Better to build pressure or short cycle gas boiler? Downfire?

DCMA1 · February 11

Burnham Independence IN6PVNI power vented natural gas boiler, 175,000 BTU/hr., one pipe steam. I've clocked it at 168,000 BTU/hr, think it's a little low just to my measurement error. Regardless, it's significantly oversized to the load. Don't have the calculations handy, but think it's around 2X, there are only 4 small and 3 larger radiators attached to this, which is the largest one in the lineup that Burnham makes. Currently, I have the Pressuretrol dialed all the way down, which when things get cold out results in it shutting off around 2.2 psi (according to the 0-30psi gauge, so not sure how accurate that is). It then short cycles once things reach steady state during a call for heat with about a 2:50 minute burn cycle followed by around a 1:20-1:30 pause (it starts up about halfway through the pause but waits somewhat over 30 seconds for draft before lighting the burner).

Usually I see people advocate keeping the pressure as low as possible but if I put a Vaporstat on this it would short-cycle even more crazily. Would it actually be better to raise the pressuretrol so it would burn longer? I'm actually not sure why the short cycling is so bad besides wear on the ignition components. I would think it would be more efficient short cycling since the temps are a little lower.

The real problem seems to be the boiler is so oversized. I've seen discussions here where people say "don't do it!" regarding trying to downfire gas boilers. In the case of the Burnham boilers, the only difference I see between small and large is the number of sections and the number of burner assemblies, which are all the same and interchangeable between the models. It seems you should easily be able to convert an IN6 to an IN4 by removing the appropriate number of assemblies and plugging the manifold. No changes to orifices or anything. Each assembly would then fire identically with the same manifold pressure, you would just have some more case iron in the mix like an older boiler. Obviously it would take a bit longer to make steam but would the lack of short cycling be a bigger benefit?

EdTheHeaterMan · February 11

One options is to put a time delay that activates on shutdown on the low voltage circuit. You see, now the burner operates for several minutes until the water is hot enough to make steam pressure. Then once the pressure is high enough to reach the limit pressure, the burner shuts off and the pressure drops fairly quickly. Then the 4.5ish minute cycle starts with 2:50 ON and 1:30 OFF. if your gas valve needed to wait 6 minutes before starting again, then the pressure in the boiler would drop to zero but the water would still be very close to boiling point. Then when the burner operates again after the 6 minute delay, the on cycle would be longer to get the pressure up to the limit pressure again.

You can also find out if the boiler manufacturer can allow you to reduce the firing rate by lowering the pressure from 3.5"wc to 3"wc in order to reduce how much the boiler is oversized. Another option that I have seen (but may not be recommended) is removing one or both of the end burners and plugging the orifice port with a blank plug. that will reduce the total input of the boiler. You must be careful, not to reduce the firing rate by too much. There are other problems that may develop by down-firing without the proper tools and combustion testing equipment.

Jamie Hall · February 11

Here we go again. There is an endless debate on this one — as @DCMA1 has already figured out. Myself I'd leave it as is. You're not going too high on the pressure to waste much fuel, if any (though it would be nice to know how high it really is going — those 0 to 30 gauges aren't very helpful), and raising the pressure will just waste fuel. And not much point in going lower.

Some would advocate a time delay, as @EdTheHeaterMan suggests— but I'd rather keep the boiler as hot as possible during the off part of the cycle, which your arrangement is doing.

ethicalpaul · February 12

It seems you should easily be able to convert an IN6 to an IN4 by removing the appropriate number of assemblies and plugging the manifold. No changes to orifices or anything. Each assembly would then fire identically with the same manifold pressure, you would just have some more case iron in the mix like an older boiler. Obviously it would take a bit longer to make steam but would the lack of short cycling be a bigger benefit?

You stated this very well. Older boilers had a ton more iron to heat up and no one complained. If the short cycles bother you (they tend to bother most people), this is something to consider.

You should make sure your main venting is plenty capacious and consult your manual but I believe most manufacturers say that lowering the gas pressure at the valve is acceptable down to 3.0" of water column.

Another thing you can do is to attach a low-pressure switch from Dwyer and hook it to a timer relay module so that when a desired pressure is reached (say 1/2 psi or whatever), then you fire the relay timer and make it delay for 10 minutes. This will let your radiators let off some heat into your rooms without burning fuel.

