Qestion re: Cycling Off On Pressure

Brad White_203

All that you have been observing are quite reasonable. The fact that you are getting to six ounces (boiler at 12) then a drop, is entirely normal.

What that tells me is that your system piping pressure drop is six ounces, not unheard of.

Some may argue that two ounces is normal but hey, you are nicely in the low zone. A traditional old-time rule of thumb is that the total pressure drop through the piping be about 20 percent of the total pressure at the boiler. I am sure it worked but I would not lose too much sleep over it, if you do not get there.

That slight vacuum is OK if for a very short time, by which point the vent cools and breaks the vacuum. Seems rational.

That the boiler may cycle off before all radiators see steam says that venting can still be improved, that air is not releasing as fast as steam can compress it.

Overall though, I would say you are doing a lot of nice work there. Any fuel consumption results to report yet or is it too soon?

As for the definition of short cycling, I would say that any firing less than two minutes (by which time combustion products become relatively stable and efficiency is near peak) is one criteria. The second would be that steam is getting to the far reaches reasonably quickly and not just hanging up in the boiler due to poor venting.

I would expect the first batch (cycle) of steam to be the shortest duration between firings, as the collapse of the steam just generated will be most rapid. Once warmed, the subsequent cycles will hold for longer periods between firings. Very general observations.

All buildings are different. Yours seems to be doing very well. I bet your co-owners are glad you are on the case.

Frank_63

Is This Correct?

This probably is a basic concept that I should I understand, but I want to make sure I do.

A pressuretrol or vaporstat will shut down a boiler based on the pressure inside the boiler, correct? And the amound of pressure elsewhere in the system, such as in a radiator or steam pipe, will depend on its proximity to the boiler. If the system is set to cut out at "X psi or ounces," it will shut down when that level is reached inside the boiler, and the amount of pressure further down the system, like a rad or pipe, will decrease the further away you are from the boiler.

Kool Rod

Frank - The pressuretrol reacts to the pressure at the point where it is attached to the system.ie: The boiler. If the system is closed (nothing happening - think a car tire) the pressure would be about the same anywhere in the system. However in a steam system there are a lot of other things going on which change the pressures locally all over the system: Condensing steam creates a vacuum, vents opening and closing etc etc. Ideally if the steam system was perfectly balanced (steam production vs need) you wouldn't need a pressuretrol and the only thing controlling the system would be the thermostat when the room reached the set temperature. As there are so many changing variables a "perfect" balancing isn't achievable with the boiler/burners we have now hence the need to control the boiler with a high and low pressure pressure on/off switch.

I'm not quite sure where you were going with your question.
I haven't found that taking pressure readings at other locations in the system much benefit other than you can detect a spike when the vents close.
- Rod

Brad White_203

That is correct

All pressure "decays", the further it is from the source, with flow present and at a constant temperature.

Edit: I just read what Rod posted and he too is correct- all of those variables illustrate the basic truths.

Frank_63

I asked because . . .

After installing a low pressure gauge on one of my radiators, I can now see that the boiler cycles off before the t-stat is satisfied, and I'm trying to figure out if my system is performing appropriately and not short cycling. There are a lot of posts on this board about short cycling, but I'm still not sure I understand the difference between short cycling and appropriate cycling off due to pressure.

During a long run, such as after a setback, the pressure in the rad, as measured in the gauge, will rise to between 6 and 8 ounces before it drops, for about 60 seconds, to less than zero. The pressure will then rise for three minutes before it drops again.

I'm pretty sure that the pressure drop is due to the boiler turning off, as opposed to some other variable, b/c I was down in the basement one morning during the cycle from the nighttime setback and noticed that the vaporstat turned off the boiler for a short time of no more than one minute before turning it back on.

I'm just trying to determine whether this cycling pattern is consistent with appropriate operation or a short cycling problem that should be evaulated by a professional.

Lastly, I asked my question the way I did b/c the boiler is at one end of the apt building, and my rad is located almost at the other end of the building, and on the second floor. So I was wondering if say the vaporstat was set for 12 ounce cut out, is it possible that the cut out could be reached in the boiler, but the pressure in the rad could not have risen that high yet.

Kool Rod

Frank -I agree with Brad, you're seem to be doing pretty well with what you've got. Having an "OFF" and "HIGH" source for heat really rather limits what you can do. As Brad mentioned you might try to speed up the radiator venting and see if that helps.

Brad - Have you heard anything more on modulated oil burners for residential boilers ? There was a rumor a year`or so ago that they were in the works.

FJL

Follow Up If I Might . . .

Brad: The "slight vacuum" to which you refer is the 60 second drop in pressure that I described, correct? And are you saying that this vacuum is caused locally, by the action of the vent, rather than a vacuum created when the boiler turns off? My assumption is that it is the latter, but obviously I'm not in the basement observing the boiler to be sure.

The risers are well vented, and it seems that the steam does travel quickly to the top of each riser. Your suggesting to vent the rads faster creates the dilemma that I have been working through this heating season. I have been trying to vent my rads slowly to minimize my overheating. Maybe the solution is the vent the rads fast and attempt to control temp with TRVs.

Rod: Can you explain what you mean by my having an "off" and "high" source for heat that limits what I can do. I'm not sure I know to what you are referring.

And thanks to both for your time.

