Also uses two chimney (though smaller diameter). Automation for cascade control is very expensive (from any manufacturer) and 2 times more expensive than automatics for weather control. Increases accordingly and project costs. Based on these data, the use of dvuhkotelnoy installation will cost almost 42% more than a boiler with a boiler. Such is the price of additional security. Of course, when implementing the "comfort" version (with optional weather-automatics), the use of dvuhkotelnoy install more profitable because the price difference drops to 30%. But we must admit that it's very noticeable.
Do you need it or you can do? In the end, everyone decides for himself, but for a decision may be guided by the following arguments: Optional backup gives "peace of mind" and more comfortable life in an emergency situations, additional comfort and "peace of mind" will cost 30-40% more expensive, experience shows that the release of boiler failure is not so frequent, and when the equipment manufacturers that are well-behaved proven – and at all unusual. The most "delicate" part of a boiler in the sense of reliability is electronics, which is replaced easily. The "body" of the boiler, particularly ductile cast iron, not for nothing is guaranteed for 5 years (most manufacturers), has no more moving parts than a brick and almost no crashes, when the most "evil" frost, well-insulated home is cooled no more than 2-3 degrees in the day, so to fix the problem you are have time to freeze (though not hot water) can always call the professionals who are per night (two) persists, the more so that suppliers always have a stock of spare parts that are more likely to come from system, may enter into a contract for service, which significantly reduces the risk of unexpected breakdowns. If you have a need for redundancy boilers must bear in mind that all of the above applies to cases redundancy of the boiler equipment breakdowns.