2 boiler setup issues

MTC

Hey All,



I've posted on here a couple times brainstorming how to deal with other issues with this crazy system. Its a one-pipe system with basically 2 mains that combine into one dry return, no traps or anything, and a boiler feed pump.



The 2 equivalent boilers are piped into a single header, and a single feed pump fills both through a loop of pumped return pipe to both boilers. Fairly simple setup.



But they were a total mess when I started messing with them. One boiler had no way of operating - it was completely separated from the controls. All you could do is turn it on and off with an outside temperature override, so it basically cycled on pressure anytime it was below 20 degrees outside. The other was on a regular programmable thermostat.



The owner almost certainly wouldn't have gone for buying new controls for them, so I had to come up with a solution. So I went and bought a Contactor, like you'd use to control a 240V HVAC system with a 24V thermostat. I wired up the thermostat to the primary boiler's transformer to power it, then ran it over to the contactor. This way the thermostat controls the on/off feature of both boilers, without any messy mixing of power sources and such. The contactor does not cut out the boiler feed pump or LWCO/feeder, for safety.



Each boiler has one of the good ole honeywell mercury vaporstats, the secondary boiler has the outside override (we plan to put one on the primary boiler as well eventually, so that we can switch boilers periodically). Before, I had just set them up so they basically fired together anytime the 2nd boiler's override was allowing it (same main/diff settings on each vaporstat). This seems to just short cycle the system (which needs some major venting work, but I'm still trying to convince the owner to let me do it).



So what I started messing with today is modulating the 2nd boiler on pressure to try to keep the primary boiler running constant. I have no experience with these kinds of systems, so I'm just learning as I go, and any input would be great. I have the primary boiler set to 4oz in, 10oz out, and after playing around with it a bit, I ended up with the 2nd one at a current setting of 5oz in, 8oz out. The second boiler kicks on and off a good bit, but the system gets more heating capacity without going out on pressure, and the primary boiler stays running for a long time.



Does this seem to be on the right track to you guys? I'm thinking that I should increase the differential a bit on both, so cut in is a bit lower, to lengthen the secondary boiler's fire time a bit.



Any other thoughts?

ttekushan_3

Do you think

Do you think the first boiler could handle the load if the 2nd boiler was allowed to drop out completely after the pick up load is satisfied?



Without modulating burners, you're just going to have a lot of short cycling on the second boiler once the pressure starts to rise.



It seems to me (without further details) that boiler 1 is a bit short of handling the pickup load and therefore runs without really distributing the heat evenly. The outdoor thermostat on boiler 2 could work if it were a hot water system, but steam doesn't work that way. Remember, steam heats sequentially, so if the boiler is short of actual load conditions, cold radiation tends to stay cold, and warm areas of the building tend to overheat.



So running both boilers initially on each cycle will distribute the steam, and once distributed, might work fine.



Here's an experiment you could try: On a cool start, run both boilers. Set the vaporstat on #1 to about 12 ounces, vaporstat on #2 at about 10 ounces. Once boiler #2 shuts off on pressure, power it off and se what happens. If heat continues to distribute evenly and satisfy the call for heat, then you might consider accomplishing this elegantly with some simple relay logic, or more crudely (and perhaps more effectively) with a simple adjustable aquastat on the return line or condensate receiver.



So, the first thing to know is if the first boiler can carry on once the system reaches saturation.



A boiler sequencer would be nice but the modern electronic ones operate on pressure sensors that don't seem to resolve in the Vaporstat range.

MTC

Thanks Terry!

For the thoughts.



That is probably a better way to go about it. The whole dual boiler setup, or modulating burners setup, is pretty new to me, still wrapping my mind around how to best use those features to my advantage.



The system was mostly uninsulated when I started working on it. I've insulated the steam and return lines in the basement that is common area, but the other half of the basement is an apartment that is using those lines as a partial heat source. It also has a separate hot water loop connected to its own water heater (not part of the boiler system). I'm still working on convincing the owner that losses through the steam mains cost way more than the heat benefit they provide in those locations, so for now I'm going to assume he won't let me insulate the steam lines in the apartment. I'd like to eventually get just the steam lines done, and leave the returns uninsulated in there to give a little bit of heat. I think the water loop could handle the remaining load just fine, and then the pickup factor would be much less for the steam system.



