My landing radiator went from 826 BTU to 6,800+ BTU. That's an 8× increase. Here's why.
This wasn't about fixing a broken heating system—it was about strategic oversizing for efficiency. The conventional wisdom is to size radiators to heat loss calculations and run at 70°C flow temperatures. I took a different approach: oversize radiators strategically to enable 50°C flow temperatures, maximizing condensing boiler efficiency.
The result: more than doubled total BTU capacity (30,376 → 68,000+ BTU), 21% heating efficiency improvement, and a system that enabled choosing a 26kW boiler instead of the 32-36kW installers recommended.
The Problem With Old Radiators
1. No TRVs (Thermostatic Radiator Valves)
Original radiators had NO TRVs at all. In a well-balanced system, all radiators come on together. The problem: Living room gets cold (draughty door), but kids' bedrooms upstairs don't need heating. When I turned heating on for downstairs, I'd waste energy heating upstairs unnecessarily.
My workaround: I deliberately restricted upstairs radiators so they'd take a long time to heat up. This gave me some control, but it was still wasteful—just less wasteful than fully heating all rooms at once.
2. No Convector Fins
Old radiators got hot but didn't heat rooms effectively. They lacked convector fins to help distribute heat. Old T1/T2 types = single/double panel only with NO convector fins. This is why they "got hot but didn't heat rooms"—poor heat distribution into living space.
3. Severely Undersized
- Landing: Tiny 54cm wide radiator = 242W (826 BTU)
- Most rooms: 135cm wide = 601W (2,051 BTU)
- Total old system: 8,900W / 30,376 BTU
They handled heating, but not efficiently.
4. Truwel Pipework Limitations
House built in 1970 during the copper shortage. Extensive Truwel pipework throughout the system really limited boiler replacement options—couldn't safely do sealed system conversion with Truwel, fear of leaks when upgrading.
The Strategic Decision: Oversizing for Efficiency
The Philosophy
- Larger radiators = lower flow temps required
- Lower flow temps = better boiler efficiency (condensing operation)
- Better efficiency = lower gas bills over time
- Return on investment through ongoing savings
Traditional vs My Approach
Traditional approach:
- Size radiators to heat loss calculations
- Run at 70°C flow temperature (standard)
- Boiler modulates to meet demand
- Adequate heating, conventional efficiency
My approach:
- Target flow temperature: 50°C (vs traditional 70°C)
- Strategic oversizing to compensate for lower flow temps
- Enable maximum condensing boiler efficiency
- Works with weather compensation optimization
- Foundation for right-sized boiler (826/26kW)
Why 50°C Matters
Condensing boilers are most efficient below 55°C flow temperature. They extract heat from water vapor in flue gases. Above 55°C, less condensing = lower efficiency. Traditional 70°C operation wastes the condensing potential.
The math: Same room heat requirement, but lower flow temp needs more radiator surface area. Larger radiators at 50°C deliver same heat as smaller radiators at 70°C, but the boiler runs more efficiently at 50°C.
How This Enables Boiler Efficiency
The 826 with excellent modulation (down to 2.5kW) works better with lower flow temps. Oversized radiators + lower flow temps = boiler runs continuously at low output. Continuous low output beats cycling on/off at high output for efficiency.
Weather compensation adjusts flow temp based on outdoor temp:
- Mild day: might only need 40°C flow temperature
- Cold day: might need 55°C flow temperature
- Oversized radiators work effectively across entire range
The Complete System
- Oversized radiators → enable low flow temps
- Low flow temps (50°C target) → maximize condensing efficiency
- Right-sized boiler (826 with 2.5kW minimum) → excellent modulation at low outputs
- Weather compensation → automatic optimization
- Result: System designed for efficiency, not just heat output
Why Installers Don't Do This
- Oversized radiators = higher upfront cost
- No immediate benefit to installer
- Customer won't see efficiency gains for months/years
- Heat loss calculations simpler with conventional sizing
- Risk of radiators looking "too big" in rooms
- Easier to stick with traditional approach
How I Actually Sized the Radiators
The plumber wanted to do a formal heat loss calculation—detailed room measurements, door openings, window openings, ceiling heights, wall construction, insulation levels. Comprehensive but time-consuming.
My Pragmatic Approach
I had architect drawings with some room dimensions. I measured all existing radiators, looked up specifications, and calculated existing BTU output. My logic: "Existing radiators DO get the house warm, so if I increase BTUs I ought to be fine."
