Expanding a Solar System – How to Add Another Collector to an Existing Mount

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Expansion of a solar system – how to add an additional collector to an existing carrier

Most households that currently operate a solar system for heating domestic hot water installed it at a time when they had a different situation – smaller family, lower consumption, limited budget. Typically, these were two collectors with an accumulator tank of 200–300 liters. After years, everything changes: another family member arrives, a swimming pool is purchased, the house is expanded, or simply the owner wants to use solar energy for preheating heating as well. And here comes a practical question we often address: how to add an additional collector to what is already on the roof?

This is not just a question of hydraulics and performance. The mechanical expansion of the carrier – that is, where and how the new collector will be physically mounted – is a step that most people underestimate. The result is then situations where the carriers do not match in slope, the new collectors do not fit in the plane with the original ones, or the whole system becomes unstable. This article focuses precisely on this mechanical and assembly aspect.

Why mechanical compatibility is more important than it seems

When a customer calls saying they want to "add a collector", the first question in the technical advisory service is never about performance or hydraulics. The first question is always: "What kind of carrier do you have now and from which year is it?" The reason is simple – manufacturers of carrier systems change structural details, dimensions of mounting grooves, pipe diameters, and fastening methods. A carrier from 2012 may not be compatible with an expansion that is now sold under the same trade name.

The second reason to focus on mechanics before hydraulics: the roof area has physical limits. On a sloped roof, it is the slope, shape, and load capacity; on a flat roof, it is the load, distance from the eaves, and possibilities for ballasting. Before any expansion, it is necessary to perform a "mechanical audit" of the existing installation.

Types of expansion – what is actually possible

Before we look at specific products and procedures, it is good to clarify that "system expansion" can mean three different things from a mechanical point of view:

Direct extension of the existing carrier – an expansion segment is added to the original base carrier, which holds an additional collector. The collectors remain in one line, on the same plane and with the same slope. This is the cleanest and recommended solution.
Installation of a second independent carrier – a completely new carrier (a basic pair or a separate carrier) is installed at another location on the roof. They can be hydraulically connected, but are mechanically independent. Suitable when the original area is not sufficient, or when the roofs have different orientations.
Replacement of the entire carrier system with a larger one – relatively rare, but sometimes it is worth it, if the original carrier was dimensioned only for 2 collectors without the possibility of expansion and the customer wants to go for 4–6 units. At the same time, it is worth checking the condition of the collectors, as their age may justify a more complex renewal.

In practice, the first scenario – direct extension – is by far the most common. And precisely this one we will focus on in detail.

Basic carrier and expansion – how it works structurally

Modern carrier systems for flat and sloped roofs are designed modularly. Typically, there is a basic carrier for the first two collectors and an expansion carrier is purchased for each additional collector. This principle applies to both sloped and flat roofs, although the structural details differ.

On a sloped roof, the base is usually two mounting rails fastened to the rafters (or via special hooks under the tiles). The collector support profiles are clipped or screwed onto these rails. Expansion is achieved either by extending the rail or by adding another mounting point and another pair of support profiles. The collector must be in the same vertical position and slope as the others.

On a flat roof, the base is usually the carrier structure itself with load-bearing tiles or concrete blocks. Expansion here means either physically extending the carrier frame using additional profiles, or adding a separate carrier placed closely next to the original one and connected to it with a connecting profile.

Products for expansion – what to look for in the catalog

In the category mounting collectors you will find products directly intended for this purpose. Key is the Carrier for mounting an additional collector – this product is specifically designed to add another collector to an existing basic carrier. It is not the basic carrier itself, but an expanding module that connects to the original structure.

For installation from scratch – that is, when you plan a system with two collectors and want to have a base from the beginning, on which you can build later – the suitable product is the Carrier for mounting two collectors. This is the basic unit to which the expansion in the form of the above-mentioned module can be directly connected.

If you are installing on a flat roof or a roof with a slope of up to 15°, the situation is a bit different. In these cases, special carriers with adjustable slope are used, because the roof itself does not provide the necessary angle of inclination for the collector (optimal 35–50° for our geographical conditions). For these cases, the KS2100 – Basic carrier for the first 2 collectors on a flat roof/slope up to 15° is available, which is a basic set with a set slope for a collector with an area of 1.82 m². This base is intended for the Mounting kit for collectors on a flat roof up to a slope of 15° and for the installation of more than two collectors on a flat roof, the Carrier for 2 collectors – flat roof is used.

