How to delay boiler from firing again right away after max op pressure is reached?

nicholas bonham-carter

Mark, maybe it's time for another try, in New York:--NBC

http://www.citylab.com/cityfixer/2016/12/new-york-city-cracks-down-on-steam-heating/508925/#site-header

PMJ

MilanD said:

Thanks @PMJ! Exchange with KC, Jamie and Hatterasguy have been nudging me to this thinking, and now your comment even more.



Jamie has brought up a point of energy use to get that sitting water to go back to 1150btu per pound of water. It's on a different recent thread here on the wall, pehaps one on wifi tstats. I'm on my phone now and can't find the link.



Anyhow, my question was what wastes less fuel: cycling until tstat is satisfied, or letting the water cool some, or rather, go from 1150 btu to some lower btu level before expending fuel to get it back to boiling and to 1150btu. My supposition is that it depends on boiler efficiency vs. time it take for sitting boiler water to lose btus and then be brought back to 1150 btu. If your boiler efficiency is, say, 80%, boiler needs to be losing 230btu per pound of water that first hour to make it a wash. If it's losing less, you are more efficient to wait to fire. If it's losing more, you better keep cooking sooner. I'm still wrapping my head around this, but I think I'm right. There is also building heat loss to consider.



What you are doing with the delay switch or the PLC (and I'd really be curious what that setup is), leads me to believe that I am correct. Controlled firing will get you close to inputting only the amount of BTU which is lost through building's heat loss, to maintain a certain temperature. On tstat satisfying burn, you basically overshoot the set temp. This means, to be equally efficient, you have to swing the temp around the set temp: for example, heat to 72 and wait to heat again until temp falls to 68. But this is 1. not as comfortable, and 2. may be wasting more fuel because you are now heating from, let's say 850 btu rich water, instead of 920, and will also burn more fuel to heat building back to 72, instead of just 70.



Am I thinking this correctly?



Thanks again! Do you mind sharing what kind of setup you have for this even firing solution?

To me, comfort is the reason to do this and while I think it is more efficient, not dramatically better than a well balanced low pressure system with conventional control that overshoots a little. I just didn't like the boiling hot radiators and then the longer off periods with cold radiators. It occurred to me that the original design would have had partly warm radiators all the time so I just set about trying to get more like that.

As far as off periods between being wasteful because the water is cooling, I find that re-firing a boiler that has been off 10-13 minutes very little has been lost and full boil returns in seconds not minutes. Even faster for me because I am in vacuum between cycles. But I wouldn't even consider cycling as causing any significant loss of efficiency. It is cycling to maintain a pressure that isn't needed that is wasteful.

I have posted quite a bit about the control I use. I'm happy to share any of it. The really nice thing about being on a PLC platform is that you can so easily change your mind and try things. You aren't locked into someone else's algorithm.

PMJ

MilanD said:

What you are doing with the delay switch or the PLC (and I'd really be curious what that setup is), leads me to believe that I am correct. Controlled firing will get you close to inputting only the amount of BTU which is lost through building's heat loss, to maintain a certain temperature. On tstat satisfying burn, you basically overshoot the set temp. This means, to be equally efficient, you have to swing the temp around the set temp: for example, heat to 72 and wait to heat again until temp falls to 68. But this is 1. not as comfortable, and 2. may be wasting more fuel because you are now heating from, let's say 850 btu rich water, instead of 920, and will also burn more fuel to heat building back to 72, instead of just 70.



Am I thinking this correctly?

I do think you are correct here. I think the BTU's lost from the structure are what need to be replaced and overshooting results in a higher net average temperature differential and loss to the outside than is necessary and higher than would be the case with more even temperature.

Again, I'm not sure this difference is that significant as cheap as gas is now and the main driver for all my work is comfort and not efficiency though I think I do get both.

MilanD

So here is something interesting to note today apropos building heat loss.

Given that it's something like 8 degrees outside today, with a wind chill of -5, I pulled the building out of 3 degree overnight set-back a little earlier in the morning, had it go from 66 to 68. It took about 45 minutes to get there (I wasn't here to monitor cycling, remote wifi tstat - but not bad). Tstat is Honeywell TH8320WF (sticker says TH, not RTH - this should matter, I think). Stat is set to its CPH pre-set '1' (for steam) heat under 'Heating Cycle Rate' (under system setup function 0240). I am not sure how this exactly works, but I can't manually set the number of cycles, it does this on its own. I have yet to figure out what exactly it's doing. It's not obvious.

