Hot Water Heating Renewal Series: Condensing Boilers
By Alan Sutton
Traditional boilers in North American heating plants are "non-condensing", which means that they discharge flue gases at a high enough temperature that all water vapour is carried out of the flue (and up the chimney, in a tall building). The water vapour is discharged with the other flue gases and never has a chance to condense to a liquid and damage anything. Of course, the hotter the flue gases, the more heat is thrown away and the less efficient the appliance.
By Alan Sutton
Condensing boilers, which are common in Europe and are becoming more common in North America, are specifically designed to handle flue gas condensate. They have had intermittent success in the past, so many designers will not use them. However, the technology is maturing, and both governments and utilities are promoting them heavily. There are several advantages:
- Performance up to 99% efficiency. Actual operating efficiency depends strongly on return water temperature, which must be below 120°F for any condensing to take place. In a retrofit situation, with existing high temperature (180°F) distribution systems (e.g. baseboard wallfin radiator), the efficiency of a condensing boiler plant will be limited.
- Simplify mixing and pumping. No mixing valves needed. The boilers are controlled directly from the reset controller with no lower limit to operating temperature.
- Easy to design with. They can handle a wide range of water flow rates and temperature rises.
- Can be quite cost-effective.
However, condensing boilers are unforgiving. They need to be installed properly and taken care of. These boilers, particularly the stainless steel technology, are only for sophisticated Owners who realize they are trading off durability and reliability for energy savings.
Condensing Technology Options
Condensing boilers are categorized by the material of the heat exchanger, which is selected to resist corrosion when exposed to liquid condensate.
Stainless steel: Small, lightweight, low capacity. There is some good performance in nrecent history, with notably poor reliability with initial market introductions.
Aluminum: Newer technology. Larger capacity range, likely more durable than stainless steel. Cannot safely produce supply water hotter than about 180F, which is a significant limitation on retrofit jobs. Commercial and multi-res owners are tending to go with this technology rather than the stainless steel.
Cast Iron: A condensing boiler with a cast iron heat exchanger has been introduced to the market. Cast iron can be corroded by condensate, but these units have a surface is so smooth that the manufacturers claim that condensate doesn’t pool or become concentrated enough to do damage. Owners would be well advised to take this into account when investing in this technology. Good performance and durability would make this a perfect boiler for many of projects.
In a retrofit application, existing systems must be flushed with water (no chemicals). If this can’t be done, it is necessary to temporarily separate the boiler from the distribution loop through a heat exchanger. Regardless of condensing boiler technology employed, water treatment during initial fill is required to control magnetite. FeSO3, which is on the walls of old pipe, will react with new calcium in the fresh fill water to form magnetite. Magnetite builds up quickly and will block many heat exchangers within three weeks. Get the air out of the system as quickly as possible, and always install an air separator. It is crucial to keep water flowing in condensing boiler systems and it is recommended that a dedicated circulator be installed and a filter, which may cause flow restricts should not be installed.
A building owner is well advised to ensure that proper flushing techniques are followed by the installer and that maintenance contracts follow manufacturer’s recommendations. A skilled installer must be engaged to set up the fuel/air ratio properly across the firing range. The building owner needs to be aware of warranty terms, particularly on the heat exchanger. She/he must enter into a long-term service contract, including water treatment and annual inspection.
Taking a little extra time and effort at the planning/design stage of your boiler plant retrofit is a sound business decision that will pay dividends for years to come. The added cost of design and analysis are miniscule compared to the capital cost savings and operating cost savings. Building owners and managers should consult an experienced heating plant designer to help them optimize your objectives for code compliance, energy efficiency, and project cost. It is our hope that this series of articles has been informative and instructive.
Alan Sutton, B.A.Sc., MBA, LEED AP, (firstname.lastname@example.org) is General Manager with Efficiency Engineering Inc. in Cambridge, Ontario. He specializes in energy efficiency and control of central plants.