The method:
- Measured all existing radiators (width × height)
- Looked up specifications for each type (T1, T2)
- Calculated existing BTU output per room
- Compared against proposed new radiators
- Ensured new radiators had HIGHER BTU output
- Factor in oversizing for low flow temp operation
- Verify physically fits in room and doesn't block walkways
Why this worked: Existing system DID heat the house (just not efficiently). New radiators had higher BTU output + convector fins. Oversizing strategy aligned with weather compensation goals. I avoided analysis paralysis from formal heat loss calculations. Good enough > perfect for this application.
The Economics That Changed Everything
The Screwfix "Great Deal"
- 5× T22 500×1200mm radiators
- £184.95 total (£36.99 each)
- Including FREE TRV and lockshield valves (worth £12.64 each, £63.20 total value)
- 6169 BTU each
- 166.77 BTU per £ - exceptional value
Additional Screwfix purchase: 1× T22 500×500mm for son's bedroom at £53.38 including free valves (2571 BTU).
Screwfix total: 6 radiators for £238.33, ALL with free valves. User's quote: "Can't overstate how cheap these were."
Toolstation Kudox Premium
- 4 radiators for £393.14 (radiators only)
- Plus 4× valve sets @ £12.64 each = £50.56
- True total: £443.70
- 2× Type 11 (500×1800mm) - Master & Guest bedrooms
- 1× Type 21 (500×900mm) - Landing
- 1× Type 22 (400×1600mm) - Living room bay window
Grand total: £682.03 for 10 radiators fully equipped (including valves)
The Game Changer
This deal made the entire strategy viable. Oversizing became economically sensible. Could afford Type 22 (highest output) for main rooms instead of compromising. Without this deal, might have needed to use Type 11 or Type 21 instead, limiting the low flow temperature strategy effectiveness and resulting in higher ongoing gas bills over 15+ year lifespan.
Room-by-Room Sizing Decisions
Here are some highlights from the 10 radiators replaced:
Landing (Most Dramatic)
- Old: T1 54×53cm = 242W (826 BTU)
- New: T21 500×900mm ≈ 2,000W+ (3,532 BTU)
- Increase: 8× output 🏆
- Landing went from cold spot to properly heated
- Shows how severely undersized old radiators were
Living Room Bay Window
- Old: T2 177×39cm = 1,236W (4,217 BTU)
- New: T22 400×1600mm ≈ 2,500W+ (6,783 BTU)
- Increase: 2× output despite being half-height
- Strategic: Half-height for bay window clearance, but T22 maximizes BTU
Master Bedroom
- Old: T1 176×53cm = 777W (2,651 BTU)
- New: T11 500×1800mm ≈ 1,800W+ (4,998 BTU)
- Increase: 2.3× output even with slimmer T11
- Strategic: T11 (slimmer profile) crucial for walking space between bed and wall
- 1800mm wide maximizes BTU despite slimmer profile
Estimated total new system output: 20,000W+ / 68,000+ BTU
Overall system improvement: More than 2× total BTU capacity
Type Selection Strategy Explained
Understanding Radiator Types
Old types (what I replaced):
- T1 = Single panel only (NO convector fins)
- T2 = Double panel only (NO convector fins)
- Key problem: No convectors = poor heat distribution
New types (what I installed):
- T11 = Single panel, single convector (slimmest, ~60mm projection)
- T21 = Double panel, single convector (medium, ~80mm projection)
- T22 = Double panel, double convector (deepest, highest output, ~100mm projection)
- All have convector fins = dramatically better heat distribution
My Selection Strategy
Type 22 (Maximum BTU): Used wherever space allowed—office, daughters, sons, dining, living, hallway. 6× radiators total.
Type 21 (Balanced): Landing only—geometric constraints required balanced depth/output.
Type 11 (Slim Profile): Master and guest bedrooms only—need walking space between bed and wall. 1800mm width compensates for slimmer profile.
The Convector Difference
Old T1/T2 had NO convector fins—just flat panels. They radiated heat but poor convection (air circulation). New T11/T21/T22 all have convectors. Convector fins create airflow and distribute heat better. This is why old radiators "got hot but didn't heat rooms."
What Wasn't Replaced
Kitchen: T21 radiator under tiled floor. Would require lifting tiles—NOT in budget. Future plan: replace during eventual kitchen renovation.
Downstairs bathroom: Under tiled floor—left for future bathroom updates.
