Cycling Steam Boiler

Steve_211

This past heating season, my steam boiler started cycling. On the vaporstat, I have the upper limit set at 12 oz, the lower at 4 oz. When it reached the cut off setting, the pressure gauge showed the pressure decreasing. Within a minute the 4 oz cut-in was reached, firing up the boiler. To lessen the cycling, I set the vaporstat to the lowest settings of my old Honeywell pressuretrol, 1 1/2 lb cut-off, .5 on the differential (8 oz). I also noticed during the season I was consuming makeup water. I don't know the amount, but I would add water to bring the sightglass from the LWCO level up to 3/4 from the maximum level. I would be doing this every day and a half to two days. The first radiator off the main is the only one hissing under pressure. It appears to be at the shut-off valve, probably the valve stem packing. All air valves on my 7 radiators were replaced last year with Varivalves. They're fine. Only about the upper halves of all the radiators were getting hot, though. The lower 1/3 were cold. Any opinions on what is happening?

Jamie Hall

Assuming that the vapourstat is not damaged or defective, that the pigtail is clear into the boiler, and that usual sort of stuff, there are only three reasons the cycle timing of a given boiler will change: either the firing rate has changed, or one or more of the main vents has changed, or the heating load has changed. If I take it from your post that the on part of the cycle is now shorter at the same pressure settings than it had been, you are looking at a reduced heating load, a stuck shut main vent, or an increased firing rate.

It is entirely normal for the pressure to drop very quickly when the burner shuts down.

Now if you are adding water, that's a different problem It could be just that leaking valve you mention. It could also be other leaks. You need to find the problem and fix it -- adding water just isn't good for the boiler. Increased pressure will, by the way, increase the leakage rate.

How much of a radiator gets hot is determined by the cycle length and the venting, on one pipe systems. However, it is quite irrelevant -- the question is is the space maintaining the desired temperature, not is the radiator hot.

Steve_211

You say, Jamie, that it is entirely normal for the pressure to drop very quickly when the burner shuts down. Isn't that the purpose of the main and radiator vents, to hold the pressure? Barring the temperature setting on the thermostat not being reached, as the air gets displaced by steam, the vents close, allowing the pressure to build until it reaches the cut-off setting on the vaporstat? The main vent seems fine. The internal valve is loose and rattles when cold ( open? ) and doesn't when hot ( closed? ). All the radiator vents were replaced the prior year; they're functioning normally. Aside from the first radiator shut-off valve, could it be the boiler itself? It's a Dunkirk ESC3100, installed October of 2010. Could an internal leak have developed in the boiler? There's no white exhaust coming out the chimney.

109A_5

Hello @Steve_211,

A well designed residential type heating system should not make much pressure, maybe a few inches of Water Column. The Sq. Ft. of the boiler should closely match the EDR. The Pressuretrol is just a safety device that should never actually come into play. Like if a 6 year old closes most or all the radiator valves in the home.

Pressuretrols and Vaporstats used to control pressure are just a Band-Aid approach to tame an oversized boiler. Since the boiler was not originally sized properly.

Where air is steam is not. With a one pipe system the radiator vents and main vents are to purge the air out of the system, then close to not let the steam out, wasting water, requiring oxygenated water to be added to the system. If the steam is all the way to the radiator vents there really is no more reason to fire the boiler any more, the radiators will be hot. You don't need any additional pressure. The steam moves fairly well with few inches of Water Column.

If there is pressure and the boiler is shut down the pressure will decrease since the radiators are constantly condensing. The pressure would drop faster if the system also has a leak.

If I had an oversized boiler that cycles on pressure I would add a delay timer (minutes) so the radiators had a chance to warm the environment and fuel is not wasted making pressure. The over all heating of the environment may be more even too. And less system wear and tear from cycling.

ethicalpaul

There are a couple of misconceptions in your post above. For starters:

Isn't that the purpose of the main and radiator vents, to hold the pressure?

No, that's not their purpose, although they do prevent steam from escaping which does help to increase the pressure. Pressure over a few ounces/sq in is not useful to your steam system. Their purpose is to let the air escape the main lines quickly so all that air doesn't have to get pushed through your radiator vents, allowing steam to arrive at more or less the same time at your radiators.

You can't hold pressure when the boiler stops firing because the steam in the system will continue to condense as it always does and it's not being replaced with new steam. Steam is 1700 times the volume of water, so this causes a rapid drop in pressure.

Depending on the vents you had before and the new ones, that change in venting rate could be causing your pressure build-up to happen sooner and more often than it did with your old vents.

ethicalpaul

You have a leak of either liquid water or steam that is causing your water level to drop. That is probably unrelated to your cycling issue. In fact, it's possibly inversely related because any leak of steam (whether through a hole in the boiler, or a failed vent, etc) will act to lower the pressure in your system.

So if it were me, I would separate these two issues in your mind and attack them separately.

neilc

I'm gonna go the inverse of Paul,
and make an assumption, or a few,
you noticed the cycling,
prior, the boiler built pressure and held it,
now with the leak, the pressure wanes quickly,
and you noticed the cycling,
leakage is confirmed by having to add water,
find the leaking vent, or valve, or boiler, or combination,
flood the boiler to the header , , ,

Jamie Hall

Moving away a little from the ideal world where, for some reason, the output steaming rate of the boiler exactly matches the rate at which the radiation condenses steam (you can get close. Most of the time. Since both rates will vary depending on other conditions, that's luck...)...

