Question on boiler pressure

Joe V_2

My residential steam system seems to work very well but I have a question,

When boiler cuts off for either pressure or thermostat, the pressure drops like a rock.  From 6oz to 0 in about 5 seconds.  Is this normal?

Chris M_2

Question on Boiler Pressure

Joe,

I'm not a professional, but I know 6 ounces is a dream pressure that most installers can't get a boiler to operate at.. It's usually between 1 and nearly 2 lbs that the typical steamer hits before cutting out.  It's hard for me to understand how going from 6 oz to 0 could be described as "drops like a rock".   6 oz is nearly 0, relatively speaking. That's just me, though.  I'm sure you'll get a flood of more professionally backed answers in a jiffy!

jpf321

not abnormal

the pressure drops pretty quickly .. mine probably takes a bit longer .. but I have a large volume boiler with a large steam chest, so that may make a considerable difference.



other Wallies may have different input, but I'm not surprised by your fast drop.



there was a discussion last spring about the time on vs. time off being important. I don't remember the ratio that was floated. Something like on for 3mins off for 1.5min seems to be my recollection. it may have something to do also with the size (oversized or undersized) of your boiler vs. the connected radiation.



it will also matter if your mains are well insulated and if they are all at steam temp.

jpf321

6oz

i operate at around 6-8oz. my theory is that i want the pressure to be as low as possible. i have a vaporstat and my furthest rad does heat evenly. 1.5-2psi is about as low as a normal pressuretrol will go. it was also the standard upper limit agreed upon by the dead men (some of which were operating at 15,30,60psi at the time). but certainly operating with a vaporstat, you can get lower pressures and be comfy and happy.

BobC

Pressure drop

I'm a homeowner and have been tweaking my system for the past couple of years. Right now, because my boiler is really too big for the job, the system tends to cycle on and off for about 10 cycles before the thermostat is satisfied. I have my vaoporstat set to cut out at 16 oz and back in at 3 oz, it takes i min 17 seconds initially and finally 2 min 5 seconds for my system to fall to the cut in point. That data was recorded last year in February, the data is in the attached pdf file.



I tried going down to 12 oz cut out but felt the system was running as efficiently there so I moved it back up yo 16 oz cut out. I never tried moving the cut out lower than 13 oz.



Are all of your basement pipes insulated?

Joe V_2

Thanks all...

Our rads are hot all across at 2-3oz and twenty minutes.  I just dialed in 6oz cut out; 2oz cut in for large night time set backs otherwise if the pressure gets higher than a pound or so, water comes out the vents and I trip on low water.

My boiler is slightly oversized because I  plan to add 15 ft of Baseray radiators plus piping.

I was curious because the drop is so fast, it would be better to not have a cut in. 

By the time the damper closes and opens again and the burner is on main flame, the pressure is already at zero.  It seems, from a control standpoint, if the switch breaks on rise at six and makes on drop at say 5.5oz 

I'm in North Carolina so once the radiators heat and the thermostat is satisfied, the boiler won't come on again for another hour.

David Nadle

Mine's the same way

Joe, 6 oz. is great. When my boiler is clean I can get the cut-out down to 8 oz. and it hits zero in less than a minute. After hitting limit pressure the first time (about 30 minutes) it'll cycle 1 minute on, 1 minute off until the thermostat is satisfied. I don't like this but I'm not sure what can be done about it.

jpf321

how long ..

how long from when it hits 0 and fires back up does it take to return to 6oz?



don't forget that pressure is bad (although it is what allows steam to flow) .. but dropping to 0 is not necessarily bad since it gives all the vents a chance to take a breath.

Joe V_2

it takes...

about five to eight minutes to trip again at six oz after main flame has been established.  I havn't actually timed it.

Joe V_2

Relatively speaking, Chris

It takes about forty minutes to reach 6oz and five seconds to drop to 0.

Jamie Hall

It doesn't sound to me

as though you have a problem here -- the pressure drops rapidly because everything is condensing the steam like mad.  If all the radiators were hot, it might be a little slower.



For reference, the pressure goes from 10 oz to cut in (2 oz) in the building I super in about 20 seconds -- the post purge/pre purge cycle time on the burner is longer than that.

