System not "taking a breath"?

Steve Nichols

Okay, so I'll admit up front that I have a newly installed  oversized steam boiler, so this might be the root of all problems.

On initial startup, the system will run for about 20+ minutes before shutting of at a pressure of 1.5.  All radiators heat.  Double G1's on main vents (one is 45' the other 30', both 2" pipe).

If Tstat is still not satisfied, boiler starts up again but now it takes only 2 minutes before the system shuts down on high pressure.

I have installed ventrite 31/33's on all my thin tube convectors.  The variety of model numbers is due to the tapping size that I currently have in the convector ends.  I can't fix that now, but maybe over the summer.

I'm not hearing any of the vents opening back up when the system is coming down off of high pressure the first time.  Being thermostatic, I'm thinking they are still above 140F, so they won't release.  Should the system breathe?  Do I have the wrong vents for this application?

Is there anything I can do to keep this thing running longer (hence improving efficiency) ASIDE from replacing this boiler?

I'm trying to look at everyone's suggestions on best Tstats for steam to see if that would help.

To me it seems that if I could put a 5 minute space in between the time the boiler trips off on high pressure and when it starts, it would run much longer to build up pressure.

Thoughts on a Sunday morning?

crash2009

Why

 would you want the boiler to "run longer"?  Sounds like it's doing it's job.  The BTU's in the natural gas have been transferred to the cast iron, then to the water, then to the steam, then to your convectors, convectors transfer the Btu's to the room.  Thats it! Job done!  Thermostat still not happy, boiler waits until the pressure drops, then sends more steam up there.  Thermostat happy now, boiler shuts off and waits.

Steve Nichols

Hey Crash

Nice to see you chiming back in on my stuff.



I was under the impression that the longer a boiler runs, the more thermally efficient it is.  The AFUE is based on continuous running, right? 

I may have confused terms, but I thought that a boiler that runs for a short time, shuts off on pressure, runs for a short time, etc. was  "short cycling.  Isn't that bad on mechanicals and a reducer for efficiency?

Unless short cycling is when the boiler heats but some rads are still cold...that has been kicking around in my brain also.

Hey, I'm all for a happy boiler!

crash2009

Well you got me there Steve

  I don't know jack about thermal effeciency or afue (hell I can't even spell it right)



All I do know is.....If the burner is not lit, the gas bill is lower.

Hap_Hazzard

Let's do a little comparison.

My system never cycles on pressure. It takes a little longer for all the vents--mains and radiators--to close. Up until that point the pressure is negligible; the needle on my low-pressure gauge wiggles, but hovers at zero. Then, after the vents close, the pressure goes up to about 5.6 ounces and stays there until the thermostat is satisfied.



When the vents are closed, the boiler, piping and radiators form a closed system. Heat is being applied at the boiler and radiated everywhere else. This heat is being transferred by steam, which is generated in the boiler and condensed in the cooler parts of the system. Once the radiators and pipes are hot, condensation can only happen at the rate at which they can dissipate heat into the environment. In my system, the boiler is just barley able to keep ahead of the condensation, but in yours, the boiler is producing steam faster than the radiators can condense it, so the pressure continues to rise until the pressuretrol cuts out.



If I were to gradually increase the firing rate of my boiler, the pressure at equilibrium would get higher--6, 7, 8 ounces--until it reached 24 ounces, or 1.5 psi, at which point the pressuretrol would cut the burner and the pressure would fall. At 8 ounces the burner would restart and the pressure would rise again, but it would take a while to replace all the steam that condensed while the burner was off. As I continue to increase the firing rate, the time required to replace that steam would get shorter and shorter, but the time required to satisfy the thermostat remains the same. That is limited by the rate at which the radiators dissipate heat, and the pressure and firing rate have no effect on that. So the higher I continue to push the firing rate, the more times the system will cycle on pressure before the thermostat is satisfied and the heating cycle comes to an end.



This demonstrates the relationship between boiler sizing and cycle length: the bigger the boiler, in proportion to radiation, the faster it cycles on pressure. If you're concerned about short-cycling, you might want to consider down-firing your boiler. There are limits and trade-offs, depending on your boiler and burner design, so it isn't always practical. Some burners are designed to be downfired, others require modification.

gerry gill

Sounds like you could use

two stage firing capability. High fire & low fire.

Steve Nichols

yep

Thanks Hap,

That's pretty much what I thought about sizing of boiler vs. pressure and connected load.

Good explanation!

This is a Burnham Independence gas boiler, so no provision for changing BTU that I am aware of.

Steve Nichols

thanks Gerry

but like I commented to Hap above, I don't believe the firing rate of this boiler can be changed.  Maybe I"ll start switching out the convectors for radiators!  These thin tube units really cool off fast!

crash2009

What do you

 have to do to install Hi-Low in a Burnham 5 Section Atmospheric.   

Steve Nichols

that's a good one

I have no idea, and I'm sure my installer has no idea either.  Burnham reps have been less than receptive to even talk to me, but I'm coming around to what you are saying Crash about BTU's are BTU's and maybe the boiler is fine without modifying.

crash2009

If it could be done

 Gerry would know.  http://www.gwgillplumbingandheating.com/   I acted on a couple of his suggestions with some incredible results.  Might be worth your while to  investigate the idea of a Hi-Lo type of setup.  

Steve Nichols

will do

Thanks again Crash.  I'll see if this is possible to do.