How much is too much water storage??

I don't know how 50KW converts to BTU's, a little numb on my part. But I'll tell you what what was recommended from the manufacturer of my tank. I have a 820gal tank. I would need to have about 250sq/ft of solar collector to support that tank(I'm located in the northern half of the U.S. was down in the low 30's last night). I'm heating my house with a 100,000btu wood boiler, eventually want to hook up solar to it. So if i was to have 1500 gal tank, I guess I would need 500sq/ft of collector to support it.? The manufacturer of my tank has also been in the solar business since the '70s'. I'm just using his knowledge.

In the cool cloudy areas of the US we suggest 1- 1.25 gallons per square foot of collector. Sunbelt and southern areas as high as 1.75- 2.25 gallons of storage.
But if the wood boiler maintains the tank at 60c or higher the solar may not add too much. I'd cover the DHW preheat with the solar first, in a separate solar tank, then send any excess to the large tank. DHW is a year around load and the best use for small thermal arrays.

Go online and use one of the solar calculators to design a system exactly to your area and conditions, every job is different depending on your actual loads.

Here is one of the demo software programs from T-Sol, click on their site, upper left hand side has a free calculator, basic but gives you the key number, solar fraction.

hr

in hot water said:

I'd cover the DHW preheat with the solar first, in a separate solar tank, then send any excess to the large tank. DHW is a year around load and the best use for small thermal arrays.

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This is my strategy. Even with 1000 sq ft of solar south roof at optimum angle with no shading, solar would not even put a dent in our space heating needs. The output of the array then gets lost in a tank sized for and shared with the wood boiler. Preheating dhw makes a lot of sense and should eliminate the need to run the boiler during the off season.

A member here has a great site that goes through this http://www.nofossil.org/

I did not explain myself well(not a big surprise!). My set up, I do not expect to be able to heat up my tank for heating in the winter from solar. But, I thought it would do well in the summer for DHW. I guess my assumption is that the tank size I have, it would take that much solar collector(250 sq/ft) to give me DHW for summer time use, but still would use the wood boiler less often.

I have not installed storage for my Tarm solo 40 yet but I would
look at what you are going to use the storage for. The more storage
you have the more flexable you can be on when you fire up the wood
boiler. If you are away from the house for 12 hours and your boilder
runs for 5 hours you would need the storage to provide heat for 7 hours.
How many BTU's does your house use in 7 hours and what is the min
temp your storage needs to be to heat the house and you should have
an idea of how much water you need. The downside for larger storage
tanks is mostly $$.

This is a question I am still trying to answer as well. I have a 5000 gallon tank I can use(not pressurised). But I can also make the tank smaller, by half or more. I would hate to cut the tank then later realize more is better.

solo40 said:

I have not installed storage for my Tarm solo 40 yet but I would
look at what you are going to use the storage for. The more storage
you have the more flexable you can be on when you fire up the wood
boiler. If you are away from the house for 12 hours and your boilder
runs for 5 hours you would need the storage to provide heat for 7 hours.
How many BTU's does your house use in 7 hours and what is the min
temp your storage needs to be to heat the house and you should have
an idea of how much water you need. The downside for larger storage
tanks is mostly $$.

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I may be not the one you're addressing, but my .02. I have a 2 story- 1800sq/ft house(plus full basement, not heated). Typical windows/doors. R-19 in walls, R-38 in ceiling. Live on the top of a hill in northern maine. 1 wonderful wife, 3great kids. Kids in mid teens(lots of showers, if they want to split wood.) My oil boiler is a weil Mclain, 80,000btu unit(not used). Baseboard heat, with the exception of staple up radiant in the kitchen floor. In the dead of winter, I can maintain my heat on about 5 to 8 hours of fire and the rest of off the storage. My routine( when it gets really cold out) is my son comes home from school, starts a fire. I check on tank temps, etc later. May refill, if needed. Go to bed. -------During the summer, i can get 5 days sometimes 6 of DHW, off 1 firing of boiler. I thought 6 cord of good hardwood would take care of my needs, but might be a little closer to 7, year round.(But I keep the house a lot warmer than before when I was using oil). I'm very happy with the whole set up. Would love to rebuild/rethink/redo my house again, but...........

