What are the drawbacks to excessive storage? Is 4000 gallons to much?

[700renegade]

I am planning a new 60x60 workshop and intend to use a EKO 40 to heat it, plus my house. My house is smaller and I calc a heatloss of roughly 40,000 BTU for it, plus 40,000 for the shop.

I can buy 1000 gal decommissioned LP tanks pretty cheap ( $200 each ), so was trying to decide whether to plumb up 2, 3, or 4 tanks. I'll have 16'+ ceilings so can line them up vertical along the back wall. An EKO 40 won't have much extra capacity to charge the tanks on the coldest days, so am I foolish to install more than 2000 gal? I estimate the cost to spray foam them at $300 each, so I'll only have $500 + plumbing into each tank. I need to use a 500 gal expansion tank ( $100 ) instead of a 250 ($50) if I go to 4000 gal. storage.

What are the drawbacks aside from the additional cost and loss of 16 sf of floor space each?

Additional info: new shop will have hydronic in the slab and will be excessively insulated. Home has 25% hydronic and 75% forced air. I'm starting with an EKO 40 as I found one used for 2 years for a decent price. If I like the gassifier concept I can always move up to a 60kw unit. I have a central boiler Maxim and 15 ton grain tank I plan to somehow incorporate into the system also ( bought and paid for - no sense in giving it away at this point now that corn is $7+/bu )

 

[jimbom]

8000 degree days, 863 hours below 10 °F, boiler output 130000 BTU/hour, design load 80000 BTU/hour at -9 °F, 4000 gallons of water in heated space.

Even at design load, you should be able to charge the tanks approximately 2 °F per hour. At 10 °F you should be able to charge about 2.5 °F per hour. No load, you should be able to charge the tanks at 4 °F per hour.

Stand them vertical and connect in series. Valve them so you only have to use the number that is needed for the season. Draw/return the hot air supply water from the tanks that best meets minimum load. Same for hydronic floor supply.

Seems like a great setup to me. Can't think of any drawbacks.

 

[Duetech]

One hypothetical draw back is a vacation in the middle of the coldest part of the winter... you might only get 4 hr cat naps between loads IF you use high density wood and if you have a large supply of well seasoned and properly sized stuff on hand. That big of a system will not respond well to high mc wood. A little back up heat source won't hurt.

 

[jebatty]

For me the relevant questions would be what you want your storage to do for you, and how will you be able to accomplish that goal with your Eco 40?

I can't tell you whether 4000 gal is too much, but I can provide some info that might help you decide. I think JimboM's calculations are a little optimistic. I have a Tarm Solo 40, which I think is rated about the same as your Eco 40, 140,000 btuh, but this is about maximum output, not continuous output. I would put average output over a wood load at about 75%, or about 105,000 btuh. This means it would take about 20 minutes to raise 4000 gal of water 1F, or about 3F/hour with no load on the system.

Also, at average output of 105,000 btuh and design draw of 80,000 btuh, you would be burning almost continuously to meet demand, with little going to storage. Of course, at warmer temperatures and lower demand, you would have many more btu's going to storage.

If your forced air has minimum usable water temperature of 140F, then raising tank temperature from 140F to 185F would take about 15 hours of continuous burn, with no draw on the system, quite a chore if you want to fully charge your tanks. At design draw of 80,000 btuh, you then also would have about 18 hours of stored heat (45 x 8340 x 4 / 80,000). At 40,000 btuh draw, that doubles to 36 hours.

If it is likely that your storage temp may drop from time to time below the minimum usable, then be sure to plumb in a storage bypass so that Eco 40 output can go directly to the high temp forced air demand, as it may take a long time to raise storage temp to the minimum usable.

For your shop, you might consider using antifreeze in the radiant floor, and possibly the Eco 40 and other lines subject to freezing, with water in the storage, and a plate hx to transfer the Eco 40 btu output to non-freezable areas. Combine this with a control to shut-off draw from storage when storage has reached a minimum temperature, say 80F, and the retained heat in storage should protect the tanks from freezing for a substantial period of time. If you use a plate hx, keep in mind that usable output drops by about 10F unless the hx is over-sized to achieve a desired closer approach temperature. If storage does drop below minimum usable, use the hour calculation of burn time required to get storage back up to the minimum usable temperature.

