Originally Posted by
BadgerBoiler MN
Almost perfect.
Except, the by-pass, or primary/secondary (P/S) pumping is required to keep minimum flow through the low-mass heat exchanger used by most condensing boiler manufacturers. The addition of a by-pass or two pump, primary-secondary pumping can be a detriment to high efficiency potential, since they may increase return water temperature to the boiler and always add the parasitic cost of the second pump, without adding anything to the performance of the boiler or system.
When replacing boilers in older homes with one thermostat (zone) here in the Twin Cities, we use IBC, Lochinvar, Triangle Tube, Viessmann, and HTP products, as they appreciate the fact that every boiler system is a custom design. By contrast, Buderus, Burnham, SlantFin and Weil McLain specifically mandate the use of P/S pumping in all application new and retrofit, voiding the warranty if P/S is not used. The simple majority of the condensing boiler manufacturers do require P/S pumping to protect their boilers from overheating and only those familiar with the technology should deviate from this "idiot proof" policy.
The high efficiency condensing boilers are a perfect match for almost any old gravity cast iron radiation system, as they were originally designed for relatively low operating temperatures and often may operate well below the original design water temperature with the reduced heat loads often found in older homes with upgraded windows and insulation. Our best fuel savings matching a "then new" Lochinvar Knight to the cast iron radiators of a 1921 bungalow in St.Paul, MN. The local gas supplier came out three times to check their gas meter as the fuel bill had gone down over 60%.
Had I been an expert witness for the contractor in the "Ohio" case, I doubt the outcome would have been the same. Regardless of application, a properly installed condensing boiler will typically operate at 88.6% combustion efficiency in the coldest weather, delivering 180°F water to the system and not condensing at all. In cold climates the design condition (coldest week of the year) is so short that the bulk of the year much lower temperatures can be used to heat the space. It is in the shoulder months that the condensing boiler saves the most on fuel in non-typical applications such as fin-tube baseboard, fan coils and cast iron radiator.
There is much confusion over the savings available from condensing boilers. For instance it is the "return" water temperature that lowers the flue gasses to dew point and recovers the energy. So any system designed to operate with a return below 140°F will likely condense a good part of any heating season. All condensing boiler available in the US and Canada today feature weather sensitive controls, which modulate the flame and consequently the output and average water temperature of the system.
Those who downplay the inherently superior performance of condensing boilers willingly the ignore the obvious advantages and real-world saving of this superior technology including, sealed combustion (shutting down the drafty chimney), weather sensitive control (lower operating temperatures on all but the coldest days and reduce flue temperatures (from 400°F to less than 150°F).
When choosing a high efficiency condensing boiler one should seek out the local expert, whom you will recognize by his certificate of competency, issued by any condensing boiler manufacturer and the sample heat load he provides since sizing any boiler is the first critical step in any hydronic heating design.
