Help, please- quick sizing of expansion tank for Windhager 262 pellet boiler + 240 gallons pressurized storage?

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Unextinguished

New Member
Aug 2, 2022
18
Vermont, northerly, USA
Greetings to all. My backstory is in my signature (cordwood gasifier, unpressurized storage, "all wood, all of the time" for a number of winters). Been there, done that, have the framed certificate of verified exhaustion from the experience. Technology and design worked nicely. Willingness to work hard suited me just fine. Adequate time, and enough flexible time at the right times, for "all the wood" was my ultimate defeat.

Venturing forward, now, with a just-purchased but not yet delivered Windhager BioWin2 262 Touch pellet boiler + 240 gallons of pressurized storage.

"Back then", and since, I geek out reading Siegenthaler articles and Caleffi idronics bulletins for fun, and have helped others plan other successful versions of "playing with fire" with solid fuel.

This new Windhager 262 pellet boiler+ 240 gallons of pressurized storage, etc. will be a self-install.

But at the moment, with all else I am planning and getting bought, delivered and in motion, including extracting a near 2,000 pound wood gasifier from my below-ground cellar to give to the friend who is helping me extract it, I could use an quick assist from the collective "hearth mind", please.

I know the topic of expansion tank sizing has been discussed in past Boiler Room discussion threads but it seems deep in the midst of other lengthy discussions about varying topics, and I could please use a quick 'cliff's notes' assist to "size" and order my new expansion tank.


My past experience in sizing expansion tanks was with unpressurized storage and so it wasn't previously a challenge to size and "get enough" expansion volume.

New system is the Windhager 262 with approximately 47 liters of internal water volume - I'll round to 13 gallons.

Plus 240 gallons of pressurized storage.

The "delivery end" of the current hydronics to my house doesn't have a lot of volume. Air to water HX in ductwork. Yes, the Air-H2O HX was wildly oversized for the past cordwood adventures, to use low temps while drawing down storage. HX worked well, should work well with the new system.

At some point I may want to put in some radiant, which will add some gallonage, on top of the Windhager's internal gallonage + the 240 gal sealed thermal buffers.

Last thing I want to do is undersize my new expansion tank and then need to chuck it or buy another one.

Second-last thing I want to do is to throw more $$$ than needed into a crazy-oversized expansion tank. That said, I also kind of navigate by overbuilding to some semi-sane degree - because I hate re-doing things.

I don't especially trust regular hydronics tips and assumptions about expansion tanks because they don't expect large water volume or frequent wide temperature swings on an ongoing basis.

I am going to try to plan and operate this system to really be able to "fully load all 240 gallons of storage with Max BTUs" [and then draw as many BTUs back out as reasonably possible, before the Windhager lights up again] so there's going to be plenty of H2O expansion and contraction going on, on a constant basis, compared to initial system fill with ~50 degree water, and also a lot of frequent volumetric change compared to run-of-the-mill residential hydronics.

Can anyone please give me a reliable, conservatively large, but not crazy large, volume for an expansion tank for this new adventure with the Windhager 262 + 240 gallons of storage?

Also, I have read, somewhere, about some people using "ordinary well pump" (cold water) expansion tanks with solid fuel + thermal storage. I think I even recall that the people that mentioned it were knowledgeable, respected people. The 'thinking' was that since the bottom of the thermal storage, to which the expansion tank should be connected, should generally be the coolest part of the system, the expansion tank will never see especially much heat on its internals. I like being frugal, but I cannot roll the dice with the risk of a malfunctioning/ failed system in the depths of winter, and who knows what kind of wait to get and install a new expansion tank, under duress, especially because the world and supply chains just seem to keep getting weirder and less predictable. Am I best-advised to use a "real" hydronic expansion tank, not a cool water (well pump type) expansion tank?

Thanks _very_ much for the assist. I plan to share the voyage of this Windhager install as I go.
 

salecker

Minister of Fire
Aug 22, 2010
1,876
Northern Canada
Is there a direct benefit of storage with a pellet burner?
I know the theory of storage for wood boilers is so they don't idle.
Where a pellet burner shuts off fuel when it reaches temp,and there is no idle phase.
 

