Summary
Adveco looks at the role of preheat vessels in hybrid water heating systems play in managing systems that cut emissions and costs
Adveco considers the critical role indirect preheat vessels play in the creation of next-generation domestic hot water (DHW) systems in commercial buildings.
As commercial buildings transition to net zero, hybrid water heating systems that combine low‑carbon sources like heat pumps or solar thermal with traditional boilers are increasingly critical. A key component often overlooked in this mix is the preheat vessel. Indirect cylinders, or calorifiers, serve as these preheat vessels, enabling efficiency, resilience, and flexibility within hybrid systems.
What Are Preheat Vessels?
Preheat vessels are insulated storage tanks, indirect cylinders, equipped with internal heat exchangers. Think of them as the battery in an efficient DHW system. They hold a reservoir of hot water, preheated upstream via a heat pump or solar thermal collector, and topped up as needed by a primary heat source like an electric boiler or gas heater. By separating heat generation from delivery, preheat vessels ensure a steady supply and greater thermal efficiency.
Preheat vessels matter in hybrid systems, not least because they optimise heat pump performance. Heat pumps produce water at moderate temperatures (45‑50 °C) efficiently, but topping up to safe DHW temperatures (60 °C) demands energy and can reduce COP. A well-sized preheat vessel buffers this thermal swing, allowing heat pumps to run steadily and improve seasonal performance.
Correctly sizing indirect cylinders is a critical element of hybrid application design. Under-sizing cylinders, or preheat vessels, can leave heat pumps or boilers struggling during peak demand, leading to cold showers or over worked plant.
Under-sized preheat vessels limit heat pump runtime, forcing higher frequency cycling or boiler override. Heat pumps working against low volume tanks must reheat rapidly, straining performance and billing instead of leveraging slow, overnight reheat cycles
A larger preheat vessel supports two hour reheat cycles overnight, ensuring readiness for morning peak, avoiding complaints of cold showers. Given electricity costs remain higher, this sizing strategy holds carbon benefit and operational control.
Correctly sizing prevents this, enabling gradual reheating during off‑peak times, and maximising preheat vessel value. If oversized, the design generates unnecessary capital investment and will lead to greater operational costs as excess energy is demanded by the application to maintain temperatures.
Enabling Hybrid Resilience Preheat vessels support hybrid setups by acting as a thermal buffer between low‑carbon preheat and top‑up sources (electric or gas). The integration of controls is critical for efficient management of heat sources, maximising energy use to lower peak power demands, operational cost and, depending upon primary heating source, the level of carbon emissions. Should primary technologies require maintenance or fail, preheat vessels deliver stored hot water, boosting uptime and reducing reliance on immediate heat generation.
Indirect cylinders/preheat vessels can connect to boilers, heat pumps, and solar thermal, supporting renewable integration. A perfect example of this is FUSION Packaged Electric Water Heating
The Adveco FUSION system integrates a preheat vessel with an FPi32 ASHP (6 – 10 kW) and electric boiler (9 – 12 kW), supplying continuous hot water up to 60°C for light commercial settings. The preheat vessel ensures the heat pump operates in its efficient zone, while the boiler handles peak topping to meet safety and anti-legionella needs.
The pre-configured preheat vessel removes sizing guesswork, balancing cylinder volume against expected usage. With capacities from 200 to 500 litres and a 10 bar rating, it addresses varied demand profiles from student accommodation to public sector sites.
An additional advantage of using an indirect approach in FUSION is the system’s ability to counter limescale formation by confining water movement through a controlled loop and the electric heating within the boiler. Limescale formation, so long the bane of electric water heating elements in hard water areas, is essentially nullified for reduced maintenance. And with minimal corrosion and low wear, system efficiency is maintained, ensuring greater reliability and a longer operational lifespan of the system.
Designing with Preheat Vessels in Mind
- Calculate Peak Demand
Usage patterns, including simultaneous draw off, are used to determine the minimum volume.
- Ensure Compatible Top‑Up Sources
Preheat vessels must pair effectively with both renewable preheat, heat pumps, and topping units. Control strategies must prioritise heat pump use while deferring boiler activation until needed.
- Size for Overnight Reheat
Aim for 2‑3 hours of reheat at base heat pump output to ensure full utilisation.
- Utilise Packaged Solutions
Off-the-shelf systems like fusion pre‑size preheat vessels, reducing complexity and installation risk
In the quest for net zero water heating, preheat vessels occupy a central role in hybrid systems, bridging renewables and backup heat, smoothing demand, extending system life, and ensuring carbon-led savings. Undersize them, and heat pumps underperform; oversize them, and capital is wasted. The expert answer? Rely on right-sized, packaged preheat vessel systems that align with your demand profile and carbon goals.
For commercial projects, the message is clear: to maximise hybrid efficiency and avoid net zero compromises, act like Goldilocks when choosing the right preheat vessel, not too small, not too large, but just right.
