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Posted by dana on May 21, 2008 at 15:43:53:
In Reply to: Re: Is historical usage data better than Manual-J for sizing boilers? posted by HeatPro on May 13, 2008 at 13:12:42:
: 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.
After mulling it over a bit I bumpe into an ebay deal on an ErgoMax E45 to use as a buffer tank (& for domestic hot water, retiring the 15 year old on demand system than never modulated well). It's on the truck- we'll see what kind of shape it's in when it gets here. (It's allegedly new, undamaged.)
I hope to plumb it in before next winter but I'm deferring replacing the boiler with a modulating version until after I've installed the other low-temperature zones.
A couple of issues: Even with buffer tank as low as 140F I'm pretty sure I can get enough heat out of the fan coil running it off the tank instead of a separate zone directly off the boiler. (Remember, the system is about 3x+ oversized for design day.) But since I already have one on the shelf, I may use an older Honeywell T775 to control the buffer tank temperature in an outdoor reset mode, maybe bumping it as high as 170F when it's wicked-cold out, so the return water temp will likely vary quite a bit based on the weather.
At 150F there's about a 20F drop from the hotcold side of the fan coil which puts the return water at near-condensing, and at 140F it'll for-sure be in a condensing range. I'm assuming no matter what I'll be adding a boiler-bypass to protect the boiler from condensing too much, but rather than hand-adjusting the flow-balance with gate valves, is there a reasonable method to do that with tempering/mixing valves for better protection control, yet adjusted downward for maximum efficiency? If the tank temp can vary by as much as 30-40F or more with an outdoor reset it seems like the optimal boiler-bypass would also be varying quite considerably, particularly when the air-handler is running.
In a year or so when I get around to installing additional radiant zones & retiring the air-coil I'd reprogram or replace the T775 for a more appropriate control loop to max out the efficiency of whatever heat source gets installed. (I'm liking the flexibility a buffer tank affords- it's generic, largely decoupling the load control from the boiler control, so the boiler only serves one master, the tank.)
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