Storage Tank Size Planning - Advice Welcomed

[DBoon]

I'd like to build a storage tank that is not too big and not too small - just right.

I am remodeling a house that I've energy modeled with a heat loss of ~20,000 BTUs/hour at 0 degree design temperature. I know that my energy model is accurate as I've done the same modeling on my existing house. Over a 24 hour period, the heat loss realistic worst case is 480,000 BTUs if the mean temperature for the full day is 0 degrees F. I don't need a massive amount of storage.

I am planning on installing a gasification boiler with lambda control. The model I am looking at has a 5 cubic foot firebox that is 22" deep. If I cut the wood for it 18" long, then I theoretically could fit 4 cubic feet of wood in it, but realistically, it will be less to account for air gaps. I am thinking that 2.5 cubic feet is probably right - I would welcome some advice here. What should I assume?

I'd like to build a water storage tank to take one full load of hard maple. The reason for this is that my wife will likely not follow "rules" about how much wood to put in the boiler. Therefore, the most simple rule is "fill it until it is full, or less". If this "rule" is followed (it would be hard to break this rule) the tank would not overheat provided a fire wasn't started until the tank temperature dropped below a certain value.

I can calculate the BTU of a wood load once I know the cubic feet of wood that will realistically fit in the firebox. Sugar Maple is 44 lbs. per cubic foot. Using Jebatty's rule of 6000 BTUs per pound, I can calculate BTUs in the wood, and multiple by the efficiency. Is the efficiency 80% for a lambda controlled boiler? If I had a good number for that, I could calculate BTUs produced per load. Example:

2.5 cubic feet * 44 lbs/cu foot * 6000 BTUs per pound *80% efficiency = 528,000 BTUs. To absorb this burn I would need approximately 1000 gallons of storage (8340 lbs. with a 60 degree temperature rise).

Are my assumptions good?

 

[maple1]

I don't have a lot of numbers to throw around, but - I measured my firebox at pretty close to 3 cu.ft.

I have 660 gallons of water.

I would have no qualms about loading my firebox full, lighting it up, and letting it burn it all out if my storage was down to, say, 140 top/120 bottom. For what that is worth.

It's a good idea to size storage for what you are talking - holding a full burn worth of heat. But I think it's even better if it can hold a couple burns of wood, to get you through that far if you want it to. And there will come a time when you will want it to - or at least would be nice if it would.

In other words, and IMO, the more storage the better.

 

[jebatty]

I wouldn't get too technical on cubic feet. Just weigh out some wood and see how full the firebox gets, adjust as necessary. Next for your wife, make a simple wood cradle which she would fill from your wood supply, and the full cradle then would be the amount of wood she fills the boiler with. Weight does not need to be exact.

For your lambda boiler I would start with 90% efficiency and move downwards as needed. How low temp of water can you use? The rule then would be do not load if storage temp is < X; if less than X then load a cradle of wood. You will need temp sensor(s) in your storage with an output display easy to read. Digital panel meter type sensors look good and are easy to read. To make it easy you will want only one sensor displayed which will be the "wood loading" sensor. Then based on experience you will need to determine based on "X" temperature how much wood to load to charge storage.

For me it has become very easy. It is a middle of tank sensor which is my guide. If I want to bring the tank up to near maximum temp, then I will weigh out the wood and burn one or more loads to achieve that result. For early season heating right now, and tank temperature gets down to about 100-110F, I just burn a load of wood and ignore the weight because the load will not over-charge storage.

I agree that storage that holds more than one load of wood is good, if you have room for the extra storage. It also makes it very easy not to overload the boiler because of the extra storage capacity.

 

[Clarkbug]

I like Jim's idea about the wood cradle. I have an old Radio Flyer wagon that I use to get wood from one side of my basement to the other, and that could easily be used as a cradle to limit the amount of wood that could go in.

That said, I like your KISS principle about planning for the firebox jammed full, since it wont get more full than that. Only issue with trying to really fine tune the storage size and calculations is that no two burns will really be the same once you get into the heating season. Moisture content of wood will vary, size of the chunks that impacts loading, the amount of ash on the bottom of the firebox, wood species etc. So I guess I would say that your calc of 1000 gallons has a very solid basis, and its a very realistic number to obtain tanks for. Put in what you can, and then adjust your burning habits as needed! And oh yeah, we like install pics!

