boiler now flows down through one channel of the plate heat exchanger; in counter-current, cool water from the base of the cylinder and/or cold mains water is drawn upwards through the other channel. The cool water is heated and DHW passes out and into the top part of the cylinder. Heating of the cylinder continues until both the buffer thermostats are satisfied.

A DHW temperature control valve modulates the primary heat flow in response to the DHW temperature observed exiting from the plate heat exchanger, in order to maintain the stored DHW at the required set point. As the DHW flowing away from the plate heat exchanger approaches the required setpoint temperature, the control valve reduces the primary flow, and vice versa.

A DHW flow setter controls the flow rate of DHW being created and fed into the cylinder. The set point of this component is defined by the size of the available primary heat source and is adjusted during commissioning. In order to achieve the required flow rate it may be necessary to adjust the plate heat exchanger pump speed as required.


The cylinder will be supplied with the plate heat exchanger and bronze pump pre-plumbed, and pre-wired with the buffering and safety thermostats. The inlet control set, expansion vessel and 2 Port energy cut out valve (parts supplied loose) should be installed as per the cylinder manufacturer’s Instructions. Expansion and Temperature / Pressure relief valves should be connected to discharge pipework (via the tundish) which must be installed in accordance with Building Regulations Document G, as described in the cylinder Manufacturer’s Instructions.

Consideration must be given by the installer to the primary flow and return temperature delta and flow rates of the boiler plant or primary heat source that is to be connected. The set up and operation of the unit assumes a delta on the Cold Water Mains in / Domestic Hot Water out sides of the plate heat exchanger of 50°C in a single pass. The resultant delta on the primary side can be in the region of 40 - 45°C and flow rates of 15 - 35 Litres/min., dependent on the power of the heat source and its flow temperature. If the connected plant or heat source cannot operate with these deltas or flow rates, suitable bypass and flow regulation equipment must be sourced and fitted by the installer upstream of the plate heat exchanger, to accommodate this.

For twin cylinder variants Only: Take note that the cold supply will need to be cross-connected to both cylinders at the cold inlet connections at their base; and a further connection made between the DHW output of the pre-assembled plate heat exchanger, and the secondary return connection of the second cylinder using the pipework & second flow setter assembly (supplied loose). The image below is indicative of the resulting assembly. Refer to Section Commissioning for adjustment of the flow setters. Note that in this instance, the total required flow rate for the system must be split equally across the two components.

Amazon HXIN Twin picture for manual.png


Amazon HXIN Wiring for Install Manual V4.png

Refer to the wiring diagram above: The two buffering cylinder thermostats, relay and plate heat exchanger pump are pre-wired. Additional boiler and heating controls can then be wired to the fitted wiring centre as shown to achieve the required installation. It is suggested that the permanent Live, Neutral & Earth for the boiler (and space heating controls if fitted) are all taken from the wiring centre in order to balance with the switched live outputs to both the boiler when called, and zone control for space heating, and provide a single point of isolation for the DHW and CH plan; however the installer may choose to derive their own solution, in which case it is imperative that suitable labelling is provided to indicate which circuits/components may remain live if the cylinder supply is isolated.

Set up and commissioning of DHW controls

Having completed the installation of the cylinder as per the manufacturer’s instructions, and filled it, the set up of the temperature and flow controls can be undertaken as follows:

  1. The two cylinder thermostats should be set as follows:
    • Dual thermostat on cylinder: 50°C
    • ICT thermostat tee’d off heat exchanger: 55°C
  2. Next, refer to either Tables A or B to establish the correct flow rate adjustment for the DHW flow on the DHW flow setter - this will be dependent on the available power from the connected boiler or primary heat source; It is assumed that an appropriately sized heat source is being connected to the cylinder being installed - if in doubt, contact your cylinder supplier.
    • The standard 50KW version of the cylinder uses a RAVK valve, whilst the 100KW version uses a AVTB valve for thermostatic control of the DHW temperature being produced – see attached data sheets for further identification.
    • The calculated temperature rise across the plate heat exchanger assumes a cold mains water inlet of 10 degrees and a DHW output to the cylinders of 60 degrees. NOTE: The stated flow rate described in Tables A & B is the total flow from the plate heat exchanger into the cylinder. In a twin cylinder installation, this flow must be divided equally between the two cylinders - e.g: for a 35KW boiler, required flow rate is 10 Litres/min. Therefore each flow setter to be set to 5 Litres/min.
    • Turn the DHW timer / programmer to create a demand for DHW from the cylinder and set the correct flow rate on the flow setter:
      Adjustment of the Taconova Flow Setters.
    “The flow measurement is based on the principle of a baffle float with return spring. The flowmeter is built into the housing. The balancing can be carried out with a screwdriver at the adjusting screw. The reading position is the bottom line of the baffle float."
  3. The DHW temperature control valve (RAVK or AVTB) is to be set such that 60°C is observed at the DHW outlet from the heat exchanger (where the capillary tube from the valve is fitted), and entering the cylinder. This equates to a setting on the adjustment knob of between 4 and 5. Final checking should be done using a suitable temperature sensor / probe on the pipework above the flowsetter.

Tables of Flow Setter values

TABLE A: DHW flow setter values Vs. Boiler output - 50KW version

Boiler output (KW) Temperature rise Flow rate (L/min)
 10  50  2.9
 15  50  4.3
 20  50  5.7
 25  50  7.1
 30  50  8.6
 35  50  10
 40  50  11.4
 45  50  12.9
 50  50  14.3

TABLE B: DHW flow setter values Vs. Boiler output - 100KW version

Boiler output (KW) Temperature rise Flow rate (L/min)
 50  50  14.3
 55  50  15.7
 60  50  17.1
 65  50  18.6
 70  50  20
 75  50  21.4
 80  50  22.9
 85  50  24.3
 90  50  25.7
 95  50  27.1
 100  50  28.6

Data Sheets

The following datasheets are sent with the Amazon HXIN Unvented DHW Cylinder, and can also be found on our Wiki site,

RAVK Data Sheet
AVTB Data Sheet
Flow Setter Data Sheet
Wilo SB30 Bronze Pump