storage btu capacity rough calculation?

[pybyr]

Hello all-- I'm no engineer or HVAC pro, although I'm try to learn as much as I can--

can you all confirm if I am on the right track in trying to get a rough idea of how many BTUs a given size water storage will hold (setting aside losses)

assumptions...

One BTU= heat to raise a pound of water 1 degree Farenheit -- means 8 BTUs raise a gallon of water one degree

then, assuming a maximum tank temp of 180 deg F and a minimum useful tank temp of 120 degree F = 60 degree swing of tank
then, assmuing a 1000 gallon tank
8 BTU/ gallon-degree * 1000 gallons * 60 degree swing
= 480,000 BTUs stored

am I on the right track here or have I committed some silly oversight?

Thanks

 

[free75degrees]

Looks correct, but you should probably use 8.34 as the number of pounds per gallon of water. That would give you just over 500k BTUs.

 

[TMonter]

Just remember that at sea level, water at 1 atmosphere holds 1 Btu/lb per degree of temperature. Your expected temperature difference between your supply and return should give you your usable energy (minus losses of course).

Your calcs look basically right but as pointed out water is 8.34 lb/gallon.

 

[Eric Johnson]

I'm always amazed at the extra usable btus you get when you have infloor radiant heat, simply because you can get by with much cooler water. It's all in the math, I guess.

 

[Nofossil]

I'm always amazed at the extra usable btus you get when you have infloor radiant heat, simply because you can get by with much cooler water. It's all in the math, I guess.

Yeah - number one advice to anyone building or remodeling: Install radiant heat if you can. Just about doubles the effective heat storage capacity.

 

[TMonter]

Most hot water hydronic floor heat is between 90 and 150 F isn't it? Never done much work with floor heating systems, just large industrial and commercial hot water heating systems.

 

[mtnmizer]

How do you calculate the storage capacity of radiant in
concrete? I understand water capacity, not sure
about radiant in concrete. MM

 

[TMonter]

I don't think there will be a large benefit in calculating the storage capacity of the concrete. It takes some to heat it, but once it's hot all the energy above ambient you would get back over time. In this case unless you have a very large concrete mass the storage medium is really the water.

 

[pybyr]

Anyone happen to know what's the minimum useful temperature for purposes of water going to a coil in a forced warm air system?

While I may someday be able to retrofit under-floor radiant on the first floor, for the near-term, my plan is to add a coil into the existing ductwork of the existing heating system

Thanks

 

[TMonter]

My experience is 130 to 140 degrees minimum temperature. 160-180 is better.

 

[SteveT]

It takes some to heat it, but once it's hot all the energy above ambient you would get back over time.

True statement. But it is true for any material (2nd law of thermodynamics).

Concrete has a heat capacity of ~24% that of water. So the calculation can be made the same way with this correction factor.