Pressurized heat storage

[kcbenson]

BioHeatUSA sells 220 gallon pressurized heat storage tanks ((broken link removed to http://www.woodboilers.com/product-detail.aspx?id=58)), but I don't see them for sale anywhere else. I know a lot of people convert propane tanks, but I think I need to stick to less DIY solutions. I also want to stick to ASME rated tanks as I'm in Massachusetts.

Before I buy from BioHeatUSA, is there any other outlet that sells similar tanks? Theirs look good, but I don't like to buy anything that has a selection of only one choice.

Ken Benson

 

[stee6043]

I bought my tanks with ASME tags from a propane dealer. But this is very hard to do if you don't have connections. The only other option I'm aware of is the link below.

http://www.ahona.com/prod_waterstorage.html

Do you have an ASME boiler? Are you planning on using "enough" storage? 220 gallons is a very, very small amount...

 

[kcbenson]

This looks interesting. Thanks.

Yes, I bought the smallest (100,000 BTU) Econoburn boiler (not delivered yet). They offer it with or without ASME certification.

I had planned on two 220 gallon tanks.

Ken Benson

 

[stee6043]

I think you'll find that 440 gallons will be less than usefull with 100,000 btu's of output. What is your goal? With an average peak heat load (20k-30k btu/hr) you're looking at only a few hours of heating from those tanks depending on what type of heat emmitors you use.

Assuming you had 40 degrees of usable heat in your tanks (180-140) you would only be able to heat for 4.8 hours from your tanks with a 30k btu/hr heat load. With 1,000 gallons and your up over 11 hours. Assuming 30k btu/hr is a peak load (which mine is, roughly) you start getting into 16+ hours of heating available from storage with the higher storage volumes on "average" winter days. That's when heating with wood becomes very low maintenance.

Not to mention how quickly you'd be able to charge those smaller tanks (1.5 hours roughly). Once the tanks max out in temp your boiler will begin to idle if you don't need the heat. Idling is the enemy of efficiency.

 

[kcbenson]

I'm not qualified to debate you on the numbers. I can only tell you that Scott Gates at BioHeatUSA said that for a 100,000 BTU boiler they recommend 440 gallons of pressurized storage or, I think, 500 gallons of unpressurized storage. I just talked to Mark Schoellig at AHONA (the link you just gave me). He said that for a 25kw boiler (which he said is equivalent to 100,000 BTUs) 400 gallons is optimal. He gave me a price on a 500 gallon tank, but they have tanks much larger.

Neither he nor Scott Gates tried to sell me a larger tank. Mark Schoellig actually gave me the price on a 1,000 gallon tank (it's considerably less than double the price of a 500 gallon tank), but followed that with "But you don't need that much storage."

In March, I visited the home of a chimney sweep close to us. He has a 150,000 BTU Tarm with 500 gallons of unpressurized storage. He burns a fire once every four days in the summer, once a day in January, sometimes up to 8 hours.

Ken Benson

 

[kcbenson]

BTW, I ordered the smallest boiler because we have a fairly small house. I did my own (admittedly amateur) heat loss calculation and came up with about 46,000 BTUs/hr (which is average, I think, not peak). How do you figure out peak from average?

Econoburn says their boilers don't even need storage. So anything's got to be better than nothing, right? Besides, 500 gallons (the propane style tank that AHONA sells) is 3' by 10'. I don't think I could really fit anything bigger in my basement.

Ken Benson

 

[stee6043]

I think this is turning one-sided. You need more opinions than mine. Do a search on this forum for "storage". I think you'll find that 80% or more of the users here that use storage have 800 gallons or more. Some push 2,000+ gallons.

I certainly wasn't trying to make you feel like you made the wrong decision. The math doesn't lie. It takes 1btu of energy to raise 1lb of water 1 degree F. So for every gallon of water you can store 8.3 btu's per degree of useful change in temperature. Most folks (again, search this forum) can operate their heating systems efficiently between 180 and 140. That's 40 degrees. 40*8.3*440=146,080 btu's available. If you're truly using 46,000 per hour you only get 3 hours of heat from storage before you have to start charging again.

Most folks want storage so they don't have to burn all day. Maybe that is not your intent. That's why I asked "what is your goal?".

For the record - you do not need storage with any gasser. It's an option for convenience only. If your boiler is properly sized for your heat load it really won't even increase efficiency. For me - I need storage so I can heat my house while I'm gone at work. I heat for 16 hours most days from storage.

I highly doubt you have 46k in heat loss, by the way. Google "slant fin calculator". I heat 3,200 square feet and my peak is just over 30k....unless it's below zero for a few days...my average is closer to 20k...less if there is some sunshine.

