Short Cycling Defined?

David Hohengasser

My boiler also short cycles, because it is oversize. All vents are properly working. Boiler runs until all radiators are hot and then builds pressure and starts to cycle. It will be off for 3 minutes and on for 2 minutes. It does this 2-4 times until thermostat is satisfied. Boiler produces 470 SF and connected load is 320 SF. Gas valve calls for .2A setting. I currently have set on .7A and obtain the above results. Do I want to move the anticipator lower to .2A or higher to 1.2A?

Eric Scheidler

Perhaps I missed it, but neither in Lost Art nor We Got Steam have I found a definition of what boiler short cycling actually is. I suspect I may have this problem, though I've never really sat there and timed it during a cycle.

So how short is short?

michael_15

The most common

The most common "short-cycling" to observe is when the boiler shuts off on pressure prior to heating all of the radiators all the way across.

-Michael

Fred Harwood

Cycling

Cycles (on to on again) should approximate three to four times an hour. If each of your rads doesn't heat on each cycle, you may need more venting. More frequent cycling adds undue stress on motor contacts, etc., and should be unnecessary.

You know you're in the zone when more of the rads get warm as the temps drop, all without a change in the cycle time. In other words, the firing moment just gets longer and the off moment shorter in each cycle as winter comes. Theoretically, at -20F or your design heating temp, the on moment becomes constant.

Adjust your cycle time with the thermostat's anticipator.

Fred Harwood

Cycling

You might try lowering the anticipator amperage setting. If your rads are heating all the way across on each heating call, then room temps probably are overshooting, just what the anticipator was designed to reduce.

Try to hit three calls an hour, with cut-off before the rads get hot all the way across. If some rads don't heat evenly, then vent them better.

What is the on to on time for the thermostat call (not the pressure cycle, but the thermotat cycle)?

David Hohengasser

Cycles per hour

Fred,
Currently there is a cycle every 45 minutes to an hour. I will lower anticipator to achieve three cycles per hour. Is this the most efficent way to operate? I would think having to fill the mains with steam on each call would increase fuel usage. Where am I wrong? Thanks for your help.

Fred Harwood

Cycling

You're not necessarily wrong, just attempting to find that sweet spot. At one cycle per hour, most mains cool off and room temps overshoot, increasing heat loss. With more frequent cycles and a still-warm main, response is faster and the burner shuts off sooner because it doesn't have to heat all of the rads all the way each time.

If at three cycles per hour some rooms don't heat, increase venting, especially on a big main. With proper venting and anticipation, you'll not hear anything but the slightest breathing at your rad vents as more of each rad warms as the outside temperature goes down.

David Hohengasser

3 cycles

Thanks Fred, That makes perfect sense.