Most installers will recommend oversizing your boiler. Finding one who'll support your efficiency-focused choice makes all the difference.
When I started getting quotes for my boiler replacement, the pattern was clear: every installer recommended a minimum of 32kW, with most pushing for 36kW or even 40kW. Their reasoning made business sense—bigger boilers mean no callbacks about cold houses or weak showers.
But I'd done the research. I knew a 26kW Vaillant 826 would be adequate for my 4-bed detached house. The challenge wasn't proving the math—it was finding an installer who'd support that choice without disclaimers or "I told you so" warnings.
This article shares what I learned about finding collaborative installers who prioritize long-term efficiency over short-term business safety.
The Industry Standard: Why Installers Oversize
Before we judge installers for oversizing, we need to understand their business reality. They're not being malicious—they're managing risk.
The Business Logic
Installers optimize for avoiding callbacks, not maximizing efficiency:
- Oversized boiler: Guaranteed warm house, happy customer, no complaints
- Right-sized boiler: Potential "not hot enough" calls, time and money lost
- Callbacks cost: Time, fuel, reputation—all eat into profit margins
- Invisible inefficiency: Customers notice weak showers immediately; higher gas bills show up gradually over years
Can you fault them for this logic? Return visits cost time and money. It's easier and safer to install a larger boiler than risk a callback.
The Technical Default
Beyond business logic, there's technical conservatism:
- Boiler sizing calculators often include generous "safety margins"
- "Better too big than too small" is industry conventional wisdom
- Detailed heat loss calculations are time-consuming
- Recommending a larger model is the easier, faster approach
Why This Wastes Your Money
The installer's "safe bet" approach protects their business but costs you:
- Higher upfront cost: Larger boilers cost more to purchase
- Poor modulation: Oversized boilers cycle on/off in mild weather instead of running continuously at low output
- Wasted energy: Cycling wastes energy reheating the heat exchanger
- Years of inefficiency: A boiler lasts 10-15+ years—that's a decade of higher bills
The Conflict
Here's the fundamental mismatch:
- Their interest: No callbacks, happy customers, protect reputation
- Your interest: Long-term efficiency, optimal modulation, lower running costs
- The gap: What's safe for them vs. what's optimal for you
- The solution: Find an installer who'll support YOUR approach
Industry convention vs. my reality: Convention said 32-36kW for a 4-bed detached. My old 16-22kW Potterton handled the house for decades. Data from 2 years with the 26kW Vaillant 826 proves every installer who recommended bigger was wrong for my situation.
What I Was Looking For
I wasn't looking for the cheapest quote or the fastest installation. I needed specific characteristics in an installer.
1. Willing to Support 826 (26kW) Without Risk Disclaimers
Not "at your own risk." Not "I told you so" warnings. Genuine support for a right-sized choice with confidence in the approach.
2. Knowledge About Smart Controls
- Understanding of weather compensation
- Experience with OEM controls vs. third-party thermostats
- Appreciation for efficiency optimization
- Not just "install and forget" mentality
3. Vaillant Experience and Warranty Extension
- Familiar with Vaillant products specifically
- Part of Vaillant approved installer scheme
- Could offer warranty extension (7+ years vs. 2-5 standard)
- Quality installation matters for long-term warranty coverage
4. Collaborative Approach
- Listen to my research and reasoning
- Explain their perspective clearly
- Discussion, not dictation
- Respect for homeowner research
5. Fair Pricing and Transparency
- Clear quote breakdown
- Transparent about what's included
- Honest about lead times and costs
- Integrity around pricing (not maximizing profit on every component)
Why These Criteria Mattered
A boiler lasts 10-15+ years. This decision affects efficiency for the entire lifespan of the system. The installer relationship matters for warranty work and potential future service. The wrong installer means fighting against my choice for years. The right installer means collaborative optimization.
