Before you buy
Replacing a combi is not only about picking a brand and kW rating. The right boiler for your home depends on property heat loss, hot-water flow requirements, water hardness, and how the system is protected and controlled. A Gas Safe registered engineer should carry out a heat-loss calculation and confirm mains gas pressure and flow rate before specification.
Key decisions
- Output sizing — Match heating kW to calculated heat loss and hot-water kW to required flow rate and temperature rise. You may need only 12 kW for heating but 31 kW or more for hot water. See sizing & efficiency.
- Storage combi vs standard combi — If multiple outlets run at once, consider a storage combi or a system boiler with a cylinder.
- Efficiency rating — Choose a current ErP A-rated condensing model. Modulating boilers can turn output down when demand is lower.
- Controls — Modulating room stats (OpenTherm where supported) help the boiler run at the lowest flow temperature needed, cutting gas use.
- Warranty & service — Register the warranty and plan annual servicing. See BoilerService.com for a professional checklist.
Recommended products for maximum life
For a combi installation to last, we recommend the following (or equivalent products):
- Controls — Direct modulating controls where possible; otherwise smart stats such as Tado or Drayton Wiser. OpenTherm controls save further when the boiler supports them. See OpenTherm boilers and controls on MyBoiler Hub.
- Central heating filter — e.g. Fernox Omega TF1 or Spirotech MB3.
- De-aerator — e.g. Spirovent RV2 or IMI Zeparo deaerator.
- DHW expansion / shock arrestor — e.g. Caleffi mechanical shock arrestor on the hot-water circuit.
- Limescale prevention — Water softener preferred; at minimum an inline magnetic limescale filter in hard-water areas.
Installation best practice
A well-designed pipework layout with isolation, flushing points, filter, and de-aerator minimises corrosion and makes future maintenance straightforward. See our detailed installation best practice guide for the recommended layout and system-preparation steps. Some boiler models need specific bypass or hydraulic arrangements — always follow the manufacturer's installation instructions and Building Regulations.
If an important component fails on a combi, you can be without both heating and hot water. Fitting an electric shower provides a hot-water backup for bathing. Keep the installation manual and service record to hand.
Further reading on MyBoiler Hub
Content adapted from guides on Hub.MyBoiler.com. The original myboiler.com/choosing-a-new-combi-boiler/ URL redirected to the hub; no standalone page existed in the myboiler.com repository.