Time to heat storage

[Quincy]

Hello everyone just a question on how long it takes for you to heat your storage from 120 to 180 degrees.I have a home built seton and a 1000 gallon storage .Was thinking of purchasing a new gasser this spring if I could heat my storage quicker,presently it takes me about 8 hours with no load .I just installed cast iron rads in my house what an awesome heat source ,low temp emitters are definitely the way to go.Thanks in advance for your reply

 

[flyingcow]

I can take my 820 gals from 115 to 165/170 in about 3 to 4 hours in the summer when I'm just charging tank for DHW. That's filling an average wheelbarrow full of wood. Driving to 180 i would guess another hour?

 

[EffectaBoilerUser (USA)]

I am able to heat up my 2 x 500 gallon stacked propane tank storage from 140 to 180-185 in the winter months in one 5 hour burn (100 lbs of 15-20% moisture content wood). This is at the same time my house, hot water and hot tub is being heated.

 

[Bob Rohr]

Hello everyone just a question on how long it takes for you to heat your storage from 120 to 180 degrees.I have a home built seton and a 1000 gallon storage .Was thinking of purchasing a new gasser this spring if I could heat my storage quicker,presently it takes me about 8 hours with no load .I just installed cast iron rads in my house what an awesome heat source ,low temp emitters are definitely the way to go.Thanks in advance for your reply

8.33 X 1000 gallons X(180-120)

8.33 X 1000 x 60 = 499,800 btu to raise that 1000 gallons 60°

over a 8 hour period you would be putting 62,475 btus per hour into that tank. A boiler with a 80,000 BTU/hr output operating at 78% efficiency would be about what you have, maybe?

A 35Kw boiler is = to 119,000 BTU/hr @ 85% efficiency figure 101,000 BTU/hr.
66,000 BTU/hr. of that to raise 1000 gallons 40° in 5 hours. So 35,000 ,BTUs per hour are available for the loads, while the tanks are being recharged.

 

[jebatty]

Bob, the math is correct but in my experience actual performance cannot measure up because the boiler does not operate continuously at maximum rated output over the entire burn period. High burn may produce the rated output, or even exceed it a little, but mid and low burn produces much lower actual output. I think using a factor of about 70% of rated output over a burn period probably is close to average output, and then apply the boiler efficiency % to that. So rated 120,000 Btuh x 70% x 85% = 71,400 Btuh average output, or at least in that vicinity, probably is a better approximation.

 

[stee6043]

One quirk that I think a lot of people fail to plan for (I know I did) when "running the numbers" is the boiler return temp conundrum. Anytime you let the bottom of your bottom tank drop below 140 degrees you're going to be losing system output/system efficiency as your mixing valve works to maintain return temps. The further below 140 your bottom temps get, the worse your system will perform while working to bring storage temps back up.

To make the most of our storage systems we should strive to work between 190 and 140 +/-. For those of us using high-temp emitters (forced air HX, etc) this is the sweet spot anyway. The low-temp guys can work all the way down to 120, maybe 100 in some cases. But then they also have to deal with the return temp losses during recharge.

All that being said, I can typically bring my 1000 gallons back up to max temp in a 6-8 hour burn while I'm heating my home and charging storage. I'm usually 140-150 bottom temps when I restart my fire. 185 +/- is where it ends up overnight, top to bottom.

 

[Bob Rohr]

Bob, the math is correct but in my experience actual performance cannot measure up because the boiler does not operate continuously at maximum rated output over the entire burn period. High burn may produce the rated output, or even exceed it a little, but mid and low burn produces much lower actual output. I think using a factor of about 70% of rated output over a burn period probably is close to average output, and then apply the boiler efficiency % to that. So rated 120,000 Btuh x 70% x 85% = 71,400 Btuh average output, or at least in that vicinity, probably is a better approximation.

I would agree, Jim. Also piping and boiler heat loss needs to be considered. Hopefully all boiler rooms have some outside air for combustion, so not all the piping and jacket loss end up in the heated space. If your wood burner is inside the home it's getting O2 from somewhere to support combustion.
Efficiency and energy actually delivered to the load is an all but impossible number to pinpoint, assumptions need to be made. Even the lab tests, performed with lab quality instrumentation, I have seen performed leave some wiggle room in the data.

