Please define short-cycle

JeffGuy

I've read a lot about short-cycling on these pages, but I've never seen a good definition of it. How short a bioler run time, and how often, would qualify as "short-cycling"?

Jamie Hall

Presumably steam or vapour?

if so -- and this is just my take on it -- it depends on when in the cycle, as well as specific timings.  Supposing going from a cold start, and not being limited by the thermostat -- say recovering from a setabck, In my view, if the boiler runs for say five to ten minutes, and shuts down on pressure, that would definetly be "short".  Upon examination, you might find that some, but not all, of the radiation was filled.  In fact, that could be part of a definition of short -- the boiler shuts off on pressure before all the radiation is filled.  On the other hand, if, towards the end of that same recovery, the boiler shuts off on pressure, waits say a minute, turns back on for two minutes, then off for one etc., that's not really "short"; what that is is an indication of a boiler which is a little too big -- but not too much too big -- for its radiation.  A somewhat different matter.



That help any?

Condoman

My short cycle story

I recently put in a second loop that was a Crown MS-40 indirect.  Since my workshop is in the basement also I am around the boiler much of the day.  After installing the MS-40 I noticed a short cycle of 10 seconds or less.



I took the cover off the L7224A aquastat put an analog 110v clock wired to the burner feed and set up a camera that takes a picture every 10 seconds.  It took several days and thousands of pictures put I found the culprit to be nothing more than a normal sequence of events.



The zones are controlled by a Taco ZVC 404.  In the series of pictures I could see the call for heat, the burner fire and then finish a normal run with the temp over the HL setting,  Still on the heat call the circulator brought the temp low enough for the burner to fire again and then 8 seconds later the call ends and everything is off.



I saw more than one of these sequences even with the indirect taking priority and passing back to the heat call.



With the clock showing seconds, the 404 LED's and the burner temp on the aquastat it was a great learning experience.

Zman

6

The generally excepted number is more than 6 cycles per hour equals short cycling. With modulating boilers I think we should be shooting for longer cycles than that.

Carl 

Jean-David Beyer

With modulating boilers I think we should be shooting for longer cycles than that.

The generally excepted number is more than 6 cycles per hour equals

short cycling. With modulating boilers I think we should be shooting for

longer cycles than that.

  

At what outdoor temperature? When it is cold enough out, my boiler does not short cycle. But as it warms up outside, perhaps to 60F outside, it does short cycle somewhat, especially in my small baseboard zone. The boiler's minimum input is 16,000 BTU/hour and the zone never really needs more than about 6000. So unless it is cold enough that both zones call for heat at the same time, there is little the boiler can do except switch on, warm up to 110F, heat the little zone until the boiler gets to 118F and shuts off at high limit. Then the boiler slowly cools down until it gets to 103F and it starts up again. I do not like to raise the differential any more because it starts go get noisy, baseboard creaking and clicking. So I put up with it that way.



It matters much less with the radiant slab at grade zone. First of all, it needs 4x more heat, and second, the copper tubing is held in place by the concrete slab, and temperatures there change much more slowly.