As I understand it, storage sizing is, like many things, a series of compromises... The bigger it is, the longer you can go between burns, but the more you will have to burn to reheat it when you do fire things up... Also because even the best insulation jobs will loose SOME heat over time, you do get a certain amount of diminishing returns where the heat in the tank can get lost before you use it if the tank is too big.

Essentially the way to figure it is first decide your usable range of temperature - high is the maximum you can get the tank up to, low is the minimum temperature you can work with before you need to fire the boiler up because you don't have enough heat - this might be your DHW, which gates around 120*F, or it might be your heating load depending on how hot a supply it requires - you can't do much about the DHW, but this implies that it can be worth modifying the heating load to run on as low a temperature as possible. Typical ranges I've seen are about 120-180*F

It takes 1 BTU to raise one pound of water 1*F, and water weighs about 7.8lbs per gallon, so you can store 7.8 X your usable range BTU's per gallon of water in your tank.

Next you need to figure out what your shortest tolerable time to run on storage will be - this will be based on your "design day" heat demand, or how much heat you will be pulling out of the tank on the coldest day of your heating season - multiply your BTU/Hr heat load times the number of hours of heat you want to have in the tank to get the number of BTU's you have to store, divide this by your BTU/Gallon value to get the number of gallons you will need on the coldest day.

As a second check, figure out the number of BTU's your boiler will put out over the length of a burn on a single full load, and again divide that into your BTU/Gallon value to make sure the tank will be big enough to hold the full output the boiler produces - this is what you will need if you fired in the summer time with no heat load to share the boiler output.

The larger of those two numbers is the ideal MINIMUM size for your storage, in actual practice getting a little over is no problem, being way over can mean that you will need to burn several loads to get the storage up to temperature...

As a practical thing, most of the systems I've seen written up on the forum seem to use between 500 and 1,000 US gallons of storage. Some of this is because it's a size that works well, and part of it is because one of the common storage tank setups involves recycling propane tanks, which typically come in 500 or 1,000 gallon sizes.

Gooserider

from your original question, my answer would be how long do you want to be heating up storage? that is the limit on the ultimate size of your storage, do you want to be burning for days non stop to get the storage up to temp? It would be great to only have to burn once every few weeks but if you had to burn non stop for a week to get there it kind of looses the advantage.

firenz said:

Thanks Gooserider, Those are useful figures. Would you have the metric equivalent of those figures? i.e. KW/hr, Celsius and Litres.

It seems there are a number of variables the calculation should enable to me to estimate though.

Regards Philip

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I don't really have metric equivalents, but the converter that I use is convert-me.com They have the site setup to convert just about any unit to any other if it can be done, with a very nice interface - they give you a list of units, fill in the value for what you have and it gives you the equivalent in the others... You might also have to do a bit of research on just how to come up with equivalent numbers - especially the units.

Another useful website that is sort of an on-line equivalent to those old pocket reference books is The Engineering Tool Box They have a lot of charts of values for different kinds of things, including conversion factors, but not as nice of an interface.

Hope this will at least get you started...

Gooserider

There's one other factor to consider. The larger your storage, the more critical it is to establish, maximize, and maintain stratification. With good stratification, there's no response time penalty and no need to heat storage all the way with any particular fire.

However, if you have little or no stratification the size of the tank will have a noticeable impact.

I have this problem to some degree when heating my 880 gallon storage tank with solar. If the tank is at 110 degrees and I dump 60,000 BTU into it, it doesn't just heat the top few inches. The top gets a higher percentage, but the heat is distributed through the tank. The top will rise about 16 degrees, and the bottom will rise a couple of degrees. 126 degree water isn't all that useful.

If I were heating just the top 80 gallons of the tank, 60,000 BTU would raise the temperature by 90 degrees! Of course, I'd start losing heat transfer capability long before getting it that hot, but there's no question that I would have a lot more usable heat. Heating a smaller tank would yield the same benefit.

from your original question, my answer would be how long do you want to be heating up storage? that is the limit on the ultimate size of your storage, do you want to be burning for days non stop to get the storage up to temp? It would be great to only have to burn once every few weeks but if you had to burn non stop for a week to get there it kind of looses the advantage.

