How to connect a low pressure hot water unit heater and avoid service call-outs – an installer’s guide

Did you know that 90% of warranty claims for low pressure hot water unit heaters are rejected?

Why? Because the root cause of the problem is rarely a factory defect. Service statistics are clear — in most cases, the “culprit” is human error at the installation stage.

The main issues fall into two categories:

Hydraulics

The most common problem is a leaking heat exchanger. However, exchangers do not fail on their own. The usual causes are:

• Over-tightening or twisting the connection stub (excessive force without counter-holding), or
  
• Freezing of the heating medium due to a lack of frost protection.
  

Controls and electrics

Burnt-out fans are often the result of incorrect controller wiring, missing overcurrent protection, or operation in conditions the unit was not designed for (e.g. aggressive environments such as pig farms when using a standard heater).

This guide walks you step by step through the installation process for HC and FARMER series unit heaters, based on hard data from the Technical Documentation.

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Key information

• The two-spanner rule – ignoring counter-holding is the most common cause of irreversible heat exchanger damage.
  
• Location matters – installing the unit too close to a wall or ceiling “chokes” the fan and drastically shortens motor life.
  
• Flexible hoses – not an option, but a necessity to protect the system from vibration.
  
• Preparation – air louvres must be opened (min. 20%) before the first start-up.
  

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Before you start: Transport and preparation for installation

Before starting work, you must be aware of one absolutely critical rule. Breaking it can damage the unit before it even leaves the box.

Never, under any circumstances, lift or carry the unit by the connection stubs.
The protruding pipes are part of the delicate heat exchanger. Pulling or lifting by them can cause internal brazed joints to fail. Always lift the unit by the casing (EPP).

Preparation for installation

Open the box and check that the set is complete. Then prepare all necessary tools to avoid climbing down the ladder halfway through the job:

• Plumbing spanners (two are mandatory – one for tightening, one for counter-holding)
  
• Drill / driver for mounting brackets or threaded rods
  
• Spirit level
  
• Electrical tools (screwdriver, voltage tester, wire stripper)
  

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STEP 1: Where and how should the unit heater be installed? (Wall vs ceiling)

The location of the unit determines whether the space will be warm — or just noisy. For the fan to draw in air freely, sufficient clearance from any barrier (wall or ceiling) must be maintained.

• Minimum clearance: 0.15 m – this is the absolute minimum for unrestricted airflow.
  
• If the unit is mounted “flush” against a surface, the fan will be starved of air, leading to reduced performance, increased noise, and a higher risk of motor overheating.

Mounting height and air throw

These parameters depend on the unit’s output. For example, for the HC 70-3S model:

• Wall mounting: installation height from 3 to 8 m (horizontal air throw up to 18 m).
  
• Ceiling mounting: installation height from 5 to 9 m (vertical air throw approx. 7 m).
  

Note: In lower rooms, use lower-capacity models to avoid occupant discomfort (drafts) in the working zone.

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Mounting options

• Swivel mounting bracket: The most popular solution. Allows easy wall mounting and horizontal air direction adjustment.
  
• Threaded rods (M8): Ideal for ceiling suspension. Typically used with “Z” hangers or directly with threaded rods screwed into rivet nuts in the casing.
  

IMPORTANT DETAIL
Before first start-up, set the air louvres to at least 20% open. Starting the fan with closed louvres generates excessive air resistance and can damage the motor.

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Cooling option

Cooling is possible with wall-mounted units when the heater is supplied with chilled water. In this case, a condensate drip tray is required. Moisture condensing on the heat exchanger must be drained to the sewer.
Remember that during cooling, the heat exchanger must be perfectly clean.

STEP 2: Hydraulic connection – how to avoid damaging the connections?

This is the stage where most warranty-voiding mistakes occur.

The “two-spanner rule”

When tightening a fitting onto the heater connection, you must counter-hold the connection stub with a second spanner.

If you don’t, the tightening torque is transferred to the delicate copper pipes inside the heat exchanger.

