College Hill Combi Boiler Installation: 1948 Home Integrated Heating and Hot Water

Case study summary: 1948 two-story home in Webster Groves’ College Hill neighborhood with end-of-life cast-iron boiler (1995-vintage) AND end-of-life gas water heater (2013-vintage) both requiring replacement within similar timeframe; replaced both with single Navien NCB-240 combi boiler providing heating to existing cast-iron baseboard radiators plus domestic hot water on-demand; preserved hydronic distribution; eliminated separate water heater installation; total project cost $11,400; completed in 3 days during November 2025. Outcome: consolidated equipment to single unit; eliminated standby losses from storage water heater; freed mechanical room space; ongoing reliability with modern equipment under unified warranty; customer satisfaction high with integrated approach.

The Home

• Built: 1948
• Style: Two-story Cape Cod with finished basement
• Location: College Hill neighborhood, Webster Groves, MO 63119
• Size: 1,880 sf across two levels plus 800 sf finished basement (2,680 sf total conditioned)
• Construction: Wood frame with brick veneer; plaster walls throughout main and second floors; finished basement with drywall walls (1990s finish work); original wood floors throughout main levels
• Existing heating: Burnham Series 2 cast-iron sectional boiler (110,000 BTU/hr 82% AFUE from 1995, 30 years old at time of replacement); chimney-vented atmospheric draft
• Existing water heater: Rheem 50-gallon natural gas water heater (38,000 BTU/hr input, 60-gallon first-hour rating, 0.62 UEF efficiency from 2013, 12 years old; standard atmospheric draft venting through same chimney as boiler)
• Hydronic distribution: cast-iron baseboard radiators throughout main and second floors; finned-tube baseboard in finished basement (1990s addition); black iron piping with copper extensions in basement zone; 3/4-inch supply and return loops
• Existing cooling: separate forced-air central AC system added 2017 (Lennox 14ACX 2.5-ton with separate air handler); not affected by boiler replacement
• Domestic hot water demand: 3-person household with typical fixture configuration (3 bathrooms, kitchen, laundry); peak demand approximately 5-6 gallons per minute during morning shower/breakfast period

The Situation

Customer contacted us in September 2025 about combined equipment replacement planning. Drivers:

• Boiler age: 30-year-old cast-iron boiler approaching end of typical 30-40 year service life; recent expansion tank replacement (2024) signaled approaching end
• Water heater age: 12-year-old water heater approaching typical 12-15 year service life; sediment buildup in tank reducing efficiency; minor water leak at cold inlet valve diagnosed during 2025 service visit
• Combined equipment opportunity: both pieces of equipment due for replacement created opportunity for integrated combi boiler approach
• Mechanical room constraints: separate boiler and water heater occupied substantial mechanical room space; consolidation would free space
• Chimney venting concerns: shared chimney venting for boiler and water heater has known issues (atmospheric draft systems can backdraft, reducing efficiency and creating CO risk); modern sealed combustion equipment eliminates this concern

Initial assessment recommended combi boiler approach:

• Manual J heating load 76,000 BTU/hr – well within combi boiler capacity range
• Peak hot water demand 5-6 GPM – matches combi boiler high-output capacity
• Existing hydronic distribution in good condition with no modifications needed
• Existing domestic hot water plumbing accessible at water heater location for new combi boiler integration

The Equipment Selection

Navien NCB-240 Combi Boiler (Selected)

• AFUE (heating): 95%
• UEF (water heating): 0.95
• Capacity heating: 110,000 BTU/hr input maximum heating; modulating 11,000-110,000 BTU/hr (10:1 turndown)
• Capacity hot water: 199,000 BTU/hr input maximum domestic hot water; provides 6.2 GPM at 77°F temperature rise (typical St. Louis groundwater temperature 55°F to 132°F output)
• Technology: Stainless steel heat exchanger condensing design; sealed combustion PVC venting through exterior wall; integrated controls managing both heating and DHW priority
• DHW priority operation: when domestic hot water demand activates, unit prioritizes DHW production temporarily suspending heating circulation; this is appropriate because DHW demand is typically short (5-30 minutes) and heating thermal mass in radiator system handles brief interruption without comfort impact
• Warranty: 15-year heat exchanger, 5-year parts
• Why selected: Navien NCB-240 is established residential combi boiler with strong dealer support in St. Louis area; capacity well-matched to home’s combined heating and hot water demand; excellent turndown ratio for modulating heating operation