Unextinguished

New Member
Aug 2, 2022
18
Vermont, northerly, USA
On my original question, I think I found my answer and I am going to post it here in case anyone is trying to search for it (this specific resource really seems to specifically account for different systems' overall specific total volumetric capacity which seems a lot better than a lot of sites I found that seem to be some sort of assumed shorthand for this kinda system or that kinda system)

"TO PROPERLY SIZE A WATTS SERIES ETX, ETSX, ETA or ET-RA NON-POTABLE WATER EXPANSION TANK"

And it's looking like I need something in the vicinity of [maybe] a 33 gallon expansion tank that has around 12+ gallons of usable acceptance @ around 12psi.

On the other question (Salecker) of whether pellets need thermal storage, at all. . .

I would be delighted if I was arriving at an answer, after trying to look into it, that I could skip thermal buffer/ storage, for a pellet boiler. Substantially less initial spend, substantially less to figure out and put together _if_ I could just have a pellet boiler light off a burn of pellets when my single whole-house ducted warm air thermostat called for heat, and turn off the burn of pellets when the T-stat signaled "warm enough"

When I first started seriously considering a pellet boiler, gradually, over the last year, I was hoping to maybe not need any thermal storage. My prior/ "still physically here" unpressurized storage seemed silly big, under any set of considerations, for a pellet boiler [but it might someday have a new life, after some adaptation, as a bulk bin for actual bulk pellets]. Unpressurized storage can work, very decently, if it is the only available option but I now consider it klutzy. Didn't really want to buy any more storage vessels, for the pellet install, unless there was some seriously-clear good reason.

Various places seemed to suggest that you could operate at least some make/model pellet boilers without storage. All "well-thought-through-and-well-articulated" sources I was finding (Siegenthaler's various works, and Caleffi's idronic bulletins, and other) seemed to repeatedly suggest, across sources, and in various dates of publication over time, that a quality peller boiler is really substantially impaired, compared to what it can/should be capable of, at economy/efficiency/emissions/ongoing maintenance, etc., without, what thermal buffer gives you- which is a huge reduction of on/off "short cycling" -and the thermal buffer's enabling of many fewer, but substantially longer, separate "on times."

With cordwood, once you light the fire, you have no "off" and an "off" or way slow-burn of still-burning cordwood will be smoldery, inefficient, generate creosote. . . . Thus, the immense benefits of -big- storage, for burning cordwood, to be able to light off, "roar", and store - and then pull BTUs, as/when needed, from the "store."

With something like a gaseous fuel and a very low mass heat exchanger, like a propane or LP warm air heater (probably even a low mass mod-con hydronic "tankless" type appliance), "short cycling" of many, frequent, separate, short, light-and-burn events, is probably a much more minimal concern- the fuel burns well and efficiently almost immediately at start, and you don't spend a lot of time and BTUs heating up much thermal mass of a heat exchanger, and you don't randomly dissipate (up the flue, etc.) a whole lot of BTUs that have been thrown into the mass of a heat exchanger/ big volume of water, once you turn off the flame.

For something like an oil boiler, especially if old school with some heavy iron/steel + plenty of water volume within the boiler, you are apparently throwing a lot of BTUs into a lot of thermal mass of the boiler structure and water volume, just to get it up and fully rolling, and apparently the burn of the oil doesn't really hit full its stride of efficiency until after some non-negligible start-up phase of the burning process. And then, after you cut the oil burner flame and stop pumping the water, you are kind of randomly dissipating those BTUs that simply brought the mass of the boiler's physical structure and water up to full temperature. And, apparently, we all were able to not care about this as long as oil was cheap and we didn't worry about impacts of burning oil. Also, the "baseline technology" of things like oil burners + boilers evolved at a time when the available/ affordable technology (especially residential) wasn't up to doing much more than turning a burner and/or associated pumps, on/off in response to the "call" of a heat zone/ load, etc., without [much, usually] attention to overall burn cycle patterns and durations - and that "traditional" level of technology and control is well known, relatively fail-proof, etc.

