please define short cycling?

Steve_168

David, it takes about 10-18 minutes for the pressure to build that high on the unit. After that the unit cycles ON and OFF constantly due to the pressure. i'd say on for 20-25 seconds and off for about 20 seconds. I know this cant be normal, but i just cant figure what is happening.
new vents on main, new vents on radiators.

I spent quite a bit of money on this install and i dont want it to break down in 3 years.

Steve_168

please define short cycling?

hi all. i have a single pipe steam heating and have a quick question on short cycling. I've been reading alot bout it ont his forum, but wondering if my problem is indeed short cycling or is it normal.

As instructed by Dan's book, i lowered my pressuretrol to 2.5 main, 1.5 differential.

Pressure builds on my burnham unit and cycles off due to limit reached. T-Stat is still calling for heat and the unit will cycle back on once it hits the low limit.
This will go back and forth and back and forth for numerous times until the T-Stat is satisfied.
Is this short cycling? is it healthy for boiler?

Thanks.

Brad White_203

Time Component

How much time elapses between the cut-in and cut-out? That is what is missing to me. If 20 seconds after cut-out it cuts in again, you have a serious steam leak or condensing issue. If four minutes, that sounds really good, knowing nothing about your system.

You could probably go lower on your pressure, in fact, I think you should.

Another general point about short-cycling: Burner On-Time should be, IMHO, at least 1.5 to 2.0 minutes just to become stable (where the boiler combustion process reaches a "leveling off" and a good balance of combustion products, not too much CO and measurably stable CO2, O2 and stack temperature, etc.) If it shuts down before this point, you have basically used a booster rocket to clear the tower, then stopped.

I think you will find lots of opinions on what constitutes a "subjective undefinable" and I for one am eager to hear what others think.

nicholas bonham-carter

short cycling

many things around us are designed to cycle on and off: automobile a/c; gas oven controls; etc. to name a few.
just the same with your boiler, as the flame is either on or off; except for those fortunate enough to have a modulating burner.
i do think you could lower your pressure to below 16 oz. with improved comfort and economy.ideally you should have a vaporstat, and a good low-pressure gauge. right now if i read your post right, the boiler cuts out at 2.5 psi, and in at 1.5 psi.
many steam vents cannot operate effectively above 2 psi, and may become paperweights above that.
on my 54 rad system, the vents have removed the air from the mains by 3 oz.[more quickly than if i had a pressure above 1 psi]. so see how low you can go. at least get a good 0-5 psi gauge [gaugestore.com] to see where you are.--nbc

Steve_168

Brad, once the pressure builds and cuts off, the unit will cycle back on after 20-30 seconds(pressure drops rather quick once the pressuretrol shuts it off) until it reaches the high limit again. This will go on and on until T-Stat is satisfied. Im worried that this will eventually break my new boiler!!
it is not a leak as i replaced this boiler last year. No steam is leaking out of chimney, so you think its a condensing issue?
how do i resolve that? is this a piping matter?

David Nadle

Wish I knew

I've got the same problem and I cut out at 10 ounces. The boiler loses it all in under a minute. While the thermostat is still calling the cycle time is then 40 seconds on, 40 seconds off. Instinct alone tells me that's wrong.

I'm experimenting with delay relays that will hold the boiler off after a pressure cut-out to (a) give the thermal mass a chance to get the job done, (b) give the condensate a chance to get back to the boiler, and (c) stretch out the off and on times to not be so hard on the electromechanical parts of the boiler.

I'd love to hear your answer to Brad's question as well. How long until you reach 2.5 the first time, and what are the off and on times afterwards?

Brad White_203

Condensing...

Good to know that the "open to atmosphere" issue is probably not a concern. That narrows it down nicely.

In a normally operating steam system, the biggest pressure drop is condensing. In the radiators, you want that! In the piping? Not so much.

How well insulated is your piping? Particularly the steam supply, less so the condensate returns as far as condensing is concerned.

20-30 seconds sounds too short once the system is warmed, but every system is different. A minute might make me "feel better", but how subjective is that? During a warm-up, a cycle or several like that would not concern me.

Once the system is warmed, I would want to see a longer off-time. But then, condensing is essential here! Without that, the steam does no useful work. Like your brother-in-law, it sits around the living room promising you the world and delivering bupkes.

Think of it this way: From a cold start and with proper/generous venting, the pressure will of course be zero to start and will be nearly so until the vents start to close. First the main vents, then the radiators, ideally at the same time, but often one by one.

Once the vents are fully closed (or if insufficiently sized, plugged or "gone missing" -another issue!), the pressure will start to build above that minimum vapor state. This will allow your first "cut-out" for this heating cycle series.