 

[jebatty]

Anytime you let the bottom of your bottom tank drop below 140 degrees you're going to be losing system output/system efficiency as your mixing valve works to maintain return temps. The further below 140 your bottom temps get, the worse your system will perform while working to bring storage temps back up.... The low-temp guys can work all the way down to 120, maybe 100 in some cases. But then they also have to deal with the return temp losses during recharge.

Yes and no. Yes because below 140F part of boiler output is heating return water, and that output is not available to the heated space, although the boiler output is still there.

But no if the entire boiler output is not needed for the heated space, then there really is no loss of system output/efficiency because the excess is going to storage, which is why storage is of benefit. One could argue that system output/efficiency actually is enhanced in this situation.

On the other hand, there are lots of other variables, like line losses, whether or not the boiler/storage are in the heated space, boiler idling episodes, and more, all of which complicate exactly where the optimum operational point may be for a particular system. With my in-floor radiant I normally run storage down to about 100-110F, and then I burn every 2nd to 3rd day during most of winter. I believe that longer burns to bring storage up to 190F, which is what I do and without any idling, is more efficient that more frequent burns to maintain a higher low point of storage temperature. This also relates to less time to tend the boiler and frees up days with no tending necessary. I do not have any hard data, however, to actually support this belief.

 

[flyingcow]

I got to chime in again. When running numbers of out put, you're figuring ideal super dry wood with no bridging.

Also, us that have unpressurized storage we don't have that much of a problem with the return temps to boiler do we? Seems my system doesn't, but i don't pay that close attention either. Also, i got the termovar loading valve on my return too. Just curious

 

[Fred61]

One quirk that I think a lot of people fail to plan for (I know I did) when "running the numbers" is the boiler return temp conundrum. Anytime you let the bottom of your bottom tank drop below 140 degrees you're going to be losing system output/system efficiency as your mixing valve works to maintain return temps. The further below 140 your bottom temps get, the worse your system will perform while working to bring storage temps back up.

To make the most of our storage systems we should strive to work between 190 and 140 +/-. For those of us using high-temp emitters (forced air HX, etc) this is the sweet spot anyway. The low-temp guys can work all the way down to 120, maybe 100 in some cases. But then they also have to deal with the return temp losses during recharge.

All that being said, I can typically bring my 1000 gallons back up to max temp in a 6-8 hour burn while I'm heating my home and charging storage. I'm usually 140-150 bottom temps when I restart my fire. 185 +/- is where it ends up overnight, top to bottom.

I only heat storage and distribute to zones from the tank. I don't very often let my storage get below 140 but that's measured at the top and I failed to place a well in the bottom of my tank and to this day I haven't decided how I want to measure the temperature so I haven't done it. I know the bottom is alot colder and the boiler protection is opening and closing but I have my pump launch temperature high enough (174*) so that the boiler temperature doesn't drop much before the circulator shuts off. In the early part of a new fire with a cold tank, the circulator cycles on and off alot. It seems to work for me.

 

[stee6043]

I got to chime in again. When running numbers of out put, you're figuring ideal super dry wood with no bridging.

Also, us that have unpressurized storage we don't have that much of a problem with the return temps to boiler do we? Seems my system doesn't, but i don't pay that close attention either. Also, i got the termovar loading valve on my return too. Just curious

Quite right, unpressurized storage will not have the return temp "system output" challenges that pressurized storage users will...

 

[rkusek]

I wish Santa would bring me a couple of Amtrol expansion tanks so I could skip my homemade expansion tank that I haven't finished yet. With this warmer December, I've only been burning when the temps fall into the 20's at night. A half load twice a week on the cooler nights keeps my well insulated pole barn in the 50's and keeps the heat pump in the house off for several hours. I don't like to let the boiler sit and idle when there is not that much load though. What really sucks is the 53 degree temps that the boiler protection valve has to "work off" in my 350'x2 of pex after everything has sat for 2 days. I wonder if that is what stee6043 is talking about? If my storage were operational that 140 return water or even 120 would keep my boiler out of idle and maximize efficiency. I wonder if I need to check out my Danfoss valve or close down the bypass more? Is it normal to go into idle for the rest of you from a cold (ie. 50 degree) start?

