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Posted by HeatPro on May 13, 2008 at 13:12:42:
In Reply to: Re: Is historical usage data better than Manual-J for sizing boilers? posted by dana on May 12, 2008 at 18:21:31:
Blower coils are usually rated at a high temperature to deliver heat with at least a 30-degree drop across the coil, so they are doing what they do. The variation in the water delivery temperature is not to their design.
The efficiency of an indirect depends on the 'slope' of the hot water on one side and the water to be heated. It is part of the construction. The final efficiency depends on the losses of the heat from the connecting tubing to and from the boiler and the quality of the insulation around the tank, so operating temperatures have less to do with it. The idea is to have well-insulated tubing and tank and whether it takes a shorter time with high water temperatures or a longer time with lower temperatures to heat the content.
Hot water makers are rated in a different test from AFUE house-heater tests. They are not equivalent.
It is not worthwhile to assume stack temperatures as exchanger design and atmospheric vs power burner operations make a great difference. That's the reason for testing, to no longer assume.
A buffer tank would be a good way to lengthen boiler cycles, provide a constant temperature for the blower coils and give known outlet temperatures for varied uses, such as a tempering valve to provide hot water at a lower temperature.
The blower coils complicate the matter of condensing boiler savings as they are designed for high temperatures; though they can be used for lower temperatures by being coils twice as large as necessary for the needed output. I am amused to recall a consultation where blower coils were completely misdesigned in a warehouse, yet worked by being oversized.
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