The building has some serious main venting issues, which I'm struggling to get the owner to let me fix. Its complicated, b/c where I need to put the vents ideally is in the finished apartment, but I have a few ideas that would at least make it substantially better. ATM, the only main vent is a single hoffman 75, the rest of the venting is through radiators and the boiler feed pump. I suppose this probably gives enough venting to make the system work, as its a wide open 3/4" pipe with dry returns to F&T traps in the boiler room dropping to the feed tank. I need to rebuild these traps, the new guts will hopefully increase their venting capacity again, it def seemed to help on one of the other buildings with 2 pipe steam where we did all the traps at the drops from mains to returns.



Ok, so lots of meandering there... but yes, I think once the system is finally vented out, filled up, etc, the first boiler can probably handle the load. I'll give your idea a try.



Would this not be a simpler way to achieve the same goal: Set #1 vaporstat to 12 with 8 diff for a cut-in of 4, and #2 to 10 with diff of 8 for cut-in of 2? That way, in theory, if #2 drops out at 10oz,, system will keep running until either #1 cuts out at 12, or it dropped way down again to 2oz (indicating the pickup isn't satisfied yet). If #1 cut out, then #1 would come back in by itself at 4, leaving #2 off. This would continue until the thermostat is satisfied, which would depressurize the system enough that #2 would come back on at next call for heat. If pressure dropped all the way back to 2oz, then #2 would cycle again to finish the pickup load, hopefully moving then into the above scenario of #1 cycling to meet thermostat. Does that make sense?

ttekushan_3

yes

You've got the idea! If that works to adequately prevent a lot of cycling on the 2nd boiler, go with it!



Your way also prevents the system from sending false feedback to the boilers due to bad venting. In other words, if the venting is inadequate, the system can reach the cut off pressure of the 2nd boiler early in the cycle because two boilers can expand a lot of steam into the system, perhaps faster than vents can let the air out. Should one boiler permanently shut down because of this, rather than genuine steam circulation, then you may as well have not started the 2nd boiler to begin with.



As for myself, I tend to write notes on tags for anyone and everyone getting anywhere near the boiler to see, explaining what's adjusted and why. So if I get hit by a wayward chunk of asteroid, at least the steam system will remain unmolested. Since I started doing that, no one's ever touched anything and it seems the clients like it well enough that they keep me on. And I haven't been hit by an asteroid yet!

jumper

different logic

My logic is different. Requires some sort of timer. If boiler A runs for a determined length of time and pressure doesn't increase enough to reach cut out; then boiler B fires up. A more sophisticated control actually figures how quickly first boiler raises pressure and on that basis decides if B should be fired.



Of course if you can determine need for both boilers on basis of outdoor temperature that's even better. Some 2 boiler setups simply depend on building superintendent. Depending on outdoor conditions he turns on either smaller boiler or larger or both. Yes that is against standard procedure for steam, but if it works ?



Standard procedure is for both boilers together to be sized to EDR. Both run together full time at lowest temperature; both run together less often in milder weather. You guys know the routine.

ttekushan_3

MTC and owner's pocketbook

MTC said,



"The owner almost certainly wouldn't have gone for buying new controls for them, so I had to come up with a solution."



===



Now, some things pay for themselves over other options. New sequencing controls might do just that. On the other hand, Brainstorming and thinking about simple ways to get decent improvements in system operation are worth considering. As Yogi Berra once said, "You can observe a lot just by watching."



I think MTC is doing the right thing by carefully observing the actual operation of the system. A lovely PID controller installed by someone who hasn't analyzed the actual system operation could be wasting everyone's time and money.



Obviously, availing oneself of a more sophisticated boiler operation logic is preferred. But only if it is applied correctly. Myself, burner modulation is a must, but I rarely get that either. An intelligent work-around can be something to behold if the person/people paying the bills are penny wise and pound foolish. If they are hopelessly cheap (and I don't mean poor- I help them out the best I can) then I fire them as my client!



Should the building owner want to spring for one, a Heat-Timer SQ could be considered. I have some experience with this control, but have only used it in a 1.5 - 2.5 psig system with a couple of steamers with lo-hi-lo power burners. It utilizes PID logic, but can also compensate for oversized systems. It will resolve to fractions of 1 PSI, and even vacuum, if you use the lowest possible (15psi) sensor. You can program just about every parameter of system operation.



