outdoor reset defined 2

Robert_15

is the way a reset is designed to run would be the circulator would run all the time based on the outdoor temp say last night was 35f it would go down to 135f then the burner would go on the circulator would continue to run till the call for the boiler low temp then burn to say 180 the aquastat setting on the high end? or should the boiler temp go to a lower temp based on the outside temp say keeping the boiler temp to 150f then in cold weather it would go to 180 on the design day?

Mike T., Swampeast MO

Reset and constant circulation are separate, but related, concepts.

If your reset-controlled supply temperature is just perfect to meet your thermostat setting the thermostat will constantly be calling for heat. Since a call for heat is ALWAYS linked with circulation, such a perfect system would have constant circulation.

In reality that perfection is extremely difficult to achieve--especially under low load conditions.

When digital control (wall thermostats, zone valves, zone pumps, etc.) are used with reset it is rather common to use constant circulation to "average" the emitter temperature between calls for heat.

When proportional control (TRVs or FHVs) are used with reset, constant circulation is assumed because those devices "want" to be constantly supplied with available heat. The flaw here, again, is with low load. Without significant buffering, the boiler short-cycles because it's attempting to meet the instant heat demand.

Robert_15

Thanks mike, the system is running with zone valves & a digital termastat, like tonight the temps around 17f here on L.I.NY the circulator runs (but not all the time)calls for heat to 170f then continues to circulate i based the outside temp to 0f if i move the outside setting to 10f (based on the outside avg.of 20f)the circulator would not continue to circulate as much so my question is should the low end be based on not the avg. temp but the posible low temp?and you said the circulator should run on temps from design to mild right?