The system is running off this one tstat and I mentioned it being in office space that, when rad is on, heats quite fast, sometimes faster than the rest of the building. It's generally set to 69 in the am (off 66 overnight set-back), and as the day progresses, I bump it to 71 and 72 at 2:45 and 3:45 (4 temp per day programming). This works well, for the most part, keeping everything comfortable. This morning I had it come to 71 at the end of that 66-68 bump (manually), and it's now holding at 71.

My plan is to, for now, add a TRV on this rad in the office with tstat, and set the TRV to shut off the rad where the room gets to 71, with the rest of the building being close behind. This is 2nd rad from the boiler, coincidentally, so it heats quickly, which makes it tricky to balance heat in the entire building while tstat is there - but, let's say I'm happy the way it is now. I can't move it elsewhere as I don't want to mess with either wires or people changing settings, or putting a lock box on it, which I guess I could do one or all of it, but am lazy for this kind of change right now (so am looking at Ecobee 3 and it's functions - more about that later). Instead (and this is why TRV valve may be a good alternative) I have a tall card board box next to the rad covering the rad and have been playing with where the box is - high-tech, I know, to balance this room's heat gain with each heating cycle. This works well, for the most part, except when someone closes the rad bc they may be too hot - or moves my box. That's another story I covered earlier and why I started this thread.

At any rate, being that's colder today, keeping tstat at 69 does not get the building warm enough on a day like today. I have it now on 71. Now the interesting part: tstat still shows 71, call is 71, and the system actually came on and cycled one cycle for about 30 minutes without tstat falling back to 70. This got all the rads hot (I just checked the last rad on this loop, hot beyond the comfortable touch), and tstat is holding on 71. One of the zones in the auditorium (also mentioned earlier in the thread) is completely off. I did not check if we had some cycling on pressure with this set-up (closed zone essentially makes the system oversized), but I am pretty sure we did not have more than a few cycles on pressure for this particular condition as is today: colder outdoor temp with windchill.

I am mostly amazed, the tstat actually cycled the boiler, without it falling from 71 to 70. I have not seen this happen prior to today, and we've had this tstat for the past year.

And, this kind of functioning, in addition to perhaps delaying immediate boiler refing on pressure rise/drop and thus cycling on steam is precisely what I was looking to achieve. So, on a colder day, with a wind chill of -5, tstat actually functioned where BTUs lost to building heat loss was replenished through a 30 min burn, without tstat moving the temperature...

Anyhow... I know, I'm OCD with this. I will now go and do something else for a few hours.

MilanD

@Gordo

I am not sure how this is achieved with 2 vaprostats. Are you saying it can be set-up to fire lower BTU and also higher BTU input based on some setting/parameter, or on higher and lower oz setting, based on some setting/parameter? If BTU input changes, this would make it 2 stage, no? How is this done?

If lower BTU input can be included, would this, in essence, also create a 'slower' steam head? Would the benefit of this be lower cycle pressure and also slower, more even dissipation of BTUs into the space, but also allow an 'oversized' boiler (like if you close one zone off, it becomes oversized), to reduce the BTU rating and thus, size down the boiler? If so, this would be fantastic!

I did see that LGB has a possibility for smaller than max BTU input on LGB, as you said, and now that you said it can change, I am wondering if it could become 2 stage... Perhaps someone else can chime in too.

I honestly don't remember installers talking to me about what the spec was when it was installed and if we are doing the max BTU input or not. And I didn't know to ask at the time. I know that we had the distributor come out, count and measure the rads, number of sections and alike, and calculate the EDR for the new boiler based of that. Now, if this burner input was then adjusted by getting the boiler to fire less than the max BTUH rating, to match our EDR, I am unaware. It makes sense that it could.

All this was before I took interest in figuring this out. Next time I have the guys over to service it, I will ask about it. I am, however, certain it has not been set-up as a 2-stage now. Flame and pitch (hum) of it all sounds the same all the time. Interesting to know it could. I need to ask more about that. I owe Bob from Midwest Mechanical a lunch, so I'll ask him next time I talk to him. He's a great guy, and these guys do more industrial installation then semi-residential like this. There aren't many people here in Cincinnati I am aware, who know how to do steam. But I also, didn't search too much as I do most of basic maintenance myself and for anything major, I would call the pros who installed (there hasn't been anything major so far, knock on wood).

