How long will your storage retain usable heat?

This is an impossible question to answer, Dogwood, without more information. A BTU is defined as the amount of energy required to raise the temperature of one pound of water by one degree F. Conversely, if you are drawing energy (in the form of heat) from a storage tank, you will be losing one degree F per pound of water per BTU that is being drawn to heat your home.

So, for example, my heat storage tank holds 850 gallons, which equals 7,094 pounds (water weighs 8.3454 lbs. per US gallon). It therefore takes 7,094 BTUs to raise the temp of my tank from, say, 150F to 151F. (Of course that presumes that a perfect transfer of heat from the boiler to the heat storage which is never the case. There will always be heat losses along the way. But for the sake of the illustration...)

In my system a while back I monitored the heat loss of my tank over two hours. Outside temp was about 25F (as I recall--cannot find my notes) and I noted that the tank dropped 10 degrees F over the two hours--a loss of 70,940 BTUs or 35,470 BTUs per hour. At that temperature (quite mild for here), if the effective useful temperature range of my tank was between 112F and 175F, then the tank has 446,922 BTUs available which at 35,470 per hour equals 12.6 hours of useful heat.

However, if the outside temp was, say, -45F (a not untypical cold snap temp), then those 446,922 BTUs are going to be used up much, much more quickly.

So sizing a tank will relate to how many BTUs you predict will be needed to provide heat to your system and etc.

Sorry if I have gone on in too much detail-but helps me as I think about my own tank sizing needs.

Barrett

Depends on surface area of the box, R-value of the insulation, thickness of the insulation, temperature difference, and the amount of water inside the box.

These are pretty rough figures, but assuming a 5 ft x 8 ft x 10 ft box of 8 inch expanded polystyrene enclosing two 500 gallon tanks of water starting out at 180 degF sitting in a 32 degF space, here's the temperature at the start of each day:


Of course the bottom of the enclosure isn't going to see 32 degF, and the bottom of the tank won't be 180 degF for long, and if the enclosure is inside somewhere then the other walls won't see 32 degF, and your R-value may be better than 8 inches of polystyrene, and you may have some interstitial insulation around the tanks, so this should be a rather pessimistic estimate.

... what I am trying to do is get a ballpark figure on how long it would take for my thousand gallon (propane -type) tank to cool off on its own, if was heated to 180-190 degrees and there was no load placed on it.

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From the chart I'm guessing a reasonably pessimistic estimate would be something like 20 degF loss after four full days. The chart is day number on the left, degF on the right.

My alternative may be to heat the tank up to temp Sunday night, have my wife heat the house off storage until the tank is down to 140 degress, and heat the home with the propane furnace until I get back.

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I think this would be the correct strategy. Instead of letting the heat leak away, have your wife use it up and then go to backup heat. The sooner you lower the storage temperature, the less heat is lost to leakage because the temperature delta is lower sooner.

I'm thinking it would take a while to get the tank back up to 180 degrees if I started out from room temperature each Thursday.

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In nearly every implementation the wood boiler heats top of storage and then load is drawn from top of storage. No matter what temperature the storage tanks are down to when you get home you should have enough hot water at the top of storage to start heating inside of a half hour from when you light the boiler, more or less.

[quote author="dogwood" date="1330314860"]If you had your storage fully heated, say to 180 degrees or more, and were away for a few days, say four, what temp would you anticipate finding your storage water at upon your return. Assuming you were heating your home in your absence with your back-up furnace/boiler, your tank is reasonably well insulated, and not outside in the cold, or even if it is in an unheated space. Conditions vary, what what would you expect in your situation with your storage tank size?

I guess I am wondering how quickly a fully heated, reasonably insulated tank will cool off to below 140 degress , if unused a few days. Don't know if there's a difference between how fast a 300, 500, or 1000 gallon tank might cool.

Mike[/quote

Our storage is 1,000 gallons with a working range of 150F to 195 F. Drawing 15,000 BTU'S per hour brings the tank down to 159 F in 24 hours, not enough for two days between firings. For the area that I live in, it would represent an average amount of BTU'S being taken from the tank. For your area, your BTU draw is going to be a lot higher.

Determine your hourly BTU draw, then if you are going the way of a boiler and storage size, your boiler should take your storage to its maximum temperature on one firing and your storage the 24 hours to your next firing.

Below is the section from the EKO manual on sizing your storage to your boiler. It's a starting point.