 

[jebatty]

Quite right, unpressurized storage will not have the return temp "system output" challenges that pressurized storage users will...

You're going to have to explain to me why this is true. For most users a goal is to keep hot water at the top of the tank and cooler water at the bottom. Are you saying that users with unpressurized storage are returning hotter water transferred from the top of their storage to their boilers so that they do not need boiler return water protection via a Termovar-type valve or loading unit? And that return water from unpressurized storage to the boiler always is 140F or higher?

All of the gasification boiler system design schematics with open storage that I have seen show that boiler output hot water goes through a copper coil in the unpressurized tank (or through a plate hx with similar function), with hot water into the top of the coil near the top of the tank and return cool water from the coil near the bottom of the tank, and a boiler return water protection device on the boiler. Hot water then stratifies at the top of the tank with cool water at the bottom of the tank. What's wrong with this picture?

 

[maple1]

Maybe it means with a cold tank, the water returning from a coil is warmer than the water returning from the bottom of a storage tank would be?

Then that would also mean the coil isn't transferring all the potential heat it could be to storage.

 

[stee6043]

You're going to have to explain to me why this is true. For most users a goal is to keep hot water at the top of the tank and cooler water at the bottom. Are you saying that users with unpressurized storage are returning hotter water transferred from the top of their storage to their boilers so that they do not need boiler return water protection via a Termovar-type valve or loading unit? And that return water from unpressurized storage to the boiler always is 140F or higher?

I guess you took it about 83 steps further than I had intended. I simply meant that those heating with a coil in an open tank will have substantially less work to "get up to temp" when the pressurized portion of their system falls below 140. Getting a pressurized coil in an unpressurized tank (holding say 10 gallons?) up to temp should take a whole lot less work than getting the bottom 250 gallons of my 1000 gallon pressurized tank up to temp. I was not suggestiing there was any difference in the need for loading valves, stratification or otherwise.

I was only talking about the time we spend with tank return temps below 140...

 

[JP11]

Maybe I'm missing the physics here. When my return temps are very low.. Like 125 or so..

The danfoss doesn't let all of the water out to storage. I can SEE this happening by looking at my digital temps that are at storage input and output elbows.

BUT.. when this is happening.. I get big delta T's. So I think the boiler is still putting out the same heat.

Example.. 24GPM with delta T of 13 to 17 degrees when return temp is above 140.

Same GPM delta T of 20 to 25 degrees when danfoss is holding back some water.

????

JP

 

[maple1]

I guess you took it about 83 steps further than I had intended. I simply meant that those heating with a coil in an open tank will have substantially less work to "get up to temp" when the pressurized portion of their system falls below 140. Getting a pressurized coil in an unpressurized tank (holding say 10 gallons?) up to temp should take a whole lot less work than getting the bottom 250 gallons of my 1000 gallon pressurized tank up to temp. I was not suggestiing there was any difference in the need for loading valves, stratification or otherwise.

I was only talking about the time we spend with tank return temps below 140...

But - if the coil is doing it's job good, and even if it did only hold 10 gallons, you wouldn't see much of a rise in coil return temps until the bottom storage tank water got up to temp. Would you?

(No experience here with storage tank coils).

 

[jebatty]

I simply meant that those heating with a coil in an open tank will have substantially less work to "get up to temp" when the pressurized portion of their system falls below 140. Getting a pressurized coil in an unpressurized tank (holding say 10 gallons?) up to temp should take a whole lot less work than getting the bottom 250 gallons of my 1000 gallon pressurized tank up to temp.

Then we are talking about completely different things. I had in mind an unpressurized storage tank, easily 1000 gallons, rather than an unpressurized "I'm not sure what" 10 gallon tank; perhaps just and expansion tank? A pressurized, or unpressurized, 1000 gallon storage tank, properly plumbed, will rapidly flow hot 160F+ water into the top of the tank to meet system demands, even while the lower portion of the tank is much cooler.

I see no difference between unpressurized storage and pressurized storage with regard to return temp "system output" challenges.