You should determine when you are hitting your 2.2 psi cutoff…as in, how full are your radiators when this occurs? If they are just starting to get steam, obviously you are in more trouble size-wise than if you reach pressure after some of the vents start to close.

LRCCBJ · February 12

These work quite well without reinventing the wheel:

https://www.supplyhouse.com/Robertshaw-700-053-3-4-x-3-4-Two-Stage-Natural-Gas-Valve-300-000-BTU

On the lower firing rate you get 60% of the full firing rate. The boiler does fine with it.

109A_5 · February 12

To me excessive pressure and/or short cycling, just seems silly, I would not tolerate either, my house heats just fine at 0.072 PSI or less. Obviously if your boiler is huge compared to the radiation you should not have much problem filing your system unless your venting is very poor.

My vote is the timer. The user can change the delay to meet their needs or disable it totally if needed. The problem with a timer, is it seems most folks can't deal with the electrical aspect to install it. And there are multiple ways to implement a timer.

Mad Dog_2 · February 12

Be very careful here: It may be "easy" to physically plug gas orifices, just be aware that without express written permission from the manufacturer, you're voiding any warranty. Worse yet, you are taking sole responsibility for assuring SAFE combustion. Are you trained? Qualified? Experienced? Mad Dog

dabrakeman · February 12

The 2 stage gas valve seems an elegant solution but likely also needs a vaporstat to control. Probably need someone knowledgeable to install it. Would it also void warranty on that boiler?

ethicalpaul · February 12

I've wondered that about 2-stage valves. Is it OK to just feed that much less gas into an existing set of orifices? Or do you have to install a second set for the lower level firing?

And what about the air intake and exhaust? Does that get modified when it's on the low level?

Long Beach Ed · February 12

https://forum.heatinghelp.com/discussion/comment/1842123#Comment_1842123

Quick question, LRCCBJ: Doesn't an atmospheric burner run really lean on the low flame 1.3" pressure? I'd think the combustion would be terrible with the flame lifting?

dabrakeman · February 12

The 2 stage are factory supplied on many of the larger boilers.

Long Beach Ed · February 12

https://forum.heatinghelp.com/discussion/comment/1842177#Comment_1842177

The big ones we work on have dampers in the air supply for the staging. Do some have a fixed air shutter for both gas settings?

Jamie Hall · February 12

I'm speculating — which is always rather fun. And I would imagine that some form of varying air flow with relation to gas flow — or vice versa — must be employed. In some other applications, though, I know there is. Let's consider an ordinary gas grille in your backyard. There, the gas pressure (and flow) is varied when you change the knob setting. That, in turn, varies the pressure drop in a venturi, which varies tha amount of air that the venturi brings in. Presto. None to accurate, but it works. Or a carburetor. There the air flow is varied by how lead footed you are, which in turn changes the pressure drop in the venturi, which in turn sucks in more or less air, as needed, from the intake. Actually works pretty well. In fuel injection engines, there is some means of measuring mass air flow (controlled by the throttle plate), and that in turn is used either mechanically (anyone remember the early Bosch mechanical units? Or even earlier GM units?) or in a computer to determine how much fuel to inject.

And so on.

So. Anyone care to enlighten me on how this is done in modulating boilers?

LRCCBJ · February 12

https://forum.heatinghelp.com/discussion/comment/1842170#Comment_1842170

Surprisingly, they do just fine. The flame does not lift at 60%.

ScottSecor · February 12

I am a little concerned with the power vent and adjusting the firing rate. If you go this route, I would suggest checking the combustion with an analyzer.

ScottSecor · February 12

@Jamie Hall as you are well aware there are many types of modulating boilers. I am referring to natural gas only below.

The simplest ones have two stage gas power burners that are typically low-high-low. Most of the ones we work on use the gas valve actuator as the "driver" that connects with a rod to the air shutter. They work pretty good, but almost always "shutter or is that "stutter" when transitioning from low to high or vice versa. Probably not all that efficient in low fire, but this setup allows for longer cycles.

The slightly more sophisticated gas power burners have modulating motors that control the air shutter and the gas valves together. The mod motor drives the gas butterfly valve and the air shutter together, at the same time. These are a big improvement over the low-high-low burners listed above.

