Factors to consider when buying a boiler
Boiler is the core component of any hydronic heating system, responsible for producing (heating) water in baseboard, radiator and radiant floor heating installations. There’s a large variety of boiler types available on the market, designed to fit individual project specifications based on building age, new or existing system type (baseboards/radiators/PEX radiant/solar, etc.), available fuel, space, venting and other factors.
Type
Heat-only boilers are intended only for producing hot water for a hydronic heating system, whether baseboard/radiator or
PEX radiant floor heating type.
Combi boilers combine hydronic heating and domestic hot water heating into a single unit and are capable of providing both heating water and hot water for plumbing needs - at the same time.
Fuel
Natural gas (NG) and propane (LP) boilers are the most common due to widespread availability and relatively low costs of gas, especially in urban and suburban areas. If a property has a gas meter, it is most likely NG. If there’s an outdoor gas tank, it’s most likely LP. Oftentimes, the same boiler model will be compatible with both fuel types and can be converted from NG to LP with an appropriate conversion kit. These boilers come in the largest variety and have the highest versatility of all types.
Oil boilers are still very common in areas where gas is unavailable or expensive to deliver. Large underground, indoor or outdoor tanks are usually an indicator of an oil-fired system. Keep in mind that oil boilers require a burner kit which may be sold separately or included depending on the model of choice.
Electric boilers, while not very common, are gaining popularity in the US and are typically used where other fuel types are not available or cost-prohibitive. They are compact, require not venting (making them 100% efficient) and can be useful in a variety of small to medium-sized projects (such as radiant heat in a basement, summer home or garage).
Wood-fired (including pellet) boilers can be a great solution where wood is abundant year-round. They may cost more than others, but offer the benefit of lower operating costs.
Efficiency
Condensing (high-efficiency) gas boilers generally utilize a secondary heat exchanger which extracts heat from flue gases, resulting in higher efficiency rating - from 90% to 95 AFUE and above. Due to lower temperature of exhaust gases,
condensing boilers are often compatible with plastic vent pipe, including PVC, CPVC, ABS and PolyPropylene.
Non-condensing (standard efficiency) boilers most commonly include traditional
cast iron gas,
steam and
oil boilers which have anywhere from 82% to 87% AFUE. Due to restrictive cost of condensing technology (at present) and proven track record of cast iron boilers, they still remain very popular for heating projects on a budget.
Mounting
Wall-hung (wall-mounted) boilers are designed for installation on a vertical masonry wall which is capable of supporting a load, and are nearly always vented though the same. These are more compact than floor standing models and are best suited for small boiler rooms.
Floor-standing boilers are designed to stand on a solid foundation. This category includes all cast iron boilers which are too heavy for on-the-wall installation.
Vent Type
Direct vent - both intake air and flue exhaust vent pipes run though an adjacent outdoor wall. This option is the most energy efficient (since it does not require conditioned indoor air) and is the safest when concentric/co-axial vent pipes are used (in the event of flue gas leakage, it will remain in the pipe and not leak into the dwelling).
Power vent - combustion air is taken from indoors while flue gases are power-vented though an outdoor wall.
Chimney (atmospheric) vent - utilizes the temperature differential between hot flue gases and cooler outdoor air, making the former naturally buoyant. Category I chimney vented boilers do not have an integral blower fan and therefore depend solely on the condition of the chimney in order for proper venting to occur. Local regulations may require a clay liner, or in addition to the latter, a stainless steel liner.
Vent Material
PVC, CPVC and ABS plastic pipes are selectively allowed only for category IV condensing, power-vented boilers. NOTE: some localities do not allow these and instead recommend PP or Category III stainless steel. We advise against using these types of pipes for boiler venting, since they were never intended for this purpose and may pose a health and safety risk.
PolyPropylene (PP) pipe (specifically,
Centrotherm Innoflue) is UL 1738 listed and approved for Category IV (condensing, power-vented) boilers. This is a time-tested, safe and cost-effective solution for high-efficiency boiler venting.
Stainless Steel (specifically
Z-Flex) is metal vent pipe of choice for Cat. III and IV boilers.
Standard chimney vent pipe or B-Vent is the common choice for Category I (non-condensing, atmospherically vented) boilers.
Additional factors to consider
Energy Star rating may qualify a homeowner for a rebate, helping to offset initial purchase and/or installation costs.
Indirect water heaters are often installed in combination with a boiler. To heat domestic water in the tank, they utilize an integral heat exchanger (coil or tank) to capture heat from hot water produced by the boiler and therefore do not require a separate fuel source on their own.