Here's a tidbit of info you might find useful. Since my setup is similar to yours (Tarm Solo 40 in shop, 1000 gal insulated storage in shop), I have found it takes 1.7 lbs of wood to raise tank temperature 1F, with assumed boiler net efficiency of 80%, assumed wood MC at 20%, stack temp in the 380-480F range, and no draw on the system.

This is fwiw opinion, gut-reaction only, but based on the above facts and my experience. I think 1000 gal of storage would be quite responsive and very usable, 2000 gal of storage still within reason, but above this for an Eco 40 may fall into the range of quickly diminishing returns and increasing effort.

I wish you good success on your system.

 

[rkusek]

Valves between the tanks would be a must I would think. I you got them up to 180* you would have a heck of a buffer though, could probably even leave town for a few days. It would also give you tremendous flexibility for when to fuel the boiler. As long as you burned enough to replace the BTUs, roughly 2 loads per day average, you should be able to maintain the hot tank temps during the winter. Nice thing is you could take a day off and burn 4 loads the next and catch back up. Once all the tanks dropped below usable temps 110-120*, I would think you would only want to charge 1 or 2 at a time initially especially in the dead of winter. It would probably be worth designing something that would charge the tanks back up to temperature adding 1 at time without requiring you to babysit the valves.

 

[jimbom]

Excellent information JEBATTY. As always, practical experience informs and educates. Jim

 

[ewdudley]

What are the drawbacks aside from the additional cost and loss of 16 sf of floor space each?

Additional info: new shop will have hydronic in the slab and will be excessively insulated. Home has 25% hydronic and 75% forced air. I'm starting with an EKO 40 as I found one used for 2 years for a decent price. If I like the gassifier concept I can always move up to a 60kw unit. I have a central boiler Maxim and 15 ton grain tank I plan to somehow incorporate into the system also ( bought and paid for - no sense in giving it away at this point now that corn is $7+/bu )

The tanks will lose heat according to temperature inside, the temperature outside, the insulation RValue, and the surface area. If you had enough dozens of tanks then at some point the surface area would increase to the point where the entire output of the boiler would be dedicated to keeping the tanks hot. If you have too few tanks then you have to limit the size of the fuel loads to avoid idling the boiler. So as you suggest the answer lies somewhere in between.

Calculating the heat loss for various configurations should be strait-forward enough, just run some numbers and decide if the constant heat loss is more than the shop would need over-and-above the minimum amount of heat you would want to supply to the floor; and also consider if the heat loss through the ceiling and through the exterior wall and decide if the extra surface area causes more heat loss than you're willing to throw wood at.

If the tanks are vertical you want them more or less right next to each other and insulated as a group. Plumbing them in series doesn't buy you anything, plus the flow velocities of a single inlet into each tank in turn will aggravate inlet jet mixing unnecessarily.

Valves to isolate the tanks from the boiler would also be unnecessary if you plumb to tees in the tops and bottoms each in sequence. The hot water from the boiler goes in one end of the top manifold of connections and the return to the boiler pulls from the opposite end of the bottom manifold of connections.

The loads pull from the opposite ends as the boiler, so the load circuit and supply circuit are both configured as reverse-return manifolds, and the storage tanks become one big hydraulic isolator. The minute hot water from the boiler becomes available it can be used to supply loads without waiting for the tanks to heat up whatsoever.

If the shop is insulated well at all then running antifreeze would definitely be unnecessary as long as some guarantee of intermittent circulation through cold spots can be guaranteed. It would take precious little passive solar gain to guarantee the interior would never get below freezing in the first place.

But to Jim's point; if the boiler is running twelve or fifteen hours a day just to keep up, I agree that it may not make much sense to have a couple thousand gallons of extra storage that rarely gets used.

--ewd

 

[Chris Hoskin]

Some really great information here as usual. I love the single 1000 gallon vertical tank as a system separator, for what it's worth. Also wanted to mention the high cost of anti-freeze for even 1000 gallons of storage might be prohibitive. Keep us posted, sounds like a cool project.
Chris

 

[700renegade]

Thanks for all the good insight so far.

My point of having the storage is to span the 8am to 4pm time period at a minimum on extreme cold days. In shoulder season, perhaps allow me to fire all-out on the weekends and coast thru a number of weekdays. I don't mind keeping a fire going, it's the cold starts that are a pain. I have no desire to do summertime DHW or anything like that.

Ultimately I'd like to upgrade the boiler to a 60 or 80kw unit, but that is down the road. With the new shop there is only so much budget and I'd rather spend the extra money now on insulation than the bigger boiler.