Caleffi 521 Mixing Valve and Supporting Components

• Mixing valve, modern circulator pump, expansion tank, air separator as with other hydronic case studies; protect condensing operation and modernize hydronic loop

Alternative Equipment Considered

• Weil-McLain Ultra 155 plus separate heat pump water heater: separate boiler and water heater approach with electric heat pump water heater; would have provided slight efficiency improvement for water heating but at substantially higher equipment cost ($3,400 boiler + $2,200 heat pump water heater + $1,800 electrical work for heat pump water heater circuit = $7,400 in equipment vs. $4,200 for Navien combi)
• Weil-McLain Ultra Series Plus tank-style indirect water heater: traditional approach using boiler heat to make domestic hot water with separate storage tank; provides better hot water capacity at sustained demand but at higher cost and complexity
• Cast-iron boiler replacement plus tankless water heater: separate equipment approach; cheaper individual equipment but lacks integration benefits

The Project Scope

Pre-Installation

• Webster Groves HPC review: Submitted for new sealed combustion through-wall venting on side-yard exterior wall (non-visible from street); approval received in 21 days
• Webster Groves Public Works permit: Pulled within 5 business days of HPC approval; both mechanical and plumbing permits required
• Equipment ordering: 10 business days for Navien NCB-240

Installation Day 1 (November 12, 2025)

• System drain-down: hydronic system drained (34 gallons); water heater drained
• Existing boiler removal: 460 lb cast-iron sectional removed; chimney venting disconnected
• Existing water heater removal
• Chimney evaluation: brick chimney top to be sealed (no longer used for any venting); chimney to remain as architectural feature
• End of Day 1: old equipment removed, new equipment ordered/staged

Installation Day 2 (November 13, 2025)

• Navien NCB-240 placement on wall-mount bracket; substantially smaller than predecessor equipment combined (Navien NCB-240 vs. 460 lb boiler + 50-gallon water heater)
• Sealed combustion PVC venting installation through side-yard exterior wall
• Gas line modifications: existing gas service adequate, minor reconfiguration at new equipment location
• Hydronic piping connections: new copper supply/return to existing distribution; Caleffi mixing valve, new circulator, expansion tank, air separator
• Domestic hot water piping connections: cold water supply from existing line; hot water output to existing distribution piping (reused water heater outlet plumbing)
• Electrical: existing electrical circuit for boiler/water heater reused (new equipment uses single 120V circuit replacing previous separate circuits)
• End of Day 2: equipment installed but not commissioned

Installation Day 3 (November 14, 2025)

• System fill: hydronic loop filled and pressurized; air bled from system at high points
• Domestic hot water system commissioning: cold inlet pressure verified, hot output temperature setting configured (120°F default), flow rate verified at multiple fixtures
• Heating system commissioning: combustion analysis (CO 16 ppm air-free, CO2 9.4%, O2 5.8%)
• Outdoor reset configuration for heating loop
• System balancing
• Customer walk-through: explained DHW priority operation, expected operation patterns, maintenance schedule
• Chimney sealing at top with stainless steel cap (sealing unused chimney for moisture protection)
• Final cleanup and customer sign-off

Inspection (November 21, 2025)

• Webster Groves Public Works inspector visit; installation passed both mechanical and plumbing inspection

