Pressuretrols keep boiler switching btwn 50% - 100%, never off?

sreja

Hello again,

I've posted before about our building (5 story, 14 apartment, 83 radiators).

We have a Peerless 211A-08, which has this 3-stage burner thing with 3 pressuretrols (4 if you count the high-pressure shutdown) which each have a cut-out and cut-in setting. The idea is that it can go on full force in the beginning and then shut down to 75% and then 50% as it gets near desired pressure.

I've been experimenting with settings trying to find the best configuration.. It's strange that there aren't better guidelines/examples for these pressuretrol settings since it must be something that people commonly have to do (actually the peerless documentation gives an example but its way up in the 10-12psi range which we all know from dan's books is wayyy too high).

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So i've been experimenting with the psi settings and i have it now basically cycling between 1.5psi and 3.25psi in the building. it basically hits 2.5psi and goes to 50% and coasts to 3psi slowly, then drops to below 2psi at which point it kicks back to 100% and goes back up to 3 psi (i'm just approximating here).

What seems good about this to me is that the boiler seems to spend quite a bit of time at 50% gas power, and seems to keep a pretty steady pressure range around 2-3psi. (i tried hovering around 2psi, but it seemed to me that maybe some far radiators weren't getting hot -- does that make sense in a building of our size?).

Ok so finally to the heart of my question:

What troubles me about my current settings, and what prompts this post is my concern that there might be a real downside or risk to the fact that with my current settings, the boiler NEVER actually switches off completely. That is, it never reaches its "operating" pressure which would cause it to shut off power. That's because i have the operating psi at 4psi, but because it cuts down to 50% when it gets near 3psi, it never gets to 4psi.

So.. should i be concerned about this? Should i change the settings so that it doesnt trigger the 50% mode, and just let it get to 3 or 3.5psi and then shutdown until psi drops to 1.5 or 2 and then turn back on? I just worry that the boiler being on continuously for weeks at a time might be stressful for it.

Thank you for your consideration and advice as always,
s.r.

ttekushan_3

firing levels and boiler stress.

Hi-

The continuous running doesn't hurt the boiler in my experience. Everything outside stays nice and dry and there's never any exposure to the thermal shock of warm-up/cool down. That doesn't concern me that much, except that in the real world it should shut down.

When I retrofit boilers to multistage firing, the thing I avoid is having boiler pressure drop when the firing rate drops. In other words, I want the firing rate to prevent the pressure from continually rising to the high limit. What happens when the firing rate drops and pressure drops, any air in the system expands and causes the farthest areas of the building to run cool (or not heat at all) even though there is a nicely boiling boiler in front of you. This is the situation you describe. The pressure gage is your guide. You don't want the pressure to fall off when the firing rate drops.

Pressure dropping at the boiler=contraction of the steam volume; contracting steam volume=expansion of air within the system. It may even admit air under some circumstances. [I know, lower pressure means an increase in volume, but when the rate of steam condensation exceeds the boiler steam output, the steam volume decreases and the remaining air expands to take its place since it is not condensible.]

The low firing rate that's not heating the whole building is wasteful. I encountered this situation when I was learning how to make these kinds of adjustments. When I raised the low firing rate so that pressure holds steady on low fire (rather than rising) all the heat complaints stopped.

As you suggested, in the absence of someone who can fine tune the firing rates at each level, you might be better off preventing the lowest firing rate. I'd prefer to see it stay at about 2 or 2.5 psi at all times. The boiler will get out of high fire quickly as soon as the piping is hot and main vents close, and the 75% fire raised to 80 to 85% so that pressure builds a little more quickly than the slow rate you have. Then the low fire adjusted upward so the 2.5 psi holds. This would be perfect, and everything would heat. That 50% rate is too low, but maybe 60 to 70% would do the trick.

Finally, once you prevent the system from contracting (due to the prevention of dropping pressure) you may find that the system will heat with an overall lower pressure. You might be able to heat with something like 1.5 psi if its a sustained pressure, for example.

sreja

Your comments are so useful Terry, thank you!

I will start experimenting a bit more with the dif pressuretrols to see what i can see.

To confirm, do i understand that you are saying that, if i can't find a good way to prevent the (albeit slow) pressure drop when it switches to lower power, that your recommendation is basically that it would be better to just let it go full power till it reaches desired top pressure (3psi or whatever), then shut off completely and let it drop down to 1 or 2psi before turning on again.

A couple of thoughts:

The multistage system is actually capable of switching between 50%, 75%, and 100%. I may be able to tweak it to go to 75% instead of 50% when it hits the sweet spot of psi and maybe it will hold steady there a little better, rather than the drop that occurs when it switches to 50%.

Or maybe i can get them to tweak the gas valves for the different multistage settings.

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One really important thing i guess that is important to realize is that there will be different loads on the boiler at different times depending on temperature and depending on the thermostatic valve states, so i'm thinking that it's really not going to be possible to find some set of pressuretrol settings that always keep the system stable as some target psi. [god forbid someone make a control that could explicitly regulate the gas precisely to maintain a target psi].

If that's true, it seems to me it may really come down to choosing between a philosophy of oscillating between low power and high power while the boiler pressure slowly oscillates between low and high (2psi and 3psi), OR basically letting it go full power till it hits high psi and then turn off till it more rapidly drops to the low level.

I'd be interested to hear what other pros think of prefering one of these basic approaches over the other.

mark_121

Objects in motion

Object in motion need less energy to stay in motion.
Same as a boiler, the name of the game is equilibrium.
In the industry we use modulating controls to operate boilers and we do have operating limits we have to test them periodically to make sure they even will work.
Some boiler run at 50% to 75% for months with out shunting off.
I installed a LOCHINVAR Knight boiler in a large house with gobs of radiators during the month of Jan. it idled between 30-40% all month.
What you are doing is great I know a few guys that cant make $3000 worth of fancy controls achieve what you are doing keep up the good work.

nicholas bonham-carter

mod-u-pack

hope you are getting closer to "equilibrium"
when you are next if front of your boiler, could you write down the gas valve numbers, and let me know what they are?
i think my next project will be some sort of 2-stage firing for the burner.
honeywell shows many different valves in the 2-stage area, and it would be useful to see what peerless chose.i think i would not have the 2nd smaller valve, just the 2-stage type.
it would be interesting to see what a combustion analysis would show on that 211-a burner running at 50%. some of the pros here doubt that an atmospheric burner can burn efficiently in all of its stages-but there is only one way to find out!--nbc

sreja

sure, i'll write down what i find and i know there is a manual on peerless site somewhere about the mod-u-pak thing.

one thing i will point out -- the boiler has a 3 stage gas thing. it's impossible for me to imagine how to use all 3 stages in the 0-2 psi range that we are all using.

so basically as it is now we are only using 2 stages. full power on startup and then cycles back and forth between full gas and half (or 75%) gas as it oscilates between 1.5 and 2.5.

can't imagine having use for 3 stages of gas (50%,75%,100%) within that range -- at least not without vaporstat and more accurage readings, and even so..