Midi-Fill Digital® Pressurisation Unit

From £1,700.00 £2,040.00 inc. vat


Midi-Fill Digital® has been designed to comply with the Water Regulations, providing Fluid Category 5 backflow protection for filling and top-up of other than a house heating or chilled systems. Filling loops with a Double Check Valve must no longer be used for other than a house primary circuits. Midi-Fill Digital is a compact & wall mounted filling and automatic make-up unit for adding water to a sealed system. The primary components consist of: Fluid Category 5 backflow prevention for initial fill & top-up, pump with anti seize daily pulse feature, high & low pressure cut-outs with boiler control output, comprehensive BMS controls, digital display, self test monitoring system alerting of pressurisation fault before heating system is shut down, electronic water meters with monthly excessive filling warning trigger, pressure transducer. The unit is supplied fully assembled and tested. Inlet and outlet stainless steel braided hoses with Serving Valves and push fit connections are provided to facilitate installation.


Filling and topping up of- Primary Heating Systems; Under Floor Heating; Refrigerating Equipment; Chilled Water Circuits; Industrial Processes.

Product Specification
  • Backflow Prevention Yes
  • Powered Yes
Specification Clause

The primary sealed heating system shall be filled and pressurised by a “Midi-Fill” Digital pressurisation unit – model MFD22[1] – as manufactured by Arrow Valves Ltd – Tel 01442 823123 www.arrowvalves.co.uk . The unit shall have a digital control panel with a pressure transducer monitoring the system pressure. The controller shall have three volt-free relay outputs for – boiler high and low pressure cut-out, general fault and warning signal. The fill rate shall be 1000[2] litres per hour @ 1 bar pressure differential. The unit shall feature a built-in electronic water meter with a volt free warning relay, which shall activate should the monthly consumption exceed 100[3] litres per month.

The unit shall be wall mounted at a serviceable height with the digital control panel accessible and visible. The automatic filling device is to be connected to the mains water supply or, where the mains supply pressure cannot maintain 1 bar dynamic, from the boosted cold water service if available. The manufacturer’s flexible hoses incorporating a servicing valve and inlet strainer shall be used. A double check valve is not required.

The high cut-out pressure shall be set upon commissioning to 0.2 bar below the safety relief valve pressure. The cold fill switch-on pressure shall be 0.2 bar more than the static head of the system. The unit shall automatically set the low cut-out to 0.5 bar below the cold fill pressure.

The outlet from the unit must be connected to the primary circuit return before the circulation pump. A 22 mm warning pipe must be connected to the overflow and discharged into a gully in a conspicuous manner.

The primary heating system expansion shall be accommodated in an air charged Expansion Vessel – model EVCP – as distributed by Arrow Valves Ltd – Tel 01442 823123 www.arrowvalves.co.uk. The vessel body shall be mild steel with red epoxy coating with a bottom entry male connection. The butyl membrane shall be suitable for 99 degrees Celsius at 6 bar (for 80 litres and larger). A separate expansion vessel ball valve with wire lock, drain tap and union – model BVEV – shall be supplied to enable draining of the vessel in order to periodically check the air charge. The vessel must be correctly sized[4]. The pre-charge pressure shall match the cold fill switch off pressure.

Amend as required
[1] 22 denotes the supply size, also available in 15 mm (e.g. for boilers less than 200 kw) code – MFD15
[2] 720 (for 15 mm version)
[3] Value between 50 – 1000 litres per month. The reading automatically resets on 1st of each month
[4] Where possible specify quantity and size of vessel(s)

Data Sheets


mfddatasheet.pdf (885.29 KB)


Installation Guide

mfdommanual.pdf (1.23 MB)


Additional Information



Approval Certificate

2209317-MFD-DF.pdf (104.68 KB)