The Pricing

• Navien NCB-240 combi boiler: $4,200 equipment
• Caleffi 521 mixing valve: $380
• Modern circulator pump, expansion tank, air separator: $480
• Installation labor (3 days, 2 technicians plus hydronic specialist): $3,800
• Hydronic and domestic water piping modifications: $480
• Sealed combustion PVC venting installation: $580
• Webster Groves Public Works permits (mechanical + plumbing): $440
• Webster Groves HPC review fee: $180
• Old equipment removal and disposal (460 lb boiler + 50-gal water heater): $480
• Chimney cap installation (sealing unused chimney): $240
• System drain-down and refill: $180
• Bacharach combustion analysis and commissioning: $240
• Domestic hot water commissioning (flow rate verification, temperature setting): $80
• Exterior siding repair at new venting penetration: $240
• Interior drywall and paint patching: $80
• Miscellaneous fittings, hardware, and consumables: $300
• Total project cost: $11,400

Combined-equipment economics: $11,400 for combi boiler vs. estimated $13,800 for separate boiler ($9,200) plus water heater replacement ($4,600); combined approach saved approximately $2,400 through equipment consolidation and shared installation labor.

The Outcome

Heating Performance

• First-winter operation (November 2025 – March 2026): System maintained setpoint reliably with comfortable distribution to all radiators
• Modulation benefits: equipment operating at 20-40% capacity during typical winter conditions matches actual demand better than previous fixed-capacity equipment
• DHW priority operation impact on heating: brief heating circulation pauses during DHW demand have no noticeable impact on radiator surface temperatures due to thermal mass of cast-iron and finned-tube baseboards

Domestic Hot Water Performance

• Capacity: 6.2 GPM provides adequate hot water for typical 3-person household demand including simultaneous shower and kitchen sink operation
• Endless hot water: tankless operation eliminates the storage tank limitation; sustained demand (e.g., back-to-back showers) produces hot water continuously rather than running out
• Initial response time: 3-8 second delay between fixture activation and hot water arrival at fixture (similar to traditional tank water heater pipe delivery time)
• No standby losses: tankless operation eliminates 50-gallon tank standby heat loss that consumed approximately 1.5-2 therms/day with previous water heater

Energy Performance

• First-winter combined gas consumption (heating + DHW): approximately 1,180 therms vs. previous winter (heating only 1,420 therms plus DHW 540 therms = 1,960 therms total) = 40% reduction in combined consumption
• Annual savings at $1.20/therm: approximately $940 reduction in combined heating and DHW costs
• Simple payback on installation: approximately 12-14 years on energy savings alone; faster considering combined equipment economics

Mechanical Room Space

• Floor space recovered: water heater location now empty; boiler footprint smaller than predecessor
• Customer use of recovered space: additional storage shelving installed in freed area

Customer Feedback

Customer reported high satisfaction with several aspects: single equipment manufacturer warranty for both heating and hot water (vs. separate warranties tracking); endless hot water capability is meaningful improvement over storage tank approach; mechanical room space recovery valued; equipment dramatically quieter than predecessor combination; reduced equipment maintenance footprint (one unit instead of two).

Ongoing Service

Customer enrolled in maintenance plan covering combi boiler including specific scope for both heating and DHW components; annual service scheduled October 2026.

Frequently Asked Questions

What is a combi boiler and how is it different from separate boiler + water heater?

