Usually, it is vital for good boiler management and safety – in either a sealed or open vented heating systems - that there is a constant minimum flow of water through the boiler. The boiler is dependent on a minimum circuit flow for safe operation and longevity of components. Primary circuit design is centred around establishing the correct flow rate with the pump capabilities and the boiler requirements.

All boilers therefore have a flow and return and either an internal or external pump that pushes water through the boiler, and maintains the flow rate, as it moves around the primary circuit, from the low loss header or plate heat exchanger to the boiler and back again.


Stainless steel boiler that has no minimum flow rates

What is a no flow boiler?

no flow boiler  does not depend on the system circuit flow for safe operation. These boilers have an internal, variable speed circulation loop within the boiler that ensures water continues to move within the heat exchanger, meaning the boiler safely regulates its own water temperature when circuit pumps are off or set to low.

Traditionally boilers modulate by monitoring their respective flow and return delta temperature hence the reason it is important to maintain a constant minimum flow rate according to the boiler’s flow / return differential range. When a traditional boiler is firing or at “over-run” cessation of flow rate will create an overheating situation. The no flow boiler in this scenario simply sees the narrowing of its delta T  and starts the internal circulation pump modulating its speed as necessary to maintain safe heat exchanger temperature.

The high-water content in a no flow boiler means it has a high thermal mass (e.g. 135 litres in each boiler), this large thermal mass absorbs latent heat inside the exchanger when system flow slows or stops.

A no flow boiler allows you to stop system circulation dead within the boiler with no risk to the components themselves as, once the controls stop the boiler, this thermal mass safely absorbs any residual heat.

What does this mean for your commercial heating system design?

A dedicated primary circuit is not required when you have a no flow boiler as, essentially, the boiler itself is your self-contained primary circuit. This means that you do not have to consider installing a low loss header, plate heat exchanger or any of the associated pumps required to maintain flow around your primary circuit. This reduces risk in terms of points of failure, and limits maintenance requirements as well as reducing purchase and installation costs.

This is a far simpler system than your traditional commercial heating system boiler, in fact despite the outputs being very large these are hydraulically similar to a domestic system.


Installation considerations

You can cascade a no flow boiler such as the Hamworthy Varmax, but flow isolation through non-firing boilers is required, as this helps the system pumps to modulate which – in turn – ensures flow through the firing boiler(s). This is not unique to the no flow boiler as isolating any non-firing boiler in any system is good practice to reduce standing losses through non-firing boilers. However with a no flow boiler that by its very nature needs a large thermal water mass, flow prevention methodology is vital to minimise energy losses when a boiler is at idle.

You may also want to consider the space available to you. Although the no flow boiler is often larger and so access and weight should be taken into consideration, they requires less in terms of peripheral components (such as hydraulic separation)  so often space is gained by having less equipment installed.

When would you choose a no flow boiler?

A no flow boiler would be a valuable choice in premises with high peaks of demand at certain points in the day and no or significantly reduced demand for long periods. A district heating scheme will tend to have a peak in the morning and then long periods where most people are out or working, followed by a larger more sustained peak in the evening. A cascade of no flow boilers would allow for all boilers to run during peak times and then a number to be closed off during the longer, low demand period, keeping power requirements to a minimum and ensuring maximum efficiency.


What options are available?

The Varmax no flow boiler is tolerant to a wide range of system water conditions as it has a corrosion resistant stainless steel heat exchanger. It is well insulated for low standby losses and matches heating system loads accurately with outputs up to 637kW from a single module and the ability to cascade multiple boilers for greater demand. The Varmax also has a simplified flue system design, and its flue gas non-return valve is built in to provide effective protection from re-circulation of flue gases through the boiler when it is connected to a flue header and is not firing.

The Varmax also comes with built-in boiler sequence controls that can control up to 16 boiler modules, multiple heating circuits and a hot water circuit using the master/slave principle. The sequencer can also be configured to be controlled by a BMS control by VFC enable, 0-10v temperature..

Its ideal hydraulic applications include split temperature systems thanks to separate high and low temperature return connections – such as a combination of radiator loops and underfloor heating – as well as medium to large district heating schemes, schools, shopping centres, offices, and sports or health facilities.


Whatever the scope of your system redesign, you can get expert support at any time from Hamworthy. We’re happy to help you with anything from sizing your heating system requirements, and identifying the most cost effective approach to upgrading your system, and from product selection to  long term spare parts availability.

If you need more advice or want someone to look at your heating system request a free site survey from our team of sales engineers who can advise on the best products.

Alternatively, Email us or visit our technical library