Plant Room Rebuild — Oversized Boiler System Downsized for Efficiency
London
Property Type
Property with plant room and underfloor heating. The existing installation had two 55kW boilers — 110kW of combined output — that were significantly oversized for the property's actual heat demand.
Existing System
Two 55kW system boilers (110kW combined) with pipework that had been adapted over years without a coherent system design. UFH and hot water circuits were not independently zoned.
Customer's Problem
The oversized boilers were short-cycling — firing at full power and shutting down within minutes, never reaching efficient condensing mode. Heating was uneven across the property, hot water temperature was inconsistent, and gas consumption was higher than it should have been. The system could not control the underfloor heating and hot water independently.
RARE Assessment
A full heat loss calculation confirmed the property's actual demand was well below 110kW. The existing boilers were operating far outside their efficient modulation range, and the pipework configuration prevented proper zoning of UFH and hot water circuits.
System Design
The system was redesigned with a combined output of 75kW (45kW + 30kW) in a cascade configuration, providing comfortable headroom while allowing each boiler to operate in efficient modulation range for far more of its running cycle. Pipework was reconfigured for independent zoning of UFH and hot water.
Installation
The plant room was rebuilt from the ground up. Both old 55kW boilers were removed and replaced with a 45kW and a 30kW system boiler in a cascade configuration. Three Grundfos circulating pumps were installed for primary, secondary and UFH circuits. All pipework was reconfigured, re-lagged and re-routed for logical system operation and future serviceability. System controls were reconfigured for independent UFH and hot water operation.
Engineering Considerations
- Correct boiler sizing based on a proper heat loss calculation — not a like-for-like replacement
- Cascade configuration allows each boiler to modulate independently for efficient operation
- Pipework reconfiguration for proper zoning of UFH and hot water circuits
- Independent system controls allow hot water priority and separate UFH scheduling
- Re-lagging all pipework reduces heat loss and improves system efficiency
- Brand selection is property-specific — we specify based on heat demand, space and requirements, not a one-size-fits-all preference
Equipment Installed
- System boiler (45kW) — Cascade primary
- System boiler (30kW) — Cascade secondary
- Grundfos — Circulating pumps (x3)
Outcome
The property now has even heating temperature throughout, consistent hot water, and independent control of the underfloor heating and hot water. Gas consumption is measurably lower because the correctly sized boilers operate in efficient modulation range rather than short-cycling at full power.
RARE Engineer's Note
Oversized boilers are one of the most common problems we find in London properties. When a boiler is too large for the actual heat demand, it short-cycles — firing up and shutting down repeatedly without ever reaching efficient condensing mode. The result is higher gas bills, uneven heating, and shortened boiler life. Correct sizing based on a proper heat loss calculation is the single most impactful change you can make. The brand we specify depends on the property — we install Vaillant, Viessmann, ATAG, Worcester Bosch, Keston and Ideal, and select based on heat demand, available space and requirements.
