Vaillant plate heat exchanger PHE S 120-70 (120 kW, 1 1/4" threaded connection)

Vaillant PHE S 120-70 plate heat exchanger (120 kW, 1 1/4" threaded connection)

Vaillant plate heat exchangers of the PHE S and PHE C series are among the key components of large professional boiler rooms, cascade arrangements of condensing boilers and industrial heating hubs. Their purpose is to hydraulically and thermally separate the primary circuit (heat source – boiler, boiler cascade, heat pump) from the secondary circuit (distribution system, underfloor heating, radiator circuits, domestic hot water heating), while heat is transferred between the circuits exclusively through a set of stainless-steel plates – without any direct mixing of the media. In practice, this principle is applied wherever it is necessary to prevent circulation pumps of individual circuits from hydraulically influencing each other, to balance different flow rates, or to separate circuits with different quality, pressure or heat-transfer medium composition (e.g. glycol mixtures in a solar or heat-pump circuit from ordinary heating water in the distribution system).

PHE S 120-70 is an entry-level, compact model of the Vaillant plate heat exchanger range with a screwed (threaded) 1 1/4" connection and a maximum transferable output of 120 kW. Thanks to its compact design (height only 281 mm, thickness 73 mm) it is intended primarily for smaller and medium-sized professional applications – apartment buildings with their own boiler room, smaller commercial buildings, business premises, or as a separating element between a cascade of two to three wall-mounted Vaillant condensing boilers and the secondary heating distribution system. Unlike the larger PHE C series models, the PHE S 120-70 is not mounted on a floor bracket – its low weight (only 10.70 kg) allows it to be hung directly on the pipework via its threaded connections.

A highly valued feature of this model is its maximum operating pressure of up to 3.0 MPa (30 bar) – significantly higher than the flanged PHE C models (0.6 MPa / 6 bar). This makes it a universally usable component, suitable not only for classic low-pressure heating circuits but also for higher-pressure industrial and process piping systems, where a safe margin against pressure spikes, water hammer or a short-term exceeding of operating pressure due to a fault in the expansion vessel or safety valve is required.

Technical data of the PHE S 120-70 model

Overview of the entire Vaillant PHE series (official manufacturer technical data)

Plate material: stainless steel. Operating temperature (plate material range): -196 °C to +200 °C. The PHE S model has a screwed (threaded) 1 1/4" connection, PHE C models have a flanged connection. Source: Vaillant installation manual "Externer Plattenwärmetauscher PHE S, PHE C", document 0020196730_00.

Construction and principle of operation

The PHE S 120-70 plate heat exchanger consists of a set of pressed stainless-steel plates with a precisely defined pattern, which are brazed (or welded) into a compact block. Channels for the primary and secondary medium are formed alternately between the individual plates – the liquids therefore never come into direct contact between the circuits, and heat is transferred exclusively through the thin wall of the stainless-steel plate. The plate pattern also creates turbulent flow, which significantly increases the heat transfer coefficient compared to classic tubular heat exchangers of the same size – thanks to this, the PHE S 120-70, despite its compact size, is able to transfer a relatively high output of 120 kW.

The connections of both the primary and secondary circuit (supply and return) are led out on the same – side – face of the heat exchanger body, which simplifies installation in cramped boiler rooms and enables easy access for future servicing without having to dismantle the surrounding pipework.

Installation guidelines (according to the official Vaillant installation manual 0020196730_00)

• Installation only in areas without risk of freezing.
• Ensure sufficient space for operation, servicing, venting and draining of the heat exchanger.
• Connections are always led out on the side, never from below the heat exchanger.
• A vertical mounting position is recommended to minimize pressure losses.
• The PHE C model must always be additionally anchored to a floor bracket with height-adjustable feet (10 – 55 mm) – it must never be left hanging only on the pipe connections.
• The primary and secondary side flows are routed in counterflow – ensuring the highest heat transfer efficiency.
• Maximum permissible forces and moments on the connections: tensile force T, transverse force F, bending moment Mb and torque Mv – see the table below to prevent mechanical damage to the connections during pipework installation.

Commissioning

1. While filling the system with heating water, the heat exchanger is continuously vented via the venting valves on the supply and return.
2. To avoid water hammer caused by entrained air, the pumps are first started against closed supply and return shut-off valves.
3. The supply and return shut-off valves are opened simultaneously and slowly, until the operating temperature is reached.
4. The primary circuit is always commissioned first.
5. After start-up, check whether pressure surges are affecting the heat exchanger – if necessary, add stabilizing (calming) sections to the pipework and close the shut-off valves.

Service, cleaning and maintenance

If signs of limescale or dirt build-up appear (typical with hard or contaminated water), clean the heat exchanger as follows: shut down the system, drain and remove the heat exchanger. Use only cleaning agents suitable for stainless steel and copper – unsuitable cleaning agents may permanently damage the heat exchanger. Flushing with the cleaning solution is carried out against the normal flow direction, after which the heat exchanger is flushed with clean water, reinstalled, and it is recommended to check the water hardness before restarting. In case of high water hardness, treatment of the heating water is recommended according to the documentation of the other system components – atria.sk offers, in the chemical and physical water treatment category, corrosion inhibitors and cleaning products from the SENTINEL and ULTIMA ranges for this purpose, which extend the service life of the heat exchanger and the entire boiler room.

In case of temporary shutdown (repair, servicing, longer outage), it is recommended to completely drain and clean the heat exchanger to prevent damage from frost, aggressive media or biological fouling. In case of permanent decommissioning, the primary circuit is closed first, then the secondary circuit, followed by draining and disassembly.

Typical use case in practice

Consider an apartment building or a smaller commercial property heated by a cascade of two Vaillant condensing boilers with a total installed output of up to 120 kW. For various reasons (e.g. different length and hydraulic resistance of individual branches, the need for independent control of several heating zones, or the requirement to separate the boiler circuit filled with well-treated water from an older, historically contaminated distribution system in the building), the investor or designer needs to separate the boiler section from the rest of the system. This is exactly what the PHE S 120-70 is for – it is connected between the manifold of the boiler section and the main manifold of the heating system, with heat transfer taking place exclusively through the stainless-steel plates. Thanks to the counterflow connection (primary and secondary flowing against each other), a minimal temperature gradient between the circuits is achieved, meaning only a minimal loss of boiler efficiency compared to a direct (undivided) connection.

In practice, this type of heat exchanger is often combined with apartment or boiler metering assemblies (e.g. the IVAR.KS, PAW or HEAT BLOCK ranges, which you can find in the same boiler and apartment metering assembly category on atria.sk) and with hydraulic separators – as an alternative, thermally separated (not just flow-balanced) method of solving the same problem in large boiler rooms: how to reliably connect the heat source with the distribution system without the circulation pumps negatively influencing each other.

Why choose the Vaillant PHE S 120-70

• Original Vaillant part – full compatibility and guaranteed functionality in Vaillant systems, unlike universal third-party heat exchangers.
• Compact dimensions – fits even into smaller plant rooms and boiler rooms where there is no space for bulkier flanged PHE C models.
• High pressure margin (30 bar) – a safe choice even for more demanding process circuits.
• Easy installation and servicing thanks to side connections and low weight (can be handled by one to two people without lifting equipment).
• Additional accessories – thermal insulation (code 0020248922) and a connection adapter for a 35 mm Cu pipe (code 0020248931) are available separately in this category, so you can configure the assembly exactly according to the needs of your project.
• Proven supplier for Slovakia – WINTNER s.r.o., a long-standing Vaillant and Protherm partner on the Slovak market.

Frequently Asked Questions