Short cycling

pocono123

I have a two pipe steam system with 590 ft of edr. The boiler is a weil McLain SGO-7 rated at 750 square feet of edr or 180K btus. When the boiler runs for an extended period it builds up pressure and pushes the water level down creating a low water situation. The boiler shuts down for a minute or two and starts up as more condensate returns and the pressure drops. Is the boiler THAT oversized? This only seems to happen if the Thermostat calls for heat and the radiators are as hot as they can get. Seems like the steam has nowhere to go. Pressure rises to about 2 psi which is enough to push down the water level in the boiler. What is the solution? Should I get someone to lower the boilers btus? It seems that when the house is at desired temperature everything works fine. Will this situation only occur when the boiler is asked to raise the house temperature a large amount? This is a second home and the heat is set way back when we are not here.

Ironman

The pressure should not exceed 2 psi and could probably be set even lower depending upon the system.

There's no real way to "lower the btus" unless the gas pressure is too high at the burner manifold.

More than likely, you have an issue with clogged returns and/or malfunctioning traps if the water level is going down near the end of the cycle.

Is there a pump on the return near the boiler or or is it a gravity return?

BobC

I agree with ironman on the possibility of partially clogged returns. Did this problem just start or is it an ongoing problem? Pictures of the boiler and the pipes around it would help us see what might be going on. Also does the system have a cycleguard LWCO on it?

Most 2 pipe steam systems function best at low pressure -below 1 PSI. Does the system have a pressuretrol or a vaporstat to control the pressure? If it has a vaporstat try lowering the cutout pressure to less than 1 PSI and check the returns to make sure they are all open.

Bob

pocono123

The system is gravity and oil fired. The returns are not clogged. It has a Hydrolevel CG450-1560 LWCO. My theory is this. If the radiators are at their maximum temperature and the Thermostat calls for heat, the boiler will comply. However if the radiators are as hot as they are going to be then the steam being created by the boiler has nowhere to go. Hence the build up in pressure. If the pressuretrol is set at say 1.5 lbs. cutout then I probably would not get the low water cutoff but I would still get short cycling. As soon as the pressure dropped, the boiler would kick back in. These modern boilers are designed to run for shorter periods. If the existing temp in the house is low, it will take a while to raise the temperature. The thermostat is going to constantly call for heat. The ability of the radiators to give off heat is limited.

Fizz

The SGO 7 fires at 240K BTU's/hr. The SGO 6 which I have is rated at 640 SF of steam, firing at 210K's. You might want to consider changing the nozzle to one used in SGO 6 which consumes 1.75 gal of oil/hr, vs 2.0 in SGO7. That being said, I converted to gas 5 yrs ago with great saving, of course oil has come down, but conversion may still be viable.

Jamie Hall

First, since it is oil fired, you can probably down fire it, as @Fizz suggested. That is the first and simplest thing, and will help with the short cycling problem -- which is due to the fact that you have a boiler producing 750 feet of steam which you are trying to squeeze into a system which can only condense 590 feet. Won't fit. No way. Yes, the boiler is that oversized.

Then, if you don't have a vapourstat, get one and set it to cut out somewhere around 12 ounces and cut back in at about 6 ounces. That will almost certainly cure the problem of water backing out of the boiler. Unless you can down fire the boiler enough, you will still get short cycling on a long recovery. It's the only way the system can make the steam production rate match the steam condensation rate.

Your problem is not slow returns. It is an oversized boiler and excessive pressure. Both of which can be fixed.

EBEBRATT-Ed

Lets see some pictures of the header and equalizer. An oversized boiler will short cycle but shouldn't go off on low water.

My guess is an equalizer,header or Hartford loop problem

Ironman

@pocono123,
If you know for certain that your returns aren't restricted, have you tried setting the pressuretrol lower? Try it with 1 psi cut off. Also, check the pig tail on it to make sure it's clear.

If the pressure gets too high, or you don't have enough "B" dimension, it could cause what you're describing.

pocono123

In response to Jamie Hall's post, The difference between installed edr and the boilers' edr is + 160. Dan Holohan in his book says that the boiler should be 1.3 times installed edr. Using that metric the boiler is right on where it should be. 1.3 X 590 = 767.

By the way all comments are Greatly appreciated !!!