Important: do not combine products from different manufacturers. If you have an original carrier from one manufacturer and want an expansion from another, problems usually arise with the profile spacing, the diameter of the mounting grooves, and the overall arrangement of the collectors. Solar collectors do have relatively standardized dimensions (e.g. flat collector 2.0 m × 1.0 m or 2.1 m × 1.0 m), but the carrier systems are specifically designed for specific collectors of a given manufacturer.

Step by step: how to proceed with the expansion of an existing installation

1. Audit of the existing installation – before ordering anything

The first step is to document what you have. Find out:

Manufacturer and type of the existing carrier (usually a type label or engraving on the profile)
Year of installation and condition of the carriers – corrosion, loose screws, shifted collectors
Dimensions and type of existing collectors (width, height, frame thickness)
Current slope and orientation of the collectors (azimuth) – ideally measure with an inclinometer and compass
Available free space next to the existing carrier (to the left, to the right or above/below?)
Fastening – to rafters, via hooks, ballast – and its condition
Accessibility for installation: scaffolding, ladder, safety anchors?

Special attention should be paid to the condition of the fastening. It is very common that the original carrier was fastened for a specific load of two collectors filled with medium (each collector usually weighs 40–55 kg). Adding a third collector increases the load by another 40–55 kg – not to mention wind and snow. The fastening points need to be re-evaluated. You will learn more in the article How to correctly fasten a collector carrier on the roof without damaging the waterproofing.

2. Verification of the hydraulic capacity of the entire system

This is not a mechanical issue, but it must be resolved before mechanical expansion, as it also determines the final configuration. The solar circuit has limitations:

Circulation pump – it is dimensioned for a specific flow and pressure loss. Adding a collector (or connecting it in series or parallel) can change the hydraulic resistance. Usually, when moving from 2 to 3 collectors, you will not replace the pump, but it must be verified by calculation.
Expansion tank – its volume must correspond to the larger volume of the medium in the circuit. A typical flat system of 2 collectors requires an expansion tank of about 10–12 liters; for three collectors, it must be recalculated.
Storage tank – a solar storage tank of 200 l covers 2 collectors with a total area of about 4 m². Adding a third collector (about 6 m² in total) means you should increase the tank volume to at least 300 l, otherwise the system will overload the tank and the collectors will unnecessarily overheat.

3. Choosing the correct expansion carrier

If you have confirmed that the original manufacturer offers a compatible expansion, you can order a specific expansion module. Ideally, this is a module directly from the manufacturer of the original set, e.g. Carrier for mounting an additional collector. This module includes all the necessary profiles, connectors, screws and expansion elements.

Attention: the new collector (if you are buying it) must be from the same manufacturer and of the same model as the original, or at least have the same frame dimensions and the same pipe diameter. Otherwise, you will have problems with the hydraulic connection and the mechanical mounting in the carrier.

4. Preparation of the work site and safety

Work on the roof requires safety measures that many amateur installers underestimate. On a sloped roof, a safety line or work platform is mandatory for any work above 3 m in height. On a flat roof, you must be careful especially when walking near the eaves and when handling long profiles.

Cover the collector with an opaque film before installation – solar collectors without medium circulation can heat up to 200–250 °C (so-called stagnation temperature) on a very sunny day. Handling an unconnected collector on a sunny day is really dangerous.

5. Installation of the expansion carrier

The installation procedure depends on the type of roof. In general:

On a sloped roof: First, extend or add mounting rails (fastened to the rafters via hooks). It is important that the new fastening point is in the rafter, not just in the protective foil or eaves. You must use the same type of hooks as the original ones, so that the carrier profile fits the same. Then install the load-bearing profiles (vertical or sloped) according to the manufacturer's documentation. The collector is mounted in the carrier and fastened with clamps.

On a flat roof: An expansion typically means adding another carrier with its own ballast, connected to the original carrier profile, which strengthens the entire system and prevents mutual movement. The ballast load must be recalculated for the entire assembled unit—not just for the new carrier alone. For this type of installation, also read the article Mounting solar collector carrier on a flat roof step by step, where the procedure is described in detail.

6. Hydraulic connection of the new collector

The new collector is hydraulically connected either in parallel or in series with the existing collectors. The recommended configuration for most flat collectors is parallel connection with hydraulic balancing—so-called Tichelman circuit. Series connection increases pressure loss and can lead to uneven flow through the collectors.

All connections must be sealed with solar thread sealant or Teflon—ordinary sealant or Teflon is not sufficient for temperatures around 200 °C and glycol. Use materials certified for solar systems.