A combination boiler integrates heating and domestic hot water functions into a single piece of equipment. Traditional separate equipment approach: dedicated boiler for heating (water circulated through radiators or radiant tubing); dedicated water heater for domestic hot water (storage tank or tankless); two separate gas appliances with separate venting, electrical, plumbing connections, and maintenance scope. Combi boiler approach (this case study): single equipment provides both functions; heating mode operates similar to traditional boiler with water circulated through radiators; domestic hot water mode operates similar to tankless water heater with cold water heated on demand as it flows through equipment; DHW priority logic temporarily pauses heating circulation during DHW demand (typical duration 5-30 minutes per shower or other DHW use). Specific advantages of combi: single equipment reduces installation cost (one set of venting, electrical, plumbing connections rather than two); reduces ongoing maintenance footprint; consolidates manufacturer warranty; smaller total mechanical room footprint; eliminates standby losses from water heater storage tank. Disadvantages of combi: limited DHW capacity vs. dedicated tankless water heater (heating capacity must be balanced with DHW capacity in single equipment); DHW priority can briefly interrupt heating circulation; equipment failure affects both heating and DHW simultaneously (vs. separate equipment where boiler failure doesn’t affect hot water); maintenance complexity slightly higher than separate equipment. For this customer’s situation, combi boiler was excellent fit: home’s hot water demand within combi capacity; both pieces of equipment due for replacement; mechanical room space premium; customer valued integration benefits.

What’s the deal with DHW priority and heating interruption?

Combi boilers cannot simultaneously make domestic hot water AND circulate heating water at full output – they handle one function at a time when demand exceeds capacity. DHW priority logic: when fixture activates (shower, sink, washing machine, etc.), flow sensor in combi boiler detects DHW demand; equipment switches to DHW mode, increasing heat output to handle cold water heating; heating circulation temporarily pauses (5-30 minutes typically); when DHW demand ends (fixture closes), equipment returns to heating mode and resumes circulation. Why this works for hydronic systems: heating distribution (radiators or radiant) has substantial thermal mass; radiator surface temperatures don’t drop noticeably during brief DHW priority periods; comfort impact is negligible for typical residential DHW usage patterns. When DHW priority becomes problematic: extremely heavy hot water use (back-to-back-to-back showers, washing machine plus dishwasher plus shower simultaneously) could extend DHW priority duration; during these scenarios, heating might be paused for extended periods; for typical residential usage, this is rare and customer doesn’t experience comfort impact. Sizing considerations: equipment must be sized for peak DHW demand to provide adequate response (Navien NCB-240 sized at 6.2 GPM matches this customer’s peak demand); undersized equipment would struggle with high simultaneous demand. Alternative: indirect water heater with separate storage tank: traditional boiler + indirect tank (water heater that uses boiler heat indirectly through internal heat exchanger); provides better sustained DHW capacity for high-demand homes; doesn’t have heating interruption; but at higher cost and complexity than combi. For this customer’s typical 3-person household, combi boiler DHW priority is non-issue; for large families with simultaneous heavy demand, indirect tank approach may be preferable.

Why not heat pump water heater instead?

Heat pump water heaters provide excellent efficiency for water heating but with different trade-offs than combi boiler approach. Heat pump water heater approach considered for this customer: 50-65 gallon hybrid heat pump water heater (Rheem ProTerra or AO Smith Voltex) at approximately $2,200 equipment plus $1,800 installation including electrical circuit; total approximately $4,000 standalone installation; combined with separate boiler replacement at $9,200 yields $13,200 total vs. $11,400 combi approach. Heat pump water heater pros: very efficient water heating (typical UEF 3.5-4.0 vs. 0.95 for gas combi); environmentally beneficial (electricity vs. gas reduces home gas consumption); standalone equipment lifecycle independent of heating equipment. Heat pump water heater cons: requires 200-amp electrical service capacity (this customer had 100-amp service, would have required electrical upgrade or separate panel installation); ambient temperature requirements (heat pump water heater performance reduced in cold spaces; basement installation usually adequate but specific verification needed); noise (heat pump operation creates fan and compressor noise audible in basement); requires more space than standard water heater. For this customer’s specific situation: 100-amp electrical service inadequate for heat pump water heater + AC + future possible electrification; electrical upgrade would have added $3,400-$5,200 to project scope; combined total approach with electrical upgrade would have been $16,600-$18,400 vs. $11,400 combi boiler; the electrical capacity limitation was the driving factor, not heat pump water heater concerns. For customers with adequate electrical capacity (200-amp service) considering equipment replacement, heat pump water heater is excellent option to evaluate against combi boiler; decision depends on customer’s electrical capacity, gas vs. electric heating preference, and overall electrification strategy.