As to vaporstat, thanks for the note on accuracy. I am uploading a video just now, of our LGB 7 and how it's set-up, since it's different than most stuff here on the wall. In it, it shows there is a slight discrepancy between 8oz setting on the vstat and the 3psi gauge on the cut-out. So yes, I've already experienced the slight lack of accuracy... wasn't sure if it's gauge or stat... Your video shows cut-in is lower than dialed in, right? Mine, I think, is on the money when it goes to cut in. Yes, so they vary.

Thanks again Gordo - and if you have more details on how to do the 2 vstat setting, please share. I may have to get the pros to do this set up, as I'm not there yet with my knowledge.

Koan

@MilanD
here is the link to our conversation

forum.heatinghelp.com/discussion/160158/best-wifi-thermostat-for-steam-heating#latest

my wiring is very simple, transformer to tstat to vaporstat to LWCO to gas valve. I don't think the neutral is switched at the vaporstat. It seems it would not matter, but any interruption would create a delay, not just one due to pressure. Like I said, I can't see where that would make a difference though.

@Hatterasguy suggests a nice device. I also read more about this on another post, Ill see if I can find it

Koan

@MilanD ok this is a bit different but very interesting!

forum.heatinghelp.com/discussion/156690/a-diy-vaporstat-alternative#latest

Koan

in fact this is suggested by the same @MarkS who made Eco Steam!

MilanD

@PMJ

Thanks - I'm getting to that conclusion - it's about BTU in BTU out balance on the heat loss... no more, and no less.

Could you point me to your posts on PLC? This may be one way to get at least my mind going in thinking about all the various solution people have mentioned here to be doing: from EcoSteam (what a great product!) to PLC and delay fire timers...

Great thread everyone and thank you so much for sharing your knowledge. What a brain-trust!

MilanD

Thanks @Koan! I'll be giving this a read tonight after kids are in bed (barring any calls for no heat from my tenants... It's bound to happen - it's getting down to 6 tonight, and wind chill is supposed to be in minus double digits... last night I had air in couple of rads on one of the units that needed letting out... fun never stops.

MilanD

And for all of you that have been patiently reading this - a bonus: here's the video of our set-up at work.

https://youtu.be/rN-3CQcdXFQ

Stay warm people - with steam heat.

MilanD

Here's the video one more time.

https://youtu.be/rN-3CQcdXFQ

PMJ

MilanD said:

@PMJ

Thanks - I'm getting to that conclusion - it's about BTU in BTU out balance on the heat loss... no more, and no less.

Could you point me to your posts on PLC? This may be one way to get at least my mind going in thinking about all the various solution people have mentioned here to be doing: from EcoSteam (what a great product!) to PLC and delay fire timers...

Great thread everyone and thank you so much for sharing your knowledge. What a brain-trust!

@MilanD ,

This link has a lot of it.

http://forum.heatinghelp.com/discussion/152286/electric-solenoid-vent-system#latest

MilanD

Hatterasguy said:

MilanD said:

I am not sure how this is achieved with 2 vaprostats. Are you saying it can be set-up to fire lower BTU and also higher BTU input based on some setting/parameter, or on higher and lower oz setting, based on some setting/parameter? If BTU input changes, this would make it 2 stage, no? How is this done?

You utilize a two stage gas valve. When the Vaporstat is below setpoint (8 oz), the 24V is closed to the gas valve and you get high fire. When the vaporstat trips at 8oz, the circuit opens and the boiler immediately goes to low fire (typically 60% of rating). At that point, the pressure would typically fall slowly. But, in your case, it could still climb even at 60%, so you'd need to use the Pressurtrol to shut the boiler down at about 1.5 psi.

It's an excellent addition for your setup as you have a boiler that is far too large when certain zones are closed.

@Hatterasguy

AWESOME!! After you mentioned this, I was thinking about it and the implication - this would be just amazing! As this is a bit above my pay grade, I'll contact our Midwest Mechanical that installed it, and talk to them about this set-up. This would be, I think, just amazing.

I'm wondering if this in combination with the time-delay refire after pressure buildup after and if the low-fire pressure rise cut-off, would significantly improve performance. You just opened another possibility that I'm going to ponder over.

Thank you so much! This is just amazing.

Gordo

MilanD: This is what I was getting at:

You may not need to worry about any delay fol-de-rol and other gingerbread.

You already have low fire capability built-in with your boiler, you just need to implement it with another vaporstat and some modest additional wiring.

