im very curious to see what the temp diffs are from top of top tank to bottom of bottom tank. Wondering how well two tanks act together as one big tank or two stratified tanks?
Installing Wood Gassification Boiler With 1000 Gallon Thermal Storage
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maple1
Minister of Fire
I've got two 330 gallon tanks, horizontal, one on top of the other. My 2 temp sensors are in the middle of the tank ends, surface mounted. Tanks are 30" diameter, with blocking between them, so say the sensors are 3' apart. Yesterday morning when I got up, the temps were 135/175. So 40 degrees difference in 3 feet. Quite satisfactory stratification.
Great to hear they should stratify well since I two was worried about stratification, limited by space and positioning options I really had no other choice though. My tanks will only be connected by that 2" pipe as shown in drawing and videos/pics. Maple1, do you happen to have any pics of your storage and how it is connected? Would be great to see.
Can anyone else speak to how well or poor my setup will provide stratification? During heating months (especially like temps now!) I don't see much of an issue since I'll be burning quite a bit, but during the summer I hope to only have to fire once and a while to prove domestic hot water.
As for the expansion/compression tank, I can get an 86 gallon diaphragm one for about $500. Relative to the rest of the system cost and labor, it seems it might just be worth me sucking up the price and getting the diaphragm style. I originally was looking at the Froling FHG until I stumbled across the Attack DPX Profi which saved me over $4,000, so in the realm of things I'm still within planned budget. Just had to fill up oil tank yesterday and that alone was over $400...boy I can't wait to stop buying oil (this is only the second tank I've every bought in my life but it still hurts).
Thanks again to all contributors.
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Also, forgot to mention that one of my students who wants to be a game designer is almost finished 3D modeling the tanks and stand. His goal is to create the entire boiler room virtually, and if so I will host it online for everyone to 'walk around' in.
maple1
Minister of Fire
Don't confuse air with O2
in regards to closed loop hydronic systems. The initial gulp of O2 in the fill water reacts with the water and caused some oxidation, in small amounts that is good as it protects the surfaces of the metal from further corrosion. Think of anodized aluminum, that "rust" layer provides the protection for the aluminum.The O2 in the closed system is "consumed" in this oxidation process and once that happen no further corrosion should occur. The system becomes O2 neutral, so to speak.
With open systems, or non barrier tubing, that allows O2 into the system, corrosion can continue, as the O2 source continues to be replenished. Good reason not to flush and add fresh water to hydronic systems, ever.
It's the O2 molecule we are talking about, if the tube leaked air, then the stuff on the inside, would end up being stuff on the outside. And you see puddles
In a perfect world with perfect water quality (for boilers) there would be no need for hydronic conditioner fluids. I don't remember ever seeing corrosion issues when I started this trade back in the 60's. I believe because we piped with steel and copper, there was no PE, PB, PAB, PEert tube back then, systems stayed O2 free for decades.
Once radiant and plastics hit the market corrosion issues came along with it.
There are other places where O2 could get into a system, like air vents for example, improperly applied they can allow small dosages of fresh air (O2) into the systems, common with high head circs pumping towards expansion tanks.
Keep in mind the O2 barriers on pex tube SLOW the O2 ingress, the barriers cannot stop it 100% like copper and steel piping. I prefer the tube with aluminum layers for this reason, but still some ingress potential at fittings.
The hotter you run the fluids in PE (polyethylene) tube the higher the rate of O2 ingress. PEX = PE that has been cross linked (X) to limit O2 ingress, that is where the name PEX comes from.
Those 180- 200° operating systems will continue to inhale higher %'s of O2 even with barrier tube. Hence the large market for hydronic conditioner fluids, they scavenge the O2 with a chemical romance. But it is a never ending romance.
Fred61
Minister of Fire
In my opinion the xlinking does not have any effect on he prevention of 02 ingress. It is the process that gives PE more integrity and improves it's ability to maintain it's shape. 02 barrier is normally a coating or a metal such as Pex al pex.
stee6043
Minister of Fire
I've got two stacked 500 gallon tanks similar to what you're installing, Sid. My tanks stratify quite nicely. Generally I'll see 40 degrees difference between top and bottom (top of top tank, bottom of bottom tank). I have dip tubes in my tanks installed to within 6" or so of the bottom.
