Reviewed by our senior heating engineer. Serving Harpenden (primary), St Albans, Borehamwood, Radlett, and Welwyn Garden City.
Picking the right boiler size isn’t about guessing a kW number—it’s about matching your home’s hot water demand, radiator load, insulation, and future plans. Use this practical checklist to avoid cold showers, short-cycling, and wasted energy—so your system feels effortless all year.
Even a powerful combi can’t overcome poor mains flow. A quick bucket test helps, but professional flow/pressure testing is most accurate.
Smart controls (Nest, Hive, Tado), weather/load compensation, and TRVs keep rooms comfortable and bills down. They won’t change peak demand but smooth day-to-day use.
Boiler Sizing Basics (What “kW” Really Means)
Boiler size is the heat output in kilowatts (kW). Combis show two numbers: central heating (CH) output and a higher domestic hot water (DHW) output. System and conventional boilers primarily focus on CH output because a cylinder stores hot water.- Combi boilers: Higher DHW kW delivers faster hot water to taps and showers (typical 24–40kW).
- System/Conventional: Size the CH output to your home’s heat loss and radiator load; the cylinder handles hot water volume and recovery.
Step 1: Count Bathrooms and Simultaneous Showers
Hot water demand drives sizing. One bathroom homes can often use smaller combis. Two or more simultaneous showers point to a system boiler + unvented cylinder.- 1 bathroom, 1 shower: Combi 24–30kW can work (subject to mains flow rate).
- 2+ bathrooms or overlapping showers: System boiler with 150–250L unvented cylinder.
- Large baths/rainfall heads: Favour higher flow or stored hot water.
Step 2: Test Your Mains Water Flow Rate
Even a powerful combi can’t overcome poor mains flow. A quick bucket test helps, but professional flow/pressure testing is most accurate.
- Target: 12–15 L/min at the kitchen tap for comfortable combi performance.
- Below ~10 L/min: Consider boosting or choose a system with storage.
Step 3: Count Radiators to Estimate Heating Load
Total radiator output (BTU/kW) should align with CH output. Avoid big oversizing; it causes short cycling and higher bills.- Rough guide: Small flat: 6–9 rads (12–15kW). 3‑bed semi: 9–12 rads (15–20kW). Larger homes may need 24kW+ CH.
- Pro tip: A quick heat-loss check beats guesswork and keeps rooms evenly warm.
Step 4: Factor Insulation and Windows
Better insulation = lower required output. Loft insulation, cavity walls, and double glazing can let you choose a smaller, cheaper-to-run boiler.- Do this first: Fix insulation where practical before replacing the boiler.
Step 5: Pick the Right Boiler Type
Type and size go together:- Combi: Compact choice for 1‑bath homes with good mains. Size by DHW demand.
- System: Family-friendly; cylinder stores hot water. Size by CH load, not taps.
- Conventional (Regular): Suits older pipework or low-pressure areas; uses cylinder + loft tank.
Step 6: Plan for Future Upgrades
Extensions, new bathrooms, or underfloor heating change demand. Allow a little headroom without excessive oversizing.
Step 7: Controls, TRVs, and Zoning
Smart controls (Nest, Hive, Tado), weather/load compensation, and TRVs keep rooms comfortable and bills down. They won’t change peak demand but smooth day-to-day use.
Step 8: Water Quality in Hertfordshire
Hard water in Harpenden, St Albans, Borehamwood, Radlett, and WGC can reduce efficiency. Scale reducers, magnetic filters, and water softeners protect your boiler and cylinder.
Typical Sizing Scenarios (Indicative)
- 1–2 bed flat, 1 bath, good mains: Combi 24–28kW DHW; CH often 18–24kW.
- 3‑bed semi, 1 bath + 1 ensuite: Larger combi 30–35kW DHW, or system boiler + 150–200L cylinder.
- 4‑bed detached, 2+ baths: System boiler sized to CH load (18–24kW typical) + 200–250L unvented cylinder.
- Older property, low mains pressure: Conventional/system with storage and possible boosting.
Common Sizing Mistakes to Avoid
- Oversizing “just in case”: Causes short cycling and higher bills.
- Ignoring mains flow: Big combi + poor flow = disappointing showers.
- Underestimating hot water demand: Two teens = two simultaneous showers; size accordingly.
- Skipping a survey: A quick heat-loss/radiator audit pays for itself in comfort.
Mini Case Study (St Albans)
A 3‑bed semi with 11 L/min flow and 10 radiators wanted stronger morning performance. We specified a 200L unvented cylinder with a 19kW system boiler. Result: two comfortable showers at once and faster reheat—no queue for the bathroom.
FAQs (Hertfordshire)
How long does installation take?
Combi swaps: ~1 day. System/conventional: 1.5–2.5 days depending on pipework and cylinder siting.
Can you size a boiler in Harpenden and St Albans?
Yes—we cover Harpenden (primary), St Albans, Borehamwood, Radlett, and WGC.
Do you test mains flow on surveys?
Absolutely. We take accurate flow and pressure readings to guide the right choice.
Will smart controls reduce my bills?
Used well, yes—especially with weather/load compensation and TRVs.
Local Help, Right Size, First Time
We carry out accurate flow tests, radiator audits, and heat‑loss checks across Hertfordshire. You’ll get the right size and type for strong showers, cosy rooms, and lower bills—no guesswork.
Call to Action
Ready to size it properly? Book a free survey and quote today. We’ll measure your mains flow, total your radiator load, and recommend the ideal boiler and cylinder combo for your home.