Condensate & high return temps

PS

Just a homeowner so please be kind.

On condensing HW gas boilers - what happens when your HWR temp is higher than the condensing effect high limit? Does the water bypass the condensing heat exchanger (if it is seperate?) or does it simply flow through to HWS heat exchanger without picking up flue gas energy? I haven't been able to determine if there are 1 or 2 heat exchangers (one extracting heat from flue gas and one transfering heat from gas burner), or does it differ between manufacturers?

Also, recent posts mention damaging condensate (acidic?) from these boilers. How much condensate is released, rule of thumb (I realize it varies) and what methods are available for disposal (I do not have floor drains).

Thanks in advance.

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You dispose of condensate by sending it down a drain...if there is no nearby drain, you use a condensate pump and nonmetallic (plastic) tubing to send it to an utility tub or other suitable place to drain it. It is somewhat acidic but it doesn't hurt to send it down the drain if you have a municipal sewer system. But if the condensate would end up somewhere else, it may possibly be a problem.

The boiler doesn't send the return water elsewhere if it's too hot; the process remains the same, but it doesn't get as much energy back out of the flue gases if there is no condensation. There is no sharp cutoff anyway; the magic number of 135F or whatever is a simplification. As you return temperature increases, there is less and less condensation and so the efficiency drops gradually. You need to be below about 100-110F for maximum efficiency.

As for how much condensate, not a whole lot. There is a value for how many pints per therm of nat gas, assuming full condensation, but I don't have it, maybe someone else does.

PS

Thanks - I understand how the condensing boilers gain their extra efficiency. I guess I was more interested in the mechanism and process. Seems to me you would need 2 heat exchangers for a condensing boiler - one just like every other boiler (gas burner heat transfer) and another to extract the heat from the flue gas, prior to cycling HWR back to the burner HE. I can't verify short of looking at every manufacturer's IOM manual - looking to understand.

Constantin

several answers...

You are correct, most boiler designs offer multiple heat exchangers to achieve condensation, though some use just one to get the job done. The Monitor FCX and the Dunkirk Quantum Leap have two distinct HX's, for example, while the Viessmann Vitodens does it all in one.

If the return water temperature is too high, most of the condensate will leave uncondensed via the exhaust - and condense outdoors. Under ideal conditions, you will condense about 1.15 gallons per Therm of gas used.

The condensate from a gas boiler/furnace is somewhat acidic (ph ~4), while the stuff coming out of a condensing oil boiler is pretty nasty (ph ~2). The latter has a lot to do with allowable sulfur contamination in the fuel. Either condensate should not be sent directly into cast iron piping, as it will rot them out quickly.

All manufacturers sell kits that allow you to treat the condensate water with marble chips, lime, whatever to become neutral and hence safe for septic systems, cast iron pipes, etc. Some Wallies simply allow the condensate to run through a 5 gallon bucket filled with marble-chips before lifting it to an overhead sewer line with a condensate pump. Such pumps are frequently used to keep AC systems happy, for example.