After the first "fill" of steam, the rooms are still cold, so the condensing within the radiators will be happening more rapidly than at any time in a cycle. This of course drops the pressure.

Then comes the random opening of radiator vents (to relieve vacuum), which at least makes the pressure more "neutral"; vacuum being the lowest pressure of all.


Then comes their re-closing, upon seeing steam, which again will allow pressure building. These occurrences will go on for a while, even once the rooms are warm.

Say by now you are fully heating your radiators and with them, your rooms, to temperature. Pressure will ebb and flow but at a somewhat slower rate.

The condensing at this point is principally within the radiators (assuming that your piping is insulated).

The condensing will continue of course, because to steam at 215 degrees, a 70 degree room does not seem a whole lot warmer than a 60 degree room. A slowing of pressure loss due to condensing, but not nearly so much as steam hitting 50 degree bare iron.

So, this sorry excuse for a ramble gets me to ask you to think about what "non-beneficial" condensing might be going on? What could be contributing to a rapid drop in pressure once the firing stops?

I see no "Right versus Wrong" here. Rather it is just seeing how your system, a unique thing, does at it's best. Within this, and the experience of others, we can see what range of "normal" we can find and how you fall within that.

pete m.

Had the same question

I am running a WM Ultra2 155, just started it up this season. It runs about 10 minutes, then shuts down for about 10-15 minutes... pretty much on the same cycle day and night. Runs a little longer when very cold out. Sixed the boiler for both future expansion and for the indirect water heater. Heat loss for the house, as is, is around 45k BTU's. Yes I know it is way oversize, but I was thinking about future load for addition and shop. Using ODR, with supply set at 140 degrees and using panel radiators with TRV's. Designed the system so that Delta T would be 30 degrees. It is about 25 degrees so far.

So am I short cycling?

Brad, I saw your picture of the Wilo in your house. What does it cost?

David Nadle

Thoughts

You may have one or more vents that are not holding. At 2.5 psi you may have condensate stacked up in the returns or equalizer, causing wet steam, which hits the vents and causes them to open and hiss or gurgle, then ping shut, repeat ad nauseam. Does this sound familiar?

Then, when you do cut out, you have either (a) vents that are not shut or not holding or (b) a lot of moisture causing rapid condensation of remaining steam and strong vacuum, or (c) both.

Even if I'm 100% correct about the causes of the rapid loss of pressure, which is a big if, I haven't found a solution for my own system. That's why I'm playing with delay relays.

I do think you should lower your cut-out to 1.5 psi. You can't make the short cycling any worse.

We should be realistic about the damage of short cycling as well. Compressors can be damaged by short cycling, but to the damper, igniter, and gas valve it's just one more go. Not the same risk. I don't know about oil burner pumps.

Fred Harwood_2

Cycling

A well matched boiler will cycle on for less than ten minutes and not shut down on pressure. Properly vented, the mains will fill in about three mins, and each rad will get steam, proportional to the rad vent and room BTU losses. Your thermostat anticipation should allow you to set your cycles to about 25 minutes, from on to on again.

As outside temps cool, the boiler will stay on a bit longer, but seldom to the point of filling a radiator, except on design day. If your boiler runs long to get steam to the rads, insulation, piping, or venting usually is the problem. If you're short-cycling on pressure, either your thermostat can't see the heat, or your anticipation is way off.

You should not hear hisses or spits, and temperatures should not overshoot by more than 2 degrees. Short cycling frequently can be traced to oversized boiler (down fire it) and/or inadequate venting or missing mains insulation.

Phil_17

Boiler could be a bit oversized too...

That's what it sounds like to me at least.

If it's oil fired you can ask your serviceman to check into down-firing the unit a bit. That will make it take longer to get to the pressure cutout point (but perhaps burning less fuel doing so) and then it will need to run longer on each cycle to reacquire that pressure setpoint. I think that an ideal firing rate would get you to the pressure cutoff at the same time that the tstat is satisfied...

Remember the old coal fired boilers ran all day long, never reaching any meaningful pressure at all (there was no way to turn the fire off even if it did) and I think that's what we would like to approximate in a perfect world (i.e. modulating controls)

bob_114

Wealth of information and all very helpful.
Thanks to Brad for getting this kicked off.
Thanks everyone for your inputs.

i will get a servicemen to check on it.

Kara

Sorry for the double post!

Kara

The obvious "cure" for a lot of the short cycling problems is a modulated oil burner. My question is: Are we likely to see one anytime soon?

My steam system is perfectly matched to the house (200 years old) but due to the cost of fuel ($7000 last year) we have shut down the heating in a lot of the unused rooms which has resulted in the boiler being way oversized and short cycling.

Mark Custis

Keep an eye on

Beckett