Heat Loss From Storage

[maple1]

I could go 4 days at the longest doing dhw in the summer with my 660 gallons. That was going from real hot (190/175) to around 135/125.

 

[huffdawg]

[quo

I suppose. Google heat transfer and see if you can find this equation anywhere other than "rough guess" home builder threads. This is not a formula engineers would use to estimate or calculate heat loss.

I'd also value some real world feedback from folks that have actually observed their tanks in a static condition. My numbers may seem appalling but I haven't seen any other "real measurements" here so far, spray foam or otherwise.

I will try and get you some after I get home , depends on the outside temps if it is fairly mild I should be able to get a reading for 4 or 5 hrs of a static storage tk temps.

 

[Floydian]

This is not a formula engineers would use to estimate or calculate heat loss.

Apparently it is. I just flipped open my Modern Hydronic Heating, 2nd edition by John Siegenthaler, P.E.

In chapter 2, on page 19 he clearly gives this formula for estimating heating loads: Q=A/R x T
where:
Q=rate of heat transfer through the material(Btu/hr)
T= temp differential across the material (deg F)
R=R-value of the material (deg F*ft sq*hr/Btu)
A=area across which heat flows (ft sq)

If THE MAN says its OK to use.....

Noah

 

[ewdudley]

If THE MAN says its OK to use.....

With all due respect, 'The Man' would be Dr. Carrier.

 

[ewdudley]

This is not a formula engineers would use to estimate or calculate heat loss.

Quite true. Engineers use U-value.

I'd also value some real world feedback from folks that have actually observed their tanks in a static condition.

Here's one data point: For modeling purposes I'm calling my three 250 gallon tanks a 30" x 90" x 90" box full of hot water. With 2" cellulose, 2" styrofoam, and 6" fiberglass bats, R-value should be about 40. Calculated temperature drop in summer when drawing for DHW comes out to 2 degF per day. I see a little less than that, about 900 milli-kelvins according to the upper tank sensors, which I chalk up to the interstitial cellulose.

 

[stee6043]

Oiy. There is clearly no way I'm going to win this one. I'm a gluten for punishment I guess.

Is there not a mechanical engineer in the boiler room that has had to suffer through a semester of a course called "heat transfer" in their fourth of fifth year? What a waste if you could have plugged the numbers into the formula above and be done with it. Poof - 4 credits earned.

Even "THE MAN" above is using a formula intended to provide gross approximations under a very limited set of conditions. An engineer not interested in rough approximations intent on using proper engineering principles to evaluate real or "very close" heat transfer across two mediums will never be able to solve for heat transfer with anything other than a differential equation (Differential Equations & Laplace Transforms = Calc 3 iirc).

I fully appreciate there are easy ways to approximate many, many things. This thread is a great example of that. I'm not saying THE MAN is wrong. I'm saying (and have been) that there are flaws in this approach even though it may be useful X percent of the time under Y conditions.

Let us consider this - the rate of heat transfer between two mediums will change as the difference in temperature changes. I assume we can all agree on that. As such, how would you solve for heat loss with an equation that doesn't account for a changing rate of loss as the two mediums become closer (or further apart) in temperature? Rates of heat transfer are not linear. The formula being tossed about in this thread with reckless abandon would provide a beautifully straight line, if graphed. Yes, the straight line might provide a very nice approximation, however.

I'm still waiting on someone to show me they are losing less than 10 degrees per day with their hyper insulated tanks too. I'd love a reason to break mine down and spring for spray foam. I really felt like my 10 degrees was pretty good. You guys have some pretty high standards!

 

[stee6043]

Quite true. Engineers use U-value.




Here's one data point: For modeling purposes I'm calling my three 250 gallon tanks a 30" x 90" x 90" box full of hot water. With 2" cellulose, 2" styrofoam, and 6" fiberglass bats, R-value should be about 40. Calculated temperature drop in summer when drawing for DHW comes out to 2 degF per day. I see a little less than that, about 900 milli-kelvins according to the upper tank sensors, which I chalk up to the interstitial cellulose.

What do you mean by "calculated" temperature drop? Are you actually measuring less than 2 degrees per day? That would be impressive (to me at least). My 10 degree # was nothing more than noting my tank temps (top, 2 middles, bottom) and returning 24 hours later to see the change on a day I had no call for heat, boiler not running. I used zero math to come up with my nice and tidy 10 degree estimate.

 

[ewdudley]

What do you mean by "calculated" temperature drop? Are you actually measuring less than 2 degrees per day?

I mean calculated according to the formula area times deltaT divided by R-value. Instead of trying to model three vertical tanks I just substituted a box big enough to hold the three tanks. According to the simplified model temperature drop would be 1.98 degF per day. What I actually observe in the summer is 0.9 degC per day, or 1.6 degF, which is not too surprising since the simplified model ignores all the interstitial cellulose.