What I Was NOT Looking For
- Cheapest quote: Price wasn't the deciding factor
- Fastest installation: Quality over speed
- "Yes man" who'd do anything: Wanted informed support, not blind compliance
- Someone who'd override my research: Collaboration, not dictation
How I Found Candidates
My Approach
Step 1: Contacted Vaillant Directly
I wanted a Vaillant boiler specifically for the OEM controls and warranty benefits. I contacted Vaillant and asked for local installer recommendations. They connected me with approved installers in my area who were pre-screened for Vaillant experience.
Step 2: Evaluated Multiple Quotes
About three installers came for quotes. All provided different recommendations ranging from 826 (26kW) to 840 (40kW). I listened to their reasoning for each recommendation.
Step 3: Assessed Their Receptiveness
How did they respond when I mentioned my preference for the 826? The range was telling:
- Gentle caution with willingness to discuss
- Strong opposition without discussion
- Interest in my research and reasoning
- Willingness to discuss modulation ranges and efficiency
Alternative Approaches You Could Use
Gas Safe Register:
- Official register of qualified engineers
- Search by postcode
- Filter by manufacturer experience
- Check qualifications and certifications
Manufacturer Websites:
- Most major brands have installer locators
- Pre-screened for product training
- Often part of warranty extension schemes
- Direct connection to manufacturer support
Personal Recommendations:
- Ask neighbours who recently had boilers installed
- Local Facebook groups
- Nextdoor community
- Real experience from people in similar homes
Trade Association Memberships:
- CIPHE (Chartered Institute of Plumbing and Heating Engineering)
- APHC (Association of Plumbing & Heating Contractors)
- Membership shows professional commitment
- Complaint resolution processes available
What NOT to Do
- Don't just pick the cheapest quote
- Don't ignore bad vibes about collaboration
- Don't accept "trust me, I'm the expert" without discussion
- Don't hire someone who won't explain their reasoning
The Father/Son Team: Why They Won
One installer stood out from all the others. Here's what made them different:
1. Didn't Push Back on the 826
Other installers: "Very strongly suggesting going bigger" (32-36kW). This team: No pushback, no risk disclaimers, no "I told you so" warnings. They were confident in the 826 for my situation.
2. Smart Controls Knowledge
The son was particularly knowledgeable about smart controls—weather compensation, OEM controls importance, app integration. He understood and appreciated the optimization strategy. This aligned expertise sealed the decision.
3. Vaillant Approved Installer Status
They were part of Vaillant's approved installer scheme, offering a 7-year warranty extension (from the standard 2-5 years). This eventually extended to 10 years through a Vaillant program. Manufacturer confidence in their work speaks volumes.
4. Fair Pricing and Integrity
- £3,180 supply + install (31 Jan 2024)
- £200 discount for self-sourcing controls (gateway, H clip, outdoor sensor)
- Transparent about the discount
- Honest about what I could source cheaper myself
- Not trying to maximize profit on every component
5. Taking Time for Quality
Installation took 2 days (Jan 16-19, 2024). They discovered a gas pipe routing issue during installation and took time to handle it properly. No corners cut—quality over speed.
6. Installation Integrity Example
They'd bought the boiler before a price increase. They could have delayed and charged me more. Instead, they honoured the original pricing despite market changes. Integrity matters more than maximizing profit. This builds trust.
Two Years Later
- Zero issues with installation
- System performing exactly as hoped
- 826 choice vindicated by data (21% heating efficiency improvement)
- Their support enabled the optimal outcome
- Would recommend them to others
Questions to Ask Potential Installers
These questions will help you identify installers who'll support efficiency-focused decisions.
Before You Even Get Quotes
1. "Are you familiar with [Brand] boilers?"
- Gauge their experience with your preferred brand
- Ask about recent installations
- Warranty extension schemes they're part of
- Training and certifications specific to brand
2. "What's your approach to boiler sizing?"
- Red flag: "We always install X size for homes like yours"
- Green flag: "Let's discuss your specific situation"
- Listen for: heat loss calculations, modulation discussion, safety margins
3. "Do you install OEM controls or recommend third-party?"
- Tests their understanding of control limitations
- Red flag: "Nest/Hive works with everything"
- Green flag: "OEM controls preserve boiler capabilities"
During Quote Visits
4. "What size boiler do you recommend and why?"
