← Selection Guides
Selection Guide
Boilers
Engineering reference for commercial hydronic boiler specifications. Intended to help engineers consider all relevant factors � system design conditions, code compliance, venting, efficiency targets, and plant layout � before submitting for equipment selection.
Condensing vs. Non-Condensing
| Factor | Condensing | Non-Condensing |
|---|---|---|
| AFUE / Thermal Efficiency | Up to 95�98% | Up to 80�84% |
| Typical Capacity Range | 400 MBH � 3,000 MBH | 400 MBH � 6,000 MBH |
| Return Water Temp Requirement | Below 130�F for condensing operation | No minimum; operates at any return temp |
| Flue Gas Temp | 100�140�F | 300�450�F |
| Venting Material | PVC, CPVC, Polypropylene, or SS | Type B, SS chimney liner, or AL29-4C |
| Condensate Drain Required | Yes � acidic condensate pH 3�4; must pipe to neutralization kit or approved drain | No |
| Heat Exchanger Material | Stainless steel or aluminum (corrosion-resistant) | Cast iron, steel, or copper |
| First Cost | Higher | Lower |
| Operating Cost | Lower | Higher |
| Best Application | Low-temp systems: radiant, fan coils, combination plants, ASHRAE 90.1 compliance | High-temp systems: steam, non-condensing replacements, budget-constrained projects |
When to use each
Per ASHRAE 90.1-2019 �6.5.4.8, gas boilers from 1,000,000 to 10,000,000 BTU/hr must achieve a minimum 90% thermal efficiency � effectively requiring condensing boilers on all new construction in this capacity range. Exceptions include:
- On-site or recovered energy provides =25% of annual space heating
- Boilers are installed in individual dwelling units
- Perimeter convective heating or radiant ceiling panels serve =50% of the design heating load
- Individual gas boilers have less than 300,000 BTU/hr input capacity
ASHRAE 90.1-2019 Efficiency Requirements
- Boilers =2.5 million BTU/hr: minimum thermal efficiency (Et) of 75% for gas, 78% for oil
- Boilers >2.5 million BTU/hr: minimum combustion efficiency (Ec) of 80% for gas, 83% for oil
- Confirm which edition of ASHRAE 90.1 is adopted by the local Authority Having Jurisdiction (AHJ)
Selection Checklist
Load Sizing
- Heating load calculation complete � block load, not sum of peaks unless diversity is accounted for
- Safety factor applied (typically 10�15%; avoid oversizing beyond this)
- Multiple boiler arrangement evaluated vs. single large boiler
- DHW load separated or combined plant considered
- Growth factor considered (typically 1% per year)
System Temperatures
- Design supply water temperature confirmed
- Design return water temperature confirmed
- Design delta-T established (typically 20�40�F for hot water)
- Return water temperature compatible with boiler type � condensing requires =130�F for peak efficiency
- Outdoor reset schedule defined if applicable
Efficiency & Code
- ASHRAE 90.1 edition confirmed with AHJ
- Minimum efficiency requirement met for boiler capacity range
- Exception applicability reviewed if using non-condensing above 1 MMBH
- AFUE or thermal efficiency (Et) documented on equipment schedule
Turndown & Modulation
- Turndown ratio specified � standard 4:1; high-performance 5:1, 10:1, or 15:1
- Staging sequence defined for multiple boilers
- Minimum fire rate compatible with minimum system load (prevents short-cycling)
- Buffer tank evaluated if minimum fire rate exceeds minimum system load
Combustion & Venting
- Fuel type confirmed: natural gas, LP, or dual fuel
- Gas supply pressure confirmed with utility
- Venting material specified per boiler type
- Vent run length and termination clearances verified
- Combustion air source identified: sealed combustion, mechanical room ventilation, or outdoor air
- Condensate drain routed for condensing units � include neutralization kit
- Local emissions and NOx requirements checked
Physical & Mechanical
- Boiler footprint fits mechanical room layout
- Floor loading verified (especially for roof-mounted or upper-floor installations)
- Rigging path confirmed for boiler delivery
- Service clearances provided per manufacturer requirements
- Piping connections sized and oriented correctly
- Electrical supply confirmed: voltage, phase, amperage
- Boiler water volume documented for buffer tank and expansion tank calculations
Controls Integration
- BMS/BAS integration protocol confirmed: BACnet, Modbus, or LON
- Lead-lag sequence defined for multiple boilers
- Outdoor air reset control specified
- High-limit and low-limit safeties defined
- NFPA 85 compliance addressed for boilers =12.5 MMBH input
Combination Plant Considerations
When the boiler plant serves both space heating and DHW:
- Evaluate a "swing boiler" concept � certain boilers assigned to heating, others to DHW, with the ability to reassign based on load
- Per ASHRAE, peak space heating and peak DHW loads rarely occur simultaneously � this can reduce total installed boiler capacity
- A combined plant improves condensing efficiency because DHW return temps are typically lower (boosting condensation rate)
- Ensure proper hydraulic separation between heating and DHW circuits
- Confirm the boiler is rated for potable water contact if directly heating DHW, or specify an indirect heat exchanger
Key Design References
- ASHRAE 90.1-2019 �6.5.4.8 Boiler efficiency requirements
- ASHRAE Handbook: HVAC Systems and Equipment, Chapter 32 Boilers
- ASME Boiler & Pressure Vessel Code, Section IV Heating boilers
- NFPA 85 Boiler and Combustion Systems Hazards Code
- Local mechanical code IMC or equivalent adopted edition