The goal is to simulate as close as possible the coal firing that your piping and radiators were designed for.

MilanD

Got it. Thank you both!

Let me ask you the following: if on, say, 60% burn the steam pressure drops below 8 oz (or whatever that low fire pressure point is) but btu is now not sufficient to replace the heat loss, how does a full 100% firing happen again? What triggers it, how does the full burn come back again if it's needed? How does it oscilate between high and low burns?

Also, how much down do you set de-rol? Is this a trial-error type thing? Obviously, I get it can't go below the sticker minimum btu input so how is this honed in, and will this require combustion analysis? I am thinking this will be like turning down the burner on the stove, so no need to do combustion analysis (afterall, LGB 7 does have open burners and is not considered hi f), right?

I'll take pics of the gas valve setup on Monday. We have 2 of them on there already. If you can tell me what to get to replace, or if I can use what's there, that'll be great! I will also test vstat voltage. I was under the impression it was full 110-120... Didn't pay attention what it was when I replaced the trol with vstat. Thick, 14 gauge wires tho.

Thank you both @Gordo and @Hatterasguy !

SWEI

Don't give up yet. I think I have a way to make this work.

MilanD

So my original post did not upload in it's entirety. I need to retype all this again from memory....

Not giving up @SWEI ... No worries.

Here is where I'll need help with getting the low firing figured out, as far as wiring goes. I'm attaching the pics of gas valves and wiring that's on the boiler.

First off, I've replaced the older Hoffman 75 on 2.5 inch loop/zone with the BJ Big Mouth. Omg! Steam got there in no time flat. Wow.

Now, there is another wonky zone with 3 rads just before this vent. It's probably 45-50 ft to the last rad on the 2nd floor. It's on a 2 inch line that reduces to 1.5 and the 1.25 for the last 25 or so feet for the last run to the rad. To help it with venting even better, I'll be adding a tee with a 3/4" nipple and a Gordon 1 vent just before the rad. This should get that steam there rather fast, I gather. Perhaps 2 Gordon's on an antler. But I'll start with one.

Now to the "I need help with" part:

I get that I need another vaporstat. I will need to be told how to wire it in (if not by just replacing the trol that's there). If it's to be used as a high limit switch, can't I just use the trol that's there already diales down to 1.5 psi cut out?

Then, I will need to know how exactly to wire burner controls for the low burn.

Then, how do I set up the low burn, and what to set it to - how do I tell it to do 70%, 65%, 60%? What % should I set it to?

2 more questions, more as a "what would you do" kind of things.

Mark Stayton and @Hatterasguy both have shared with me the delay relay setup with a fairly cheap timer. I know this is different than the low burn setup, but I am not sure how better in terms of fuel consumption overall? So, could I ask for a show of hands, if this were your personal setup, what would you do? Timer or downfiring?

Which brings me to my last point: there was a thread about downfiring a 300k btu oversized boiler and expert advice was basically against it. How is LGB 7 different to allow it and this boiler wasn't? I'm mostly curious how some boilers can be and other cannot be down fired or derated? (And as I mentioned in my video, I get LGB 7 has a max and min btu input allowance and am wondering why it does and some other boilers do not.)

Here's that thread:
http://forum.heatinghelp.com/discussion/159797/downfiring-a-300-000-btu-hr-american-standard-steam-boiler

Along those lines, no one mentioned having to adjust anything associated with combustion, so I am assuming this will be a simple "simmer down" setup.

Thanks everyone. You have taught me so much!

MilanD

And now the pics.

Oh, there is an outdoor cut out switch. It's JCs A350R-like, but with a simple temp dial to set at what ever outside temp you want boiler to turn off. That is A350 without an "R", I believe.

Koan

@MilanD
That thread was about my boiler. The gas guys such as @Tim McElwain explained that down firing more tha 20% can mess with combustion and cause issues. I am not knowledge about this, but I do know carbon monoxide is really dangerous. If you are downfiring I think it is super important to have a combustion test. We down fired my boiler, but not much and did run a test to make sure it was safe. It is completely possible your setup is made to accommodate a lower burn that mine is not, just want you to be safe. I encourage the combustion test. Note that the fact that my boiler is 1.88 times larger that it should be is also an issue. 750 EDR boiler / 399 EDR total radiators! Another factor us that my boiler is from 1969.

If I do anything with my setup it will be a time delay off the vaporstat.

MilanD

@Koan

Thanks for responding too!