Jamie Hall

Ah... well, no. If you are using the boiler's stated EDR, that takes into account the 1.3 factor already. You only use that factor if you are playing with the net BTU output of the boiler vs. the net BTU demand of the system.

pocono123

I have posted a few pix of the boiler setup. In response to Ironmans' post, the "B" dimension is 31". Due to existing factors (beams, conduit, soil line, and water pipes etc.) could not run it higher.

pocono123

Sorry, San Diego zoo pix by mistake.

pocono123

Sorry, San Diego Zoo pix instead by mistake.

pocono123

In response to Jamie Hall, I did not know that. Very helpful, Thank You. Could have bought a slightly smaller version and saved a few bucks, Seems simple enough to downfire. Any pitfalls to look for on that approach?

Jamie Hall

I'm sure someone will know just how far down the SGO-7 can be down fired -- but I'm quite sure it can be. The main thing when doing that, though, is to make sure that the right nozzle flow rate, angle, and type is used -- and that the combustion is set up by the digital meters. You can't do it by guesswork or by eye and come even close!

Fred

As has been said, the boiler is well over sized but there is the issue of setting the thermostat back and then causing the boiler to run for a very long cycle to bring the house up to temp. If you are going to use a set back, I wouldn't do more than about three degrees, especially knowing that the boiler is over sized. That will minimize the short cycling. The other thing to try is a good skim. Has there been any new piping (or boiler) added to the system? If so, was the boiler ever properly skimmed from a skim port above the boiler water line? Draining water and/or adding chemicals won't do the trick. Oils from any new piping and boiler will sit on top of the water in the boiler and make it hard for steam bubbles to break through the surface of the water. When that happens, water tends to get pushed back into the returns, water level drops, boiler shuts down on low water, after shut down, water quickly flows back into boiler and it fires up again. I had this problem years ago and after several good, slow skims, my problem was resolved. It may take several skims to clear all the oils off of the surface of the water but I suspect that may be your problem. Set backs don't help when the boiler is over-sized either but that will usually cause the boiler to shut down on pressure, not low water.

Ironman

I'm not sure what effect it will have, but the equalizer is not piped correctly. It should tie into the top of the Tee on the boiler return and the wet return should enter the side of the Tee. In other words, they're backwards.

If your "B" dimension is 31", then you cannot exceed 1 psi on your pressure. 8oz would probably be better.

nicholas bonham-carter

With the backwardly piped equalizer/Hartford loop, the returning condensate will have to stack up in the wet returns a couple of inches before it can return to the boiler. That may be the straw which broke the camels back.
Are all the wet returns at the same low height, or is there some rise in them beyond the wall?
This system should be able to operate at a couple of ounces, with no setback, while you are there.--NBC

EBEBRATT-Ed

It's why I suggested pictures. The backwards Hartford loop could be the problem

nicholas bonham-carter

When I look at the header on that boiler, I see that it is not the ideal arangement to return any water blown up into the supplies immediately back to the equalizer. At least half of any wet steam will have to take the long way around through many feet of main, to get into the wet returns.
Modern boilers with their smaller water content are much more sensitive to improper piping than the old ones they replace.
I would suggest redoing the risers and supply with a big diameter drop header, and new equalizer. This should feed the main at what is now the equalizer connection, with the old original tee plugged. The new equalizer can be finally have a correct Hartford loop. Luckily the original piping doesn't look so old as to be a real chore to take apart where needed.
So first give it a good skimming, and down-firing, and if, when winter comes back, the symptoms of low water persist, be ready to repipe. Frequent low water shutdowns are quite hard on a boiler from thermal shock.
The somewhat recent installer may be willing to give you a discounted price to correct his mistakes.--NBC

Fizz

Jamie is absolutely right when it comes to downfiring. Have a pro do it who is familiar with the process. When we switched to gas, we fired the boiler at 186K, slightly over 12% decrease, with no issues.

nicholas bonham-carter

Another potential source of problems with pressure pushing up the water level in the returns is bad main venting. In a two-pipe system, the venting is often handled by crossover traps which, if failed closed requires excessive pressure to squeeze out the air from the supplies before steam can enter the radiators.
Look at your system, and map out how the air escapes, when steam is rising. Make sure that it has no impediment. This can be measured using a good low-pressure gauge, (0-3 psi, Valworx). During the venting phase, as steam is pushing out the air, the back pressure should be only a couple of ounces. Any higher pressure, and you have a restriction.--NBC