If you're replacing a boiler, the first decision is bigger than “Worcester or Baxi?” — it's which type of boiler. Combi, system or regular (also called “heat-only” or “conventional”).
Pick wrong and you'll be stuck with weak shower flow, expensive bills or wasted loft space for the next 10–15 years. Here's exactly when each one wins, in plain English.
The 30-second version
- Combi — 1–3 bed homes, one bathroom, no loft tanks, mains-pressure hot water on demand. Cheapest to install. Most popular.
- System — 3–5 bed homes with 2+ bathrooms. Hot water comes from a stored cylinder (no loft tank). Brilliant flow at multiple taps simultaneously.
- Regular — large homes (5+ beds), older properties, low water mains pressure. Has a hot-water cylinder AND loft tanks. Most flexible but most expensive to maintain.
Combi boilers — by far the most common in 2025
About 80% of the boilers we fit in Lancashire are combis. They heat your radiators AND your hot water on demand, all from one wall-mounted unit. No tanks, no cylinders.
Pros:
- Cheapest to install (typically £1,550–£2,800 fitted)
- Frees up cupboard space (no cylinder), loft space (no cold-water tank)
- Mains-pressure hot water — no loft-tank gravity-fed dribble
- Hot water on demand — never “runs out”
- Smaller footprint — fits in a kitchen cupboard
- Most efficient (no stored hot water = no standing heat loss)
Cons:
- Flow rate drops if two showers run at the same time
- No backup if it breaks (no cylinder of hot water sitting there)
- Needs decent incoming mains water pressure (1.5+ bar) to perform well
- Can't pair with most solar thermal panels
Best for: 1–3 bed flats, terraces and semis with 1 bathroom (or 2 bathrooms used at different times). Most Burnley, Padiham and Nelson properties fit this profile.
System boilers — the larger-home choice
System boilers store hot water in an unvented cylinder (typically Megaflo, Telford or Heatrae Sadia), kept at 60°C and ready to go. The boiler itself is wall-mounted like a combi, but it doesn't make hot water — the cylinder does that.
Pros:
- Mains-pressure hot water at every tap — no loft tank
- Run multiple showers simultaneously without flow drop
- No cold-water tank in the loft (frees up space, no winter freezing)
- Compatible with solar thermal panels and heat pumps
- Very reliable — fewer moving parts than a combi
Cons:
- Need cupboard space for the cylinder (typically airing cupboard)
- If you run out of stored hot water, you wait 20–30 mins for reheat
- More expensive to install (£3,500–£5,500 fitted including unvented cylinder)
- Annual servicing slightly pricier (cylinder needs a G3 safety check)
Best for: 3–5 bed homes, two bathrooms used simultaneously, en-suites, and any household with multiple morning showers running at the same time.
⚠️ G3 qualification matters. Unvented cylinders MUST be installed by a G3-qualified engineer (it's a legal requirement). Every Gas Tech engineer is fully G3 qualified.
Regular (“heat-only” / “conventional”) boilers
The traditional setup: a boiler plus a hot-water cylinder in the airing cupboard, plus a cold-water tank in the loft, plus an expansion tank. The full Victorian works.
Pros:
- Best for very low mains pressure (gravity-fed showers still work)
- Multiple bathrooms can run simultaneously
- Cheapest like-for-like swap if you already have one (no system change)
- Pairs easily with solar thermal, biomass, and immersion backup
Cons:
- Most expensive to maintain (more components = more things to go wrong)
- Loft tank can freeze in winter, leak through ceilings, breed bacteria
- Slowest hot-water recovery (waiting for cylinder to reheat)
- Takes up the most space
- Lower efficiency than modern combis or systems
Best for: period properties, very large homes (5+ beds, 3+ bathrooms), homes with poor mains water pressure, or anyone replacing a regular boiler with another regular for cost reasons.
Decision table — by bedroom count
- 1 bed flat or terrace: Combi (Main Eco or Baxi 600 — £1,550–£1,900)
- 2 bed terrace/semi: Combi (Baxi 800 24kW or Worcester 2000 — £2,000–£2,400)
- 3 bed semi (1 bathroom): Combi (Baxi 800 30kW or Worcester 8000 30kW — £2,250–£2,800)
- 3 bed semi (2 bathrooms): System with unvented cylinder OR a high-output combi (36kW+) depending on demand — £2,800–£3,800
- 4 bed detached (2 bathrooms): System with 180–210L unvented cylinder — £3,500–£4,500
- 5+ bed (2+ bathrooms): System with 250–300L unvented cylinder — £4,000–£5,500
- Period property, low pressure, or 3+ bathrooms: Regular boiler with cylinder + tanks — quote per job
What about converting from one type to another?
Three popular conversions in Lancashire:
- Regular → Combi (most common): Free up loft and cupboard space, lower bills, cheaper maintenance. £2,800–£4,000 fitted.
- Regular → System with unvented cylinder: Keep the cylinder option but lose the loft tanks. £3,500–£4,800.
- Combi → System (rarer): Family expanding, second bathroom going in. £3,500–£5,000 depending on radiator changes.
Cost to run — annual energy comparison
For a typical 3-bed Lancashire home heated 6 months of the year:
- Modern combi: ~£950–£1,150/year (most efficient — no standing heat loss)
- Modern system + unvented: ~£1,000–£1,200/year (slight loss from cylinder)
- Regular boiler + cylinder + loft tank: ~£1,150–£1,400/year (more standing loss, lower efficiency)
Numbers based on 12,000 kWh annual usage at 6.5p/kWh (2025 Ofgem cap). Real figures depend on insulation, controls and household habits.
FAQs
Can I run two showers at once with a combi?
Usually yes if you fit a 36kW+ combi (Baxi 800 36, Worcester 8000 35), but flow will drop slightly when both run. For genuine simultaneous use without compromise, fit a system boiler with unvented cylinder.
What size combi do I need?
Roughly: 24kW for 1 bed, 28kW for 2 bed, 30kW for 3 bed, 36kW+ for 4 bed or bigger. Heating output and hot-water output are different specs — your installer should size both based on radiator count and bathroom count.
Will a combi work in my old house with low water pressure?
If your mains pressure is below 1 bar, a combi will struggle. Get a system boiler with unvented cylinder (you can fit a pump if needed) or stick with a regular setup.
Do I have to remove my hot-water cylinder if I switch to combi?
Usually yes — but it can sometimes stay as a heated towel-rail loop or be repurposed. We assess on-site.
Is a system boiler the same as a regular boiler?
No. A regular boiler needs a cold-water tank in the loft. A system boiler doesn't — the expansion is handled internally and water is fed from the mains. Big difference.
Are heat pumps better than gas boilers?
For new-build, well-insulated homes — yes, often. For older Lancashire stock with traditional radiators — usually no, not yet. We can advise on a heat-pump suitability survey if you're curious.
Not sure which to pick? We'll tell you straight
Send us your postcode, bedroom count, bathroom count, and current setup — we'll come back with a recommendation, a fixed-price quote, and an honest reason for our choice. No commission, no upsells.
Get a free fixed-price boiler quote → · or call 01282 914 044.