So it seems like these tanks are going to stratify just fine according to what you and others are posting for temps.
Another big question I've been having in regards to stratification is how to return the water from load. Nofossil shows in his diagram (which I have been pretty much been planing to use) that he is returning water from load only to the bottom of bottom tank. If my return water is let's say 20 degrees less than supply, wouldn't that only let me get about 20 degrees stratification (when returning to bottom of bottom tank)? I've read post about properly 'loading' and returning to the storage and remember something about using diverter valves (i think) to match return temps to that of which is currently in the tanks. The European storage tanks engineered for thermal storage have many ports all the way down the tank, possibly for this reason?
Basically, I feel like with radiators (and possibly not enough of them) the return temps will be too high to send right to bottom of bottom tank, and when I do I will be selling myself short on overall storage capacity due to mixing.
Any thoughts?
stee6043
Minister of Fire
I am living what you're describing and I would say it's not worth investing in more controls to optimize this return temp quandri. Return your load to the bottom of your bottom tank and live happily knowing you're sacrificing such a small amount of efficiency you'd never be able to measure it.
Unless your load is running 100% of the time you'll still get the stratification even if you're pumping higher temp water in through the bottom. It's like magic....or physics...or thermodynamics. You pick.
maple1
Minister of Fire
When I got up this morning, my readings were 129/170. So 40° again in 3 feet of tank height.
My return is to the very bottom.
A big part of maintaining stratification is how big your load flows are. I swapped a 15-58 3 speed circ out for an Alpha after my first winter. And I also then played with the ball valves on my zones and currently have them throttled about half way. So I have pretty slow flows through my zones. Which widened my delta-T quite a bit and gets me a bit farther on storage. I haven't precisely measured it, but with the stratification I see it must be pretty wide. That works for my particualr baseboard arrangement - the house is still staying warm and the rooms on the ends of the zones (mainly smallish bathrooms) are still staying comfortable. If my kitchen was on the end of its zone rather than the beginning of it, it might not work so good. My tank tappings are also horizontal where they go in & out of the tanks. Just the way I did it, didn't really think about the effect it might have on stratification at the time.
My return is to the very bottom.
A big part of maintaining stratification is how big your load flows are. I swapped a 15-58 3 speed circ out for an Alpha after my first winter. And I also then played with the ball valves on my zones and currently have them throttled about half way. So I have pretty slow flows through my zones. Which widened my delta-T quite a bit and gets me a bit farther on storage. I haven't precisely measured it, but with the stratification I see it must be pretty wide. That works for my particualr baseboard arrangement - the house is still staying warm and the rooms on the ends of the zones (mainly smallish bathrooms) are still staying comfortable. If my kitchen was on the end of its zone rather than the beginning of it, it might not work so good. My tank tappings are also horizontal where they go in & out of the tanks. Just the way I did it, didn't really think about the effect it might have on stratification at the time.
Clarkbug
Minister of Fire
You can get a thermal diverting valve that would send your return water around the loop again if you dont want it coming back to storage. Might be more trouble than its worth, but helps stretch things out some if you dont need 180 to get heat from your emitters.
Thanks all for the real life results/replies. I was going to just go for it anyways since this installation is dragging on and on as it is, but now I feel much better knowing things are actually going to work ok. I've been told that maybe I should consider adding an additional 2" port and then I could use these as two separate stratified tanks, kind of like Clarkbug's avatar pic. I don't feel like adding another fitting at this point and I feel I'd be happy with the results you all are posting.
ewdudley
Minister of Fire
Ever since I started studying the topic I've accepted the idea that [pressurized] storage tank stratification is quite important and that avoiding storage tank mixing is an important design goal.
