Setting Hi/Lo fire rate

Abracadabra

Wondering how to decide where to set the lo fire rate on a steam boiler. First system:

19 Unit building, HB Smith 28A-5 with a powerflame burner.

Boiler is rated for 3646 sq.ft of steam.  Powerflame is a CR1-G-12 

Total load here is 3574 sq.ft.

Hi rate is about 1.5MBtus 

lo rate is about 750,000 btus

I've currently have the vaporstat to kick lo at 10" and back to hi at 4"

On a 30 degree day I measured temps at the furthest apartment from the boiler:

12:42 boiler on 68.2

12:46 2.5"

12:49 3" main vent hot and shut off

12:55 3" 

12:58 4"

1:00 4" 68.4

1:05 5" 69.2

1:10 6" 69.4

1:13 7" 69.6

1:14 9" 69.8

1:15 10" vaporstat trips to lo

1:16 4" trips hi 70.2

1:18 10" trips lo 70.4

1:21 4" trips hi 70.8

1:22 10" trip lo 71

1:37 72.6

1:44 72.8 boiler off



I have a second location with a WeilMclain LGB-14 also 19 unit bit larger units though, with a vaporstat set 6" lo 12"hi.  This boiler sometimes can continue to raise pressure beyond 12" even on lo fire.  Or what should be low fire.  Haven't seen it happen while in front of the boiler, but owner has an IP camera setup to watch the gauges and he's shown me recorded footage of the pressure going beyond the 12" the vaporstat is set for.  Not sure if the vaporstat is failing to kick lo occassionally or if the lo-fire is still too high. I still need to take some measurements.

I guess my question is, how to determine the lo fire rate?





Thanks!

nicholas bonham-carter

hi-lo burner settings

looks from your table as if the back-pressure during venting could be a little bit lower. you have 2.5 oz. and i have 1-1.5 oz. on 55 rads [18 gorton#2's].

on the second building, the rise of pressure, if at the beginning of firing, could be due to inadequate main venting.

there must be a lower limit for the burner settings, but the ideal would be if the low fire just maintains the lower pressure in this cool, but not cold weather. when the temperature is at design temp, then of course you will have some lo-hi switching back and forth.

is that farthest apartment on the top floor? generally, the aim would be to get the steam [not the air temp] arriving at all the rads on a floor simultaneously. if this were a tall building, you might need some riser vents to do this. this could explain the slightly higher venting back-pressure as the air is removed from the risers by the radiator vents.--nbc

Abracadabra

Re: Setting Hi/Lo fire rate

nbc, thanks!







#1 - first question i guess is, how much pressure, when boiler first starts, should i be seeing until the main vent closes up? I'm guessing none would be optimal, but what's reality?

#2 - buildings are 3 story (plus a basement) about 35' high.  risers to the 3rd floor would be about 21-22" long (8 foot ceilings, 1 foot joists between floors).  The first building would have about 67 radiators. (3 room units, 1 rad in LR, 1 rad in BR, 1 rad in kitchen, 1 rad in bath).  Some BR radiators have been removed and I've been slowly trying to add them back in as the units turnover. I've been checking radiator vents to make sure that they are sized properly to the radiator.  Radiator vents range from Gorton #C to some bigger ones on the 3rd floor down to a 4 for the smaller rads in the bathrooms. 

#3 - On the 2nd building I guess i need to clarify something.  It will occassionally work back and forth between the vaporstat limits 4" to 10", but sometimes it will peg the gauge to over 16" when it should be dropping.  I've read about the newer (non-Hg) honeywell vaporstats sticking, but it wasn't clear if they were sticking hi or lo?  Kind of hard to test since it hasn't happened in front of me, but I've watched video of the gauge working like it should, and also a video of the gauge pegged.

#4 - If I may ask my original question a bit differently.  How fast should pressure build from the lo setpoint to the hi setpoint  of the vaporstat once the hi got hit once and it's started coming back down?

Steamhead

A couple things

First, how long are your steam mains and what vents are on them? I'd look at maybe increasing the main venting so it happens more quickly at 1-2 ounces.



Second, having it trip to low fire and back to high is preferable to having it shut off on pressure. Most of these larger commercial burners have Code-mandated pre-purge and post-purge cycles, which can blow a lot of heat up the chimney. So you want to avoid stopping the burner because then it must purge.



I bet the boiler water in the second building is dirty.  That would cause such erratic behavior. Flush it out then skim it and see if it still does that.

Abracadabra

Re: Setting Hi/Lo fire rate

Steamhead,







There's 2 main vents, one off a short "stub" and a main mainline that's vented with a single Gorton #2.  I've been thinking about making a coupler of antlers with 3-4 #2s off it.  The other stub has a small dole (can't think of the model, it's a small mushroom, no float, just a bi-metal

The HBSmith has a power burner, so ya, it would do a purge if flame went out.

I'll check the water on the second building and see.

Any ideas on the other questions I had?

Thanks!

Steamhead

That's a Dole #4

which is probably way too small- no more useful than the acorn it looks like. Measure the length and diameter of all your mains and stubs, and we can tell you what's needed. For a more in-depth discussion of venting, there is no better resource than Gerry & Steve's e-book, which you can get here:



http://www.heatinghelp.com/products/Steam-Heating-Books/25/146/Balancing-Steam-Systems-Using-a-Vent-Capacity-Chart-by-Gerry-Gill-and-Steve-Pajek



The time spent in lo-fire while cycling between hi and lo can vary. Remember that steam is condensing in the system and that will bring the pressure down. The times you posted don't seem out of line, but you might try increasing the differential so it won't cycle so quickly.