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You don't have to wait for a heat storage tank to be depleted of heat to fire the boiler.

It's like having twin 34 gallon tanks in your pickup. You don't need to wait until the engine is sputtering on empty to hit the pumps (though I probably would from lack of attention). You could drop a couple of twentys in there to top it up most of the time and only use the full capacity of the tanks when you're hauling a trailer on a long trip.

If you had a job where you were away from home for a couple days on a regular basis it might be handy to be able to let that big tank cruise occasionally. Yeah, when you get back you'll need to burn continuous loads until it is back up but that might be only once in a while for your own convenience. You burn the same amount of wood either way.

But bigger tanks are more money and if you're talking propane tanks it gets pretty interesting trying to decide how to stack or make room for them and hook them together. I agree with you that at some point one convenience is gained at the cost of a lot of inconvenience. Not a point really but something like a fog bank of diminishing returns.

firenz said:

Thanks again. I managed to work through it and convert back at the end. The burn time I got from the manufacture based on the volume of wood and energy value-

The 50 Kw = 170700 BTU's

The fire burning for 5.5 hours max produces 938850 BTU's (170700 X 5.5)


Divide 7.8= 119980 gallon per 1 deg F temp change

Say the water temp to be changed was 15 C - 80C (60- 180 F) = 120 deg F
change

119980/120= 999 gallon of water will be brought up from 60 to 180 in a 5.5 hour firing.

If I am correct it would would seem be sensible to have the storage about this amount.

Potentially with the solar the cooler temperature could in fact even be
higher.

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Looks pretty much OK, except that I think you are picking way to wide of a range for your storage tank temperatures. By the basic rules of thermodynamics, in order to transfer heat, the source has to be hotter than the target. Thus you need to have a tank that is hotter than the things it's trying to heat - i.e. your DHW tank or your heating system. Here in the US, the expectation is that you will get about 110-120*F out of a faucet, and anything under body temperature (98.6*F or 37*C) is considered warm at best. However for safety reasons our building codes require that faucets don't put out more than 120*F, either by limiting the setting on the water tank thermostat, or by using a tempering or mixing valve that mixes cold water into the hot in order to bring it down to 120*F.

Similarly, your heating system will have a certain temperature that it needs to have sent to it if it is going to do a reasonable job of heating your house. Radiant floor systems usually want 100-120*F water, other types of heat often want even hotter... Thus if your storage tank gets much below 110-120*F, which I think is around 50-60*C, you are going to need to heat it back up, so your "useable range" is going to be more on the order of 55-80*C - you will have to heat up the tank with several burns when you first get it filled, but you presumably would never again let it cool down that much unless you had to work on it.

As NoFossil said, thermal stratification is also an important factor, which will let you bring part of the tank down lower, but that doesn't have a big effect on the storage sizing figures. It does suggest though that it is good to have a separate DHW tank that charges off the boiler or solar to the highest possible temperature, and then lets any remaining heat go to the main heat storage tank. If you go to NoFossil's website (see his sig) he has an excellent setup for charging his DHW tank off the boiler, and IMHO one of the best configurations for stretching the amount of useful hot water from his DHW tank by using two stages of mixing valves, and preheating the feed water to the DHW tank. For the solar setup, what I would do is run the panel output water through a heating coil in the DHW tank to transfer as much heat as I could into that, and then run the water from there into a tank exchanger before heading back to the panels. Remember the bigger the temperature swing you can get out of the panels, the better they work.

Gooserider

firenz said:

Thanks again. Are there any rules for stratification, presumably the size is bette tall and skinny is better than shallow and flat?

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Theory says that tall and skinny is better, but that can be a challenge to build / buy as a lot of tanks are built with a horizontal install in mind. From an engineering standpoint a short and wide tank is a bit easier because the pressures at the bottom of the tank are lower. The experience of the users here seem to suggest that it doesn't make a huge difference. What seems more important is do water input / output designs that do as much as possible to discourage mixing - i.e. use a diffuser that has lots of little holes going sideways or something like that.

If you are doing coils try to arrange the coils so that they will not encourage a lot of convection mixing, etc.

Gooserider