 

[Floydian]

Hi Dboon,

IMO, that is a pretty small heat load to justify the expense of the system you are talking about. Of course it would be super efficient and as convenient as burning cord wood can be.
I know from another thread that you have received some ridiculous quotes on mini split HPs but I cant help but think that along with a small woodstove a mini split is a great option. Even in central NY I believe you would see a seasonal COP of over 2.5 or maybe even 3. And if your design load is in fact only 20K btu/hr, your avg heat load is probably half that.

Estimating annual your annual heat load, figuring 8000 HDD for central NY and a 20K btu heat load, I am coming up with about 35MM btus/yr. You would only get to burn less than 2 cords/yr with that sweet setup-how disappointing . Of course you could probably add another cord or so for year round DHW.


Noah

 

[rkusek]

Noah makes some good points. My home's design load is about 70k btu/hr and I would agree that the average load is half that maybe even a little less. If you got the money and want the gasser go for it if you are not concerned about the payback period. I'm also heating a 30x60x14 building (where the boiler is) which helps justify what I've spent (over $12k) and I quit keeping track a long time ago.

 

[flyingcow]

I've mentioned before in other threads, I've got 2 mitsu's air sourced heat pumps, just installed this summer. I have a gasser with 820 gals of storage. I'm using the HP's for my main source of heat this time of year. Also, note i am in the northern half of Maine. Once the deep winter sets in i will be using the boiler more, but I really like these units. Extremely efficient.

If i was to rebuild I would have a house with R-40 walls and R-60 ceiling and HP's for heat and maybe a pellet or wood stove for the sub zero nights.

 

[flyingcow]

I installed my boiler about 6 yrs ago. Before that it was oil heat, i would use about a 1000 gals a yr.

 

[leon]

Why build when you can buy surplus and simply plumb it together
with your system?

One option would be to purchase a used bulk milk tank from
a milk equipment dealer as the tanks are all insulated.

If you purchase a round one they are fully insulated and a flat
tank will only need a plywood box lid with styrene insulation
sandwiched between two pieces of pressure treated plywood
or what ever size you need to make the cover.

it will not be hard to find either a used bulk milk trailer and or bulk
milk tanks from 10 wheel trucks sitting in junk yards.

Lots and lots of storage and they are insulated very well.

 

[nrcrash]

The bigger the better, the tighter the sweater

 

[hiker88]

My heat load is a little bit more than yours, but not much. In the summer I would let my tank get down to 45c in between fires. This would take me from Sunday to Sunday. I would get up each Sunday and burn 100 lbs of wood. My goal was to get top of storage to 85c. (85-45)X2.5=100. Through a little bit of experimentation, I found that 2.5 was the multiplier that worked for me.

100lbs was a bit to much for me to fit in one load. I would go back after an hour and put the rest. I can usually fit around 75 or so lbs, but I burn a mix of woods; if you were burning all maple, it wouldn't take as many pieces, so you might get it all to fit.

Now that heating season is back, it's usually just one load unless I have a piece or two that is knotty or something.

 

[jebatty]

An important piece missing from this discussion is how hot you need your water. If you need 160F+ water, you're going to find that you will need a lot of storage to accept the btu's from a load of wood or to provide heat between burns, unless you just want the storage to serve as a buffer tank, which also is useful. If you an use water down to 100F (my situation), then storage provides a lot of flexibility in loading and burning and in most cases a lot of time between burns.

Example with 20,000 btuH heat load, 160F water needed, 1000 gallons of storage:
Assuming you can burn your boiler to heat the tank, top to bottom to 190F, maximum amount of stored btu's = 30 x 1000 x 8.34 = 250,000, which is about 41 lbs of wood, a pretty small load, and the boiler would need to be burned twice a day to meet the demand. The storage is functioning pretty close to a buffer tank to take excess boiler btu's which aren't immediately needed and a little like a storage tank. You also would have to give close attention to the amount of wood you are burning, as you could fairly easily have a burn with more btu's then the storage could accept, with boiler idling a probable result.