You might want to vent the risers to the third floor too. Size these at 2 ounces. The easiest way to do this is to use Gorton C or D vents- then all you have to do is drill and tap 1/8" pipe thread holes in the risers, right below the second-floor ceiling so the steam will almost reach the third floor rads when the riser vents close. Like this.

Abracadabra

riser venting

Steamhead,









I can't really vent the riser like that since the pipe is in the wall.  I was thinking of maybe adding the venting required for the riser to the third floor radiator.  In other words, if I would normally vent the radiator with a #5, I'd bump it up to a #6 on the 3rd floor rad. I know this isn't optimal, but what do you think?







P.S.  I've done a bit more work on this project and have posted here about it:

http://www.heatinghelp.com/forum-thread/139049/Calculating-Main-Venting/success

Long Beach Ed

Top of Riser Vents

There are a few ways you can vent the tops of risers that might work for you. 



Remove the radiator valve on the top floor and replace it with a reducing tee.  The radiator gets mounted to the bull of the tee with a straight valve and a vent goes on the end of the tee sticking up from the riser. 



The other way is to drill and tap the boss on the valve end of the radiator and add a second vent. 



We've had great success with both methods. 

Abracadabra

venting the risers

What is wrong with adding the venting required for the riser to the radiator vent?







I've seen the Tee -> straight valve and a vent.  The problem is that the vent side of the radiator is up against a wall, so I don't have room to move the radiator away from the riser.

What is meant by the "valve boss"?

Dave in QCA

Clock the Meter First

Before you do any adjusting, you need to clock the meter first to see what you are burning on the hi fire and on the low fire mode.  The LGB should have a honeywell 2-stage valve and the firing rates are adjustable.  But don't adjust until you know what it's doing in the first place.  



Also, you need to phsically observe the vaporstat and see that it is set correctly.  The calibration may be off.  It usually is.  I set mine by actual performance, not by the scale.



It is my recollection, and I could be wrong, that the LGB runs at a good efficiency, even on low fire.  Low fire is typically 50-60% of hi fire.



It sounds like the building on the Smith boiler is working quite well, except I must ask, what is controlling the system and do you normally 4 degree swing in the space temperature?  Or, was that just during a prolonged firing that you set up for the test?

jpf321

one thing I have seen that maybe someone can explain ...

my house .. and other places where I have a v-stat/snubber ..

with vaporstat (0-16oz) and snubber ..

  fire is burning .. pressure is climbing .. slowly slowly .. v-stat trips around 6-8oz ..

  THEN right AFTER FIRE-OFF the pressure SHOOTS up RAPIDLY.. another 6-7oz

  it's not causing any real problems and it doesn't stay there very long ..



has anyone else observed this? thoughts? is it the back-pressure of the v-stat diaphram/spring that finally breaks and allows a post-fire pressure spike?



is it the calming of the water surface that somehow releases more steam after fire?



I only mention this because you report seeing higher-than-vstat pressure .. but you don't clearly indicate whether you know or not if the vstat has actually tripped and what level the fire is at (off/lo/high) .. you only state that you see higher than v-stat pressure. (at least if I'm reading you correctly)

Dave in QCA

from the diaphram

JP,  I observe something similar.  I am not using a snubber, but probably should.  I observe that when the vaporstat trips, it causes at precisely the same time, a momentary spike, or flutter, in the pressure reading on the gauge.  It immediately returns to where it was before, but as I said, no snubber.



I also occaisionally see a really wild spike in the guage accompanied by an audible snap sound.... sort of a mini water hammer, which I am sure is caused by steam creeping into the pig tail.   I am guess that the amount of displacement possible within the combinination of the vaporstat and the gauge is enough to push the water in the pigtail far enough that the steam coming in snaps. 

jpf321

the rise i see...

(with a snubber) .. is more like a 30-45sec rapid rise .. then drop .. i remember the times before the snubber when the needle would jerk a little after diaphram/spring trip... but this is more of a rapid rise and fall .. but maybe that's the same jerk but snubified.

Abracadabra

hi-lo rates

Dave,











I clocked the meter on the HBSmith boiler after I replaced the gas valve with a hi-lo version.  The burner originally had an on/off version only.  On high fire it's doing 1350MBtu/hr on low fire I set it to 750MBtu/hr. 

I was going to try to adjust the LGB lo-fire, but the low fire adjustment won't go any lower.  I'm thinking the LGB is oversized for the building.  I cannot downfire the low-stage anymore. The owner recently replaced all windows with thermopanes. On low fire, especially in the morning, after trying to recover from night setback, the LGB reaches 2PSI.  During the day, since the cycles isn't as long, it usually won't run up to 2psi unless it's really cold. I was thinking of adding a 2nd vaporstat. The first vaporstat would control hi (10oz) / lo (4oz).  The 2nd vaporstat would kick boiler off at 12oz and allow it to come back on at 2oz. Hopefully this bandaid will help with the over-firing.  I have not clocked the LGB hi/lo yet since I cannot downfire the low stage anyways.  With regards to the vaporstat settings, I know quite well the problems with the microswitch vaportstats.  I've adjusted the hi/lo setting on the vaporstat with a gauge.  It's about 1.5oz off from the vaporstat.

With regards to what's controlling the boilers, it operates off the average of the apartment temps.  It's this: http://www.rdcontrolsystems.com/steam-200-series.php

I'm thinking to limit the average temp rise to less than 1 degree.  The coldest apartment has 4-5 degree swings unfortuantely.  it's got 3 exterior walls and the windows stink. That was a normal cycle.





The control 

Abracadabra

pressure rise after burner shutoff

jpf,







I've seen this happen sometimes on the HBSmith boiler (which I think has more "mass") On the LGB, the pressure just sort of stays where it was when burners shut off for about 30 seconds then starts to drop.