 

[maple1]

Then we are talking about completely different things. I had in mind an unpressurized storage tank, easily 1000 gallons, rather than an unpressurized "I'm not sure what" 10 gallon tank; perhaps just and expansion tank? A pressurized, or unpressurized, 1000 gallon storage tank, properly plumbed, will rapidly flow hot 160F+ water into the top of the tank to meet system demands, even while the lower portion of the tank is much cooler.

I see no difference between unpressurized storage and pressurized storage with regard to return temp "system output" challenges.

I thought the 10 gallons mentioned was w.r.t just the volume of the coil itself.

 

[Fred61]

I'm very confused

 

[Nofossil]

Wow - freshman thermodynamics all over again, in a single thread! In my experience, my boiler can be counted on to produce an average output over the course of a fire of about 75% of the rated output. My 80,000 BTU EKO is good for about 60,000 BTU on average, although it does reach 80,000 BTU or more when in full 'afterburner' mode.

Regardless of tempering valves, storage, zone demand, inlet temperature, or anything else, that heat is going somewhere. Only two choices: raising the boiler temperature, or heating the water that's passing through. Once the boiler is up to temp, it's ALL going into the water passing through. The temperature rise through the boiler depends on the flow rate through the boiler, which in my case varies depending on how many zones are open. I run 25 to 30 degree rise through my boiler for most of the fire.

Here's where it gets interesting. I have unpressurized storage with an immersed copper coil. As storage warms up, the return temperature from the storage coil starts to rise. Eventually it gets to around 160, at which point the boiler outlet will exceed 180 if the boiler is still running anywhere near normal output. This makes the boiler go into idle. For that reason, it's really hard to get storage anywhere near 180 in my case.

To answer the original question, I can get my 880 gallons from 120 to 160 in 4 or 5 hours if I have no other loads. Anything above that is a balancing act of matching the fire's burnout to the heat transfer available into storage. At the end of the day, I'll idle a few times towards the end of the fire as storage gets hot.

 

[goosegunner]

I think what he is trying to say is; With a coil in a tank it will only give up so much heat. If a coil is designed to give up 20 degrees would the return temps stay higher than bottom of the tank temp?

Boiler supply 160
Coil Delta T 20
Boiler return 140

The tank could be below 140 on the bottom but depending on the flow rate, the water in coil might not drop to the bottom of tank temp.

vs

Pressurized storage,

Boiler supply 160
tank bottom = return temp

Obviously it is not that simple depending on coil design and flow rate but it would be a possibility.

gg

 

[stee6043]

I thought the 10 gallons mentioned was w.r.t just the volume of the coil itself.

Yupper, this is what I was talking about.

It seems we're having four different conversations at the same time in this thread!

 

[mole]

In the summer i heat my 1250 gallon tank from about 115 to about 175 in 8 hours.

 

[jebatty]

Nofo's description is very good. It is all about freshman thermodynamics.

Anytime a heat exchanger, coil or plate, is used to heat a storage tank, it is nearly impossible to achieve an output temperature from the heat exchanger which is equal to the boiler supply temperature. The heat exchanger output temperature will be less than the boiler supply temperature. This difference is called approach temperature. A somewhat standard design results in a "normal" approach temperature of 10F; that is, if boiler supply is 185F then output from the heat exchanger will be 175F. This also means that the maximum temperature that can be reached in a storage tank when using a heat exchanger will be about 10F less that the maximum boiler supply temperature. It is possible to design a heat exchanger to obtain a closer approach temperature than 10F, and of course the cost to do this also goes up. I am well aware of a system using a Garn WHS3200 that has an approach temperature of 5F while moving about 70 gpm through a plate hx.

A heat exchanger usually is required when using open storage, and one is not usually required when using pressurized storage. This difference means that it is possible to obtain maximum temperature in a storage tank about equal to the maximum boiler supply temperature. This can be important, especially in systems which require high temperature supply water, and it also is important because it can extend the time between boiler firings by having a greater quantity of hot water available from storage.

Through my several posts on weighed wood burns I have demonstrated with pressurized 1000 gallon storage that it is possible, even quite easy, to load my storage tank to 193F water, top to bottom, and 193F being about the maximum supply temperature of my Tarm before the controller shuts it down to prevent overheat, and moreover to accomplish this without any boiler idling.