What if the combi boiler fails – do we lose both heating and hot water?

Yes – that’s the trade-off for equipment consolidation. Failure mode analysis: combi boiler failure affects both heating and DHW simultaneously; separate equipment failure affects only the failed equipment; for this customer’s previous separate setup, a boiler failure left heating offline but water heater continued to work, and vice versa. Failure probability with combi boilers: modern condensing combi boilers have reliability comparable to separate equipment when properly maintained; warranty coverage typically aligns with components (15-year heat exchanger, 5-year parts for this equipment); manufacturer warranty support generally responsive to combi customers. Risk mitigation strategies: enrollment in maintenance plan (catches issues before catastrophic failure); priority dispatch for emergency calls; emergency repair scope coverage; established relationship with installer reduces emergency response time. Backup heating options if combi fails during cold weather: electric space heaters for primary living areas (similar to other emergency heating scenarios); family relocation to alternative housing if extended outage. Backup hot water if combi fails: limited – cold-water-only alternatives unless extended emergency justifies temporary tankless water heater rental ($200-$400 per week typical). For customers concerned about combined equipment failure risk: separate equipment approach (separate boiler + water heater) addresses this concern; cost premium of $2,400-$3,800 over combi approach; whether premium is worthwhile depends on customer risk tolerance and emergency response options available. For this customer with established maintenance relationship and family flexibility for emergency response, combi approach was acceptable risk; for some customers (elderly, medical equipment dependence, etc.), separate equipment redundancy might be the better choice.

Is the combi boiler’s hot water capacity really enough?

For typical residential demand, yes – but household profile matters. Navien NCB-240 capacity analysis: 6.2 GPM at 77°F temperature rise (55°F St. Louis groundwater to 132°F output); 5.4 GPM at higher 87°F rise (45°F coldest winter groundwater to 132°F output); reduced capacity during cold winter compared to summer due to colder inlet water. Typical residential fixture demand: shower flow 1.8-2.5 GPM (low-flow shower heads); kitchen sink 1.5-2.2 GPM; bathroom sinks 1.0-1.5 GPM; washing machine 2.5-4.0 GPM during fill cycle; dishwasher 1.5-2.5 GPM during fill cycle. Simultaneous demand scenarios: 2 showers running simultaneously = 4-5 GPM (within capacity); shower + kitchen sink = 3-4 GPM (well within capacity); 2 showers + sink simultaneously = 6-7 GPM (at or slightly above capacity, may experience temperature reduction at fixtures). Household profile considerations: this customer’s 3-person household typical morning routine: 1 shower at a time, kitchen activity, occasional simultaneous use; combi capacity is more than adequate for typical usage patterns; only extreme simultaneous use (3 showers simultaneously) would exceed capacity, which isn’t a realistic scenario. For larger households (5+ people, multiple bathrooms in use simultaneously): combi boiler may be inadequate during peak morning usage; indirect water heater with storage tank provides better sustained capacity; assessment during quote development determines whether combi capacity matches household needs. For homes with high-flow fixtures (rainfall showerheads at 4-6 GPM, jetted bathtubs requiring 60-80 gallons): combi may struggle; specialized equipment selection needed. For this customer’s typical residential profile, NCB-240 is well-matched to actual demand patterns.

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Contact Purisync Heating and Air

For your combined heating and water heater replacement, combi boiler installation, or integrated heating-and-DHW equipment evaluation in Webster Groves, contact our 325 N Kirkwood Road office at (314) 338-5111. Initial consultation includes combined system assessment, household hot water demand analysis, equipment options across combi boiler, separate equipment, and heat pump water heater alternatives, and written quote with itemized pricing.

• Emergency Line (24/7): (314) 338-5111
• Address: 325 N Kirkwood Rd #245, Kirkwood, MO 63122
• Email: info@purisyncheatingairconditioning.xyz
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