Result: Twisted manifold and leakage.
Service verdict: Mechanical damage caused by the installer. Warranty rejected.

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Connection parameters and piping layout

• Connection size: standard 3/4″ (HC and FARMER).
  
• Maximum medium parameters: 120°C and 1.6 MPa.
  
• Counterflow connection: To achieve full heating output, connect the supply at the bottom and the return at the top of the exchanger.
  

After pipe installation

• Fill the system with water.
  
• Bleed the system (the heater has an air vent at the highest point of the manifold).
  An air-locked heat exchanger results in a drastic loss of output.
  
• Perform a leak test.
  

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Why should flexible hoses be used?

Rigid steel pipes are not ideal for air-blown devices. Flexible hoses allow you to rotate the heater freely on the mounting bracket.
The fan generates micro-vibrations, which flexible hoses absorb, protecting the heat exchanger from leaks at the connections after years of operation.

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STEP 3: Electrical installation and control (AC and EC motors)

Control selection depends on the type of heater. Always check the nameplate.

Identify your motor type

• HC-3S / FARMER HCF IP54: Classic 3-speed AC motor. Requires a simple controller or speed regulator.
  
• FARMER HCF IP66: Single-speed AC motor with increased ingress protection. Simplest control: ON/OFF.
  
• HC-EC: Modern electronically commutated (EC) motor, controlled via a 0–10 V analogue signal. Requires a dedicated controller (e.g. HMI EC) but offers smooth speed control.
  

Want to learn more about the difference between AC and EC motors? See our case study.

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Electrical safety

• Installation must be carried out by a qualified electrician, in accordance with good practice and local regulations.
  
• Earthing: The green-yellow conductor is essential for safety.
  
• Protection: Use overcurrent and RCD protection in the distribution board.
  
• Service isolator: Installing a local disconnect switch near the unit is recommended for safe servicing.
  
• Cabling: Minimum cross-section is typically 2 × 1.5 mm² or 3 × 1.5 mm², depending on the controller and cable length.
  

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Wiring

• Use only the manufacturer’s dedicated control equipment. Improvised substitutions are a common cause of burnt windings.
  
• Always verify connections against the electrical wiring diagram in the technical documentation.
  
• After start-up, check that the fan responds correctly to control changes (speed, temperature) and rotates in the correct direction.
  

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How to look after the unit? Servicing and heat exchanger cleaning

Dust and dirt on the heat exchanger act as an insulating layer, reducing heating performance.

Standard cleaning (HC series)

The recommended method is blowing with compressed air. Direct the airflow along the fins, not across them.

Warning:
Do not use pressure washers or stiff brushes. Aluminium fins are very delicate — bent fins will restrict airflow.

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FARMER HCF specifics (aggressive environments)

These units operate in poultry houses and pig farms, where dust is greasy and corrosive.

• FARMER heat exchangers are coated with a special LCE (Gold Epoxy) coating.
  
• Combined with motors rated IP54 or IP66, this allows the unit to be washed (in accordance with the instructions), which is essential for hygiene in livestock buildings.
  

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Post-installation checklist for installers

Before packing up your tools and issuing the invoice, tick off the following points — this is your insurance against call-backs:

• Is a minimum 15 cm clearance from wall/ceiling maintained?
  
• Are the air louvres open (min. 20–30%)?
  
• Were the connection stubs tightened using the two-spanner method?
  
• Has the system passed a leak test?
  
• Is the installation fully bled (air vent at the highest point)?
  
• Does the fan rotate freely (no contact with the EPP casing or grille)?
  
• Do the fan and controller operate correctly at different speeds?
  

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Summary

Correct installation of a low pressure hot water unit heater requires precision and adherence to a few key rules that are often overlooked. By following the two-spanner rule, maintaining proper mounting clearances, and using the correct control equipment, you provide the investor with peace of mind and low operating costs — and yourself with the reputation of a professional who gets recommended.

It’s better to spend 15 extra minutes on installation than to waste hours on service visits.

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