View Full Version : What is boiler short cycling

01-02-2010, 12:56 PM
I'm assuming that the boiler only runs a short time. But could someone explain what a boiler does when it shortcycles and why?

01-02-2010, 07:35 PM

There is a wealth of information out there if you google it.

Basically this occurs when a boiler is oversized for the heating demand. If you have multiple heating zones and only 1 or some of them are calling for heat the boiler fires and gets to temp quickly. This happens over and over causing ineffiecient firings.

The longer the boiler runs the more efficient it is.

There are outdoor resets, heat managers to help with these conditions. Basically advances computers controllers to throw some smart at your boiler.

Or if you are replacing your system you can fix the problem with a properly sized boiler and or a modulating one that can fire only at the BTU required to satisfy the heat demand at that point in time.

01-02-2010, 08:44 PM
Think multiple short, low-speed trips in your car in the winter. You'll get lousy gas mileage verses a long highway trip at constant speed. Same idea with the boiler, or any heating device. It works best running constantly. Now, why it would be short cycling is the real issue.

01-03-2010, 09:04 AM
One site said that short cycling is less then 3 minutes...accurate ?

Is the time length different based on the heating method ?
IE: hot water, air etc & by fuel ? - gas, oil, propane etc ?

I have oil fired hot water...I've never timed it
But depending upon weather & Temp change it can fire for maybe 15 minutes to an hour or longer - when turning the heat from 60-68

Usually once the house is at heat - saturated - it may only come one once every 90 minutes
Colder weather more often

01-03-2010, 11:46 AM
Don't think that there is a specific time interval that defines 'short-cycling'. Depending on the system, it will have some minimum 'on' time to reach it's full efficiency. It just makes sense to run long enough to reach that point and stay there. That is why sizing a heating system for the real load is so important, not only for efficiency, but for comfort as well. Having a constant heat input that offsets the losses means more comfort rather than a hot blast followed by a cool-down. WHen you shut a system off, all of the heat you put into it is not moved into the load side, and is lost; either up the flue, or in just sitting there warming your utility room (not necessarily bad, but probably not all that useful either). This is where a modulating heating system that can adjust to the load can come in and be much more efficient.

On the cooling side, it is even more important, especially where you want to control the humidity...an oversized unit means cool, clammy space. This is where an undersized unit, running full tilt will be more comfortable since it will pass the humid air across the heat exchanger more often, and extract more moisture, lowering the humidity, and making things much more comfortable.

01-03-2010, 03:00 PM
I dont think the boiler is short cycling. This is what it does, the thermostat calls for heat the boiler fires and gets up to temp really fast. The outdoor reset isnt doing much right now because it like 5 degrees out right now. So it gets up to temp clicks off and climbes on average 5 degrees above the high limit. The zone circulator continues,if it continues long enough and the boiler losses temp it will refire sometimes after 10 minutes of shutdown. If the zones are satisfied the boiler will not turn back on at all. So I think i'm not short cycling.?

01-03-2010, 03:30 PM
When a call for heat is active, how long does the boiler run? Ideally, it would run continuously if it was sized properly on the cold design day. That rarely happens, since not every day is the design day. That's where a modulating boiler comes in, it can adjust it's output to the day. So, the on time verses off time is the relevant info. AN outside reset helps (if set up properly) by adjusting the supply temp to allow things to run more often.

01-03-2010, 03:51 PM
It runs for about 5-7 minutes.

Doherty Plumbing
01-03-2010, 07:30 PM
It runs for about 5-7 minutes.

If you're temps are getting back to where they should be then such a short run time would be indicative of an over sized appliance for the heating load it.

01-03-2010, 07:36 PM
If that's the case when I add a zone in the basement hopefully the boiler won't be oversized.

Doherty Plumbing
01-04-2010, 09:51 AM
If that's the case when I add a zone in the basement hopefully the boiler won't be oversized.

Depending on the load that zone puts on the system I doubt you'll see much improvement on the run times. But hopefully you get up to about 10 minutes or so per run.

10 minutes isn't so bad!