A couple points:
> I sense agreement that the tanks ( however many ) should be piped in parallel, not series. Each will have isolation valves. I love ewdudly's balancing via simple piping.
> I see no need for glycol, as I will never walk away for enough days to allow the interior slab to freeze.
> I am trying to figure a unique way to insert my Central Boiler Maxim corn boiler ( 180k btu ) into the system. That boiler is completely automated and lights itself when the water temp drops below the setpoint, shuts down if the temp exceeds the upper setpoint, and feeds itself from my 15T bin. Corn and pellets are more $ than wood, but it seems to be an ideal way to lend a hand when the EKO 40 can't keep up. I'll start a new post on this as it is a unique topic vs. storage.
> If I could find straight thread weld-o-lets, I'd weld in about 8 taps for 4500w water heater elements into the tanks. According to my math I can make enough BTU in the 12 hrs of off-peak electric ( $0.047 / kwh ) to meet my daily demand and it isn't terribly expensive as a back up source. I wonder what the best vertical position would be for those taps? ( again probably a new post )
> I still have the oil furnace and a fireplace in the house which I now use for shoulder season, and is a perfectly good backup. I am not a purist who loses sleep just because I burnt a gallon of oil.

 

[mr.fixit]

Here's what I heat in Wisconsin with a EKO 40- 1350sqft shop with infloor radiant,560sqft storage building and 2000sqft house all well insulated.I have 500gal storage.
I can say that during colder weather there just isnt alot of extra btu's to charge the storage unless you're there to feed the boiler every 4-5 hours.
My storage is really more like a buffer tank but its just enough to carry me through from the last stoking of the night to the AM.
For your set up I would use 1000 gallons,If you increase the boiler size in the future it would be easy to ad another 1000 tank if you have the room.
Another thing is it takes dry wood and ocassional heat exchanger tube cleaning to really get optimal performance out of the boiler which it sounds like you'll need.
With the storage tank on end and plumbed as a huge hydraulic seperator couldn't you tie the pellet boiler in that way? Maybe someone with more knowledge on that could help.

 

[ewdudley]

> I am trying to figure a unique way to insert my Central Boiler Maxim corn boiler ( 180k btu ) into the system. That boiler is completely automated and lights itself when the water temp drops below the setpoint, shuts down if the temp exceeds the upper setpoint, and feeds itself from my 15T bin. Corn and pellets are more $ than wood, but it seems to be an ideal way to lend a hand when the EKO 40 can't keep up. I'll start a new post on this as it is a unique topic vs. storage.

I had an additional solid-fuel boiler that I plumbed in in parallel with the wood boiler. Here's some pictures:

https://www.hearth.com/econtent/index.php/forums/viewthread/68776/

The coal boiler is not shown in the plumbing schematic, but if you look below 'ReturnInjectCirc0' in one of the pictures you can see a cross fitting that feeds return water over to the coal boiler, and the hot water out of the coal boiler just feeds up into the same horizontal pipe that the wood boiler feeds up into (up inside the flue base mounting compartment) and back over to the top of storage. Need to be sure to have the check valves and/or Flo-Cheks to guarantee that one boiler doesn't flow through the other when the other is off-line.

For the coal boiler control I used a Ranco ETC aquastat at the top of storage manifold that calls for heat from the coal boiler when heat up there is depleted. As an enhancement I added a delay-off timer to the coal boiler call-for-heat signal so that when the coil boiler runs it sees a demand for at least 30 minutes (or whatever) at a time.

> If I could find straight thread weld-o-lets, I'd weld in about 8 taps for 4500w water heater elements into the tanks. According to my math I can make enough BTU in the 12 hrs of off-peak electric ( $0.047 / kwh ) to meet my daily demand and it isn't terribly expensive as a back up source. I wonder what the best vertical position would be for those taps? ( again probably a new post ).

YMMV but I saved a fair amount going with forged steel half-couplings as opposed to the weld-o-lets.

As for position, to do it right maybe study how the pros in Europe do it with their purpose-built highly integrated storage/solar/electric tanks. Basically they provide a guide pipe inside the tank that takes the heated water to the top with the heating element on the bottom and the boundary between hot and not-so-hot water seeks its own level. But simply putting the elements on one side of the bottom of any given tank should work pretty well; the hot water would rise up one side and the not-so-hot would fall down the other. If your manifold pipes were fat (say 1.5") it might work well to have all the resistance heating elements in the bottom of one tank and then let the heated water seek its own level across all the tanks.