Most common mistakes when expanding the carrier system

After years of consultations and dozens of customer projects, we repeatedly see the same mistakes. Here are the most common ones:

Different slope of the new collector compared to the original ones – this occurs when the expanding carrier is mounted without accurately measuring the existing slope. Result: an unattractive appearance and unbalanced flow.
Insufficient anchoring of the expansion – the new carrier is attached only to the original carrier without its own anchoring to the roof structure. In stronger winds, the entire assembly can be subjected to large forces that the original anchoring is not designed for.
Change in orientation of the new collector – if there is not enough space on the original roof, the customer sometimes wants to place the new collector on another part of the roof with a different orientation. This is hydraulically problematic and in most cases requires a separate circuit.
Uncontrolled overheating after expansion – a larger collector area → higher performance → if the storage tank is not enlarged, the system will fall into stagnation and the glycol will degrade much faster.
Lack of air venting in the new circuit – during hydraulic reconfiguration, air gets into the circuit and must be removed. Without a functional air vent at the highest point of the system, the flow will not be correct.
Overlooked change in pressure ratios – after adding a collector, the volume of the solar circuit changes. The expansion tank must be recalculated and possibly replaced or supplemented.

About other problems with mounting, you can read in the article Common problems with mounting solar collectors – loosening, corrosion, leakage at joints.

Special case: system expansion on a flat roof

A flat roof has specific requirements. Collectors here are not placed on the flat roof but are mounted on inclined carriers at an angle typically of 35–45°. Each collector on such a carrier has a much larger wind-catching area than a collector lying on a sloped roof. Therefore, when expanding the system on a flat roof, it is crucial to:

Maintain minimum spacing between rows of collectors to avoid shading (rule of thumb: minimum distance = 2.5 times the height of the shading obstacle for winter sun)
Ensure correct orientation relative to the prevailing wind direction – the long side of the collector should be perpendicular to the prevailing wind
Provide sufficient ballast – approximately 15–25 kg/m² of collector area, more in exposed positions
Check the roof's load-bearing capacity – adding a collector with ballast increases the load by 100–200 kg per collector including the carrier

On a flat roof, it is also important to ensure that the collector connections are not under tension (e.g., if the pipe route on the roof is shortened by a collector further away). Flexible solar hose connections of 0.5–1.0 m in length between collectors are a good solution, as they absorb thermal expansion.

When expansion is not worthwhile – and what to do instead

Not always is the expansion of an existing system the optimal solution. There are situations where it is better to consider another approach:

The original collectors are more than 15–18 years old – the glass surface of the collector degrades, the selective layer loses absorption, and the seals dry out. Adding a new collector to old ones is economically questionable.
The existing carrier is not compatible with the expansion – the manufacturer has stopped production, spare parts are not available. In such cases, it is sometimes simpler to replace the entire carrier system at once.
The roof does not have enough space or structural capacity – in this case, you need to look for another location: façade, garage, standalone pergola. More about this option can be found in the article Carrier on a flat roof vs. sloped roof vs. wall – comparison of collector mounting systems.
You want to add more than 1–2 collectors – if you are considering a larger expansion (e.g., from 2 to 6 collectors), it may be worthwhile to completely reinstall the system with a larger storage tank and new circuits instead of a patchwork solution.

Dimensions and technical parameters – what to note before ordering

If you want to order an expansion carrier or consult it, prepare the following information:

Parameter	Where to find it	Example value
Collector manufacturer	Label on the collector frame	e.g. KS 2100F
Type/model of carrier	Installation documentation, engraving on the profile	e.g. KS2100
Collector width (mm)	Measured or from technical data sheet	1 000 mm (typically)
Collector height (mm)	Measured or from technical data sheet	2 070–2 100 mm (typically)
Roof type	Visually, by measuring the slope	Sloped 35°, flat (slope <5°)
Carrier orientation	Measurement on the roof, photo	Collectors on width / on height
Number of existing collectors	Directly from the installation	2 pcs

The orientation of collectors (horizontal vs. vertical mounting) and how it affects the choice of mounting system is discussed in a separate article Horizontal vs. Vertical Collector Mounting – When to Use Which Mounting Orientation. If you are unsure which type of mounting system you need at all, the overview Which Collector Mounting System Do I Need – Roof Slope, Collector Type, and Surface Dimensions will help you.

Practical example from practice: expansion from 2 to 3 collectors on a sloped roof

A specific situation we often deal with: a two-story family house with two flat collectors 2.0 × 1.0 m installed on a south-west sloped roof (slope 38°). A 200 l storage tank, circulation pump Grundfos Solar 15-65. The owner bought a swimming pool (8 × 4 m) and wants to use the solar system for its heating as well.

