Heat Loss Calculations--Does this sound right?

While I continue to think about how I will replace my beloved Tarm Solo Plus 40, I am wanting to track my BTU needs by observing heat loss from my storage tank, and I just wanted to make sure that I was not missing anything in my calculations.

So... I have an 850 gal., non-pressurized heat sink. This morning at 11:00 am the top temp. was 138.4°F. Two hours later it was 128.3°F. (Outside temp was fairly warm, rising over the same time from 15°F to 24°F.)

So a total loss of 10.1°F over two hours. 850 gallons of water weighs 7089lbs. So BTU loss over 2 hours is 7089 x 10.1 = 71,599 or 35,899 per hour.

I fully understand that a two hour test does not tell you much. But is my thinking and my math correct? (For the record, the boiler was not circulating, as it was 'cold' after finishing its burn from the last night.)

My intention is to try and do a number of these observations, seeing how outside temps affect the BTU draw. It is a bit challenging, as the timing does not always work out to delay firing the boiler up while I take readings. But it strikes me as the most accurate way to see what my actual BTU demands currently are.

Thoughts?

Thanks!

Barrett

chuck172 said:

I don't believe your entire tank (850 gallons) dropped 10.1* in temperature. Just the top layer.

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The bottom layer (I have top, middle and bottom sensors) dropped from 126.5°F to 117.5°F. I did not record the middle layer, but they were all pretty much in sync.

As to actively heating, a good question. I believe it was, but that is my gut, not tested. Mind you, 'actively heating' at 15°F is a different challenge than actively heating at -40°F!

hobbyheater said:

Have you thought of approaching this from a different direction ?

You have a very good boiler that has a lot more BTU output potential that what your tank can store. If you have the space, add another tank or replace your current tank with another one double the size or even larger. Putting in a larger tank may be cheaper as you already have all the coils.

Just a thought.

http://www.americansolartechnics.com/ A source for tanks.

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Thanks for your thoughts, Allan. I am looking at the American Solar Technics tanks--or more likely building my own tank, as I could fit close to 1500 gallons in the space that I have if I make my own. American Solar Technics will make custom liners or I can apply SaniTred to a plywood 'box' tank. (Check them out--they appear to offer an excellent product: http://www.sanitred.com/.)

As for the BTU output potential, undoubtedly true, except when the real winter arrives. This year has been quite mild by comparison to most--but there are still at least 3 months of serious cold possible. It is not unusual to have 3-4 weeks of temps not going above -30°F and as low as -50°F (or more). When we hit 0°F or lower, I cannot get enough heat out of the Tarm to prevent the oil boiler from needing to come on before I wake up in the morning or return home from work. A typical scenario would involve loading the firebox when I get home (say at 6pm). The storage tank might show 120F (kept there by the oil boiler). By 9:30 most of the 6pm load has been consumed and I re-load the firebox. The heatsink is showing perhaps 135F. I show up again at 6am the next morning. Heatsink is 120F again, with some likelihood (depending on the outside temp, of course) that the oil boiler has had to cycle on once or twice.

I have not been able monitor closely enough over time when it is cold to see how a high heat demand is drawing down the heatsink, but it is pretty clear that the cold weather demand is only slightly less than what the Tarm is able to put out. This would be OK if the Tarm had a bigger firebox, perhaps. My 'gut' again is that I need only slightly more BTUs--but with a bigger firebox and a larger storage tank.

(I should add that this issue developed after I added about 1200 sq.ft. of heated space to what the Tarm was originally sized to heat.)

My thoughts, for what they are worth.

hobbyheater said:

This is a question outside the thread of your posted topic, but of interest to me.
Is perma-frost an issue in construction where you live? The reason I'm asking is that we have friends that live in Norman Wells and heavy boilers and storage tanks create some unique problems in construction .

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Permafrost is not an issue in Whitehorse. There are pockets around, but serious permafrost begins about an hour or two drive's north. In Dawson City they face real issues with melting permafrost, leaving buildings and infrastructure sagging and sinking.

Barrett

Hunderliggur said:

Are you sure you need more storage? It looks like you are using 8*850*(135-120)=100K BTU about from storage. If you brought the storage up to 150 you would double your stored capacity. I don't know how high your storage will go, I can get mine to about 185 (but that is a steel tank). I expect the lined tanks could handle up to about 180 without problem, particularly since you are not looking at long term 180F storage. Raising it to 180 is 4 times the stored energy you started with. More BTUs from the boiler - most likely. More storage - don't think it would help. Or am I missing something with your operating temps?

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Appreciate your points. In fact, my old EPDM-lined tank is vulnerable to degrading if the heat gets above 175F. The current liners from American Solartechnics have no problems going above 200F and the Sani-Tred coating is rated to 250F.

As I see it, there are two issues--the amount of BTUs I can store and the ability to load those BTUs into the storage tank without needing to be reloading the boiler every 3-4 hours.

Barrett