 

[woodsmaster]

I agree with mr. fixit 1,000 gallon storage is plenty for your load and a eko 40. Any more than that will be a struggle to heat in the middle of winter. It would be pretty easy to add more storage when you get a bigger boiler. Maybe buy and plumb up if your sure your going to trade up in the future.

 

[700renegade]

The sense I am getting is to install 2000 gal at this point. It probably is overkill with the EKO 40, but the cost to get someone in special later on to spray foam, etc tells me to put in two tanks right away. I'll reserve space for a couple more in the future if I find I ever need them.

ewd - where did you find NPSM half couplings for the electric elements, or will they thread to a NPT? Also - my Maxim pellet is an open system and everything else will now be closed. Tying in won't be as simple as a cross fitting...... For sure I'm deploying a plate or shell/tube exchanger somewhere. First question is - do I insert heat from Maxim into the primary loop between the EKO and tanks, or on the secondary loop. If it is backup only, then my logic says to insert it on the secondary loop and find a way to control it so that whenever the temps in the secondary loop fall below 140*, I wake up the Maxim and put it to work. I can't get my head around a way to scavenge the heat back out of it's 80 gal built in water jacket once It is no longer called for. Seems a waste to let 80 gal of 180* water to go up a flue.

 

[ewdudley]

ewd - where did you find NPSM half couplings for the electric elements, or will they thread to a NPT?

Oops, didn't know what I was talking about. I don't know of a source for what you need.

Also - my Maxim pellet is an open system and everything else will now be closed. Tying in won't be as simple as a cross fitting......

For sure I'm deploying a plate or shell/tube exchanger somewhere. First question is - do I insert heat from Maxim into the primary loop between the EKO and tanks, or on the secondary loop. If it is backup only, then my logic says to insert it on the secondary loop and find a way to control it so that whenever the temps in the secondary loop fall below 140*, I wake up the Maxim and put it to work.

I can't get my head around a way to scavenge the heat back out of it's 80 gal built in water jacket once It is no longer called for. Seems a waste to let 80 gal of 180* water to go up a flue.

I've been pleased with feeding the secondary boiler to the top of storage from the same side as the wood boiler. I just let it heat a layer a few inches thick at the top of storage and when that's gone the boiler runs again for a while.

For scavenging -- just thinking out loud -- maybe a heat exchanger near the storage tanks. Situate the HX near the floor with the hot side going to the top of storage with a tee going into the side of one of the tanks half way up. To scavenge the Maxim, just leave the HX pumps running and as the Maxim cools the storage side of the HX would start feeding into the midway port of the tank as the temperature dropped. A manual timer for initiating the scavenge cycle would probably work OK since the end of backup mode implies that somebody is standing there firing up the wood boiler.

 

[thecontrolguy]

Some really great information here as usual. I love the single 1000 gallon vertical tank as a system separator, for what it's worth. Also wanted to mention the high cost of anti-freeze for even 1000 gallons of storage might be prohibitive. Keep us posted, sounds like a cool project.
Chris

Hey Renegade;

Cost prohibitive, for sure, but you should also consider the cost of properly flushing the system during commissioning (lots of fresh water!) and then the amount of corrosion inhibitor chemicals you will need to properly treat all that additional water in those tanks. Yikes! We are using Nalco 7357 at work that ends up being about $1200.00 PER 20 litre pail. Very concentrated, for sure, but still...

Cheers, and good luck.

 

[jebatty]

I can’t get my head around a way to scavenge the heat back out of it’s 80 gal built in water jacket once It is no longer called for. Seems a waste to let 80 gal of 180* water to go up a flue.

To me, unless it was a "freebe" in the setup, this is not worth pursuing. First, you describe the Maxim as a backup (to be used when the Eco can't keep up), not likely to be fired a lot. Second, if 140F is the minimum usable temperature, then the btu's in this 80 gallons are 80 x 40 x 8.34 = 26,700, which is about 20 minutes of design load heat. In equivalent electrical use, it is about 8 KWH, and at $0.10/kwh = $0.80. If the cost of parts and time to achieve "scavenge" were as little as $50, that would be 63 scavenges, and if you're scavenging that often, then the Maxim really is more likely your primary boiler. Time to rethink your entire adventure.