Thermal time loss on water storage

[a31ford]

Was wondering what would be the calculation for a water storage tank's thermal loss rate, based on the below.

1) 160 gal.imperial.
2) water at a temp of 150-f.
3) galv. steel tank with "R-60" insulation (all sides, top & bottom).
4) surounding air temp of 45-f.

How long before the water is at 80-f ?

 

[stevenw]

160 gallons = 21.4 cubic feet of h2o

Approximate size of tank
2.8 feet x 2.8 feet x 2.8 feet = 21.95 cubic feet

Surface area of insulation
2.8 feet x 2.8 feet x 6 sides = 47.04 square feet

Is this this correct so far?

 

[a31ford]

Yes, I would have to assume so, as when it comes to thiese things I'm quite dumb... Now get me behind a down Server...

 

[PacificSteve]

The answer: a long long time. R-60 is a whole lot of insulation. This would be equal to about 18 inches of fiberglass.

Are you sure this is the right number? If so you would lose far less than 1 deg f per hour at the start, and as the tank cooled, the loss rate would lessen.

Probably, you would lose more heat through less thoroughly insulated attached piping and gauges.

PS

 

[a31ford]

Thank you, for picking up the thread...

The 1 deg f per hour, is this based on an external temp of 45 f ?? Would it be the same, based on minus 10 f ?

The system will be a thermal reserve for a hydronic heating system. (wood fired) and the Idea is to NOT "smolder" the fire, rather 2 or 3 complete burns a day, and use the stored heat as the demand requires (3000 sq ft of heated space)

And yes, R60 (was a lot of work to do it)

 

[PacificSteve]

OK, I wasn't sure if this thread was still alive, so here is a little more info:
1) Actual calc on a cylindrical tank was based only on conductive through the shell losses, and was .1 deg F per hour at the onset. So with some allowance for radiative losses, and convective losses, I still feel comfortable with saying less than 1degF is safe.
2) If your tank is outside, in the wind, the losses could be much higher. In an outside shed would replicate what I did.
3) I guess you will find out real soon as winter sets in. We are getting for winter in Hawaii. It gets so cold here that you have to close the window at night.
4) I hope your house is well insulated and tight, since my gut says this is not a huge about of BTU storage, say 80kBTU assuming a "working temp of 80F in your heat exchanger". at three fired per day, 240kbtu, or roughly the same output as 2 gallons of heating oil. Can you get by with 2 gal of heating oil per day?
5) The losses at -10F would be proportional to the difference between the storage temp and the ambient, with perhaps a little extra loss due to increased convective effect, as well as radiative. In other words, still less than 1F or therabouts.
6) You will likely have greater losses through attached piping, and what everyone seems to call "thermal siphoning" meaning that hot water rises to the highest point of your system, i.e. your water will be moving around whether you like it or not. Solenoid activated or motorixed valves can mitigate this if it is a real problem.
7) Get ready to buy another tank, the R60 is probably overkill, and a pre0insulated R30 would probably be the best economic choice, based on gut.

 

[a31ford]

Thank you !, sorry it's taken so long to reply, as I've been busy with work. The "r60 thing" is an approx. as we have insulated the tank ourselves. The operating temp. is 140f, all pipes & such are r12 at least, and yes, the tank is in a shed (with the "boiler&quot(wood fired heater). I came up with some numbers as follows: 140f for high side of contents, 100f when system demands to be heated (tank to boiler), there are 3 "circuits" of pumping going on, and one is "tank to boiler" only (with 4way valve, so it acts like a mixing system, when not in boiler heat demand). The thermal drop across the fan-coils and domestic side arm, should be roughly 5f at any given time (except the coldest days, they should be around the 10f mark). Given this, I am of the assumption that the whole thing should work within the 2-3 firings a day thing. (yes the house & shop are well insulated, this is Canada you know... lol! having to close the window at night ?? we NEVER dare open them during the winter ! minus 40f !!
Greg