Hot Water Storage
For the best results, the use of a heat storage tank is recommended. A properly sized storage tank
can cut wood consumption by as much as 40%. Hot water storage allows the boiler to run at optimum
capacity regardless of the demand for heat from the building being heated. The stored heat can be
recovered later, both stretching the time between boiler re-fuelings, and as an additional source of
heat on very cold days. Generally, the bigger the storage tank the better, as the boiler is most efficient
when running at full capacity. The rule of thumb on tank sizing is that 13 gallons of water can store
about 1KW of boiler heat. Using the EKO 25 as an example: 13 gal/1KW x 31KW = 421 gallons, 421
gallons x 91% efficiency = 383 gallon minimum size water tank. A hot water storage tank connected to
an EKO boiler can also be used to store hot water from a solar water heater.

dogwood said:

Well Eliot, I misread your chart, mistaking hours for days. I am more encouraged now.

I was looking at HR's recent Iodronic's post and it had a schematic as you describe with all boiler output going directly to storage, and from there recirculated to the loads. This is certainly a simpler arrangement. Do you think it would be advantageous?

Mike

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In our system the boiler charges the storage. There are two heat exchangers in the storage tank - one for heat and the other for DHW. If your storage has ample reserve, it works well.
If our storage goes much below 120 F that's the bottom end for a comfortable shower!

Here's another question for you about heating off the top of the tank. The plumbing schematic I am using from Tarm Biomass has a bypass plumbed in so you can circulate directly from the boiler to the load.

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I didn't know that any of the Tarm reference designs for pressurized storage had a provision to isolate storage from the wood boiler. Are you sure you're not looking at an unpressurized 'heat battery' system?

I was looking at HR's recent Iodronic's post and it had a schematic as you describe with all boiler output going directly to storage, and from there is circulated to the loads when there is a call for heat. This is certainly a simpler plumbing arrangement. Do you think it would be okay to go that route (and leave our the bypass)?

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Whether you go from boiler to storage with one set of tank fittings and then from storage to load with another set of tank fittings, or with a pair of lines with tees going off to storage as in the 'Simplest', it doesn't make a whole lot of difference hydraulically. Whichever works out best for your situation is fine. If you want to have the most hot water a litter faster after firing the boiler then the 'Simplest' would work a little better because you're not heating a layer of hot water across the top of the tank, but again, it would be just be somewhat faster, no big deal.

dogwood said:

Eliot, my current Tarm schematic is their PT3 diagram on page 21 of: http://www.woodboilers.com/admin/uploads/public/WoodBoilerPlumbingSchematic0111Web.pdf
HR’s schematic is Figure 6-21 on page 35 of: http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf

The PT3 has a bypass to the storage going directly to the load. Maybe that is too much trouble for what it worth. What do you think?

Mike

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If I'm looking at the right drawing, PT3 from version 12-10 dated 2010-12-07 has no bypass. For some reason there's ZV-1, but it serves no purpose, so I'm assuming it is a pasting error by the draftsman.

Hydraulically the PT3 and Figure 6-21 are nearly identical. But neither has a fossil fuel boiler integrated, so you still have to deal with that.

--ewd

dogwood said:

it seems you can heat from the boiler directly to the load and back again in the PT-3 arrangement.

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I'm just saying that the hot water will float across the top of the tank nicely in the Figure 6-21 design, in effect 'bypassing' the tank. True, the PT-3 system will bypass somewhat more effectively, and it's the way I did mine, but either way is fine. Again, in PT3 ZV-1 doesn't do anything and it doesn't help-- or make possible--storage tank bypass.

FWIW- my first fire in the Innova-30, i brought 820 gals/unpressurized storage from 52 degrees to 180 in about 12 hours. This was on a night where the house got down to about 50f. We had taken the house offline to hook up system. It was -10 outside with a stiff breeze. The boiler heated the tank and brought the house up to temp in no time. The house was first in line for the heat. Then heated tank and house together. The house is a pretty big load. I don't think you're Innova-50 will be long bringing your tank back up to temp.

Also, i couldn't load the Tarm schematics, so bear with me. If your layout has a Termovar valve to divert heat to the house, if the house is calling for heat(while your wood boiler is running) I would use it. On my unpressurized storage system layout, it works great. Within 45 minutes of throwing match I have 165 degree water running thru my baseboard. In 60 minutes it's 190ish and climbing. And there is heat going to storage. This is a nice plus if house is cool and tank is down below it being useful.

I have 550 gallons of pressurized storage supplied by American Solartechnics. I believe they sell at least a porion of the output to the Tarm folks in NH. When I was setting up the controls, I charged up the tank a couple of times with no way to remove the heat. I came up with a rough estimate of about 3 degrees loss per day but you would have to do some calculus to figure it out exactly as the amout of heat transfer is directly realted to the temperature difference bewteen the inside of the tank and the outside of the tank. Think of its this way, if the suroundings are at the same temp as the tank it will not cool down. If the tank is at the maximum difference between the inside and the outside it will cool down the quickest. To further confuse the issue the tank top will give up more heat per amount of surface area than the walls due to convective air flow and the tank bottom will give off substantially less heat than the top as it most likely is sitting on the floor. Now add in that the temperature of the inside of the tank is not the same due to stratification with warmer water sitting on top and it get more complicated.