Joe V_2

Thanks again for the feedback

I guess, from a control stand point, I was thinking there is a better way to control steam like using a PID loop to  modulat gas valve to maintain pressure until thermostat is satisfied.    It would save on wear and tear of damper motor, ignitor etc

jpf321

yes simmer is good

simmer mode on a burner would be good, there are ways to do it, but not always possible given the existing equipment, you need a professional.

Jamie Hall

Exactly

how the old coal-fired boilers used to do it -- there was a nifty arrangement which varied the draught over the fire in inverse proportion to the pressure.  Worked great, so far as coal economy and control went.  Lousy so far as total efficiency, though.



There are such things as hi-lo burners and burner controls for larger units.  They do work well, but they aren't usually available in smaller sizes (and only very rarely available for oil).  The problem is, of course, maintaining an even flame pattern in the fire box at large turn down ratios.

Dave in QCA

Modulating Burners are Great!

A modulation burner can work great, but, it is a much more expensive proposition and available on only large commercial equipment.

In the hospital complex where I used to manager building operations, we had 2 338HP boilers.  Burner cut-out occurred at 75 psi.  Cut-in was set at 50 psi.  The burner modulated on a 6-1 turn-down ratio, controlled by a modulating pressure stat, with high fire set at 55 psi and low fire set at 65 psi.  On very light load conditions, the pressure would still rise to 75 psi and the boiler would shut off.  On medium load conditions, the burner would modulate up and down very slightly as the load shifted, but continuous fire kept things running smooth and steady.  On very extreme conditions, the boiler would remain in high fire for prolonged periods of time.

I doubt if this type of set up as available on smaller residential systems, although hi-low-off firing certainly is.

The apartment building that I currently own and am working on / restoring, has an old vapor system.  I have installed a vapor stat, and fire shuts off at 7 oz.  All of the radiators are piping hot!  It only takes about 45 - 60 seconds for the pressure to drop to 2 oz where the boiler fires up again.  I have an atmospheric boiler, so there is no delay in staring the burner.  Then, it will take about 4 minutes to get back up to 7 oz.    It takes my system about 45 minutes to get to cut off pressure, and actually this normally does not happen unless it is coming out of a set back situation and it is VERY cold outside.  Remember, when the system is fully heated, there is steam in every part of every radiator and pipe.  The is a huge condenser!  I am thinking about installing a hi-low-off gas valve operated by a second vapor stat.  If perfectly balanced, when it drops into low fire, it would set there slow and easy, steam supply in perfect balance with the system, until demand was satisfied.

davide

rwf 40 siemens

Hi,



I have a painting plant and the oven are heated with burners. For

command all of plant I have a PLC.



can I command the RWF40 instrument with plant PLC by 4-20mA

signal for regulating burner's gas valve? or is possible only by 3-point

regulation?

Joe V_2

Simmer Mode

I like that name!

I agree, on a small residential unit, like mine, it will probably be too expensive.  The return on investment would be long-not to mention involve more parts that can fail.  Best to keep it simple.

Rod

RWF40

Hi - I can't answer your question as I'm not that familiar with the RWF40. However I have some RWF40 manuals if that would be of any use to you.

- Rod

LarryC

How about a variable time delay?

..........I am not a heating professional......

 

How about a variable time delay relay between the vapor stat and the thermostat?  When the vapor stat opens on high limit, the TDR will prevent the restarting for two to five minutes ( user determined), to allow the entire system to cool enough, so that when it restarts, it burns for an extended period and therefore reduce the number of cycles?

 

Obviously this will extend the initial heat up time, if the initial heat up takes more than one pressure cycle.  It will reduce the number of pressure cycles.

 

Larry C

Rod

Delay Relay

Hi Mark-

Thanks for the circuit info. How long a delay interval are you using?

I was using ICM relay last year that was adjustable to a max of ten minutes. I had to scrap the idea and remove the relay before I came to any conclusions as I was going to be away from home and didn't want to confuse any tech guy my wife called in an emergency. My main problem is that I shut down half my house during the winter (with TRVs) and this makes my boiler way oversized so that it short cycles a lot. Please share with us your thoughts and conclusions as your experiment progresses.

- Rod