I used siggy's equation

MikeJ

to come up with the buffer tank size, that equation does not compestate for the boiler modulating. I would think if any one had a equation to compensate for the modulating. It would be siggy. I have some his books but could not fine a equation for it. What I want to do it calls for a 80 gallon tank according to the equation. But by gut feeling (plus NRT_Rob post) is saying a 30 would suffice,

As far a buffer tank size, too big shouldn’t be a problem.

Plus with the Ramp delay, gradient limiting. You would think you could get a min 10 minute run time with a 30 gallon buffer tank.

CC.Rob

try these

Just rearrange the terms. Play with them in Excel.



To get a runtime based on various inputs, use



t = (Vbt*500*dT) / (Qh - qload)



To get the input (modulation) required for a given load, runtime, dT, and buffer size, use



Qh = ( (Vbt*500*dT) / t ) + qload



For example, you want to know the modulation that gives a 10 minute runtime with a 30 gal buffer, 20 dT and load of 10000 BTU/hr.



Qh = ((30*500*20)/10) + 10000



Qh = 40,000 BTU/hr



Say you want to double the runtime. Now the modulation is down to 25k BTU/hr.



There's not a simple way I can think of to model a continuously modulating input, but do enough playing in excel and you will see what changing the variables does and hopefully find your answer.