Hidden costs people miss when budgeting for a new boiler
Most people don’t object to paying for a new boiler. What frustrates them is when the final quote is hundreds of pounds higher than the number they had in their head.
Usually it happens after a quick Google search for things like “new boiler cost”, “boiler installation cost”, or “cost of installing a new boiler” — and then the real-world quote lands and it feels like a different planet. That gap is nearly always down to the bits people don’t see until an installer actually looks at the property.
The reason is rarely dishonesty. It’s usually because boiler installations uncover real-world issues that aren’t obvious until someone looks properly. We see the same surprises over and over again across London homes.
You’re not “just buying a boiler”.
You’re adapting your existing home to safely run a modern one.
1. Gas pipe upgrades (very common in London)
Modern boilers need more gas than older models. Many London properties — especially Victorian houses and older flats — still run on undersized gas pipes.
- Old 15mm or 22mm gas supplies can’t meet demand
- Boiler may work, but won’t run safely or efficiently
- Manufacturers can void warranties if this isn’t corrected
Typical extra cost: £100–£400 (more if floors or walls need opening)
2. Flue changes and plume management kits
A boiler flue isn’t just a pipe out of the wall. Its route, length, terminal position and plume direction all have to meet regulations.
- Vertical flues through roofs cost more than rear-wall exits
- Flats often need longer or shared flue routes
- Plume management kits are sometimes mandatory near windows
Typical extra cost: £120–£350+
3. Boiler relocation costs
Moving a boiler sounds simple. In reality, it’s one of the biggest labour drivers — and it’s one of the quickest ways a “cheap” new boiler installation cost turns into something very different.
- New gas, flow, return and condensate routes
- New flue hole or roof penetration
- Additional safety clearances
Common moves include kitchens → lofts, bedrooms → cupboards, or old airing cupboards → kitchens.
Typical extra cost: £300–£750
4. Condensate drain work (often overlooked)
Modern boilers produce acidic condensate that must drain correctly. Poor routing causes freezing, leaks and shutdowns.
- External runs may need insulation
- Long internal routes take labour
- Pumps may be required in some flats
Typical extra cost: £80–£250
5. System cleaning: chemical flush vs powerflush
Old pipework holds sludge. Installing a new boiler onto a dirty system is one of the fastest ways to shorten its lifespan.
- Chemical flush: lighter contamination
- Powerflush: heavy sludge, cold radiators, old systems
Typical extra cost:
Chemical flush: £120–£250
Powerflush: £300–£750
This is one of the most common “costs you miss” because it isn’t glamorous, but it protects the boiler long-term. If you want more examples like this, see costs you miss when getting a new boiler.
6. Magnetic filters and inhibitors
Many boilers now require a magnetic filter for warranty compliance. It protects the heat exchanger from metal debris.
- Usually installed on the return pipe
- Needs annual servicing
Typical extra cost: £120–£180
7. Controls and thermostat upgrades
Basic controls are often included. Better control equals better efficiency and comfort.
- Smart thermostats improve modulation
- TRVs help zoning and fuel savings
Typical extra cost:
Thermostat: £100–£250
TRVs: £35–£80 per radiator
8. Making good (honest truth)
Installers are heating engineers, not builders. Some making good is included — some isn’t.
- Basic pipe boxing may be included
- Decorative plastering or tiling often isn’t
A good installer will tell you this upfront, not afterwards.
9. Parking, access and time on site (London reality)
Congestion, parking permits, stair-only access and restricted working hours all affect labour time.
It’s rarely listed as a “fee” — it simply makes the job take longer, which pushes up boiler installation cost in London compared to simpler areas.
How to avoid surprises
- Get a fixed-price quote after photos or a survey
- Ask what’s included — and what isn’t
- Confirm gas pipe size, flue route and cleaning method
If you’re trying to get close to a realistic average cost of a new boiler for your property, you need the quote to be based on your layout, not “best case assumptions”. That’s also where boiler replacement cost UK numbers get messy — because “replacement” can mean anything from a straight swap to a relocation with upgrades.
Most “hidden costs” aren’t hidden at all — they’re just things comparison sites don’t explain.