Air to water heat exchanger question

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raydias
raydias Solar Expert Posts: 68 ✭✭✭✭
in General Solar Power Topics #1
Has anyone done a project that uses and air to water heat exchanger in a house attic to heat/pre-heat water.

airtowater4.jpg

After reading the threads on attic fans etc it occurred to me that maybe that maybe the heat in the attic could be directed by a solar fan through an air to water heat exchanger in order to pre-heat water before entering the water heater.

any thoughts?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: Air to water heat exchanger question

    Maybe... But is it probably not worth the expense...

    If you are in a humid area, you may need to put a catch pan under the coil to prevent water damage to the ceiling below.

    In general, there is just not that much energy in "warm" air. People for years have been looking at (for example) clothes drier exhaust as a source of heating too... In that case, can cause condensation and lint accumulation--its own set of (fire) hazards.

    If your attic is correctly vented, then there should not be that much warm air in there in the first place.

    Placing a thermal solar collector out in the sun is going to collect a whole bunch more energy.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    #3
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    Re: Air to water heat exchanger question

    I put up 300 feet of 3/4" pex three years ago. Worked nice. Will try to get some numbers from the year.

    Removed it in winter, have not reinstalled it to a big OOPS. Left it in basement by the window. Sun + PEX = trash.:cry:
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  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
    #4
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    Re: Air to water heat exchanger question

    I saw a while ago an Australian company that has just such a thing for heating pools.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
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  • LucMan
    LucMan Solar Expert Posts: 223 ✭✭✭
    #5
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    Re: Air to water heat exchanger question
    raydias wrote: »
    Has anyone done a project that uses and air to water heat exchanger in a house attic to heat/pre-heat water.

    airtowater4.jpg

    After reading the threads on attic fans etc it occurred to me that maybe that maybe the heat in the attic could be directed by a solar fan through an air to water heat exchanger in order to pre-heat water before entering the water heater.

    any thoughts?

    A flat plate solar panel on the roof will do a much better job. And you can make it yourself also if you wish.
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #6
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    Re: Air to water heat exchanger question
    LucMan wrote: »
    A flat plate solar panel on the roof will do a much better job. And you can make it yourself also if you wish.

    Looks like the heat exchanger that I had installed in my hot air furnace for an outside wood heater. When using the wood heater you need the water to be heated to at least 180 degree to be effective. S0larvic
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #7
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    Re: Air to water heat exchanger question
    solarvic wrote: »
    Looks like the heat exchanger that I had installed in my hot air furnace for an outside wood heater. When using the wood heater you need the water to be heated to at least 180 degree to be effective. S0larvic
    It looks like the one I had also, I worked great until the water turned to steam and the pop off was in the wrong place and it had enough force to to blow the whole blower out of the furnace. I chalked that one up to a lesson learned.
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  • russ
    russ Solar Expert Posts: 593 ✭✭
    #8
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    Re: Air to water heat exchanger question

    Like LucMan said - To easy to build a solar thermal panel - far easier than a solar PV panel.

    Heat exchangers are all about delta T (temp difference) between the two sides. The attic temp may seem warm to you but to a heat exchanger it is nothing of any use.

    The heat exchanger - You would spend you money and get nothing measurable in return.
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  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    #9
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    Re: Air to water heat exchanger question
    russ wrote: »
    Like LucMan said - To easy to build a solar thermal panel - far easier than a solar PV panel.

    Heat exchangers are all about delta T (temp difference) between the two sides. The attic temp may seem warm to you but to a heat exchanger it is nothing of any use.

    The heat exchanger - You would spend you money and get nothing measurable in return.

    A slight addition to this: Heat exchangers are all about delta T, and the heat capacities of the fluids on either side.

    In this case you have a small delta T, with the "hot" fluid having a very low heat capacity and the "cold" fluid having a very high heat capacity. That makes for a very, very ineffective heat exchanger.


    The best way to transfer heat is to either condense or boil a fluid on one side of the heat exchanger. You can create a massive thermal flux with a very small heat exchanger that way. It's the principle behind refrigeration and heat pipes (used in electronics cooling and commercial solar-thermal systems).
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  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    #10
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    Re: Air to water heat exchanger question

    The delta T maybe low, but with a large surface area & low flow can makeup for that.

    300 ft of 3/4" PEX is a lot of SQ. IN.
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  • russ
    russ Solar Expert Posts: 593 ✭✭
    #11
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    Re: Air to water heat exchanger question
    n3qik wrote: »
    The delta T maybe low, but with a large surface area & low flow can makeup for that.

    300 ft of 3/4" PEX is a lot of SQ. IN.

    That would not be what the poster was talking about. Totally different topic.

    Russ
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: Air to water heat exchanger question
    russ wrote: »
    That would not be what the poster was talking about. Totally different topic.

    Russ

    No, it isn't.
    Although the OP attached an image of one type of heat exchanger he was making a general inquiry about using attic heat to pre-heat water. This is exactly what n3qik was doing with the pex. There has been no mention of how much temperature rise is needed/wanted. In some cases source water can be very cold, and even bringing it up a few degrees may be worth the effort.

    There is a question of how much cost & trouble vs. the rise that is a function of Delta T.
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  • russ
    russ Solar Expert Posts: 593 ✭✭
    #13
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    Re: Air to water heat exchanger question

    OK but I do hope you are not serious.

    Exposed to the sun many, many meters of black pipe are required to heat water any appreciable amount.

    In the attic you will be looking at kilometers to accomplish the same thing.
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  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    #14
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    Re: Air to water heat exchanger question

    In summer, my well water is about 65 deg. F.

    Have a 40 gal. pre-heater tank.

    On a good summer day, that 40 Gal. tank would go from 70-80 to 100-110 Deg. F.

    As a pre-heat system, the attic will work.

    As a DHW system, must have the panels/tubing on the roof in direct sun.
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  • raydias
    raydias Solar Expert Posts: 68 ✭✭✭✭
    #15
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    Re: Air to water heat exchanger question

    thank you everyone for the information.

    To help clarify:

    My parents live on a farm and their house is very old with a large attic. They are all electric and their water source comes from a well (water temp is ~69f).

    What I was looking for is a way to cheaply bring that temp up so as to not need as much electricity in the DHW heater.

    I am also evaluating the DIY solar water heater projects that are all over the net.

    thanks for the info so far.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #16
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    Re: Air to water heat exchanger question

    Some suggestions from the Working Beginner's FAQ thread:

    Solar Shed and other Solar Thermal Links
    Trying to make this thread a jumping off place to further information (links, data, etc.).

    Following up on Russ' Energy Star Suggestion:

    www.energystar.gov
    Energy Star Hot Water Heaters

    Links to a couple of manufacturers from Solar Guppy (see above links for more):

    GE Heat Pump Water Heater
    Ruud Heat Pump Water Heater

    For a quick discussion about heat pump based hot water heaters, see this thread (starting at post #12):

    Guidance for Grid-Tie

    Heat Pump based hot water heaters are probably a great idea for those in warmer climates (especially if using electric power or oil for heating).

    If you are in a cooler climate (below ~55-45F area where heater will be installed)--check out the details to ensure it makes sense where you live--and I may very well be overly pessimistic on my suggestions--others disagree with me.

    Home Power Magazine is also a great place for conservation ideas:

    Home Power Mag

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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