Do I set the High fire / low fire control to hold a constant pressure or oscillate?

SD2

I live in a 20 unit 6 story coop in NYC and I take care of the boiler. Half of the building has 2-pipe heat (with returns emptying into a condensate tank) and half has 1-pipe (with wet returns). The boiler is a 12 section cast iron HB Smith with a Carlin 1150 burner. It is fitted with 2-4 Gal per hour nozzles and 1-12 Gal per hour nozzle. When the cycle begins, it takes about 12-15 minutes to reach 12oz of pressure, where low-fire mode starts. Once the burner enters low-fire mode, the pressure stays steady. It neither rises nor falls.



What should we set the pressure to? Is 12 oz too low? Should we try to keep the pressure constant or set things up so it oscillates? Do we need different nozzles? (perhaps a faster rate of burn in high-fire mode so the pressure in the system builds more quickly, and a lower rate of burn in low-fire mode so the pressure can drop).

arches

constant is probably ideal

A constant pressure means that the steam is being produced and condensed at nearly identical rates....that's ideal for a steam heating system. In terms of whether you want to hold a lower constant pressure - that's hard to say...so long as pressure is high enough for steam to reach the ends of the system, lower seems to be better (and would save you some fuel by cutting short the run time of that monster high-fire nozzle). It may be something of a trial and error process.

As an aside, I live in a 25 unit, 5 floor co-op in NYC...Like you, I am the boiler's "keeper" - an interesting, but sometimes unenviable task! W have an 8 section Smith with ~14 gph Carlin  burner (2 x 7 gph nozzles - one each for low/high)...which is grossly oversized. Unless your 20 units are unusually large, a 12 section with high fire of 20 gph sounds...just...wow. Have you ever compared it to the EDR of the installed radiators? I'd love to trade some fuel consumption data if you are willing. Do you keep track of that?

SD2

will constant pressure work on the 1-pipe side of the building?

Oversized is a relative thing - you should see the boiler that heated (powered) the building from 1890 to 1985 – brick, 15’ wide X 20’ deep X 12’ high. I haven’t checked in the last few years but I think we are running something like 20K gals per year. On the other hand the building has 12,000 sqft floors and 3 of the 4 wall are exposed (facing streets or alleys).



I understand that constant pressure is best for the 2-pipe side of the building, but I’m wondering if the 1 pipe side needs oscillating pressure. Also I think part of the reason we use so much oil is that the high-fire 20 gals per hour is too low for the boiler/burner combination we have and its taking too long to fill the system (and reach pressure). I was thinking if we had bigger high-fire nozzles, the system would fill with steam faster and all of the spaces would have more even heat – and we could reduce the overheating.

Jamie Hall

Constant pressure

is ideal -- sort of the nirvana of steam.  And 12 ounces should be ample.  You do not need to have the pressure oscillate to get steam into a one pipe radiator; as the steam that gets in condenses, the pressure in the radiator will drop slightly, and pull in more steam (and it may also open the vent, if the vent is cool enough) and so on.



And it seems to me that 12 to 15 minutes is amply quick.



As to whether you are burning too much oil... not qualified to say!

arches

whew...

...ok, you're dealing with a much larger building...our 5 floors are only ~4500 sq feet each. If you're burning 20k gals, you're doing better than we are on a per sq basis...our annual consumption (including hot water heating) is in the mid-17k gallon range....which has always seemed high to me. 

nicholas bonham-carter

uneven heating

i vote for an even 12 oz, or lower. can you tell when your main vents have closed [pressure wise]?

when you say uneven heating, do you mean wide temperature swings in general, or unevenness in temperatures, from one unit to another?

what are you using to control the boiler? some control systems can use several indoor sensors, taking the average.-nbc

ttekushan_3

even pressure will equalize better

In my experience of doing just what you are talking about, the low pressure is a great idea but keep in mind that you don't want to be to close to the range where the pressure drop across the system leaves you with zero net pressure anywhere. Oscillating at low fire can cause just this to happen, where the most distant or "difficult" radiators run cold at the bottom of each oscillation.



I adjust the low fire for a very, very slow creep upward in pressure. I know that perfectly even is ideal but absent full burner modulation, this has turned out to be the best way for me. Unusual weather or temperature pick up conditions can cause a greater demand for steam and therefore create a very slow decrease in pressure instead. This is where a problem like that mentioned above can surface. A slight tendency for the pressure to increase at low fire prevents these problems.



-Terry

SD2

Some more detail

1. I was thinking we might need oscillating pressure because the 1-pipe system required the pressure drop to free and clean out any water that gets trapped in the radiators and pipes.