So here's the numbers from a simple realistic model:
Code:
175 starting_temperature
300 gallon storage_per_zone
10000 btu/hour zone_load
15 feet zone_length
3 zone_gpm
t_begin=175.00 t_end=164.70 t_drop=10.30 btu=15448 duty=0.647 lap_minutes=154 t=154
t_begin=164.70 t_end=155.67 t_drop= 9.03 btu=13552 duty=0.738 lap_minutes=136 t=290
t_begin=155.67 t_end=147.74 t_drop= 7.93 btu=11889 duty=0.841 lap_minutes=119 t=409
t_begin=147.74 t_end=140.79 t_drop= 6.95 btu=10430 duty=0.959 lap_minutes=104 t=513
85533
t_begin=145.41 t_end=138.74 t_drop= 6.67 btu=10001 duty=1.000 t=544
90647Calculations are done for a single zone of baseboard emitter, with a certain amount of storage allocated to just the one zone for modeling purposes, and likewise for zone gpm and zone emitter length.
Using manufacturer's data for btu per hour per foot of emitter as a function of water temperature it's pretty straightforward to figure out what the temperature drop is for a given millimeter of emitter at any given gpm.
In the example above we start out with 175 degF water that returns to the tank at 164.7 degF. The thermostat calls for heat 64.7 percent of the time, and it takes 154 minutes before the top of storage is at 164.7 degF.
On the next lap the water temperature is lower, so the thermostat calls for heat 73.8 percent of the time. And so forth until the the supply temperature is too low to keep up even when pumping 100 percent of the time.
Total elapsed time 513 minutes. Total btu extracted from storage 85,533 btu.
Then we assume there is a mixing valve that lowers the supply temperature such that the thermostat calls for heat 100 percent of the time and the zone load is perfectly satisfied.
Total elapsed time 544 minutes. Total btu extracted from storage 90,647.
Nine hours between fires instead of eight and a half, not a compelling case for getting all worked up about stratification.
Then again, if we have one or two loads that need higher supply temperatures (for instance DHW or a single baseboard zone when all the others are over-sized radiators), then it could definitely make sense to implement strategies that will keep the top of storage hot for as long as possible, so YMMV.
Here's the code if you'd like to review the methodology:
Code:
use strict;
my $gallon_tank = 300.0;
my $t_0 = 175.0;
my $gpm_pump = 3.0;
my $load = 10000;
my $l = 15.0;
my $t_end = $t_0;
my $t_begin = 0.0;
my $t_begin_was = 0.0;
my $t_drop = 0.0;
my $btu = 0.0;
my $btu_tank = 0.0;
my $duty;
my $gpm;
my $lap_minutes;
my $t = 0;
sub btu_per_foot {
my $degF = shift;
return(2.0 * (($degF * 6.5) - 592));
}
sub t_drop_btu {
my $inc = 0.1;
$t_drop = 0;
$btu = 0;
for (my $foot = 0; $foot < $l; $foot += $inc) {
my $btu_per_foot = btu_per_foot($t_begin - $t_drop);
my $t_drop_delta = ($inc * $btu_per_foot) / ($gpm_pump * 500);
$t_drop += $t_drop_delta;
$btu += $btu_per_foot * $inc;
}
$duty = $load / $btu;
$gpm = $gpm_pump * $duty;
$lap_minutes = $gallon_tank / $gpm;
$t += $lap_minutes;
}
sub p {
printf("t_begin=%5.2f t_end=%5.2f t_drop=%5.2f btu=%5.0f duty=%5.3f lap_minutes=%3.0f t=%3.0f\n"
,$t_begin
,$t_end
,$t_drop
,$btu
,$duty
,$lap_minutes
,$t
);
}
sub main {
$t = 0;
printf("\n");
printf(
"%5.0f starting_temperature\n" .
"%5.0f gallon storage_per_zone\n" .
"%5.0f btu/hour zone_load\n" .
"%5.0f feet zone_length\n" .