Same example but 100F water needed:
Maximum amount of stored btu's = 90 x 1000 x 8.34 = 750,000, which is about 125 lbs of wood, probably two good loadings of wood in a long burn (4-6 hrs about), and the boiler would need to be burned about once every three days to meet the demand. The storage is functioning as a storage tank, and you have lots of flexibility in loading and burning. Let the tank get down to the lower 100's, throw in a full load of wood, and walk away. Storage can take it all without weighing or measuring the amount of wood to burn. Maybe burn once per day or every other day.

Your plan is a gasification boielr with lamda control. I can't speak for all boilers of this type, as I have personal experience only with the Froling. The Froling has a range of output and modulates the burn to maintain efficiency over this range of outputs. The modulation slows the burn down and works to eliminate or minimize idling. This ability increase flexibility in the first example, and in the second example lets the boiler roar through the burn in its high output and get the tank quickly charged.

My situation is the 2nd example, and I normally let the tank get down to about 120-130F and do a burn every other day. I load the tank up to about 185F with weighed wood burns rather than try to squeeze in the maximum possible btu's. If I'm going to be away for several days, then I can load the tank to 190-193 top to bottom with a careful weighed wood burn.

 

[Floydian]

Good points Jim.

DBoon, if you are going to invest in a gasser+storage it will definitely be worth your while to invest in some "over sized" emitters so you can run that storage as low as possible. Another nice feature would be a mixing valve with outdoor reset. This modulates the supply temp based on the outdoor temp which is really nice for maximizing storage. I designed for 120supply and in avg winter temps I supply around 105. More info in the link in my signature.

Noah

 

[DBoon]

Hello Everyone,

Thanks for all the advice and comments. All are welcomed and appreciated.

It's a good idea to size storage for what you are talking - holding a full burn worth of heat. But I think it's even better if it can hold a couple burns of wood, to get you through that far if you want it to. And there will come a time when you will want it to - or at least would be nice if it would. In other words, and IMO, the more storage the better.

Hi Maple1, I wish it was this easy...a couple of burns of wood would be a lot of BTUs...more than I care to size for. My house heat load will be not so large, so my target is to handle one full burn. One burn could take me through two days.

Hi jebatty, thanks for the detailed info. I've been meaning to post on your bike trip - sounded awesome.

Just weigh out some wood and see how full the firebox gets, adjust as necessary. Next for your wife, make a simple wood cradle which she would fill from your wood supply, and the full cradle then would be the amount of wood she fills the boiler with.

I'm realistically super-focused on "design for the user", and if the user is my wife, I have to know how she will use it. Here is what I know - if she has to fill up a cradle, then unfill it and put it in the boiler, and this is moving 80 to 100 lbs. of wood twice, she won't do it. So I'm looking for some simple, fail-safe ways to make it easy.

For your lambda boiler I would start with 90% efficiency and move downwards as needed. How low temp of water can you use?

Would you consider 90% to be total efficiency in getting heat to the storage? I am planning on sizing my radiators for use down to 120 degrees - I'll get some heat from them at less than that, but they'll product 20,000 BTUs at 120 degree water.

For early season heating right now, and tank temperature gets down to about 100-110F, I just burn a load of wood and ignore the weight because the load will not over-charge storage.

From what I recall, you burn mostly softwoods, correct?

So I guess I would say that your calc of 1000 gallons has a very solid basis, and its a very realistic number to obtain tanks for. Put in what you can, and then adjust your burning habits as needed! And oh yeah, we like install pics!

Hi Clarkbug, I'm planning on unpressurized storage. I am likely 1-1/2 to 2 years away to a full install, so it will be a while before I have some pics.

 

[DBoon]

Hi Dboon, IMO, that is a pretty small heat load to justify the expense of the system you are talking about. Of course it would be super efficient and as convenient as burning cord wood can be.

Hi Floydian, convenience is key. I have a small wood stove right now. We like the heat from it, but I need to be around too much to get what I want from it, and my wife would prefer more zoning and ability for "anytime" heat.