Apparently some modern commercial-scale fossil fuel heat systems do use some extent of thermal buffer for efficiency's sake, although it may also do dual-duty as a big hydraulic separator to allow multiple smaller boilers to join in, or not join, take turns, etc. in meeting relative thermal load under varying conditions, rather than a single big fossil boiler that spends most of its life under only part throttle.

From what I think I have picked up from Siegenthaler, idronics, and various other things: Apparently pellets fall somewhere between the "wood" scenario and the "oil" scenario above. Apparently, burning pellets most efficiently (and, apparently, cleanly) doesn't hit its stride until you are "some substantial non-negligable while" "into" a given boiler-burn-session (and, apparently, this is a notably longer time than an oil burner), and, apparently, the turn-off-and-burn-down portion of the cycle of a pellet boiler, repeated, over extended time, consumes some non-negligible amount of pellets while not (in the ramp down) delivering a lot of useful BTUs that can be transported to your heat load. Plus, if you can do fewer, longer, burns, you are "calling on" (and using up the useful life of) your pellet boiler's igniter fewer times, over the same calendar span. Thus, apparently, the benefits (even with pellets, which you can turn off) of some extent of "thermal buffer" storage that lets you do fewer, but longer, separate light-offs and burns of pellets. But, since you can turn pellets off, unlike cordwood, there is some volume of storage, smaller in total size, than with wood, where your benefits of longer burns of the pellet boiler become outweighed by the fact that any storage, even insulated, is lossy, and (storage) is at least (usually) a bit random about when and where it dissipates the water-storage-stored BTUs. So to some extent, more than with wood, it is better to keep a higher proportion of available BTUs in the fuel not the thermal storage. This overall approach to pellets (aiming for fewer, longer burns) also kind of makes sense to me because for a lot of physical and electrical systems and devices, there is substantially more wear/strain in each distinct transition from off to on than in "run" and a good pellet boiler is a substantial investment.

From what I have tried to dig into, it seems like the pros are recommending about (and apparently, specifically, not less than) 240 gallons of buffer storage for my size boiler and approximate heat load and that works out to a twinned pair of fairly common 119 gallon tanks. Apparently less volume would be OK if I had a substantially smaller heat load and smaller boiler. I don't generally try to just wing it on details but the 240 gallons seems to consistently come up as a 'right size' for what I am trying to do, and anything bigger would get hard in my available, and kinda funky, horizontal and vertical available space.

Do I think anyone is 'wrong' if they already did or now do a pellet boiler without thermal buffer? No, and not my place. I concluded that the recommendations I kept seeing and the rationale, for a buffer with a pellet boiler, seem to make sense, to me/ for me.

Truly hope none of the above is pedantic. I am not an engineer or a hydronics pro and I don't want to seem like I pretend to be. For some reason I really enjoy digging into this stuff and trying to figure it out. It's kind of astonishing how much so many facets of "practical physics" makes modern life work, and we somehow forgot to marvel at it - and we have become incredibly dependent on it. It's kind of fascinating the extent to which it seems like we could maybe make "medium" design/operational/control refinements/thinking a bit differently that may, even without radical total innovation = less dollars, less pollution, etc.

I like that this forum is inhabited by people that are both do-ers and think-ers and I hope to tap into the collective knowledge here, and if I can [accidentally?] share anything or advance options in any way (which I do not assume) that'd be great. My colliding with constraints in my last biomass adventure helped me learn some things I certainly didn't know before, some of which are helping in this next effort, and it'll probably be the same this time, although I am trying really hard (maybe some wisdom from life's scrapes and blunders?) to I.D. the constraints I can try to steer around or mitigate before I start paying for many more major parts or wielding wrenches and a torch.
 