How do you find out if you have adequate main venting? Oddly, it's not a bit difficult -- but it does require a sensitive pressure gauge, calibrated in ounces per square inch or inches of water, and a little patience.

To do it, let's consider a residential steam system, starting cold. The burner fires. At first, you will see no pressure change at all. This is the time that it takes to bring the boiler water -- and the boiler -- from cold to the boiling point. It's not a fixed time -- "cold" is rather poorly defined -- but there's nothing to be done about it anyway. Now when the boiler starts to actually make steam, you will note a very slight pressure rise (may be hard to see, particularly if the gauge isn't damped, or is on the boiler rather than on the header, where it belongs). This very slight pressure is needed to persuade the steam to move into the header and steam mains, and the air to move out. Over a period of time, which depends on the size of the system, especially the piping, and the pipe insulation, this pressure will increase. Depending entirely on the system, however, it should not increase much above a few ounces, and in some smaller systems it may not increase much over an ounce or so. Then, the pressure should stabilise, or very nearly stabilise. Furthermore, you main vents or crossover traps should be closed. If the pressure does not stabilise, or very nearly stabilise, your main vents are too small (it's unlikely in the extreme that your crossover traps are too small). After some period of time, which will depend on how closely matched your boiler is to the rest of the system, the pressure may start to rise again. This is the point at which all the radiation is condensing at its maximum rate, and any further pressure increase is useless. Therefore, this is the point at which your control (NOT safety) arrangement should shut the boiler off. If you are using a mercury vapourstat, which is very precise, I'd suggest setting it to allow that pressure to increase to perhaps two ounces over the running pressure. A mechanical vapourstat or pressuretrol, which is much less precise, should be more than that -- say four to six ounces. Further, if the control device is on the boiler rather than the header, it may need to add another ounce or two to allow for pressure variation within the boiler.

And there you are.

Now if the pressure doesn't stabilise, but keeps steadily rising, you need more main venting. If it is stable or nearly so, you don't.

As @ethicalpaul pointed out, once the burner stops the pressure will drop very rapidly -- typically less than 15 to 30 seconds. At this point one runs into a difference of opinion. Should the boiler refire as soon as it can (oil burners, particularly, have a post purge and pre purge time out which limits that time out), or should you use a timer to hold the boiler off until the radiation has cooled, or for some other interval. This debate has been going on ever since I started frequenting The Wall. In my opinion, the reason it keeps going is that either strategy is quite valid, depending on how closely the boiler matches the load, and on how closely the installed load matches the heat loss of the building. Clearly if the three are very closely matched, forcing the boiler to hold off for any length of time is a poor choice. Let the poor thing run, and if -- through a slight mismatch between the boiler and the radiation a short time out is needed, let your pressure control manage it. On the other hand, if the radiation is wildly oversize for the building, holding it off for a while to let the radiation cool may well make good sense. Likewise, if the boiler is seriously oversize for the radiation, using a timer or other device to reduce its effective output is quite valid (just how long the total cycle length should be is another topic entirely).

As I've said before, and I'll say again, not two systems are going to be the same -- and therefore no one control strategy is going to be optimal for all systems.

And the debate will roll on...

At some risk of muddying the water, I will note that in one pipe steam systems, if the vents on the radiators are used to balance the system, or thermostatic vents are used, the system much cycle off and drop to atmospheric to maintain control.

Steve_211

Could the boiler, indeed, have developed a leak? During the past heating season, there was no water on the floor and no telltale exhaust coming out the chimney.

As info, it's a one pipe steam system, 1250 sq.ft. house, 3 radiators on the first floor, 4 on the second, none in the attic. The Varivents replaced the Gorton's and are used to balance the system.

ethicalpaul

There are only two kinds of boilers: ones with leaks and ones with no leaks yet. It is every boiler’s destiny to rot out. 

Unless they are foolishly ripped out and replaced with forced air.

reggi

@Steve_211 I didn't notice if it was asked but did you have any additives introduced to your system? 
Is your water purple ? 

Steve_211

Answering your question reggi, the water isn't purple. No additives were added, ever.

So, how do you go about determining that it is the boiler that is leaking?

STEAM DOCTOR

Fill the boiler, with water, to above top of boiler. Look underneath for water. 

Steve_211

Steam Doctor, you say to fill the boiler with water above the top of boiler. Neilc said in an earlier post to fill the boiler to the header. How do you know when you've reached that point? Do I remove the cap on the skim port nipple, or the pressure relief valve, or is it a guesstimate?

exqheat

Watch the rate of rising water inthe glass. This will give you an idea of how long to leave water on to fill the boiler.

ethicalpaul

If you're nervous, you can remove your main vent so that if you over fill it you'll see the water spill. Better there than at your radiators.

bburd

If you keep your hand on the steam riser coming out of the top of the boiler, you will feel a change in temperature when the water gets there. Then turn the water off.