Gas boiler condensate

With high-efficiency condensing water heaters growing in popularity, Rinnai has installation advice on dealing with gas boiler condensate.

Rinnai say they are finding more and more customers selecting high-efficiency condensing water heaters for both residential and commercial applications. For example, the recently opened Britomart 5-green-star hotel uses Rinnai INFINITY N56 boilers for its water heating system.

The effect of condensate needs to be understood when installing both condensing and standard-efficiency appliances.

When gas is burned, carbon dioxide and water is produced. In a standard-efficiency appliance, all the water exits the flue terminal as a vapour. In a condensing appliance, sufficient energy is removed from the products of combustion for some water to condense and drain as a liquid from the appliance.

Condensed water from a boiler will generally have a pH between 3 and 4 (lower pH = more acidic). By way of comparison, vinegar has a pH of approximately 3.

This mildly acidic water can impact installations with both condensing and non-condensing appliances in the following ways. Flue gas impinging on metals

Where the exhaust hits cooler surfaces, water will condense. This will corrode many metals over time. Galvanising, in particular, will be removed, exposing steel substrate to rust. Marine-grade stainless steel offers reasonable protection. Some epoxy powdercoat may offer protection (refer to the coating manufacturer to confirm suitability). Some anodized aluminium may be affected.

Rinnai have seen installations where boilers have been installed in car parks. “While these boilers are open on the sides and may meet the outdoor area definitions in Appendix I of AS/NZS 5601.1, in these installations we often see corroded metal building elements in the areas above and in front of the units,” say the Rinnai Commercial Hot Water team. “The best solution for an application of this kind is to use internal water heaters, such as the Rinnai INFINITY N56. This can be fitted with an economical exhaust-only horizontal flue.” Condensate dripping on concrete

The mildly acidic condensate will dissolve cement over time. You can see the result of this in the photograph on this page. The cement is gone, leaving the aggregate exposed. If the flue will terminate horizontally over concrete or other material that won’t tolerate the condensate, make sure the flue is pitched well back to the boiler and drain the condensate to waste.

Condensing boiler considerations

There are other considerations that particularly apply to condensing boiler installations. Plumes

Some of the water vapour in the flue exhaust will condense as it passes into cold surrounding air. This means the flue products may be particularly noticeable in a condensing appliance as a visible plume around the flue terminal. Consideration of this phenomenon should be given when locating flue terminals, particularly where neighbours may raise concerns. Condensate drain lines

A condensate drain line must be fitted—normally to a gully. Drain lines must not be in copper. Copper will corrode away very quickly. Rinnai have seen some instances of copper condensate drain lines above copper gas supply pipes. PVC is the preferred material, or 316L stainless steel is acceptable.

Condensate is mildly acidic and will dissolve cement over time if it drips onto concrete.

To ensure the drain line empties and will not be subject to freezing, an air-break should be provided, and the line pitched preferably 50mm/m.

Condensate will normally be drained by gravity. Where this is not possible, a condensate pump can be fitted. Condensate pumps are commonly available for heat pumps. Ensure the system selected is specified as suitable for gas boilers. Condensate pumps usually incorporate a small reservoir. There is normally a float that will trip an upper limit switch if the pump fails and the reservoir is close to overflowing.

“Most modern condensing boilers such as the Rinnai INFINITY N56 have the facility to connect to this upper limit switch,” say the Rinnai team. “When the boiler detects the switch closing, it will cease operation to avoid condensate overflow and prevent damage. An error code identifying the issue is then displayed.”

Where the exhaust hits cooler surfaces, water will condense, corroding many metals over time.

Condensate treatment

Boiler condensate can be treated by passing it through a container of lime flakes. “While treatment is not normally necessary, some water heaters, such as the Rinnai INFINITY EF26, incorporate condensate treatment inside the appliance,” say the team. “The lime flakes will need to be replenished over time and the container flushed with water to remove accumulated sludge. Treated condensate should still be drained to a sanitary sewer rather than the customer’s prized begonias!”

Benefits of condensing gas appliances

Condensing gas appliances offer significant running cost and emission reduction benefits to consumers. Many are futureproofed for the introduction of hydrogen blends. Once the industry is fully converted to hydrogen, all appliances will likely be condensing. Take the simple steps outlined above when planning installations to ensure reliable, long-lasting performance.

About the author: The Rinnai Commercial Hot Water team are available to assist with specifying residential or commercial condensing water heaters. Please contact the team on 0800 RINNAI; 0800 746624.

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