Kool Rod

HI Frank- What I meant is your boiler has only 2 heat settings: "Off" and "High". This is rather limiting. Think how it would be to cook spaghetti on a stove that only had these two settings. You turn on the heat to "High" till the water boiled too violently (You can't move the pot! ) and then you'd have to turn it to "OFF" and wait until the water almost stopped boiling at which time you turn it back to "High" again. You would repeat this on ("High") and off cycle until the spaghetti was cooked.Your boiler is making steam the same way with a High /Off cycle.

This "Off"/"High" system wouldn't be acceptable in the kitchen. As anyone who has cooked spaghetti knows you turn the heat on high till the water boils and then turn the heat down (to "Medium Low"?) to just enough heat to keep the water boiling. The other possibility is to just the pan on the stove and set it to "Medium Low" and after a longer period of time the water will finally boil.

Ideally a steam boiler would have adjustable heat just like your kitchen stove. It would have a high heat setting to quickly produce steam and then have the ability to regulate the heat to a point where the water is kept just boiling.
Until we get a residential steam boiler with modulated heating we're stuck with the "On"/"Off" cycling which makes our options rather limited.

I think I'd try maximum venting on the radiators and use TRVs. At least that way you're getting the steam to the radiator surface as quickly as possible so it can heat the room and the TRVs are an automatic way to control the room heat. The only downside is that you may get some "spitting" if you vent too fast but that depends on each individual radiator and/or the dryness of your steam. You might try a Danfoss TRV and attach a Heat Timer Varivalve to itand see how tht works for you. Get the straight model Vali Valve. Here's a link to the Danfoss model for one pipe steam:

http://na.heating.danfoss.com/PCMPDF/RA2000 1PS-DS.pdf

I have TRVs on most of my radiators (Remember don't put one in the room that has the thermostat) mainly so I can shut off the heat in rooms we're not using without worrying about the pipes freezing (The TRVs have a ("star") anti freeze setting which will keep the room at 43 degrees)
Been very happy with them.

- Rod

FJL

Thanks

Now I understand your point. I've noticed that I get less moisture in the valve when using smaller valve and venting slowly, although none have ever gotten to the point of spitting water, I've seen maybe a droplet once and a while on the valve outside the hole. My perception of moisture in the valve is based on the sound of the valve as it vents, if that makes any sense.

Kool Rod

Hi Frank - The Heat Timer Vari Valves are very aggressive vents. They are adjustable though and at a minimum settling they are larger than most vents but at a maximum, their venting capacity exceeds a Gorton #1. You might just want to try one vent and see how it works for you.

BTW if you don't have one already, I'd get Gerry Gill's & Steve Pajek's booklet on venting. Lots of good info and formulas in it, including the venting capacities of different model vents.

http://www.heatinghelp.com/shopcart/product.cfm?category=2-138

You're right about slower venting results in less "spitting"
I think this is because the air on a fast vent, moving at a greater velocity, sometimes picks up water (condensate) as it enters the radiator. I find it happens on some radiators but not on others so I think it must be something to do with how each individual radiator is piped or something like an internal bump on the radiator casting which causes the water to be splashed into the escaping air.

I "fixed" one "spitting" vent by making a shield for it out of a plastic water bottle that I cut down. I drilled a hole in the base of the bottle for the vent connection so that now the vent is surrounded and the plastic "cup" catches any water that sprays out. In winter our humidity is so low that any water collected evaporates quickly.

- Rod

Brad White_203

That slight vacuum

Hi Frank-

Yes, that slight vacuum, that which is held in the radiator for 60 seconds, likely by action of the radiator vent not yet releasing, is what I was referring to.

I agree, you probably are using conjecture as to what is happening at the boiler at the same time, but your conclusions are reasonable. (If the boiler pressure were to actually be lower, there is no way the pressure at your radiator would be higher.)

The "downside" of such vacuum is that it will draw in another breath of steam to fill that. ("Nature abhors a vacuum" as Dad drilled into me.)

Some older systems, from the coal-fired days, operated this way quite well and by design. (They had no choice given that coal fires do not have an off-switch short of a lot of cold water which is just dumb.)

The TRV solution has some merit- understanding that the TRV will only close if the room temperature is satisfied which is your objective.

Once satisfied, the vent will open, but even if not, the TRV vacuum breaker (insist on that!) will, guess, what, break the vacuum.

Frank_63

Thanks Guys

Great stuff. Thanks to both of you.

Rod: I have the Gill/Pajek booklet, so I am familiar with the venting rates of the vents for each brand. It has great information in it.

Funny you should mention the style of radiator as a factor that influences spitting, because I've found that to be true. The one rad that is the noisiest and often has a moisture sound when it vents has columns that are slimmer and spaced closer together than the others that heat faster, quieter, and with less moisture. When I get home I'll try to post a picture.

I think for now I'm just going to observe my system for a while and learn about how it operates. During the second run after the setback this morning, the pressure rose slowly inside the rad and remained close to 5 ounces, dropping an ounce or so, but never dropping to zero as describe above, until the t-stat was satisfied. I want to see what kind of variation there is day to day.

Also, one thing that I noticed this morning when the boiler fired for the first time after the setback is that the rad got fully hot for the most part on one ounce or less of pressure in the rad. About 30 minutes into the recovery, about 15 minutes after the steam hit the rad, is when the pressure started to rise in the rad to above one ounce.