Viessmann System Boiler Essex: A PDHW Installation Case Study
- 7 days ago
- 4 min read
Updated: 1 day ago
Improving efficiency, control, and performance with a correctly sized system boiler
Replacing an outdated fireplace back boiler with a modern heating system can present a number of design challenges. One recent project involved relocating the boiler from a fireplace back boiler arrangement to a first-floor cupboard, where we installed a Viessmann Vitodens 100-W 19kW system boiler designed around a Priority Domestic Hot Water (PDHW) setup.
The goal was not simply to replace the existing heat source, but to create a more efficient, better-controlled heating system that could deliver reliable hot water performance while accurately matching the heating requirements of the property.
The system design: 4-pipe PDHW configuration
The installation utilised a 4-pipe system boiler configuration, allowing the boiler to independently manage both:
Primary Domestic Hot Water (PDHW) production
Central heating demand
This arrangement provides greater control over energy usage by allowing the boiler to prioritise hot water production when required, without compromising heating performance.
To achieve correct operation, this type of setup requires several additional components, including:
A cylinder demand box
An external temperature sensor (or equivalent 10k resistor where applicable)
Correct commissioning of the boiler controls
The system was configured with weather compensation set to heating curve 1.5, enabling the boiler to automatically adjust flow temperatures based on outdoor conditions. This reduces unnecessary energy consumption by avoiding the traditional approach of running the heating system at maximum temperature regardless of demand.
Why a Viessmann19kW boiler was selected instead of a larger model
During the design stage, alternative proposals suggested installing a 30kW boiler using an S-plan configuration. While this approach may appear attractive due to the larger output rating, it was not the most appropriate solution for this property.
The decision to install a 19kW boiler was based on the actual requirements of the system.
The existing 210-litre unvented hot water cylinder had a coil rated at 19kW, meaning the cylinder’s ability to transfer heat into the stored water was limited to that output. Installing a significantly larger boiler would not increase hot water recovery performance because the cylinder coil would still restrict the amount of energy transferred.
In addition, the calculated heat loss of the property was approximately 8kW, meaning the heating demand was considerably lower than the boiler’s available output.
The Viessmann 100-W Vitodens 19kW system boiler was therefore correctly matched to the requirements of both the hot water cylinder and the buildings heat loss.
Dual-temperature operation: efficient heating and hot water
One of the key advantages of this setup is the boiler’s ability to operate at different temperature requirements for different demands.
For domestic hot water production, the boiler can operate at a higher temperature to heat the cylinder efficiently, delivering approximately:
65°C cylinder temperature
80°C boiler flow temperature for hot water demand
For central heating, the system operates at a lower temperature, automatically adjusted throughout the year using:
Weather compensation
Load compensation
Seasonal temperature changes
This approach improves efficiency because the heating system only produces the temperature required to maintain comfort, rather than continuously operating at unnecessarily high temperatures.
Correct boiler sizing: matching output to system demand
A common misconception when specifying PDHW systems is that a larger boiler will always provide better performance. In reality, correct system design is about matching the heat source to the limitations and requirements of the entire installation.
By selecting a boiler that matches the cylinder coil rating, the system is able to meet the highest demand requirements effectively while avoiding excessive cycling and reduced efficiency.
A correctly sized boiler can often outperform an oversized boiler because it operates within its optimal performance range for longer periods.
Commissioning considerations for installers
For engineers carrying out this type of installation for the first time, it is worth allowing additional time for:
Control wiring
Understanding the PDHW strategy
Setting up weather compensation
Correct commissioning and testing
The configuration can initially appear more complex than a traditional heating system, but once understood, the benefits become clear. The result is improved efficiency, better temperature control, and a more refined heating experience for the homeowner.
Helping customers understand smaller boiler outputs
One of the biggest challenges when specifying PDHW systems is customer perception. Many homeowners naturally assume that a larger boiler will provide better performance.
However, a 19kW PDHW boiler is still capable of delivering 19kW output for both domestic hot water and heating requirements. The difference is that the system intelligently manages where that energy is directed, prioritising demand when required rather than relying on excessive boiler capacity.
Educating customers on correct sizing is an important part of delivering modern heating solutions. A smaller, correctly matched boiler can provide outstanding performance while improving efficiency and reducing unnecessary energy consumption.
Learn more about Priority Domestic Hot Water (PDHW)
For a more detailed explanation of PDHW systems, including how they work and why they are becoming increasingly popular in modern heating design, read our full guide:
Professional Boiler Upgrades and PDHW Installations Across Essex
At LHS Plumbing, we specialise in designing and installing high-performance heating systems across Essex, helping homeowners upgrade from older, inefficient boiler setups to modern, energy-efficient solutions.
From replacing traditional back boilers and upgrading ageing heating systems to installing advanced Priority Domestic Hot Water (PDHW) configurations, we work with homeowners throughout Essex to deliver heating systems that are correctly designed, accurately sized, and commissioned for maximum efficiency.
Our experience covers properties of all types, including older homes where careful consideration is required when relocating boilers, upgrading controls, and improving hot water performance. Whether you are looking to improve energy efficiency, increase hot water availability, or modernise an outdated heating system, our approach focuses on designing a solution around the property’s actual requirements rather than simply installing the largest boiler available.
We provide professional heating and plumbing services throughout Essex, including areas such as Loughton, Buckhurst Hill, Epping, Chingwell, Harlow, Chelmsford, Brentwood, Billericay, Basildon, Ingatestone, Shenfield, Rayleigh, Southend-on-Sea, and surrounding areas.
Every installation is carefully planned, from heat loss calculations and boiler sizing through to system design, wiring, and final commissioning. This ensures homeowners receive a heating system that delivers reliable performance, improved comfort, and long-term efficiency.
If you are considering a boiler upgrade, system conversion, or a more advanced PDHW installation in Essex, our team can help design a solution tailored to your home.
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