"%5.0f zone_gpm\n"
,$t_0
,$gallon_tank
,$load
,$l
,$gpm_pump
);
printf("\n");
for (my $lap = 0; $lap < 10; $lap++) {
$t_begin = $t_end;
t_drop_btu();
($duty > 1.0) && (last);
$t_end -= $t_drop;
$t_begin_was = $t_begin;
p();
}
$btu_tank = $gallon_tank * 8.333333 * ($t_0 - $t_end);
printf("%.0f\n", $btu_tank);
printf("\n");
$t_begin = $t_begin_was;
for (my $i = 0; $i < 10000; $i++) {
t_drop_btu();
($duty > 0.9999) && (last);
$t_begin -= 0.001;
}
$t_end = $t_begin - $t_drop;
my $supply_duty = ($t_begin - $t_end) / ($t_0 - $t_end);
$btu_tank = $gallon_tank * 8.333333 * ($t_0 - $t_end);
$t = 60 * $btu_tank / $btu;
p();
printf("%.0f\n", $btu_tank);
}
main();
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I am planning to use the wood boiler for DHW in the summer time and hope to prolong in-between burn times. Do you think it is then worth it?
ewdudley
Minister of Fire
I'm thinking that if you have two or more loads and some load(s) can work with lower temperatures than the other(s) then we could make a case for minimizing the return temperature of the lower temperature load(s) in order to extend the time that unmixed hot water remains available at the top of storage for the benefit of those loads that require higher supply temperatures.
For summertime DHW there wouldn't be any other loads to cause mixing problems.
Just ordered chimney last night, went with Champion Stainless Steel double wall. I decided to go with the 316L stainless inside liner over the 304L stainless inside liner (both have 304L SS outside). Total increase in cost for my 4 x 4' lengths (16' total going up through bedroom and out through cathedral) was less than $200. 316L stainless has better corrosion resistance and even though not required for solid fuel like my wood boiler, it's nice knowing I can hook anything to it I want in possible future. Also, though I don't think condensing will be much of an issue due to my relatively short and relatively warm (located inside house) setup, it's still nice knowing there is the higher quality steel in there. JMO
maple1
Minister of Fire
Not sure what you have lined up for heating your DHW exactly - but a sidearm doesn't work very good at all in the summer. It will mess up your stratification and your storage will be pretty well useless once it gets below 150 or so. I started out with one, but then added a 20 plate FPHX pumped slowly both sides. Which works very well, I burn every 7 days or so in the summer for DHW. Having said that, I may or may not do it again next summer. Depends what I have for junk wood laying around - it only costs us $25-30/mo for electric DHW.
ewdudley
Minister of Fire
I think a lot of us have backed away from using the wood boiler for summer DHW. In recent years I've been cutting up a pile of dead wood from around the dooryard and keeping it out of the rain until laundry day and then disposing it with the boiler between stints with the lawnmower.
Bad Wolf
Minister of Fire
Welcome aboard Sid. Just saw your thread here. Looks like you are off to a good start, asking all the right questions (and people). I'm in Colchester too, I went with a 1000 gallon non pressurized system so I would be interested in seeing yours sometime.
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Sure thing Bad Wolf, message me sometime and I'd love to have you see what I have so far and plans. Any advise from someone with experience is greatly appreciated.
Since we have just moved into this house, I'm not sure how much that oil boiler will need to run for DHW in summer and cost. It's just my wife and I and a new born on the way. Throwing scrap wood in there once and while and starting a fire sounds like not too much of an issue, especially in shoulder season. I'm sure a few super hot months during summer I may not feel like starting a fire, but that's why I hope the way I plan to pipe and control (acording to Nofossil's pinned thread [https://www.hearth.com/talk/threads/simplest-pressurized-storage-system-design.16567/] ) will allow for the oil boiler to come on for DHW in summer, and not have to heat all of storage every time.
maple1
Minister of Fire
If your only other source of DHW is an oil boiler say with a tankless coil in it, that's another story. The $25-30/mo I mentioned above is for an electric hot water tank. Before that, before I put my new system in, I had a tankless coil oil boiler and it used 3/4 gallon a day for DHW. Or I should say, for DHW & heat sent up the chimney. Which is why I got rid of the thing all together and the oil tank it was tied to and went with an electric boiler for backup heat. Another factor is keeping an oil boiler hot all summer adds heat to the house & can add load to your a/c. If you have a/c.
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