Heating Degree Days Calculator — NH Regional HDD Estimator for System Sizing
Estimate annual Heating Degree Days (HDD base 65°F) for New Hampshire locations and calculate heating system sizing requirements based on your building's heat loss characteristics.
NOAA 30-year normal. Editable for custom locations.
Standard base is 65°F. Some systems use 60°F or 70°F.
99% ASHRAE design temp for your region (auto-suggested below).
From Manual J load calculation or blower door test estimate.
AFUE for furnaces/boilers; COP×100 for heat pumps (e.g., COP 2.5 = 250%).
Cost per therm / gallon / kWh / ton.
Formulas Used
1. Annual Heat Energy Required:
Qannual = HDD × 24 × UA
where UA = Qdesign / ΔTdesign (BTU/hr·°F)
and ΔTdesign = Tindoor − Toutdoor design
2. HDD Base Temperature Adjustment:
HDDbase = HDD65 × (Tbase / 65)
3. Annual Fuel Consumption:
Fuel = Qannual / (BTUper unit × η)
where η = system efficiency (AFUE or COP as decimal)
4. Annual Fuel Cost:
Cost = Fuel × Costper unit
5. Recommended System Capacity:
Capacityrec = Qdesign × 1.10 (10% safety factor per ACCA Manual S)
Assumptions & References
- HDD values are NOAA 30-year climate normals (1991–2020) for NH stations.
- Monthly HDD distribution based on NOAA Concord, NH normals — proportionally applied to all regions.
- ASHRAE 99% design temperatures from ASHRAE Fundamentals Handbook, Chapter 14 (Climate Data).
- Fuel energy content: Natural Gas 100,000 BTU/therm; Heating Oil 138,500 BTU/gal; Propane 91,500 BTU/gal; Electricity 3,412 BTU/kWh; Wood Pellets 16,500,000 BTU/ton.
- System sizing follows ACCA Manual J (load calculation) and Manual S (equipment selection).
- The UA method assumes a linear relationship between outdoor temperature and heat loss — valid for well-insulated buildings.
- Internal gains (occupants, appliances, solar) are not included; actual fuel use may be 10–20% lower.
- Heat pump COP should be entered as efficiency % (e.g., COP 2.5 = 250%) and represents average seasonal performance.
- NH Energy Code (IECC 2021) requires Manual J calculations for new construction and major renovations.
- For precise sizing, consult a licensed NH HVAC contractor and perform a full Manual J calculation.