Installation and Common Issues of Instant DHW Boilers

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Installation of condensing boilers must be carried out by a qualified and authorized company.
Condensate from the boiler is an acidic by-product requiring appropriate drainage solutions.
Flue gas exhaust can be done via the facade or through an existing chimney.
Drainage materials must be resistant to acidic condensate (e.g., plastic piping).

Who should install a condensing boiler with instant DHW?

Installing a condensing boiler with instant domestic hot water is not a do-it-yourself task. Only a professionally qualified company that holds the necessary authorization for the installation and servicing of gas appliances should perform the work. After installation, always ensure you receive an inspection report and a commissioning certificate as proof of proper execution.

How to manage condensate from a condensing boiler?

Condensing technology produces acidic condensate with a pH of approximately 3 to 5. For typical boiler outputs, this condensate can be drained into a nearby waste point such as a sink or siphon. However, for higher outputs—like in cascade connections or large installations—a neutralization unit is necessary to manage the acidity. The drainage system must be made of plastic piping, which is resistant to long-term exposure to acidic liquids. Metal or cast-iron pipes are not suitable for this purpose.

What are the options for flue gas exhaust?

The most common solution for instant-heating boilers in apartments is a concentric exhaust through the facade. This involves a double-wall pipe with a short penetration through the perimeter wall and does not require a traditional chimney. However, the outlet must be positioned at a safe distance from windows, doors, and the boundary of neighboring properties to ensure safety and compliance with regulations.

When exhaust through the facade is not feasible, an existing chimney can be used. In this case, a plastic or stainless steel liner suitable for condensing boiler operation is required. You can find appropriate components in the flue gas exhausts and flue pipes category.

Fluctuating Hot Water Temperature from a Gas Boiler

Issue: Inconsistent Temperature in Domestic Hot Water

When using a boiler with instant DHW heating, you may notice that the temperature of the hot water supplied to your taps or shower fluctuates. This issue is more common in systems where the gas supply is not optimally configured.

Cause: Undersized Gas Pipeline

The root cause often lies in the gas pipeline's diameter being too small for the boiler's peak output. Boilers that provide instant hot water require more energy during short bursts of high demand compared to those used only for space heating. If the pipeline is undersized, it cannot deliver sufficient gas quickly enough, leading to unstable hot water temperatures.

Solution: Ensure Proper Gas Supply Setup

To prevent this problem, the gas pipeline must undergo a valid inspection and a completed pressure test before being connected to the boiler. Additionally, the pipe diameter should be correctly sized according to the boiler’s specifications. This ensures stable gas flow and consistent hot water temperature during use.

Electrical and Safety Requirements for Instant-Heating Boilers

Issue: Unsafe or Inadequate Electrical Setup

Even if the boiler is correctly sized and connected to the gas supply, improper electrical installation can affect its performance or pose a safety risk. Many boilers installed in compact spaces like bathrooms or utility rooms are not given the required electrical attention.

Cause: Lack of Proper Electrical Infrastructure

A key requirement for these boilers is a dedicated 230V socket with correct grounding. Without this, the boiler may not function reliably or could be a safety hazard. Additionally, the absence of a residual current device (RCD) can leave the system vulnerable to electrical faults.

Solution: Correct Electrical and Safety Installation

Ensure the boiler is connected to a properly grounded 230V socket and install an RCD for added safety. This helps protect both the equipment and the people using it. During commissioning, the installation company should also bleed the system, set the heating water pressure (usually 1–2 bar), and configure the operating parameters according to the specific type of heating system in place.

Frequently asked questions about DHW heating issues

Why do I feel a cold burst when reopening the hot water tap?

This is caused by cold water left in the pipe and the boiler's burner lag. Using a boiler with a pre-heating (micro-accumulation) function or simply not closing the tap fully can help reduce the effect.

What causes temperature drops when using multiple hot water taps at once?

If the total flow exceeds the boiler's maximum capacity, the output temperature or pressure will drop. To avoid this, use one tap at a time. For regular high demand, consider a boiler and tank system.

Why is the hot water flow weak or inconsistent?

Common causes include a clogged cold water inlet filter or limescale buildup in the heat exchanger. Clean the filter yourself at least annually. Descaling the heat exchanger should be handled by a service technician during the yearly check-up.

Why does the boiler keep turning on and off when using hot water?

At very low flow rates near the boiler's minimum threshold, this is normal. However, if it happens with the tap fully open, the flow sensor is likely faulty and a service inspection is required.

Why does the water smell or look odd after being unused for a while?

After a long period without use, let the water run at all taps for a few minutes to flush the system. If the issue continues, a service technician should inspect the heat exchanger for potential contamination or damage.

Scheduling an annual service is the best way to prevent most of these issues. If you're in the market for a new boiler, explore the full selection in our condensing boiler range.