The analysis showed: the original 2 collectors (4 m² gross in total) are not sufficient even for water heating for a 4-person household, let alone for the pool. The recommendation was to expand to 4 collectors and increase the storage tank to 400 l (with a solar-combi control), with the pool being connected via a heat exchanger as priority #2 after DHW heating.

Mechanically: the original roof had rafters spaced 90 cm apart, which allowed direct fastening into the rafters without the need for a base plate. The existing mounting system was compatible with the expansion – two additional brackets were added to the same mounting rail for the third and fourth collector. The new fastening was solved by adding two hooks to adjacent rafters. The entire mechanical intervention took about 4 hours with an experienced team of installers.

Hydraulics: 4 collectors in a parallel Tichelmann configuration, a new 400 l storage tank with two heat exchangers (solar + boiler), expansion vessel increased from 10 l to 18 l. Result: summer coverage of DHW by solar energy increased from 68 % to 91 %, the pool was heated without external energy from May to September.

Frequently asked questions (FAQ)

Can I connect a collector from another manufacturer to an existing mounting system?

Theoretically yes, but in practice it rarely works without complications. Mounting profiles and brackets are designed for specific frame dimensions of a particular manufacturer's collector. If the new collector has the same frame width (typically 45–50 mm) and the same overall width as the original, it may work – but hydraulic connections (diameter, position of inlet/outlet) may differ. We recommend always consulting with the seller before placing an order.

How many collectors can I connect to one base mounting system?

It depends on the manufacturer and the specific product. Most systems for sloped roofs allow expansion to 4–6 collectors in one row. For flat roofs it is usually 2–4 collectors in one assembly. More collectors in series result in higher pressure loss – and from 3–4 units, a parallel hydraulic connection is practically necessary.

Do I need a building permit to expand the mounting system?

In most parts of Slovakia, the installation of solar collectors on an existing family house and its expansion falls into the category of notification obligation or is even exempt – depending on the scope, municipality, and type of building. If the expansion does not exceed 50 % of the original capacity and does not change the static parameters of the roof, you will usually only need to submit a notice to the building office. For larger projects (commercial buildings, area over 25 m²) we recommend consulting with the building office in advance.

Why is my system heating less after the expansion than before?

The most common cause: an unbalanced hydraulic circuit – the medium flows preferentially through one (shorter) circuit and the other is under-supplied. The solution is hydraulic balancing (throttling valves on each branch) or switching to a Tichelmann circuit. Another reason may be air in the system after reconfiguration – it is necessary to bleed the entire circuit and refill the pressure to 1.5–2.5 bar at cold temperature.

What is the correct order of collectors in a row – new one in front or at the back?

In a parallel connection, the order does not matter – each collector receives the same temperature of the medium. In a series connection (not recommended for more than 2 collectors), the medium from the first collector is preheated and enters the second one at a higher temperature, which reduces its efficiency. In a Tichelmann circuit, the flow direction through each collector is reversed, ensuring even flow distribution without adjustment.

What if there is no space on my roof next to the original collectors?

You have several options: adding a collector on another side of the roof (requires a separate circuit or collector array), installation on the house facade (vertical mounting, lower annual yield, but a realistic alternative), or installation on a garage, porch, or pergola. Each of these solutions has its own specifics – more in the article How to Choose the Right Mounting for Solar Collectors – Flat Roof, Sloped Roof or Wall.

Conclusion – step by step, not all at once

Expanding an existing solar system with an additional collector is a technically manageable task if approached systematically. The key is not to start from the end – that is, not to buy a new collector and then be surprised that it does not fit on the existing mounting system or that the pump is not sufficient. The correct sequence is: installation audit → hydraulic calculation → selection of a compatible expansion → mechanical installation → hydraulic reconfiguration → pressure tests and bleeding.

The mounting system is the basis of the entire mechanical integrity of the installation. If the expansion is implemented with the same products from the same manufacturer (e.g., Mounting for installing an additional collector to the existing Mounting for installing two collectors), the result is predictable and problem-free. Improvisation with incompatible parts may save money at the time of purchase, but at the cost of problems that will appear during the first storm or during the first system overheating.

If you are unsure about the choice of a specific mounting system or compatibility with your existing system, take a look at the full range in the category collector mounting – and feel free to contact us with photo documentation of your existing installation. A specific photo of the mounting profile and the collector label will reveal more than an hour-long description over the phone.

Do you have a question about this topic?

Not sure how to proceed or dealing with a specific situation in your household? Write to us – we are happy to help.