Small copper-tube boilers need special consideration for zoning.
A frequent problem is using boilers that hold little water in a system that holds little water. Many small boilers are able to provide 100,000 btuh to a system. While a system uses all 100,000 btuh there is little problem; however, many 2000 sq ft floor area houses today require only 25 btuh per square foot due to the good insulation required to save energy since 1992. This means that a house that used to require 40 btuh per sq ft in the 1960's is no longer made, so 'rules-of-thumb' learned by the previous generations no longer apply. A proof of that is the design of radiant-floor heated homes which usually can only supply 30 btuh from each sq ft of exposed floor.
If the system is designed with no zoning to stop flow to portions of the system, then there would be little need for a buffer tank. A series loop, multiple series loops, one-pipe, or two-pipe systems with all the radiation ready to give off heat wouldn't have a problem.
Low-water-content systems with 100,000 btuh boilers that are zoned and have indirect water heaters achieve zone control by shutting off portions of the heating load as zones no longer need heat or are 'set back.' While a burner applies 100,000 btuh of fuel to the boiler, it can happen that there is only one zone of 20,000 btuh calling for heat. This could heat the small content of water so rapidly that there is sizzling and banging in some of the boiler tubes. A method to prevent that would be to use a buffer tank. Earlier suggestions include using a bypass around the boiler, but a bypass without a buffer tank will have the problem of sizzling and banging because the boiler controls can not react quickly enough. The buffer tank is usually a necessity because just whizzing a small amount of water around doesn't solve the problem, more water mass requires more time to heat.
In this zone-valve multi-zone system a balancing valve is in the primary loop along with the buffer tank to make a pressure for all the zones to force water through the zone valves and return water to the lower-pressure side of the primary circuit. The length of the primary loop is not important, so a primary loop could be run around the basement so supplies could be taken off one side of the room and returns off the other side with the balancing valve between them. It would operate like diverter valves in a one-pipe system. the benefit here is that one circulator can provide primary loop pumping AND provide flow through the zones when they are open. The primary loop includes the buffer tank, so the boiler will have the mass of the water in the tank to give the controls time to sense and shut off the boiler. The boiler burners might be controlled by an outdoor reset.
Some prefer circulators to provide flow to zones. As multiple circulators provide the required zone flow, the balancing valve is not necessary and the zones can be piped as primary-loop with a circulator and the zones as secondary loops off the primary circuit.
Another idea is to use the buffer tank as an indirect domestic water heater. Such tanks come with copper tubes to heat the domestic water passing through as in tankless coil boilers. A small, copper-tube boiler is inexpensive, though if used to make hot water as an instantaneous heater, they can lime up fast in most areas. Even then, as in Europe in hard water areas, their lesser cost makes them easy to replace. With a tank that uses just the water in the sealed heating system, the liming does not progress as long as the system is sealed.
They work best with a tank that will hold more water so no matter what flow is out in the system with multiple zones sometimes holding back water with the zones closed, the tank can make sure the water inside the coils doesn't overheat.
The tanks from:
ErgoMax. (www.ergomax.com) | www.thermo2000.com (Canada) | Everhot up in Mass.
Have instant hot water coils inside, so the boiler (most are over 80,000 btuh input) can provide heat without corroding, while the tank provides hot water. Both at over 80% efficiency with simpler controls.
