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A Condensing Pre-heat DHW Economizer?

Not sure if this is the right terminology however. Since the topic of the plate & frame heat exchangers as a pre-heat came up (I see Brad White's Logic as the water coming into the exchanger assuming it's flowing will be much cooler compared to diluted tank water), I thought up, however it's probably not a new idea, but perhaps something that no one has yet to do...

Does anyone make, or know of a coil that you might put in the exhaust of a condensing boiler, perhaps made out of stainless steel for obvious reasons, that domestic hot water would flow through as a sort of "pre-heat" before it goes into an indirect (or heat exchanger). Basically as the (condensing) boiler ramps up to 160-180 degrees to heat your indirect, and all that latent heat is going up your flue because it's generally not condensing or condensing very much. If there was a pre-heat coil in the flue or perhaps some kind of adapter, where 50-80 degree domestic water was present, that latent heat would condense like crazy on this coil or flue to water heat exchanger and perhaps allow some "condensing" in non-condensing mode.


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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Sounds pretty crazy to me...

    Yeah, I did it.

    I used a GFX copper heat exchanger usually used for drain waste heat recovery, and stuck it on the exhaust stack of my 80K modcon, and piped the incoming cold water feed for the DHW system into the shell side of the heat exchanger. I was worried about the acidic condensate eating a hole through the copper DWV pipe. More on that in a minute.

    The whole shebang would typically preheat the water 30 to 40 degrees F depending upon flow and other variables, and it increased the efficiency of the boiler by 5 points (from 86% to 91% on high fire at 180 degree water temp).

    I ran it this way for two years, and finally chickened out and pulled it out for placement as a true waste heat recovery heat exchanger at Hydronicahh. Upon inspection, there was NO signs of acid attack on the inside of the copper DWV pipe. None.

    I had it set up so that the condensate would not flow back into the heat exchanger of the boiler just to avoid the possibility of copper causing corrosion in the modcons HXer. I had a drip tee on the end, and sent the condensate it generated to my neutralizer.

    The GFX unit was 5 feet of 3" copper with 1/2" copper tightly wound around the outside of the 3" DWV.

    I like Brads idea of putting a flat plate heat exchanger into the boiler return and piping it in series with the incoming cold water supply to the DHW system. I have some "spare" FP heat exchangers lying around up at Hydronicahh. Maybe I will throw one into the Knight I am fixing to install up there this weekend :-)


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  • Jack
    Jack Member Posts: 1,047
    Used to build these for wood stoves

    6" smoke pipe inside, made a 6" dia wood shaft about 18" long. Router with 7/8" round bit to route spiral channel. Fill 3/4 soft copper with fine sand, Bend over ends and wind it into the channel. Fight like a fool to get it off (don't rout to deep...brilliant) bend the ends to penentrate the outer sleeve which you then install, and before you know it, Bob's yer uncle! 

    What is your run time on the boiler? Average btuH input during that time. In your app, I don't think it is worth doing. However, given the frequency of how often I am wrong, and if you have a strong desire to try it, let us know how it works out for you. 
  • Brad White
    Brad White Member Posts: 2,398
    Spare BPHX's

    I probably am not qualified to tell you, but to make sure that your BPHX has not been used for anything other than potable water. I know that you know this, but do not want anyone to get sick.

    Here in MA, the plumbing code requires double-wall vented heat exchangers where potable water interfaces with boiler water. You would be surprised how much this is ignored. I am not sure if residential installations are governed by this because nearly every indirect I have seen is single-wall, (standard Super-Stors for example, knowing that HTP DOES offer a DW model.

    But my obvious point is, your BPHX might have been used to pre-warm pig urine for one of your incredible experiments, Mark. Let me know how THAT one worked out, would you?

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
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