System Design

When fitting a hydronic (wet) central heating system you have to consider how the warm air will permiate your living space. Here Alde UK give our advice on designing your bespoke Alde central heating system.

  1. Siting the heating circuit
  2. Plumbing
  3. Convector/radiator output
  4. Commissioning

Siting the heating circuit

The correct siting of the heating circuit, the pipes, convectors and radiators, is fundamental to the performance and efficiency of the central heating system. It should be your main design consideration.

  • Fit convectors/radiators around exterior walls, this creates an even temperature without hot and cold zones.
  • Fit convectors/radiators under each window. This will reduce heat loss, cold draughts and condensation.
  • Fit at least the minimum amount of convector/radiator for your living space. This will greatly improve responsiveness (how quickly and adequately the living space is heated) and economy (an overall reduction in fuel consumption).
  • If your boiler produces 5.5 kW of power but you only have 0.8 kW of convector/radiator, the boiler must run for longer to permiate the living space with 5.5 kW of heat.

  • Fit ventilation in furniture. Air must be able to flow freely around convectors and radiators, wherever they're sited. Vents should be located in the bottom-front and top-rear of bunks, luton seats, cupboards, etc., so cool air can be drawn under the furniture across the convector/radiator and then rise up the wall behind. The gap between floor and furniture (cool air in) should be at least 40 mm high, and between wall and furniture (warm air out) at least 25 mm wide.
  • Likewise, vents should be located in the bottom-rear and top-front of overhead louvres to avoid cold and damp pockets along the ceiling.

  • Fit ventilation in the floor. The regulations governing floor vent area vary according to country and depend on the type of vehicle and number of occupants it is designed for. Consider locating vents under convector/radiator wherever possible.
  • A correctly fitted Alde aluminium convector output ≈400 W per metre.



It's easy to get too focused on convectors and radiators, or the rubber connectors needed, or the boiler itself. It's all plumbed into the one heating circuit.

  • DO use a single-pipe heating circuit. This keeps the system simple, simpler to design, simpler to fit, whilst cutting down on weight and cost.
  • (A single-pipe heating circuit isn't a disadvantage in the Alde system because pipe runs are short and all on one floor.)

  • DO ensure all plumbing is clean and undamaged before fitting.
  • DO locate air bleeds at all high points in the heating circuit, especially at drop-offs.
  • DO locate drains at the lowest points in the heating circuit, preferably near to the Alde boiler.
  • DO fit the expansion tank 400 mm above the highest point in the heating circuit (recommended, not min requirement).
  • DO NOT use less than a 700 mm flue length (recommended, not min requirement).
  • DO NOT use SpeedFit fittings for the heating circuit in a road-going installation.
  • DO NOT store pipework on the floor where it may gather corrosive particles or be damaged.
  • DO NOT cut pipework with a hacksaw, use a pipe-cutter.
  • NEVER use copper parts. The primary heat exchanger in the boiler is alumunium, and copper parts will cause corrosive galvanic action in any aluminium parts of the system.

When using the 2928-420 or 2928-433 circulation pump specifically, ensure it's fitted so that the pump's impeller sits above the Flow pipe. Don't screw the tank cap too tight or the impeller will clatter against the expansion tank.

22 mm aluminium tube should be lodged 30 mm into Alde rubber connectors and securely fastened at 10-15 mm on the connector.


Convector/radiator output

Without convectors or radiators, how will heat transfer from the heating circuit into the living space? Using Alde aluminium convector as a standard, we can produce a rough guide* to the minimum amount needed to adequately heat your living space.

Minimum amount of convector/radiator
Living space area (m²) Total length of aluminium convector (m) Output (kW)

*This is only a rough guide to get you started. You must also consider how well ventilated the heating circuit is, how well insulated the living space is, how the boiler will be controlled and the end user's habits. Ultimately any central heating system should be tested in situ in a cold chamber if possible.



G13 Spec antifreeze

It's crucial to pressure test the heating circuit before it's filled. The tester should maintain 1 bar of pressure for 30 seconds. Remember, it will be twice the work to correct mistakes at this late stage, so care and attention to detail during the build will save time later on.

When recommissioning a system, ensure it is flushed with hot water to remove the old antifreeze solution and other detrius.

To fill or purge a system, use the Alde service pump to avoid trapped air. Trapped air is a nuissance, and furthermore if trapped in the internal heat exchanger it may cause serious damage to the boiler.

After the heating circuit is filled with a mix of 50% G13 Spec antifreeze and 50% deionised water, performance testing can be carried out in a cold chamber. Alde International Systems AB offer free of charge cold chamber testing, by appointment, at their facility in Sweden.


Proudly Made in Sweden emblem Gas Safe logo QEC logo
Alde heritage

© Alde International Systems AB 2019. All Rights Reserved. Designated trademarks and brands are the property of their respective owners. Est 1949.
Web Development Team | Terms and Conditions | Privacy Policy | Cookies Policy