- Listen to their reasoning
- Ask about modulation ranges
- Discuss old system performance as baseline
- Test receptiveness to your research
5. "I'm considering [smaller size]. What are your thoughts?"
This is the critical question that reveals their flexibility:
- Red flag: Immediate strong opposition without discussion
- Green flag: "Let's discuss your reasoning"
- Pay attention: Are they listening or dictating?
6. "How do you handle situations where you disagree with the client?"
- Reveals their collaboration style
- Red flag: "I'm the expert, trust me"
- Green flag: "We discuss concerns and find solutions together"
7. "What warranty do you offer on installation? Any extensions available?"
- Standard: 1-2 years on labour
- Manufacturer: 2-5 years on parts (varies)
- Extension schemes: 7-10 years possible with approved installers
- Shows quality confidence and manufacturer relationships
8. "Can you walk me through your installation process and timeline?"
- Tests their transparency
- Reveals their attention to detail
- Shows realistic expectations
- Do they plan properly or wing it?
9. "What's included in your quote? What's extra?"
- Flush and inhibitor
- Upgrades to pipes/radiators if needed
- Controls installation
- Disposal of old boiler
- Any building work (cutting/patching)
10. "What happens if issues arise during installation?"
- Tests their problem-solving approach
- Do they absorb issues or charge extras?
- Shows planning thoroughness
- Identifies communication style under pressure
11. "Can you provide references from recent similar installations?"
- Particularly: Similar boiler model and size
- Homes with similar characteristics
- Ask for contact information
- Follow up: Actually call those references
After Installation Questions
12. "What ongoing maintenance do you recommend?"
- Annual servicing expectations
- DIY maintenance tasks
- When to call for help
- Optimization support offered?
13. "Do you offer servicing or should I use someone else?"
- Some installers offer ongoing relationships
- Others prefer to install and hand off
- Knowing upfront sets expectations
The Warranty Extension: Why It Matters
Standard Warranty Landscape
- Basic manufacturer warranty: 1-2 years parts
- Extended manufacturer warranty: 2-5 years with registration
- Labour warranty: 1-2 years from installer typically
- Total coverage: 2-5 years standard
Approved Installer Schemes
Manufacturers like Vaillant offer approved installer schemes:
- Vaillant Advance: 7-10 year warranty extensions
- Requirements: Training, certification, quality standards
- Benefit to installer: Marketing advantage, support access
- Benefit to homeowner: Extended coverage, quality confidence
Our Experience
- Started with: 7-year warranty extension
- Eventually: Extended to 10 years via Vaillant program
- Coverage: Parts and labour through approved installer
- Value: Peace of mind, reduced long-term risk
Why This Indicates Quality
- Manufacturer confidence in installer
- Installer confidence in their work
- Lower callback risk (trained properly)
- Access to manufacturer support
- Continuing education requirements
The Financial Value
- Boiler repair costs: £200-500+ typical
- Extended warranty: Covered for 7-10 years
- Risk reduction: Significant over boiler lifetime
- Worth considering in installer selection
Red Flags: Installers to Avoid
Immediate Disqualifiers
1. Not Gas Safe Registered
Legal requirement in UK. Check their registration number. Verify on Gas Safe Register website. NO EXCEPTIONS.
2. Won't Provide Written Quote
Professional installers always provide written quotes. Verbal estimates aren't binding. Written quotes protect both parties.
3. Require Full Payment Upfront
Deposit reasonable (10-30% typical). Full payment before work: RED FLAG. Payment on completion is standard.
4. Pressure Tactics
- "Discount expires today"
- "I have materials in van right now"
- Pushing for immediate decision
- Making you feel stupid for hesitating
Concerning Behaviours
5. Dismissive of Your Questions
- "Don't worry about technical details"
- "Just trust me, I'm the expert"
- Annoyed by your research
- Won't explain their reasoning
6. Can't Provide Insurance Documentation
- Public liability insurance essential
- Professional indemnity coverage
- Should provide proof readily
7. No Fixed Address/Website
- Van and mobile phone only
- No business address
- No online presence
- Can't be traced if problems arise
8. Quotes Wildly Different From Others
- Much cheaper: Cutting corners or inexperienced?