A "much improved" version came out about twenty five years ago. Honeywell came out with a system that was able to vary the air and the gas flow separately. Fireye came out with their version around the same time. For the first time in my career we could actually get a power burner within spec for the entire firing range. This was the greatest thing since sliced bread, or so we thought. After installing two of the first ones in our area, we leaned that the software was not really designed for steam boilers. We were married to the job, constant failures and problems. Thankfully, the manufacturer came out with a solution and the system is still working today.

Finally, today most if not all condensing boilers have the fan(s) control the air and the gas flow with the Dungs style gas valves. In most cases we can get the combustion numbers to meet the spec in the entire firing range with zero or almost zero CO. Shame we can't have these cool burners on good old cast iron steam boilers…

Jamie Hall · February 12

"Shame we can't have these cool burners on good old cast iron steam boilers…"

Indeed. No demand, I suppose…

Thanks, @ScottSecor

Long Beach Ed · February 12

Those high/low fire gas valves are used on cast iron atmospheric steam boilers.

I've seen them in the wild.

I just wonder what the 40% overage in combustion air does to the combustion quality/efficiency/etc.

DCMA1 · February 12

Thanks EVERYONE for your responses! They were all great!

Better gauge is on it's way.
Boiler is out of warranty, no concerns there.
If I decide to go the relay route, electrical isn't a problem for me. I don't even need an extra pressure switch, I'd use a delay relay interlocked with the pressuretrol or vaporstat.
@ethicalpaul , the radiators fill fine. I have the big one in the room with the thermostat choked up a bit so it doesn't heat as fast. The smallest radiators are actually vented the most because they are farther from the boiler and also heat the rooms with the largest heat loss. Mains are also vented well.
@109A_5 , I HAVE to tolerate either excessive pressure or short cycling, unless I can downfire the boiler!

@ScottSecor , @Mad Dog_2 - the combustion is my big concern. Obviously if anything changes, a combustion analysis needs to be done.

I'm not actually not sure what adjustments can be made to these boiler to affect combustion? The Honeywell valve has a single adjustment to affect the output pressure. Per Burnham, you should set this to 3.5"WC but you can adjust it +/- 0.3"WC to change the gas burn rate to meet the target of 175kBTU. No other adjustments are mentioned in the I&O manual, nor do I see any way to implement them. How would anyone doing a combustion analysis do any adjustments?

Burnham seems pretty adamant about not going below 3.2"WC, which is why downfiring through a two-stage valve like the one @LRCCBJ linked seems unappealing to me. To me that seems much more likely to affect combustion, plus would require a significant amount of gas repiping. Removing the burners seemed less likely to affect combustion because I would think each burner would still see the same draft and same gas pressure?

It is powervent and not atmospheric, but I think the same principles would apply there.

I'm still haven't heard if there is a real downside to short cycling besides wear? I spoke to a person who does building heat transfer (insulation not boilers) and doesn't see a downside because the heat from the boiler still goes into the rooms so efficiency is unaffected? The only heat you lose is out the flue when transfer efficiency goes down when the boiler is too hot, like building pressure.

Jamie Hall · February 12

There isn't a real downside to shorter cycling, other than wear and a small hit on efficiency with power burners from the pre-purge and post-purge.

Problem is, it seems to really bug some people…

ethicalpaul · February 12

I'd use a delay relay interlocked with the pressuretrol or vaporstat.

I was always very reluctant to do this because I wanted my safety pressure control (pressuretrol in my case) to be fully separate from anything I added to the pressure control system. This way if my stuff failed due to my own fault, or component failure, the pressuretrol was still there unmolested to shut things down if pressure climbed.

LRCCBJ · February 13

Burnham is adamant about many things…………..none of them based upon facts or tests. What they don't know means that they don't want you doing it. IMHO, they are the worst company out there.

109A_5 · February 13

" I HAVE to tolerate either excessive pressure or short cycling, unless I can downfire the boiler! "

Why ? So with a normal thermostat cycle your pressure goes to 2.2 PSI instantly ? I suspect not. I suspect the pressure starts to rise significantly when the some smaller radiators are full and the vents start to close. You did not mention the time duration from when the thermostat first calls and when the short cycling starts.