I followed your post quite intensely because of my experience with our LGB boiler. It is a commercial boiler and it, as the sticker on it and the literature from the manufacturer shows, can be tuned between the min and max btu input range. My questions to the brain trust here was precisely what makes LGB 5+ able to do this and boiler like yours not. I explained on your post that we had an LGB 11 which had the burners removed as they rotted away... this seemed to have worked. From this time distance and knowledge, I also think that the boiler would have lasted for some time more as when it was removed, sections were in quite good shape, aside from the burner assembly which on this boiler is also the base. It was all rotted out. We could have attempted to detach the header, raise the boiler and replace the base. But, we'd still have had to deal with the oversized boiler.

So, I think that unless the guys here say it's an issue, LGB model is meant to be set up to accommodate various EDR ranges within the same boiler size/capacity. LGB 5, or 6, or 7, etc, should all be able to be both undefined and 2-staged. I am still curious about why other boilers can't and this particular one can.

I know we don't discuss pricing here as each market conditions are different. I can tell you that the range of costs to replace our old boiler was between 20 and 38 k greenbacks, so maybe that's why. It's an expensive piece of commercial equipment.

How's your system performing now? I think if you can vent the living life out of your main and rads, even by putting some Gordon 1s in front of each rad like I plan to do with that one branch I desribed, and then delay cycles, you may be able to make it work quite well for you even with the double sized boiler.

SWEI

I would fire at the lowest rate which sustains clean combustion (which could well be 100%, BTW); quite likely adding a double-acting barometric in order to get that clean combustion. After that is done, I would start messing with PWM.

MilanD

Where does double acting barometric damper wire into? Then, pmw controlls the gas valve?

Is wiring pretty standard in this?

Finally, I would need to use combustion analysis to tune it, correct?

All this is done to keep efficiency high, correct? To prevent sending more heat than necessary up the chimney, right, and keep the combustion clean?

SWEI

A barometric damper regulates draft to keep excess air down to a minimum -- but no less. This facilitates proper combustion across a range of temperature, wind, and building stack effect conditions. The double-acting variety are set up with spill switches on the "out" side, wired into the safety string, making it much safer than a draft hood. The barometric is adjusted using a draft gauge.

MilanD

Ok, thanks @SWEI. I need to learn more about this. Our draft hood is enormous on the back of the LGB. Where would this instal, on the vertical run above the existing draft hood? And, is there a gauge I can use for this that's not too costly for a one time user? I suppose I could call Midwest Mechanical and have them put one in.

I'm still waiting on an answer here regarding gas valve dial back and if this means running the combustion analysis with the newly instead barometric damper in place.

Sorry I have all these questions. As a new person to all this business, I need to make sure I understand the "why" as well and the "how".

Thanks again for a great tip!

MilanD

@Hatterasguy

So I should forget the barometric damper and pwm setup and do just the straight downfire with the current setup? And this will work 99% of the time? Sounds simple enough. That's why I was asking about damper and combustion analysis. I am also of a belief that the simpler it can be, the better. I can live with a percent or two of efficiency loss at lower firing rate vs. cycling on pressure or firing delay.

Speaking of firing delay, if you had to choose for your setup, which one would you do: downfire or delay?

Second question: will I be able to do this downfire setup my self. I figure I'm a quick study, unless there are special tools I need that would be prohibitively expensive to get.

Then, can I use the pressuretrol or do I need to get the second vaporstat?

SWEI

Downfiring absolutely requires proper training and an analyzer. How much (if any) of it you can get away with is determined entirely by the numbers.

Modifying an appliance puts you in the position of owning the consequences (potentially all available remedies under law) if anything goes wrong.

Proper training and a decent analyzer will set you back a bit more than $2k. A sound investment for someone in the trades, but...

MilanD

@SWEI

So I'm now thinking 2 pros with 2 opposing views.

Here's nkw my logic.

We are talking LGB 7 by Weil McLane. Sticker has max and min btu capability 780,000 btu and a min 380,000 (or there about, I don't have it off the top of my head right now - video above covers it). Says nothing about changing draft hood or anything as such. I will call the distributor here in town for clarification for sure as will also call the controls guy I know pretty well from the company that installed the current boiler in 2010. From what questions I asked and from what answers I got so far, it looks like the thing is engineered to do it under the curent setup without changing things other than what @Hatterasguy says too. Given I'm not a pro, I am gauging answers and educating myself so that my decision on what to do is made with confidence.

