Condensing boilers are sold on high efficiency ratings, often in the mid-90 percent range. Many owners discover later that their gas bills did not drop as expected. The boiler is usually working fine. The problem is that the heating system around it returns water too hot for the boiler to condense, so it operates much like the conventional boiler it replaced.
Getting the efficiency you paid for depends less on the boiler than on the system it is connected to. This guide explains why, and what designers, contractors and operators can do about it.
How condensing works
When natural gas burns, it produces water vapour along with carbon dioxide. In a conventional boiler, that vapour leaves up the flue, carrying its latent heat with it. A condensing boiler has a heat exchanger large enough to cool the flue gases below their dew point, so the vapour condenses and releases its latent heat into the boiler water.
For natural gas, the flue gas dew point is typically around 55 C (130 F), depending on excess air. The boiler's heat exchanger surface must be below that temperature for meaningful condensing, which means the return water entering the boiler must be cooler than about 55 C (130 F). The cooler the return, the more condensing and the higher the efficiency.
| Return water temperature | Typical condensing boiler behaviour |
|---|---|
| Above about 60 C (140 F) | Little or no condensing; efficiency similar to a good non-condensing boiler |
| About 50 to 55 C (122 to 130 F) | Partial condensing |
| About 30 to 45 C (86 to 113 F) | Strong condensing; efficiency in the low to mid 90s |
| Below about 30 C (86 F) | Highest efficiency |
Exact efficiencies depend on the boiler, firing rate and excess air; manufacturers publish efficiency curves against return water temperature. Lower firing rates generally improve efficiency because the heat exchanger has more surface per unit of heat.
Why many installations underperform
Systems designed for high temperatures
Many existing Canadian heating systems were designed for around 82 C (180 F) supply and 71 C (160 F) return. Replacing the boiler without changing anything else leaves return temperatures far above the condensing range for much of the year.
Small design delta-T
A system designed for an 11 K (20 F) delta-T returns warmer water than one designed for 17 to 22 K (30 to 40 F) at the same supply temperature.
Bypasses and three-way valves
Three-way valves and open bypasses send hot supply water straight back to the boiler, raising return temperature. Primary-secondary piping with a high primary flow can do the same if boiler flow exceeds system flow. This is the heating equivalent of low delta-T syndrome.
Mixing with non-condensing boilers
Plants that combine condensing and non-condensing boilers sometimes pipe them so that condensing boilers see the warmest water, or so that return water is blended with hot supply.
Fixed high supply setpoints
Running supply water at 82 C (180 F) year round when the building needs only 50 C (122 F) in mild weather keeps return temperatures high.
How to make condensing boilers condense
1. Reset supply temperature with outdoor temperature
Outdoor air reset is the single most effective control measure. Supply temperature falls as outdoor temperature rises. Because Canadian heating seasons have many more mild hours than design hours, a well-tuned reset keeps return water in the condensing range for most of the season. NECB includes requirements for hot water supply temperature reset on many systems.
2. Design for lower temperatures and larger delta-T
New systems should be designed around lower supply temperatures, for example 60 C (140 F) or below at design, and larger delta-Ts. This means more terminal surface, such as larger coils or more radiation, but it also prepares the building for future heat pumps. See heating hot water system design.
3. Eliminate bypasses and use two-way valves
Convert three-way valves to two-way, control or remove bypasses, and use variable speed pumping so flow matches load.
4. Pipe boilers correctly
Many condensing boilers have low water content and specific minimum flow needs. Follow manufacturer piping guidance. Where primary-secondary piping is used, avoid boiler flow much greater than system flow. Where condensing and non-condensing boilers share a plant, route the coolest return water to the condensing boilers.
5. Sequence for efficiency
Because condensing boilers are more efficient at low firing rates, running several boilers at low fire often beats running one at high fire, within their turndown limits. This differs from conventional boilers, where fewer boilers at higher fire is usually better. See boiler plant design and sequencing.
6. Commission combustion
Set up combustion according to the manufacturer's procedure with a calibrated combustion analyzer. Excess air that is too high lowers the flue dew point and reduces condensing.
Installation requirements
- Venting: condensing boilers typically use Category IV positive-pressure venting. Vent materials must be certified for the application; plastic vent systems in Canada are certified to ULC-S636. Installation must comply with CSA B149.1 and the manufacturer's instructions. See CSA B149 gas installations and HVAC equipment.
- Condensate: flue condensate is acidic. Most manufacturers and many municipal sewer bylaws require neutralization before discharge.
- Water quality: many condensing boiler heat exchangers, especially aluminium or stainless designs, have specific pH and chemistry requirements. Following them is often a warranty condition.
- Combustion air: sealed combustion with outdoor air intake is common; intake and exhaust terminations must be located to avoid recirculation.
Verifying performance
- Trend supply and return temperatures, outdoor temperature and boiler firing rate.
- Calculate the fraction of operating hours with return water below about 55 C (130 F).
- Compare gas use to heating degree days before and after changes.
- Check combustion analysis records for flue temperature and oxygen.
- Confirm condensate is actually being produced; a boiler that rarely drains condensate is not condensing.
Frequently asked questions
Is a condensing boiler worth it in a high-temperature system?
It still offers good part-load efficiency and modern controls, and with outdoor reset it will condense in mild weather. The full benefit comes when the system is adapted for lower temperatures.
Do condensing boilers last as long as conventional boilers?
Life depends on water quality, installation and maintenance. Heat exchanger fouling and poor water chemistry are common causes of early failure.
Key takeaways
- Condensing requires return water below about 55 C (130 F); the colder, the better.
- High supply temperatures, small delta-T and bypasses prevent condensing.
- Outdoor air reset and two-way valves are the most effective fixes.
- Sequence condensing boilers to run at low fire, and commission combustion properly.
- Low-temperature design also prepares buildings for heat pumps; an energy audit can show how much efficiency is being lost.
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