Primary & secondary loop capacity minimum or ratio recommended?

Al Roethlisberger

As I've read more about primary/secondary loop systems, injection, etc.... I have begun to wonder if there are any recommended water capacity ratios between the primary and secondary(system) loops that should be maintained at a minimum to both satisfy the system demands efficiently and protect the boiler.



For example:  If one has a relatively huge single zone system water capacity(such as with an old gravity system); to satisfy that system/secondary loop while still providing boiler protection, it would seem that if the boiler/primary loop's water capacity were "too small" it may take a longer time to heat up that secondary loop as compared to a primary loop that had more heated water available to inject into the secondary while still providing enough warm water back to the boiler for  protection.



Even when using intelligent mixing controls, it seems like there would be an ideal ratio of primary loop water capacity as compared to the secondary loop that would balance/maximize a speedy heating of the secondary circuit while at  "maximum mixing/injection" provide minimum boiler return temps in the primary loop.



Is this an issue, are there any recommended ratios or general rules, or is it really a non-issue in small/medium applications?



Thanks,

Al

Jean-David Beyer

If it is a mod|con boiler ...

... in the primary loop, then the capacity of the circulator in the primary loop must be sufficient to keep the boiler's heat exchanger cool enough. This means it must circulate enough to remove the maximum BTU/hr that the boiler can produce. My small boiler wants about 6 gallons per minute in the primary loop.



You really want the secondary loop to be able to sink the heat sourced by the boiler. That depends on the radiation of all the heat emitters (radiators, baseboard, tubing in slab, etc.) you have, and flow rate is only a part of the calculation. Also, if the secondary loop has more than one zone, you must deal with which zones are on, which probably changes flow rates. In my system, there are two heating zones controlled by circulators, and either one or both can operate at the same time. If both circulators are running, the flow in the secondary loop is more than that in the primary loop. If the downstairs only runs, the primary and secondary loops have about the same flow rate, and if the upstairs only runs, the secondary flow rate is quite a bit less than the primary flow.



It is my impression that if you have more flow in the secondary circuit, the water temperatures there might be less than you desire -- and the boiler will run longer than it might otherwise. OTOH, if you have less flow in the secondary circuit, you will probably not sink all the heat the boiler produces and thus make it cycle more often. All this changes depending on how the boiler is controlled. On mine, the boiler has a supply and a return temperature sensor in the secondary circuit, so the boiler fires at a somewhat higher rate if both zones are running at the same time.