 

[kcbenson]

Thanks for bringing it down to my level. I can't argue with the math. I'm guessing the people who are selling storage are selling the minimum I need to burn efficiently, not what I need to burn comfortably.

I hadn't thought too much about my goal. I would like to be able to get through the night without the oil burner coming on. In our last house we had a Tulikivi (masonry heater) that would heat a 2500 sq ft house (unevenly) with two burns per day. So I'd be happy with two burns per day. And if I could get DHW in the summer I would be even happier.

I haven't made any decisions about storage yet, so I'm glad we're having this discussion.

I dug up the Slant Fin heat loss calculator and measured everything again, and it shows that I need 62,000 BTUs/hr, heating to 70 at 0 outside (see attached). Should I be using a different outside number? If I recalculate it at 40 degrees outside, it's more like 34,000, but I want to know the maximum heat loss, right? This is a 33 year old garage-under ranch with 4" walls. There's also a large sunroom on piers in there that skews things considerably. That said, I should probably double-check my numbers. This heat loss calculation wasn't a whole lot different than the first one I did, except that it came up 16,000 BTUs higher. Takes a long time to measure a whole house and all its windows and doors.

I'm going to have to think hard about whether I can fit more storage down there.

Ken Benson

 

[Nofossil]

There are a lot of people who wish that hey had more storage, and virtually none who wish they had less. As far as I'm concerned, the only downside to lots of storage is that you'll have a bit more standby loss. I'd do 10,000 gallons if I could, though I'd divide it into smaller tanks to preserve stratification and peak temperatures when I'm not charging all of storage.

 

[in hot water]

Check out this article for some ideas

www.pmmag.com/Articles/Column/ac719ac235fc7010VgnVCM100000f932a8c0____

I visited John's home in upstate NY. The interior wall of his home is 10" wide, from basement to rafters, two levels. he had me tap that wall and it had a ring to it. It's actually a water storage tank that was welded on suite. it could be warmed passively, with solar, or with a wood stove Hx. a great place to store low temperature BTUs.

hr

 

[kcbenson]

I just talked again to Mark Schoellig from AHONA. He basically agreed with the numbers (roughly, that 500 gallons represents about 150,000 stored BTUs which is heat for 3 hours at 50,000 BTUs/hr heat loss), but pointed out to me that at peak load (January, with people taking showers) the boiler will be burning for longer than what's needed to simply heat the tank. At peak load, I'll be firing the boiler for several hours a day just to heat the house. So that 3 hours between burns is more like 6 or 8 hours when you include the actual burn time. Spending a lot of time with the stove when it's 0 outside is what I'm used to. It gets easier when the temperature gets up to 40 outside...

The clincher, though, was that he said that the stackable 500 gallon tanks can be bought separately. I can buy a 500 gallon tank now. Later on I can add another 500 gallon tank and set it (not sure how, engine hoist maybe?) on top of the first tank. For someone who was originally contemplating no storage at all, I'd rather try out 500 and see.

Thanks again for the AHONA link.

Ken Benson

 

[Chris Hoskin]

Hi Ken,

as you are discovering, tank sizing is a bit of a black art. A 4 - 500 gallon recommendation for 100,000 BTU boiler is, in our experience, a good balance between space (footprint AND height), usefulness, and budget. Most people are using our 220 gallon tanks because of height or doorway restrictions (they are 30" x 77" so fit in most basements), however, if you can make it fit, our single 400 gallon tank at 36" x 95" is more cost effective. Regardless of which tank you choose, you should have no problem setting it up to be able to easily add additional storage at a later date.

We do have a preference for vertical tank orientation because of the benefits of stratification. In a perfect world, everyone would have a single, vertical 1000 gallon tank, but that would make for some pretty unusual basements! One of our dealers did this kind of thing hooked up to one of our Solo Plus 60's - I believe the tank is 16' tall!

Chris

 

[kcbenson]

Is that a pressurized 400 gallon tank? I don't see it on your website.

Your vertical tanks would be easier for us. I will have to open up a doorway and a wall to get a horizontal tank in.

We do have a preference for vertical tank orientation because of the benefits of stratification.

That's funny. I've had the exact opposite argument made to me (by Mark Schoellig at AHONA), that in a horizontal tank the strata are larger. IOW, if the top 6" of the tank is at 180 degrees, that 6" will be 10' long in a horizontal tank but only 30" long in a vertical tank. How does stratification work better in a vertical tank?

Ken Benson

 

[stee6043]

There have been countless discussions on how tanks stratify on this forum. Some are pretty good reads on fluid dynamics! Ultimately, one big advantage for vertical tanks is that you tend to get less mixing. As such, you maintain better seperation.