- Much more expensive: Overcharging or gold-plating?
- Outliers deserve explanation
9. Badmouths Other Installers
Professional respect matters. "Everyone else is incompetent" is a red flag. How they talk about competitors reveals how they'll talk about you.
10. Vague About Timeline or Scope
- Can't give clear installation timeline
- Unclear about what's included
- "We'll sort it out as we go"
- Lack of planning = problems during installation
Green Flags: Installers to Pursue
✅ Asks YOU Questions
- About your hot water usage patterns
- About home insulation status
- About your heating habits
- About your long-term plans
- Shows: Tailoring solution to YOUR needs
✅ Discusses Trade-Offs
- Explains pros/cons of different options
- Doesn't push one solution
- Helps you understand implications
- Empowers informed decision-making
✅ Transparent About Limitations
- "I haven't installed that model before"
- "That's beyond my expertise, you'd need X"
- Honesty about what they can/can't do
- Better than overconfidence
✅ Takes Time for Site Survey
- Proper survey, not rushed quote visit
- Measures and notes details
- Identifies potential issues upfront
- Shows: Thorough planning approach
✅ Explains Reasoning Clearly
- Why they recommend specific model
- How they calculated sizing
- What they considered in recommendation
- Shows: Thoughtful approach, not cookie-cutter
✅ Provides Detailed Written Quote
- Itemised breakdown
- Clear scope of work
- Exclusions noted
- Payment terms clear
- Timeline specified
✅ Happy to Provide References
- Recent installations readily shared
- Encourages you to contact previous clients
- Confidence in their work
- Nothing to hide
What to Do If No One Supports Your Choice
If multiple installers are pushing back on your preferred boiler size, it's time for a reality check.
Option 1: Reassess Your Research
- Are multiple professionals raising the same concerns?
- Is your reasoning sound or wishful thinking?
- Any blind spots in your analysis?
- Consult online forums (CrossThread, etc.)
- Consider: Maybe they're right?
Option 2: Expand Your Search
- 3 installers isn't exhaustive
- Try different geographic area
- Manufacturer forums for installer recommendations
- Online communities for regional referrals
- Keep looking for collaborative match
Option 3: Compromise on Size
- Maybe 832 instead of 826?
- Smaller than they recommend, bigger than you wanted
- Still better modulation than 840
- Meets them partway
Option 4: Accept "At Your Own Risk"
- Some installers will install your choice with disclaimer
- Not ideal, but possible
- Ensure warranty not affected
- Document your decision in writing
- Be prepared to own the outcome
What I Would Have Done
If no one supported the 826, I would have:
- Reassessed my reasoning
- Consulted online communities for validation
- Expanded search to neighbouring areas
- As last resort: accepted disclaimer with installer I trusted otherwise
The Humility Balance
- Respect your research AND professional experience
- Don't be so stubborn you ignore universal concerns
- Don't be so deferential you accept poor efficiency
- Sweet spot: Collaboration with informed support
My Installation Team Two Years Later
The Outcome
Performance:
- ✅ 826 works perfectly
- ✅ Zero reliability issues
- ✅ 21% heating efficiency improvement (weather-normalized)
- ✅ Every installer who recommended bigger: wrong about my situation
- ✅ Their support enabled optimal outcome
The Relationship:
- Supportive throughout installation
- Quality work evident in performance
- Warranty extension provides peace of mind
- Would recommend to others researching Vaillant
What Made It Work:
- Their willingness to support unconventional choice
- My research and clear reasoning
- Collaborative approach from both sides
- Shared goal: Long-term efficiency, not just warm house
The Validation
- Two years of data proves 826 adequate
- Mild weather modulation superior to larger models
- DHW performance fine for our needs
- No regrets about installer choice
- No regrets about boiler size
The right installer doesn't just install equipment—they enable optimal outcomes. They support informed decisions. They bring expertise AND flexibility. Finding them is worth the effort.