For 750 gallons that's about 10,000 btu per day, which is about one third again as much as the 30,000 btu of heat needed for DHW, not counting heat loss in the piping to get the heat to the reverse indirect heater tank.

 

[ewdudley]

Let us consider this - the rate of heat transfer between two mediums will change as the difference in temperature changes. I assume we can all agree on that. As such, how would you solve for heat loss with an equation that doesn't account for a changing rate of loss as the two mediums become closer (or further apart) in temperature? Rates of heat transfer are not linear. The formula being tossed about in this thread with reckless abandon would provide a beautifully straight line, if graphed.

Yes the rate of temperature drop will of course decrease as the days go by. In situations where temperature drop needs to be taken into account I use a program that iterates a second's worth of temperature drop each time around, feed the new temperature into the next loop, 86,400 loops per day of simulation, and call it close enough. I just can't get used to how fast today's computers are.

 

[jebatty]

Is there not a mechanical engineer in the boiler room that has had to suffer through a semester of a course called "heat transfer" in their fourth of fifth year?

I was holding back, but no more. I'm the mmaann! Heat transfer occurs rapidly from my wife to me via the ether. That's it, no more, no less. Sorry it took so long to add heat to the boiler room.

 

[Spartan]

I was holding back, but no more. I'm the mmaann! Heat transfer occurs rapidly from my wife to me via the ether. That's it, no more, no less. Sorry it took so long to add heat to the boiler room.

No....NO....NOOOOOO You don't want to go there. Nope. Not unless you want me to be banned.

Right now, yet another unusual -22 C weather and my wife complains about lowering the thermostat 3 degrees. And my usual answer is?

(It has to do with body part and creating friction!)

 

[huffdawg]

Yep women usually get what they want.....

 

[heaterman]

I get quite a bang out of the hair splitting on this site.

Here we are debating the finer points of heat loss/transfer on a forum devoted to burning WOOD.....
I don't see this much passion about saving, utilizing, conserving energy on a lot of the blogs or sites inhabited by gobs of HVAC professionals, engineers, and system control people.
If the mainstream folks would get with the program like the people here do all energy problems, real or perceived would disappear.

I love it.

 

[BrotherBart]

I don't see this much passion about saving, utilizing, conserving energy on a lot of the blogs or sites inhabited by gobs of HVAC professionals, engineers, and system control people.

I used to drop in on some of them. All that goes on is people in the profession stomping on DIYers and Harry Homeowners.

 

[Spartan]

I get quite a bang out of the hair splitting on this site.

Here we are debating the finer points of heat loss/transfer on a forum devoted to burning WOOD.....
I don't see this much passion about saving, utilizing, conserving energy on a lot of the blogs or sites inhabited by gobs of HVAC professionals, engineers, and system control people.
If the mainstream folks would get with the program like the people here do all energy problems, real or perceived would disappear.

I love it.

'Cause they ain't doing the chopping and hauling.

I use to burn 3 tons a season and even if the wood was free, that was a lot of work. Plus it was handled...once at work to squirrel away, once to cut it down, once to get it home, once to put it away, once on the stove....and the ashes. That's now 15 tons of handling. Probably sunk 150+++ hours through the entire cycle.

Every btu not wasted is time and money not wasted.....

 

[Newvigasowner]

I think I might have more heat loss out of my flue than I lose from my storage. Spartan mentioned he had been monitoring my system temps through my public display link and he noticed that when my boiler was at idle my flue temps mirrored my storage top temps. Has anyone else had this problem. I'm guessing its quite a significant amount.

Huff

I was wondering the same thing, as i see my boiler temp and stack temp the same when my boiler is finished it's cycle . How much cold air is coming down flue and cooling boiler water and making it's way to storage tanks next to boiler ... Maybe not much?

 

[huffdawg]

I was wondering the same thing, as i see my boiler temp and stack temp the same when my boiler is finished it's cycle . How much cold air is coming down flue and cooling boiler water and making it's way to storage tanks next to boiler ... Maybe not much?

With mine cold air would come in through the fresh air vent through the primary and secondary openings and then up fire tubes to the flue. also I think the boiler must thermosyphon because it stays the same temp as the storage also.

 

[maple1]

Mine goes cold after the fire goes out. Well, not exactly cold, but drops pretty close to room temp.

 

[jeffesonm]

Thanks, this is helpful data for someone planning their setup.

 

[hobbyheater]

1,046 imperial gallons of storage in no draw situation .At 195F to 120 F takes 20 days .,I believe that is a heat lose of around 1,000 to 1,200 Btu,s per hour !lose .