  • Like
Reactions: Patti

Unextinguished

New Member
Aug 2, 2022
18
Vermont, northerly, USA
PS, I also self-answered my question about a possible "try" of a non-hydronic "cold" (well pump type) expansion tank. I am not saying anyone is wrong who tried it, or tries it- hoping it works long and well, if you do. But I decided, that, for me, with this effort at a really reliable efficient whole new start/system, that'd be a really un-smart place, for me, to try to economize.
 

salecker

Minister of Fire
Aug 22, 2010
1,876
Northern Canada
Thanks for your reply
It lays out the reasoning behind a buffer tank/storage in your case
It will be great reading for others that are looking for information on pellet boilers and installs
 

Patti

Member
Mar 4, 2013
69
On my original question, I think I found my answer and I am going to post it here in case anyone is trying to search for it (this specific resource really seems to specifically account for different systems' overall specific total volumetric capacity which seems a lot better than a lot of sites I found that seem to be some sort of assumed shorthand for this kinda system or that kinda system)

"TO PROPERLY SIZE A WATTS SERIES ETX, ETSX, ETA or ET-RA NON-POTABLE WATER EXPANSION TANK"

And it's looking like I need something in the vicinity of [maybe] a 33 gallon expansion tank that has around 12+ gallons of usable acceptance @ around 12psi.

On the other question (Salecker) of whether pellets need thermal storage, at all. . .

I would be delighted if I was arriving at an answer, after trying to look into it, that I could skip thermal buffer/ storage, for a pellet boiler. Substantially less initial spend, substantially less to figure out and put together _if_ I could just have a pellet boiler light off a burn of pellets when my single whole-house ducted warm air thermostat called for heat, and turn off the burn of pellets when the T-stat signaled "warm enough"

When I first started seriously considering a pellet boiler, gradually, over the last year, I was hoping to maybe not need any thermal storage. My prior/ "still physically here" unpressurized storage seemed silly big, under any set of considerations, for a pellet boiler [but it might someday have a new life, after some adaptation, as a bulk bin for actual bulk pellets]. Unpressurized storage can work, very decently, if it is the only available option but I now consider it klutzy. Didn't really want to buy any more storage vessels, for the pellet install, unless there was some seriously-clear good reason.

Various places seemed to suggest that you could operate at least some make/model pellet boilers without storage. All "well-thought-through-and-well-articulated" sources I was finding (Siegenthaler's various works, and Caleffi's idronic bulletins, and other) seemed to repeatedly suggest, across sources, and in various dates of publication over time, that a quality peller boiler is really substantially impaired, compared to what it can/should be capable of, at economy/efficiency/emissions/ongoing maintenance, etc., without, what thermal buffer gives you- which is a huge reduction of on/off "short cycling" -and the thermal buffer's enabling of many fewer, but substantially longer, separate "on times."

With cordwood, once you light the fire, you have no "off" and an "off" or way slow-burn of still-burning cordwood will be smoldery, inefficient, generate creosote. . . . Thus, the immense benefits of -big- storage, for burning cordwood, to be able to light off, "roar", and store - and then pull BTUs, as/when needed, from the "store."

With something like a gaseous fuel and a very low mass heat exchanger, like a propane or LP warm air heater (probably even a low mass mod-con hydronic "tankless" type appliance), "short cycling" of many, frequent, separate, short, light-and-burn events, is probably a much more minimal concern- the fuel burns well and efficiently almost immediately at start, and you don't spend a lot of time and BTUs heating up much thermal mass of a heat exchanger, and you don't randomly dissipate (up the flue, etc.) a whole lot of BTUs that have been thrown into the mass of a heat exchanger/ big volume of water, once you turn off the flame.

For something like an oil boiler, especially if old school with some heavy iron/steel + plenty of water volume within the boiler, you are apparently throwing a lot of BTUs into a lot of thermal mass of the boiler structure and water volume, just to get it up and fully rolling, and apparently the burn of the oil doesn't really hit full its stride of efficiency until after some non-negligible start-up phase of the burning process. And then, after you cut the oil burner flame and stop pumping the water, you are kind of randomly dissipating those BTUs that simply brought the mass of the boiler's physical structure and water up to full temperature. And, apparently, we all were able to not care about this as long as oil was cheap and we didn't worry about impacts of burning oil. Also, the "baseline technology" of things like oil burners + boilers evolved at a time when the available/ affordable technology (especially residential) wasn't up to doing much more than turning a burner and/or associated pumps, on/off in response to the "call" of a heat zone/ load, etc., without [much, usually] attention to overall burn cycle patterns and durations - and that "traditional" level of technology and control is well known, relatively fail-proof, etc.

