Hot Water Boiler Short Cycling

Brad White_9

can be a bit subjective/interpretive.

Consider that any time a boiler's burner turns on, it takes about two minutes for it to reach peak efficiency. Warming the flue passages, stabilizing the flame, balancing emmissions (CO, CO2, NO2 etc.).

If a boiler stops before that period is up (due to reaching setpoint or for any other reason) that would definitely be short-cycling in my book. It is like an 18 wheeler in stop and go traffic. By the time you get up to speed or worse, just get going, you stop again. What fun is that?

When you have an on-off boiler (no modulation) this becomes more prevalent and the more the boiler is over-sized, the more often this will occur.

That you are over 200% over-sized, I find it amazing that it is not worse than it is, modulation included. Remember, if correct, your calculated heat loss occurs 1% of the season or less in terms of total hours. Most of the time you will be well above that, hence cycling even more. But as you said, the boiler holds to firing until the house is satisfied so hard to argue that you are short-cycling.

I presume your modulation is holding you in the firing mode longer, backing off on input rather than shutting down on high limit. Such is the beauty of modulation.

Efficiency? Are you condensing consistently? That is the first thing I would look for. Calculating true efficiency as you may read from these other postings is science bordering on art.

My $0.02,

Brad

Trevor Baptista

What exactly is short cycling? Is it the boiler turning off and on due to the supply water temp reaching its set point? Or is it the heating system as a whole turning on an off from frequent calls from the T-Stat. I have a WM ultra 230 for hydro air heat. I am significantly oversized, the IBR rating is 183,000 btu/h and my heat loss calc is around 70,000 btu/h. Anyway, I have read the manuals inside and out and changed a lot of the parameters on the boiler. So now, when there is a heat call the boiler fires and stays on at the correct water temp until the call ends. In my opionion, I don't think I have a short cyclling problem even though the boiler is greatly oversized. Also, I believe that I am still getting the effeciency from it that I am supposed to. Is this correct? All comments are welcome!

Jack P.

When you state the boiler is shutting off, do you mean both the burner and the circulator? If the circulator is still pumping water and the stat is still calling you have reached the aquastat’s high set point. The circulator will continue to run until the stat is satisfied or the water cools to restart the burners. That is not a short-cycle. If both the burners and circulator stop and you haven’t reach the stat’s set point you are short-cycling. Correcting that would take a higher adjustment to the stat’s anticipator setting. As far as efficiency, the best is to set the aquastat as low as you can and still have enough heat to satisfy the stat. That setting could be changed up and down as the weather gets warmer and colder with the season. That way you are only making enough heat that is needed without over-firing and letting the heat sit in the pipes without being used.

Ron Schroeder

My definition of short cycling is when the BURNER has not run for a long enough period of time that the average efficiency during that burn is noticably less than the steady state efficiency.

Other parts of the system can short cycle too, like circs. turning on and off to often but that mostly effects lifetime, not fuel usage.

Trevor Baptista

The boiler runs until the t-stat is satisified. It can hold the required outlet (supply) water temperature for as long as the call from the T-stat is with out shuting off due to it hitting the off differential. A typical call is a minimum of ten minutes as well. Also, the same holds true when ever the DHW tank calls. So I just wanted to stick it to a lot of people who told me that the boiler would be to big and it would short cycle a lot. In regards to condensing, I am not sure about that however, the condensate pump does usually turn on and pump for each call.

Ron Schroeder

Hi Trevor,

Since the WM Ultra is low mass and modulating, it has downfired itself to prevent short cycling. A smaller Ultra would be only slightly more efficient (it could modulate to a lower output) but the main differance would be the cost of the Ultra 230 vs. an Ultra 80 or 105.

Trevor Baptista

Actually, the boiler sometimes goes over the set water temp by 4 to 6 degrees but it can modulate low enough to bring the temp back down with out shutting off, it is quite remarkable. The cost of the different boilers was not a big deal for me since I got the boiler at cost and did most of the install work myself. I plan on at a minimum doubling my house and adding heat in the basement and other things so I wanted to insure myself I would never run out of BTU's. And I must admit with the amount of oversizing I did I too am shocked that the boiler does not short cycle.

Ron Schroeder

A few degree overshoot is no problem.

Remember, throttled down, it's only a 45,000BTU boiler.