Outdoor Reset and Short-Cycling

Brad White_9

I see you have a good point.

In my experience most of the times I used outdoor reset it was in conjunction with an overall control system (Honeywell Centra, Tekmar for example). Each had a "minimum burner on time" feature which I usually set for two minutes maybe a tad more. Less if it approached high limit. This is what kept the cycling down. Of course the radiation loop side of the system was de-coupled from the boiler-side by a 4-way mixing valve, so this naturally "banked" any excess heat from an over-fire condition and metered it out to the radiation until the next firing cycle. The relationship of boiler temperature to outdoor temperature was indirect: The radiation loop temperature was what was varied and the boiler temperature was set 10 degrees (adjustable) above that to keep it in control.

My current boiler, an on-off firing Monitor MZ condensing boiler, does cycle a lot as you suggested it would. No modulation. With their add-on OD Reset (made for them by Tekmar) it actually cycled more in mild weather so I de-activated it. In colder weather (below the high 20's) I re-activate it. When I set the minimum burner on-time, it would reach high limit to be manually reset at lower ODT's and that is the real reason I defeat it in mild weather.

But your point is well taken. I think if you have enough boiler mass and or system mass, a minimum burn time control will keep your cycling down. If you do not have a decoupling means (4-way valve or secondary loop) these spikes from over-firing will find their way into your radiation. Overall I think you will not notice it too much and you will save fuel on average.

Tom Wilcon

Outdoor Reset & Short-Cycling

Just a question that I have been pondering...hmmm.
If you have a non-modulating, non-condensing boiler without outdoor reset, (stayed at one temp all the time through the year) and another boiler with outdoor reset. Wouldn't the fixed capacity boiler (non condensing) with the OD short cycle more, especially in the spring and fall, because lower water temp is called for and fewer BTU's are needed in the home? So, wouldn't you be better off to not use OD on a non modulating, non condensing boiler?

Ron Schroeder

Cycle length depends mostly on differential rather than setpoint so if the differential stays the same or increases, short cycling will not substantualy increase as the boiler temperature is lowered.

hydronicsmike

I like Ron's reply.

All tekmar Outdoor Reset Boiler Controls have an Automatically adjustable Boiler Differential that changes based on the load to minimize short cycling.

Hope this helps.

Mike

[Deleted User]

Agree w/ Mike &

Ron. Takes some explaining when guys, who are used to others' controls, see the wide temp swings & longer cycle time w/ a Tekmar in the Automatic mode.