2. Like any old building where the windows have been replaced, the radiation is grossly oversized. Some middle floor apartments complain of overheating even when all their radiators are off (just the risers getting hot).



3. Our heat-timer is set up to start the cycle when the pressure (at the header), reaches 10 ounces. Typically this takes about 12-15 minutes. All the risers in the building are hot 5 minutes into the cycle and the radiators (even on the 6th floor) start getting hot after 7 minutes. If we set the heat timer to C, the building is comfortable on warm days (40 degrees or higher), but on cold days the heat-timer needs to be set to H. I’m thinking that we need to start the cycle earlier (using a thermostatic sensor placed in the middle of a radiator. That way the heat timer would start its cycle maybe 9-10 minutes into the run. If the heat timer was set somewhere around I or J hopefully we will be comfortable on both warm and cold days. Is this the right track?



4. I am also thinking that a higher burn rate in high-fire mode might reduce the warm-up time even further, allowing us to use a shorter cycle on really warm days.

SD2

Interesting

I thought we were pretty inefficient. However we don't have hot water off the boiler/ so its entirely shut down in the summer. I guess the bigger floor area means more interior space and less opportunity for the heat to leak out through the walls

SD2

even more detail

There is a single outdoor sensor, but its in a shady place and seems quite accurate. On a really windy day the building is hard to heat, but I guess there is not much to do about that short of putting a sensor into one of the apartments (which has other issues)

Mike Kusiak_2

Cycle start sensor

I am surprised that the cycle start sensor is activated by pressure in your system. Heat- Timer usually suggests that the sensor be located at the furthest radiator and be set up to start the cycle when that radiator starts to heat. By choosing the location of this sensor carefully, you should be able to get better control of temperature without constantly playing with he letter setting.



What model Heat-Timer control are you using?

SD2

Thanks

The Heat-Timer is a MPC Gold - the dry return pipe in the boiler room doesn't get hot at a consistent time and the radiators in the apartments get shut off from time to time. The pressure method works fine, but it can only be adjusted to start the cycle when the system is fully charged.



The radiator in a first floor stairwell seems like the only other option. I am hesitant to put the sensor on the end of the radiator (rather than the middle) because the radiators in the building are so over sized that on a warm day if the radiators get fully hot, everyone will be over-heated.

Mike Kusiak_2

Pressure sensor setting

What if you reduced the cycle start pressure sensor setting to a lower value? Wouldn't that start the cycle timing earlier and reduce the overheating? 

SD2

Thanks

It doesn't seem that consistent. The pressure rapidly builds (in less than 5 minutes) to about 8 ounces, then it stays there for a long time (7 or 8 minutes), then it rapidly builds to the point where the high-fire/low fire control kicks in.

nicholas bonham-carter

bad idea

using pressure to determine the timing for the start of cycle seems like a bad idea. if the system has bad venting [and many systems do], then the start of cycle will start even before mr. texaco has pushed the air out of the constipated little vents! i have 55 radiators, and the air is out before 3 ounces, and almost never see the 12 ounce cut-out mark. i doubt whether the heat-timer setup would be able to sense the low pressures that are so ideal for our systems.

timing by the start of steam arrival, determined by temperature, is the only reliable method. surely there would be some dry returns near the boiler to get the temperature from. if your mains are filled, then your risers are almost there. i am not familiar with the heat-timer setup, but am sure that it probably follows closely the apparent industry leader-tekmar in the timing of the cycle from radiation cool down to heat up, with outside temperature controlling the duration of firing. looks like a better control could pay for itself in a couple of days with your consumption!--nbc

Mike Kusiak_2

Pressure sensor

Since you say the pressure builds within 5 minutes to 8 oz, I would try setting the sensor to 8 oz and see what happens. You mention that all radiators are heating within 7 min, so this setting would would be pretty close. You could then set the letter setting a little higher to compensate. It would certainly start the cycle earlier and have a big effect on overheating in mild weather.



If this doesn't work, then locating a temp sensor on a suitable radiator seems like a good solution.

c.t.kay

Heat-timer

The H-Timer will work with the pressure sensor. I would try a H-T return line sensor placed on the longest steam pipe run ,placed on the steam supply,on the floor below the last radiator..Make sure the riser is vented (all of them should be,independant of the radiators).When this is done the sensor now senses pipe /steam tempature at this location. Play with the XYZ dial to set the correct temp.setting.The closer to the point of use the sensor is the more accurate the control is.In the past we have found this to be the best location with the upside of lower fuel bills.H / L burner operation can be done with vaporstats and very small adjustments.Carlin burners can be fired at different rates,but this takes a "good" service co. ,there are not too many good ones out there.  Thanks.