Replacing a 30-Year-Old Oil Heating System with a High-Efficiency Heat Pump
155m² Detached Bungalow | Epsom, Surrey
The Brief
Our client wanted to replace a 30-year-old oil boiler and ageing gravity-fed heating system with a modern, energy-efficient heating solution that would reduce running costs while improving comfort and reliability.
Rather than simply swapping the heat source, we designed the entire heating system around the property to ensure it would operate efficiently for decades to come.
BEFORE: 30-Year-Old Oil Boiler
AFTER: Vaillant aroTHERM Plus
Designing the Right System
Every project begins with data, not assumptions. We carried out a room-by-room heat loss calculation, which revealed a total design heat loss of 7.01kW—equivalent to 44W/m² for this well-performing 155m² detached bungalow.
This allowed us to confidently specify a:
- Vaillant aroTHERM Plus 7kW Air Source Heat Pump
- Vaillant uniSTOR 300L Unvented Hot Water Cylinder
The system was designed to operate at a 45°C flow temperature, maximising seasonal efficiency while maintaining comfortable room temperatures throughout the home.
Upgrading the Heating Distribution
Operating at lower temperatures means the heat emitters must be correctly sized. Our design identified that three existing radiators would not provide sufficient output at 45°C flow temperature, so these were replaced with larger radiators to ensure every room reached its design temperature without increasing flow temperatures.
During commissioning we uncovered another hidden issue that had not been identified during the survey. Eight radiators had been connected using an old 15mm single-pipe distribution system, which restricted circulation and prevented the flow rates required for efficient heat pump operation.
To maintain the required 5°C design temperature differential (ΔT) across the system, we replaced this section of pipework with a modern two-pipe arrangement, significantly improving hydraulic performance and ensuring the heat pump could deliver its full efficiency.
Removing decades of accumulated sludge to protect the new system.
From Gravity-Fed to Pressurised
The existing gravity-fed hot water system was completely removed and replaced with a modern unvented hot water cylinder, providing:
- Higher hot water flow rates
- Better pressure at outlets
- Faster recovery times
- Increased reliability
The result is a heating and hot water system designed to modern standards rather than adapting outdated technology.
Solving Hidden Plumbing Issues
One of the benefits of carrying out a complete system assessment is identifying issues unrelated to the heat pump that could affect long-term reliability. During the installation we also resolved several plumbing problems, including:
- Replacing seized isolation valves
- Correcting multiple plumbing defects
- Reducing incoming mains pressure from approximately 7 bar to 3 bar, helping to protect the customer’s water softener and extending the lifespan of the plumbing system
These improvements were not part of the original brief but added significant value to the overall installation.
Water Quality Matters
A heat pump is only as reliable as the water circulating through it. To protect the new system we carried out a full power flush, removing decades of accumulated sludge and debris from the existing pipework.
The system was then filled with demineralised water to comply with VDI 2035, reducing the risk of corrosion, scale formation and long-term performance issues. For additional protection, we installed an Inta magnetic filter to continuously remove circulating magnetite and safeguard the heat pump’s internal components.
The modern Vaillant uniSTOR 300L unvented hot water cylinder.
Intelligent Control
The entire system operates using weather compensation. Instead of producing the same flow temperature regardless of outside conditions, the heat pump continuously adjusts its output based on outdoor temperatures, delivering:
- Lower electricity consumption
- Improved comfort
- Longer compressor life
- Higher seasonal efficiency
System Summary
Property: 155m² Detached Bungalow | Epsom, Surrey
Existing Heating: 30-year-old oil boiler & Gravity-fed hot water
New System: Vaillant aroTHERM Plus 7kW & uniSTOR 300L Cylinder
Design Heat Loss: 7.01kW (44W/m²)
Design Flow Temperature: 45°C
Predicted SCOP: 4.3
The Outcome
The homeowner now has a heating system that is correctly designed for the property rather than oversized, resulting in exceptional efficiency and comfort. The customer is expected to reduce heating costs by approximately 35–40% compared with the previous oil boiler.
Thanks to the new £9,000 Boiler Upgrade Scheme (BUS) grant, the transition to renewable heating became substantially more affordable.
Engineering First. Equipment Second.
This project highlights why successful heat pump installations are about far more than replacing a boiler. By combining detailed heat loss calculations, hydraulic improvements, emitter upgrades, high water-quality standards and intelligent controls, we delivered a complete low-temperature heating system—not just a new heat source. The result is a system engineered to perform efficiently for many years to come.
Replacing oil or LPG heating?
Oil and LPG properties are often the strongest candidates for a heat pump, but only when the system is designed around the building. See how we handle heat pump installation in Epsom, read about our bespoke low temperature heating design, or find out why it matters that we are a Heat Geek Assured installer in Surrey.
More from our work: from a failing boiler to a future-proof heat pump system, and from an oversized quote to a precision-engineered system. Browse all of our case studies, read our customer reviews, or get in touch to book a survey.