Should the budgetary situation change or if the building owner can be convinced that such a control can improve comfort and efficiency if properly installed and programmed based on the system's actual characteristic behavior, then it's the way to go.



http://www.heat-timer.com/zhFiles/ProductDocument/manuals/SQElite8S059076B.pdf

MTC

Thanks again guys.

Your insights are quite helpful.



I believe that the situation we have in this building, were the rest of the system to magically work correctly, is that one boiler is undersized, but both is signficantly oversized. They're same BTU rating, so can't switch between them, its half or full steam ahead, neither of which is ideal.



Terry, you're right on about the venting situation. Right now, having both boilers run seems to cycle them out on pressure too much. I think once adequate venting is approved by the owner and installed, the whole system will work better, but right now it definitely tends to short cycle if both boilers fire together. This is what led me to experiment with the settings that I described above, to keep one running and let one short cycle... seems an improvement, but still not ideal. I will def try your method and see how it goes.



Do you think there's value in running that system with the outside override still in place? Like is there really need for the 2nd boiler when its 40 degrees outside? Wouldn't it be better in that situation to just let the undersized one run a bit longer to meet the lower demand?



As far as owner, I wouldn't say he's cheap. He watches money closely, and it has served him quite well. He's willing to spend money when its proven to him that it is worth it.



The problem is, he doesn't know steam systems, and has had a whole string of knuckleheads through his buildings that have sold him all sorts of BS that he didn't need. So he's very wary of "new" things now, even though the system in this building is miserably screwed up.



I'll read more about the Heat Timer... can you give me the cliff notes of what it's benefit would be? He uses a Heat Timer EPU-CH in another building that is a little bit larger on one giant boiler with a 2 pipe system. It seems to work pretty well - after insulating mains, changing all rad and F&T traps, etc, I was able to turn the timer setting down 2 letters. I figure this is about a 25-30% savings. Anyway, I'm hoping that as he sees the effects of this for the end of this winter, that over the summer he'll give me a little more free reign to get this 1 pipe system in order. Maybe even get some good controls...



But for now, I just want to use what I have to work with the best I can, and focus more on getting proper venting and insulation and such in place. Then we can really get down to work on tweaking the system to operate its best. It has all sorts of balance issues, but strangely the furthest away radiators on the highest floors get much more heat than the ones directly below them, 2 floors down. The first floor on the opposite side of the boiler room is the biggest problem area, for some reason. We're still struggling to figure that one out, but I figure proper main venting and insulation will at least be a good start towards balance. Maybe just some radiator vent tweaks will be able to dial it in after that, but so far that has only been marginally successful.

MTC

One other thought on this system...

while we're discussing its venting issues a bit.



The system is single pipe parallel flow, 2 steam mains go out and join together into one dry return with no traps (this joining point to the return is also where the 1 Hoffman 75 is) and come back to the boiler room, drop through an F&T trap to a boiler feed pump.



I hate sending steam through dry returns. Would I be better off, if I can convince owner, to add a bunch of venting at current location, and there's another spot just a little ways down the return where I could reasonably easily add some more venting, and take the thermostat out of the F&T trap and plug it so it just works as a condensate trap? That way the steam should theoretically stop at the last vent and not heat the majority of the return line, reducing the pickup load, correct?



I'll attach a sketch, hopefully that makes it easier to follow.

jumper

beauty of steam

This is what I like about one pipe steam. Despite all the piping errors it still works.

Just like the building where the super decides which boiler to run each day.

ttekushan_3

outstanding point!

I think many people just assume that all the problems that they associate with steam heat are really problems associated with a system that continues to operate reasonably well while malfunctioning!

MTC

Yeah

this system certainly has its fair share (and then some) of problems. It doesn't work as it should, but it does work. Owner had installed a setback thermostat, and that was REALLY screwing up the balance. It works much better now that we only have it set back a few degrees in the off times. But much better is a VERY relative term. Its still all out of wack.



But, it does work.



Try getting your HVAC system to work with similar types of problems...

MTC

BTW

forgot to mention that it'll be a bit before I can get over there again and play with settings. In the middle of a ton of other work for this guy and other random stuff. Will update you how it goes after I get a chance to try it out.



Any thoughts about the venting question above?



Thanks again for the advice!