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I often select makeup air handling units for buildings using Greenheck's CAPS tool. When selecting a model, you first select the unit based on your airflow needs. Then it will provide some choices for burners which it gives in units of Btus per hour. It is my understanding that this is a function of the unit to deliver that amount of heat over a period of time based on the volumetric flow that was selected. This would generally follow the rule of thumb for air of approximately: $$\dot{Q}_{Btu/hr}=1.08\cdot\dot{V}_{CFM}\cdot\Delta T_{^{\circ}F}$$

Does this mean that when looking at the burner selection you would have to adjust for a lower air flow rate and subsequently less output or does the temperature delta change accordingly that the heat output for a given burner would still be effectively the same even at the reduced airflow? On any of the software packages, I can't change the flow rate AFTER selecting a unit to see how the numbers change so that I could back into the answer to check for myself and if I simply pick a lower air flow rate, it gives me entirely different burner choices.

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  • $\begingroup$ Sounds like you're trying to use a tool for purpose it's not designed for. Step back and look at the physics of heating air - simply flow rate * heat capacity * delta T = power. (In other words, if you keep power constant and change flow rate, delta-T will change). Easy to do in a spreadsheet if you can't find better tools. (Easier still in SI units - convert back to steam age units afterwards if you have to). Once you know what you're looking for, come back to the burner chooser tool. $\endgroup$ – Brian Drummond May 20 '17 at 9:30
  • $\begingroup$ I guess my question was more related to how burners are specified. Is the power output of the burner constant and you will end up with a larger delta T at lower airflow or are the burners usually sized such that you would not be able to use the maximum output of the burner when cutting the airflow in half? $\endgroup$ – Secundus May 20 '17 at 15:17
  • $\begingroup$ If the burner is not directly consuming some of the air you need to deliver to the use point, then the burner performance will not be affected by a smaller air flow; the same power output into a smaller volume of air will lead to a larger delta T for the air. $\endgroup$ – J. Ari May 25 '17 at 12:49
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I went back and worked through it, and I was somewhat right. If you request the manufacturer to maximize the burner, you may get a different size burner even when the base unit is the same because typical units are temperature limited. So by reducing the airflow in half, you would double your delta T for a given input, to balance the equations, but the actual selections will not match simply because of manufacturer limits in place to prevent damage to the units.

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  • $\begingroup$ This makes sense. I had only just seen this question now; but I would have suggested going to the supplier and just asking (as you seem to have done). Odds are you could find a salesperson more than willing to either explain it, or look into it and get you an answer. $\endgroup$ – JMac May 25 '17 at 11:18

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