Anyway the timer is triggered when the system reaches a given pressure, say 8 Oz. (or what ever you like) the timer disables the burner for selected time duration, the duration of what the short cycling would have been.

So no short cycling and no excessive pressure. No unneeded wear and tear from the short cycling no wasted fuel to make unneeded pressure.

Another way, since your boiler is oversized even when the outside temperature is below design day temperatures the boiler has a duty cycle, that is less than 100%. A timer could be set up to mimic that duty cycle but with shorter run times to limit the pressure. No need to down fire or mess with combustion issues.

PMJ · February 13

There is no plus side to creating enough pressure in single residential systems to trip a pressure device. Show me a system that has just satisfied its thermostat with pressure in the system and I will show you one that is in the process of overshooting the temperature target a lot more that I will put up with.

If you aren't particularly interested in evening out your heat then read no further. Let the system bounce off a pressure device limit until the tstat says uncle. Running that way is not going to wreck the efficiency or wear out any equipment. If that works for you comfort wise I say just relax and let her rip. That is the way the 100 year old homes around me still running steam are doing it. It gets the job done. It's not breaking the equipment or the bank either. But we all have big boilers and extra intalled radiation that was never intended or needed to run full. So letting the boiler fill it up or nearly so is overshooting plain and simple. It's a roller coaster I moved into 35 years ago and quickly decided I wanted no part of.

But if you are interested to run without pressure (an absolute requirement for even heat) and with significantly improved comfort, the control system must somehow control the length of the burns and the time in between them. There are many ways to do much better with this that are actually quite simple. The standard control isn't one of them.

Using a timer to delay the next firing only after a pressure stop misses the point. To tame a big boiler and produce even heat the control needs to both limit burn times to well short of making enough steam to produce pressure, and then also space the burns out more evenly. Accomplishing that really isn't difficult.

ethicalpaul · February 13

I love your thinking @PMJ but I think you are putting a pressuretrol in the same bucket as a low pressure switch and I think that is a mistake.

My boiler shuts down at about 6" of water column for 10 minutes. There is no overshoot and I hope we can agree that 6" of W.C. (.22 psi) is a pretty gentle wall to bounce the system off of

DCMA1 · February 13

So, the consensus seems to be that the shorter cycling isn't THAT bad from a gas use standpoint. Just that people prefer it not to, which I understand. That makes it easier for me to live with.

Most are advocating for better controls vs. changing combustion. I'm still curious how one would even adjust combustion with this boiler if they had an analyzer?

@ethicalpaul wrote:

I'd use a delay relay interlocked with the pressuretrol or vaporstat.
I was always very reluctant to do this because I wanted my safety pressure control (pressuretrol in my case) to be fully separate from anything I added to the pressure control system. This way if my stuff failed due to my own fault, or component failure, the pressuretrol was still there unmolested to shut things down if pressure climbed.

Yes, to be clear, that is what I meant by interlocked. The added relay(s) do not control the burner directly - instead, the signal from the pressuretrol to the burner is routed through their contacts. All the added components can do is turn the boiler off - if a contact welds, all that actually does is disable the added relay functionality, and you're back to the standard pressuretrol or vaporstat. If a coil in the relay fails, the burner can't turn on, but that is a safe condition, just as if the pressuretrol turned off the boiler.

@109A_5 wrote:

" I HAVE to tolerate either excessive pressure or short cycling, unless I can downfire the boiler! "
Why ? So with a normal thermostat cycle your pressure goes to 2.2 PSI instantly ? I suspect not. I suspect the pressure starts to rise significantly when the some smaller radiators are full and the vents start to close. You did not mention the time duration from when the thermostat first calls and when the short cycling starts.

IF I'm starting to follow what your saying, you're also advocating for timers/relays - my comment was that with the provided controls I either need to build pressure or short cycle, I see that as either/or. Unless I'm missing something. Yes, with some added controls I can avoid the short cycling. I don't know the timing of the first cycle, obviously it also depends on how long the boiler was off/how cold it is outside.

@PMJ wrote:

But if you are interested to run without pressure (an absolute requirement for even heat) and with significantly improved comfort, the control system must somehow control the length of the burns and the time in between them. There are many ways to do much better with this that are actually quite simple. The standard control isn't one of them.
Using a timer to delay the next firing only after a pressure stop misses the point. To tame a big boiler and produce even heat the control needs to both limit burn times to well short of making enough steam to produce pressure, and then also space the burns out more evenly. Accomplishing that really isn't difficult.