In a motionless tank I dare say the orientation would not matter. But our boiler systems are far from motionless. If you really wanted to go engineering crazy you can prove that a horizontal tank will bleed heat faster than a vertical tank. But it is not something that would be measurable...just a fun fact.

 

[ewdudley]

Is that a pressurized 400 gallon tank? I don't see it on your website.

Your vertical tanks would be easier for us. I will have to open up a doorway and a wall to get a horizontal tank in.

We do have a preference for vertical tank orientation because of the benefits of stratification.

That's funny. I've had the exact opposite argument made to me (by Mark Schoellig at AHONA), that in a horizontal tank the strata are larger. IOW, if the top 6" of the tank is at 180 degrees, that 6" will be 10' long in a horizontal tank but only 30" long in a vertical tank. How does stratification work better in a vertical tank?

Ken Benson

There are two reasons a vertical tank will perform better stratification-wise.

First is that all things being equal, the thickness of the thermocline layer will be the same for both orientations, which means a larger volume taken up by the thermocline in the case of the horizontal tank and therefore less volume available for useful heat storage.

Second is that the loss of stratification is accelerated by conduction of heat down the metal sides of the tank, and the amount of metal available for this heat conduction is a lot greater in the case of a horizontal tank.

So if Mark is telling you that horizontal is better for stratification he's simply mistaken, which is highly unusual.

The good news is that water is such a wonderful liquid for establishing heat stratification that horizontal tanks will perform quite well if implemented and managed properly.

Cheers --ewd

 

[tom in maine]

All tanks will stratify. What happens in the tank as far as heat stratification is a matter of speculation unless you do a lot of testing.
When you are pumping 10-15 gpm from a boiler to any tank or some lower flow from the tank to the heating load, stratification will be disrupted to some degree.

My point is that stratification is not as big a deal unless you are really maxed out on your heatload/storage design.
I suspect this is a major factor in 80% of the systems that are discussed on this forum.

All somewhat idle speculation on a hot humid day in Maine.

 

[Chris Hoskin]

Yes, Ken, it is a pressurized 400 gallon tank - just a larger version of the 220 gallon tank.

Horizontal tanks have one significant advantage in that they allow you to get a larger single tank into a room with height restrictions; this is often reason enough to choose a horizontal tank. They do not, however, provide better stratification. Stratification, after all, is fundamentally a vertical phenomenon.

The benefits of stratification are not just in the setting up of a thermal layer, but in MAINTAINING that layer. When you move water through a tank you create turbulence in the tank that can disturb the layering. Having the supply and return ports far away from one another vertically is a good way to minimize the effect of this turbulence on the layering. European manufacturers' tanks (which are all vertical by the way) go to extensive measures using baffling and diffusers, etc. in order to maintain stratification. Thermal storage is not just about cramming BTU's into the tank, it is also about being able to pull those BTU's back OUT of the tank. Good stratification allows you to pull more USABLE BTU's out of a tank because the top of the tank is always at the highest possible temperature. This frequently comes up when people think it would be a good idea to 'stir' their tanks but, a tank that is mixed to an even temperature of 120 degrees throughout the tank is not able to provide usable BTU's to a hot water baseboard heating system, whereas that same tank well stratified with 150 degree water at the top of the tank and 100 degree water at the bottom of the tank would still be providing useful temperatures and BTU's to the heating system. Both of these tanks might have the same number of BTU's stored, but only one is able to provide useful energy to the heating system as you can see. (don't quote these numbers, I only use them to illustrate the concept).

As far as the 6" layer goes......in order to store X BTU at Y temperature, you must, by definition have Z gallons of water at a given pressure. So, a 6" layer of water at the top of a ten foot long horizontal tank is in no way comparable to a 6" layer at the top of a three foot wide tank in terms of the amount of energy stored. The key is the number of gallons; you can have a long skinny 100 gallon layer or a tall, narrow 100 gallon layer, they both contain the same number of BTU's, because that is a function of gallons not thickness. The tall, narrow layer in a vertical tank is better, however, because there is a smaller surface area at the layer boundary so less of the hot layer is disturbed/mixed.

Well that turned into a bit of a novel, sorry. Hope it was helpful, I am sure others will chime in too.

Chris

 

[kcbenson]

Thanks, Chris. It was a novel I needed.

Ken Benson

 

[kcbenson]

single 400 gallon tank at 36" x 95"

I'm guessing it's not a cylinder, more a pill shape. Is it, by any chance, 91" or less 7" from center? IOW, I've got 91" headroom to the bottom of the joists, but there's plenty of room between joists; would I be able to stuff the crown of the tank up between joists? There's probably something illegal about that...

Ken Benson