Apparently some modern commercial-scale fossil fuel heat systems do use some extent of thermal buffer for efficiency's sake, although it may also do dual-duty as a big hydraulic separator to allow multiple smaller boilers to join in, or not join, take turns, etc. in meeting relative thermal load under varying conditions, rather than a single big fossil boiler that spends most of its life under only part throttle.

From what I think I have picked up from Siegenthaler, idronics, and various other things: Apparently pellets fall somewhere between the "wood" scenario and the "oil" scenario above. Apparently, burning pellets most efficiently (and, apparently, cleanly) doesn't hit its stride until you are "some substantial non-negligable while" "into" a given boiler-burn-session (and, apparently, this is a notably longer time than an oil burner), and, apparently, the turn-off-and-burn-down portion of the cycle of a pellet boiler, repeated, over extended time, consumes some non-negligible amount of pellets while not (in the ramp down) delivering a lot of useful BTUs that can be transported to your heat load. Plus, if you can do fewer, longer, burns, you are "calling on" (and using up the useful life of) your pellet boiler's igniter fewer times, over the same calendar span. Thus, apparently, the benefits (even with pellets, which you can turn off) of some extent of "thermal buffer" storage that lets you do fewer, but longer, separate light-offs and burns of pellets. But, since you can turn pellets off, unlike cordwood, there is some volume of storage, smaller in total size, than with wood, where your benefits of longer burns of the pellet boiler become outweighed by the fact that any storage, even insulated, is lossy, and (storage) is at least (usually) a bit random about when and where it dissipates the water-storage-stored BTUs. So to some extent, more than with wood, it is better to keep a higher proportion of available BTUs in the fuel not the thermal storage. This overall approach to pellets (aiming for fewer, longer burns) also kind of makes sense to me because for a lot of physical and electrical systems and devices, there is substantially more wear/strain in each distinct transition from off to on than in "run" and a good pellet boiler is a substantial investment.

From what I have tried to dig into, it seems like the pros are recommending about (and apparently, specifically, not less than) 240 gallons of buffer storage for my size boiler and approximate heat load and that works out to a twinned pair of fairly common 119 gallon tanks. Apparently less volume would be OK if I had a substantially smaller heat load and smaller boiler. I don't generally try to just wing it on details but the 240 gallons seems to consistently come up as a 'right size' for what I am trying to do, and anything bigger would get hard in my available, and kinda funky, horizontal and vertical available space.

Do I think anyone is 'wrong' if they already did or now do a pellet boiler without thermal buffer? No, and not my place. I concluded that the recommendations I kept seeing and the rationale, for a buffer with a pellet boiler, seem to make sense, to me/ for me.

Truly hope none of the above is pedantic. I am not an engineer or a hydronics pro and I don't want to seem like I pretend to be. For some reason I really enjoy digging into this stuff and trying to figure it out. It's kind of astonishing how much so many facets of "practical physics" makes modern life work, and we somehow forgot to marvel at it - and we have become incredibly dependent on it. It's kind of fascinating the extent to which it seems like we could maybe make "medium" design/operational/control refinements/thinking a bit differently that may, even without radical total innovation = less dollars, less pollution, etc.

I like that this forum is inhabited by people that are both do-ers and think-ers and I hope to tap into the collective knowledge here, and if I can [accidentally?] share anything or advance options in any way (which I do not assume) that'd be great. My colliding with constraints in my last biomass adventure helped me learn some things I certainly didn't know before, some of which are helping in this next effort, and it'll probably be the same this time, although I am trying really hard (maybe some wisdom from life's scrapes and blunders?) to I.D. the constraints I can try to steer around or mitigate before I start paying for many more major parts or wielding wrenches and a torch.
Wow! That was a very well-researched, well-written, comprehensive response!
I was thinking along the same lines as Salecker, as I formerly had a pellet boiler.
But based on your explanation, it does make sense!
Thanks for taking the time to share all that information!
Man! The stuff I learn on here!!! 😂
Patti