I know from another thread that you have received some ridiculous quotes on mini split HPs but I cant help but think that along with a small woodstove a mini split is a great option. Even in central NY I believe you would see a seasonal COP of over 2.5 or maybe even 3. And if your design load is in fact only 20K btu/hr, your avg heat load is probably half that.

Yes, absolutely correct. I did get a good quote on a Fujitsu 15kBTU unit. I'm having that installed shortly. That and the boiler install is for a house I am renovating over a 3 year (or so) period. The Fujitsu will pretty much pay for itself using electricity from an (under-utilized) PV system I am in the process of installing. When I move into the renovated house in 3 years, I'll need most of the PV capacity to power the house.

You would only get to burn less than 2 cords/yr with that sweet setup-how disappointing . Of course you could probably add another cord or so for year round DHW.

Yep, I do figure about two full cords and am also planning on DHW heating with this setup as well. I am likely to not use it as much as some, but it gives me the heating flexibility my wife and I would like.

I've mentioned before in other threads, I've got 2 mitsu's air sourced heat pumps, just installed this summer. I have a gasser with 820 gals of storage. I'm using the HP's for my main source of heat this time of year. Also, note i am in the northern half of Maine. Once the deep winter sets in i will be using the boiler more, but I really like these units. Extremely efficient.

This is pretty much the model I'll be following. We have about 8000 HDD here in an average year, so I can't really 100% on the mini-splits.

One option would be to purchase a used bulk milk tank from a milk equipment dealer as the tanks are all insulated.

Hi Leon, whatever I use for storage has to fit through a 30" doorway. I don't think the milk tanks will do that...or am I missing something?

My heat load is a little bit more than yours, but not much. In the summer I would let my tank get down to 45c in between fires. This would take me from Sunday to Sunday. I would get up each Sunday and burn 100 lbs of wood. My goal was to get top of storage to 85c. (85-45)X2.5=100. Through a little bit of experimentation, I found that 2.5 was the multiplier that worked for me.

Hi Hiker88, thanks for the detail. This fits with a lot of my assumptions.

An important piece missing from this discussion is how hot you need your water.

Hi jebatty, I am going to size my radiators to produce 20,000 BTUs down to 120 degree F water temperature. They could produce heat at less than this, of course, so I could see a situation where I run them at 100% duty cycle with <120 degree F water and run the tank temperature down more. I could go up to 190 degree F tank temperature. So I am going to be more like you and am also aiming for burns every other day.

Another nice feature would be a mixing valve with outdoor reset.

Yes, I'll be considering this also as I get closer to total system design.

 

[Chris Hoskin]

DBoon, yes, your assumptions in your original post are solid.

 

[DBoon]

Hello All,

Again, thanks for all of the advice and help.

I think 800 gallons of storage will likely work fine for me. I'm going to make some adjustments to my wood cutting to reduce the amount of wood that can fit in the firebox at one time. I'll cut to 16" length instead of 20" or 18". That will mean the following:

5 cubic foot firebox (22" deep) = 3.6 effective cubic feet with 16" long wood
With air gaps, I'll fit ~2.25 cubic feet of wood
Assuming hard maple at 44 lbs./cubic foot, that will be ~100 lbs. of wood. If it was Black Cherry or Soft Maple (~33 lbs./cubic foot), then that would be ~75 lbs.
Following jebatty's "rule" of 6000 BTUs/lb, and his recommendation of 90% efficiency, then that is 540,000 BTUs (hard maple) or 405,000 BTUs (cherry, soft maple).
800 gallons heated from 120 degrees to 190 (max) OR 110 degrees to 180 (normal) - either way a delta T of 70 degrees F would give me:
800 gallons * 8.34 lbs/gallon * 70 BTUs/lb = 467,000 BTUs. Pretty fail safe. And 467,000 BTUs would be a days worth of heat worst case, more likely two days.

If I decide to bail on the boiler (for whatever reason), 16" wood fits in any wood stove. And the wife will be happier not having as heavy a piece to pick up put in the boiler, if that is the way we go.