I did notice that the copper piping used to charge the tank are very effective at thermosyphoning heat. On the connection to my side wall coils, I ran the pipe down with a couple of elbows before heading up to the ceiling. This should stop the thermosyphoning.

There seems to be a couple of different questions here - how much standby loss do we get, and how long can the tank heat the house. The answer to the first is not much, if you're well insulated. I have the passive "heat battery", a 4x4x8 rubber lined plywood box, insulated with 2 2" layers of foil faced foam. The tank hugs an outside cellar wall on one of the long sides, an interior wooden uninsulated wall on a short side, and it sits on the concrete cellar floor. We've had some stretches in the summer when we were away for a few days, and the tank leaks very little heat when it's just sitting there. In my climate, the buffer is enough to get through most of the coldest winter days with two fires and no restoking.

lots going on here. First, ZV1 is closed only when there is no call for heat from the zones. This assures that all flow goes through storage and no ghost flow through the zones. Given the hydraulics of drawing PT3, probably not strictly necessary, but can't hurt. If you are incorporating a back-up boiler, please see drawings PT1 or PT2.

The OP asked about standby loss under no load, if I understand correctly. Mostly a theoretical exercise, right? Can't imagine a real-world situation where you would need this info? If you are looking to know how long your thermal storage will last between firings, you need to determine the load and then use the math around BTU that others have already shared to show how many BTU you have stored and how long that will last.

In our experience, and purely anecdotally, systems that allow for 24 hours of coast on the coldest days of the year between boiler firings have been intentionally over-sized. For example, something like a 50,000BTU load with a 170,000BTU boiler and 1000 gallons of storage (just an example, don't bother doing the math). We have had people take three days of continuous boiler firing to charge/re-charge 5000 gallons of thermal storage, but that is very unusual, of course.

hope that helps shed some light, good luck with the project and don't hesitate to give us a call with questions.

Chris

Any time you have "pumped" flows in a tank there will be temperature blending. The water doesn't always do what the color in the drawing shows, hot top, cooler bottom.

With a hydraulic separator or separator tank you can define that temperature blending. It changes depending on how many GPM both sides of the tank are flowing.

If the boiler is flowing 8 gpm, and the distribution is 8 gpm, then the temperature on in and out ports will be about the same. If either side gpm flow changes, so does the mixed temperature. this graphic shows what goes on inside the tank. Also the formula to calculate the mixed temperature.

I think an open, unpressurized tank, with coils inside probably stratifies better as long as the water in the tank is not pumped, hot water rises, cooler water drops to the bottom.

The same concept with your home water heater, hotter water on top. Unless you add a recirc DHW pump, then the tank temperature blends, depending on where the pump ties into the tank.

hr

dogwood said:

I wish this was a theoretical exercise for me. I am away from home from early Monday morning to late Thursday night. I am trying to figure out how approximately how fast an insulated 1000 gallon tank might drop off in temp if I put no load on it for those four days.

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I believe the point is that figuring out how fast the no-load tank would lose temperature is a theoretical exercise because there is no scenario where it could offer any advantage.

To minimize the amount of load on the propane system just go ahead and draw down storage for as long as it is available, then switch to propane, then light a fire as soon as you get home. Within a half or three quarters of an hour you'll have heat from the wood boiler and the propane system can be shut down.

For as long as you do 'double duty' heating the house and charging storage after you get back home, the heat going in to storage will be heat that would have come from propane had you switched to propane when you left, and over and above that will be the heat you didn't lose to leakage due to leaving storage setting idle. Kind of a use it or lose it situation.

--ewd

dogwood, your last post reminds me of one of the plumbers involved in hooking up my system. With the DHW heat coil in my tank, I had the plumbers remove my boiler-mate from my furnace. One of the plumbers thought I was nuts, and he went on about all the oil it would take to heat the storage if I ever got hurt and couldn't build my fires. He couldn't seem to grasp the point that all the heat going into the tank was going to end up heating my house or DHW anyways, so it wasn't really an issue.

Standby loss on a pressurized, insulated tank in a heated space is very low at moderate temperatures. My system is setup to operate as follows on a fully charged tank, Tarm has burned out: draw down storage to radiant floor to maintain shop heat at 61F floor temperature. When storage has dropped to 80F, shut down radiant draw from storage. Shop will continue to be heated at 50F air temp from an electric space heater. Storage tank will remain 60F+ for at least 2-3 weeks. If I take a winter vacation, that is the scenario.