Upstairs bathroom: Radiator capped off during boiler installation. Connected to downstairs toilet + kitchen (couldn't isolate), Truwel pipework concern, wife wanted electric towel rail anyway.
The phased approach:
- Year 1 (2021): Loft insulation
- Year 2 (2022): Radiators + pipework (10 radiators)
- Year 3 (2024): Boiler upgrade
- Future: Kitchen + remaining radiators during kitchen renovation
Shows realistic home improvement timeline—not an all-at-once massive project. Strategic patience avoids redoing work.
The Results: Does Oversizing Work?
Heating Performance
- All rooms heat effectively at 50°C flow temperatures
- No cold spots (landing went from coldest to properly heated)
- Even heat distribution throughout house
- TRVs provide individual room control (game changer)
- Can heat downstairs without wasting energy on upstairs
- System responds well to weather compensation
Energy Efficiency Results
- Year 1 baseline (post-loft): 18,539 kWh/year
- After radiators + boiler: 10,115 kWh/year (most recent)
- Improvement: 21% reduction in heating energy (weather-normalized)
- Total improvement: 45.4% reduction (all improvements combined)
- £154/year savings from boiler upgrade alone (includes radiator synergy)
Flow Temperature Reality
Operating range: high 20s to low 30s°C (mild weather) up to low 50s°C (cold weather). Weather compensation automatically adjusts. Oversized radiators work effectively across entire range. Boiler runs continuously at low output (efficient). Minimal cycling (extends boiler lifespan).
The Validation
Pragmatic "bigger than old" approach worked. More than doubled BTU capacity. System runs at target 50°C flow temps. Real-world data confirms 21% heating efficiency improvement. No regrets about oversizing.
Lessons Learned: How to Size Radiators for Efficiency
1. Start with existing system performance: Measure all existing radiators, look up specs, calculate BTU output. If existing system heats adequately, ensure new radiators exceed existing BTU. Pragmatic baseline beats complex heat loss calculations.
2. Understand your efficiency strategy: Traditional = size to heat loss, run at 70°C. Efficiency = oversize radiators, run at 50°C. Lower flow temps = better condensing boiler efficiency.
3. Factor in convector fins: T11/T21/T22 all have convectors (better heat distribution). Old T1/T2 had NO convectors (poor distribution). Don't just compare BTU numbers—convectors matter.
4. Balance BTU with room function: Type 22 where space allows, Type 21 where depth matters, Type 11 where walkways constrained. Consider furniture placement and room use.
5. Look for deal opportunities: Multi-supplier strategy. Bulk purchase pricing can be exceptional (£36.99 vs £99.74 each). Free valves add significant value. Calculate BTU per £ to compare true value.
6. Accept budget constraints: Can't replace everything at once. Kitchen/bathroom under tiles left for future. Phased approach over multiple years. Strategic patience avoids redoing work.
7. Calculate system-wide impact: I went from 30,376 → 68,000+ BTU total (2× increase). This enabled 26kW boiler instead of 32-36kW. Right-sizing boiler with oversized radiators = efficiency.
8. Consider long-term ROI: Higher upfront cost for oversized radiators. Lower ongoing cost from efficient operation. 21% heating efficiency improvement. £154/year savings compounds over 15+ year lifespan = £2,310+ savings.
Conclusion
My landing radiator went from 826 BTU to 6,800+ BTU. That's an 8× increase.
This wasn't about fixing a broken heating system—it was about strategic oversizing for efficiency. The old radiators heated the house, but at 70°C flow temperatures and poor heat distribution. The new radiators heat the house at 50°C flow temperatures with excellent heat distribution.
The economics made it possible: Screwfix deal at £36.99 per radiator (including FREE valves), £682.03 total for 10 fully-equipped radiators, 166.77 BTU per £ for the main batch. Without this pricing, the strategy would have been different.
The system approach: Oversized radiators enable low flow temps, low flow temps enable boiler efficiency, 826/26kW boiler with excellent modulation, weather compensation optimization. Result: 21% heating efficiency improvement.
The pragmatic method: Measured existing radiators, ensured new radiators exceeded existing BTU, balanced BTU with room function, visual consistency over marginal BTU gains, budget constraints drove phased approach.
Would I do it again? Absolutely. The oversizing strategy works. The lower flow temperatures work. The energy data validates the approach. And the landing is no longer a cold spot.
Sometimes bigger IS better—when it's strategic oversizing for efficiency, not just raw heat output.