So, PMJ, I was intrigued by your signature, "Custom PLC control". Is that what you would advocate? Certainly in some ways it makes things easier. And potentially you could incorporate the actual boiler temperature into the process control. But in my reading, I could accomplish most of what you're after by switching to a Vaporstat from a Pressuretrol, setting that setpoint way down, and then adding some appropriate relays. I think I'd prefer that approach to make it easier for future plumbing professionals to service the system, since they're unlikely to have PLC experience.

Thanks again everyone! Such a great forum!

PMJ · February 13

https://forum.heatinghelp.com/discussion/comment/1842364#Comment_1842364

No argument there @ethicalpaul . .22psi would be way different than 2.2psi with regard to everything including overshoot. For sure calling for a stop and wait at .22 is way better than doing nothing.

I attach in case you are interested a plot of my system pressures at around zero degrees outside cycling right around 2 per hour. The max I ever see is about .1psi. The system was calling for heat 90% of the time(long calls are evidence of even heat) and the boiler actually firing only 37% of the time (big boiler for the job), vent open with system in positive pressure about 12% of the total time. Visual evidence of the idea I'm trying to explain. Bigger boiler, no pressure, evenly spaced burns, minimal variation in radiator conditions…very even heat.

Pressures1.jpg

Pressures2.jpg

ethicalpaul · February 13

I don't even need an extra pressure switch, I'd use a delay relay interlocked with the pressuretrol or vaporstat.

I should have quoted more. It was your statement that you wouldn't need an extra pressure switch that confused me. I use a separate pressure switch, connected to my own relay, which interrupts the call for heat (the thermostat connection).

Jamie Hall · February 13

The whole question of how best to modulate a system — and what pieces of it to modulate — to achieve the desired power output is so fascinating.

And every system is different.

It's a lot easier if the system load is reasonably constant to begin with, and if the various components of the system are correctly engineered and selected to work together and match the load…

Dream on!

Oddly enough (or perhaps not?) space heating or cooling systems are right up there with the hardest ones to do. Good thing it is possible to get a pretty good approximation with pretty simple systems and controls…

PMJ · February 13

@DCMA1 ,

I've been experimenting with this for a long time. I am on a PLC platform because it provides the easiest way to try different things and input analog sensors. But then I run many PLC's in my factory and have lots of experience with them. At least 90% of the code in my PLC is about collecting and displaying data about what is going on. The control itself is simple and relatively easily hardwired. I have resisted doing anything complicated that couldn't easily be hardwired. I read outside temperature, for example, but don't use it to alter the behavior of the control. My recommendation is whatever you do simply provide a switch to bypass the whole thing returning the control to standard which everyone understands.

I started very simply with one delay off/delay on timer where on/off times could be adjusted independently and went from there. There are many ways to do this. I do not claim to have the best way. I'm happy to discuss at any time.

Jamie Hall · February 13

Only 90%, @PMJ ? When I was programming data entry and management systems, I'd consider myself doing well if 5% of the code did something useful, while the other 95% tried to idiot proof things…

109A_5 · February 14

" IF I'm starting to follow what your saying, you're also advocating for timers/relays - my comment was that with the provided controls I either need to build pressure or short cycle, I see that as either/or. Unless I'm missing something. Yes, with some added controls I can avoid the short cycling. I don't know the timing of the first cycle, obviously it also depends on how long the boiler was off/how cold it is outside. "

Yes, Very true. If you have to pick one I'd go with the short cycling.

However, IMO why would you leave it that way, when you can do better for not much money. Sure you could also replace the boiler with a properly sized boiler (no pressure and no short cycling), but that would be $$$$ and seems like a waste of money to me.

PMJ · February 16

I don't know the timing of the first cycle, obviously it also depends on how long the boiler was off/how cold it is outside.

Exactly @DCMA1 . The amount of steam you would like to add to the system each burn depends on the starting place and differs by multiples. But you are stuck with only one possible rate - full blast, and no stopping until your tstat is satisfied.