Again, it is interesting that we have 2 opposing views here from professionals in the field. One must be correct.

Now, my experience with our old LGB 11 and the slow removal of the burners done by our oldtime boiler man did not seem to affect either the combustion or the old boiler's performance other than taking longer to cook steam for the volume of water. But this also meant slowing down the steam head, no? Analogous to downfiring, less burners = less btu input. If anything, it actually improved performance of then greatly oversized LGB 11. We did not change the draft hood, and I tend to think it makes sense what @Hatterasguy says about the excess air.

That old timer boiler man, btw, is now well in his 80s and has learned with his dad who ran one of the largest HVAC outfit in Cincinnati from 1940s on. His dad started in the 20s. At that time in the 40s, he had 11 men on 11 trucks working full-time. Old timer who worked on our boiler is also a college trained man and was one of the guys who set up the trades track at a local HS here in the 60s, who then came back to HVAC work after his retirement from teaching for another 20 years. He was the man who brought back to life that old LGB 11 after it sat idle for 8 years while the building was empty. Only a few years ago did he finally hang it up for good. Body couldn't keep up with the mind.

Anyhow, that's my thinking. I will call and confirm with the distributor and the installer, and will then need diagrams on how to set up the 2-stage firing.

Thanks!

MilanD

@Gordo @Hatterasguy

Can you point me to 2-stage firing diagram for the LGB 7?

MilanD

@Gordo @Hatterasguy

Can you point me to 2-stage firing diagram for the LGB 7?

ChrisJ

Hatterasguy said:

I don't disagree with Kurt. You really should check the combustion on low fire to be sure there are no anomalies with the venting system causing an issue that needs to be addressed.

That being said, the chances of an atmospheric draft boiler, designed for low fire operation with significant excess air, giving you a problem are exceptionally small.

True,
But what will it's efficiency be?

At what point does the poor efficiency outweigh the cycling?

SWEI

Even if the boiler was offered/sold with a two-stage gas valve, proper operation can only be verified using combustion analysis. Natural draft appliances with no combustion air control will burn cleanly only across a rather narrow range of rates.

ChrisJ

SWEI said:

Even if the boiler was offered/sold with a two-stage gas valve, proper operation can only be verified using combustion analysis. Natural draft appliances with no combustion air control will burn cleanly only across a rather narrow range of rates.

When I swapped the gas valve on my boiler, it ended up burning at 131,000 input rather than the rated 125,000. The "preset" Honeywell gas valve wasn't preset very well in my opinion.

I guess the absolute minimum an atmospheric appliance needs is either the gas meter clocked or the manifold pressure checked.

Gordo

In the boiler room, with every piece of heating equipment and appurtenances thereupon, there should be present at all times operating manuals, schematics, parts lists, etc. etc. That's the code.

There might even be an operations log book and an SOP sheet.

It is a REALLY good idea to start a log book in the boiler room.

The bigger and more complex the system, the bigger the paper pile binder in the boiler room should be.

All too often, the only manuals found are from three boilers ago!

On the boiler manufacturer's web site, there are pdf files with the relevant electrical diagrams.

SWEI

ChrisJ said:

I guess the absolute minimum an atmospheric appliance needs is either the gas meter clocked or the manifold pressure checked.

It's actually simpler (yet not necessarily easier) than that. When the CO and O2 numbers are good, it's good. Otherwise, it's wrong. Factory settings can be wrong, even at 'official' altitudes.

SWEI

Hatterasguy said:

It's not an ideal situation for efficiency but it will rarely compromise safety.

Flue blockages are indeed rare, but with a draft hood they can easily become deadly.

ChrisJ

The beauty of a drafthood is it allows appliances to run safe with zero draft or even a downdraft.

It's part of it's design.

In theory, chimney draft, or lack of will have no effect on the burner's performance at least not a significant amount.

ChrisJ

SWEI said:

Hatterasguy said:

It's not an ideal situation for efficiency but it will rarely compromise safety.

Flue blockages are indeed rare, but with a draft hood they can easily become deadly.

See my response above.

With a drafthood a flue blockage should not cause any problem with the burner's performance, not until the O2 in the room starts dropping anyway.

The Spill Switch should come into play long before that happens.

SWEI

A barometric damper provides draft control, not combustion air control. CAC generally requires an electrically or mechanically actuated air shutter (a big part of why modulating power burners work as well as they do.)

MilanD

Ok then. I'll need to sleep on this and then reread. My head is spinning right now, just like our gas meter.