There is endless debate on the subject because these systems change hands and new owners experience the same frustration you are. They come to know even without any technical background that their boiler should stop firing sooner than it does on virtually every burn cycle because they can feel that there is already enough steam in the radiators to get the job done and will soon - many minutes before their Tstat knows it. They get told here that their boiler is too big, which usually is a show stopper and so they just live with it. I don't think my boiler is too big at all (it would trip on pressure with the standard control) and knowing what I know now would never choose to live with it that way.

If you are interested, here is what I am currently doing with my big boiler and have been for at least 10 years and like a lot. I am quite sure it would work fine with an even bigger boiler than I have now.

I installed a heat switch on the feed pipe of my most remote radiator to be able to know on every burn when new steam gets that far. On a cold start that could take 25 minutes and in continuous cycling less that 4 minutes. So first off every burn always runs straight as long as it takes to get steam to the radiators. I then have a fixed timer that I can adjust (but haven't in years now) which extends every burn to a point where radiators are filled to about 1/3 and stops the burn there. When that same switch opens again and I know that a significant amount of the steam I just made has condensed and its heat now in the rooms I start another fixed timer - again one I haven't changed in years - which delays and starts the next firing if there is still a call in progress. I'm betting you can see that how long it takes to get steam to the radiators and how quickly that steam condenses is directly related in real time to the conditions. So in this way total burn times and total wait times change themselves as the conditions do. The rate at which natural vacuum is created between burns is also an indicator of how cold it is and I use that too for the burn time adjustment but I'll leave that out for now.

So with this setup you can take full advantage of the speed of a big boiler to recover or fill to the level you want quickly but without filling the system up and creating pressure ever. Its output is in even pulses and all that is required is to find a fill level very slightly above what is required to stay ahead of design day cold by adjusting the fixed timers. For me that fill level is around 1/2 full radiators.

Unfortunately no contractor is going to do it because there is no commercially available control doing this available. That's a shame. It is way better than the standard control. I've run both so I know.

LRCCBJ · February 16

@PMJ

I truly admire all the work you have done over the years to perfect that system. Nobody has achieved he results you have with a boiler that is 2.5X as large as necessary.

That being said, such a system requires a huge amount of development by an individual thoroughly experienced with PLC and the programming of same.

As you mentioned, no contractor is going to even think about such a system. Anyone who does it is totally on their own. Even if the burner need attention or support, most heating companies will just walk away.

This is, unfortunately, not viable for 99% of the population. I put myself in the 99% because I just do not have the time for it at a remote site.

So, with full cognizance of the above, I offer the commercial equivalent: The Tekmar 279.

The unit functions on outdoor reset, has the capability to specify cycle time, the capability to specify burn percentage at design, the capability to specify WWSD, and the capability to hold off the start of the cycle timer until steam reaches the end of the main.

I offer it as a readily made device that addresses all the issues that you have observed without the necessity of designing and supporting it by the homeowner.

If I was installing it in your place, I'd set the burn percentage at design to 40% and the cycle time at 40 minutes. I believe it will duplicate your results without a thermostat and without the PLC. It will certainly eliminate any short cycling by the large boiler and it will absolutely eliminate the typical overshoot with a thermostat.

The only thing I don't like is the current price of it. However, if I calculate the time to duplicate it with a PLC, it's probably cheap.

PMJ · February 16

@LRCCBJ ,

Thanks much for your comments.

I will point out that I'm quite sure the PLC is not necessary for this. It can easily be hardwired with low cost components.

That said, I'm sure Techmar is a very good unit. The cost, however, probably well past what will work for the average homeowner.

It is true that I have spent a long time observing and experimenting. But I have kept the actual control part very simple on purpose. I have no interest is selling anything but I wanted the end result to be accessible to more people.

PMJ · February 16

@LRCCBJ

I will say that knowing what I know now assuming Techmar solves the same issues I am(and I do think it would), if my choice was buy Techmar or do nothing I would pay a contractor to install one.

LRCCBJ · February 17

https://forum.heatinghelp.com/discussion/comment/1843079#Comment_1843079

Installing it is fairly simple. Power and two wires from the sensors. If you wish to get more elaborate, you can install one or two room sensors which will further refine the program if you don't quite have the curve correct.

Like any mod-con, it takes awhile to tweak the curve to match the heatloss in the building at various outdoor temps.

Better to build pressure or short cycle gas boiler? Downfire?

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