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  • #16
    Originally posted by Ampster View Post
    As I may have mentioned I have purchase 3 HPWHs and only had issues with the GE one which will not be an issue for you because GE sold that business. The Rheem's have worked well with no issue. Regarding condensation, you will need a condensate drain that your plumber can install when he does the rough plumbing. The cold air coming out of the unit is very dry because any moisture condenses on the coils, hence the need for a condensate line. I have not experienced an issue with condensation on the ductwork with the two Rheems. The energy savings is worth the risk that might arise from the complexity of their operation. They have the same basic mechanisms as refrigerators.

    Regarding your concern about capacity I would have your plumber do the rough plumbing for two water heaters in tandem.. You can install one and see how it goes. If you go with two you can always set the temperature on the first one lower so you don't waste energy keeping two tanks at high temperature. That will also improve the recovery rate on the primary tank.
    The condensate drain you write of is necessary and provided with a good, standard installation, but that has nothing to do with what I was writing about.

    I was referring to condensation on the OUTSIDE of a duct that can occur in moist climates or when the ambient dewpoint is high and the cooler air INSIDE the duct causes the duct temp. to drop. If/When that duct temp. gets below the ambient air's dew point temp., the ambient air in contact with the cooler surface will be cooled. If that cooled air's temp. gets to its dew point temp. moisture will begin to condense out of the air and on to the OUTSIDE of the duct (and just like on a condensate coil - remember the drain you wrote about needing ace ?).
    Exterior duct condensation may or may not be a concern depending on the application. For many reasons I believe I know a bit about, it may not need to be addressed by a design consideration or change in the design. Most folks don't bother with it because they are usually ignorant of the possibility as your post seem to serve as a good example. But that doesn't mean it doesn't need to be considered. In the OP's case, I'm assuming he's still in HI, and that seems a pretty warm and more importantly, a fairly moist/humid climate. He also mentioned a warm climate. I raised the duct condensate issue - particularly if ducting might be run through enclosed/inaccessible areas as I mentioned - because it might be something worth considering. And, the OP did ask for comments or further suggestions.

    The potential problem also gets a whole lot more insidious when if/when porous insulation such as fiberglass is used without a very good vapor (read VERY tight) barrier on its exterior and subsequently gets condensate on its interior fibers, and that condensate goes unnoticed. Forget the loss of insulating value. Now its a health issue.

    The OP can sure take or leave my or any other information as he sees fit., but IMO, you are unaware of potential problems or even what I was writing about.

    Even though the state of the cooled air is completely irrelevant to what I was writing about, I must address what I feel is a misleading statement you made about the cooled air's moisture content.

    The humidity or moisture content of the air coming out of an operating unit (which, again, has nothing to do with the possible condensation issue I was writing about), has an absolute moisture content will either be the same as it went in - if the ambient air's dew point is < the cooled air's outlet temp., or, if the ambient air's dewpoint is > the air's outlet temp., the outlet air will have a vapor pressure that will have a sat. temp. somewhere between the coil temp. and the air outlet temp. meaning the cooled air will be pretty close to saturation or near ~ 100% rel. humidity. In such cases, the air can't hold any more moisture. I don't think that qualifies as "very dry". At that point, HVAC folks don't think so either. It might be dryer than it went in, and so has a lower absolute humidity, but it won't be very useful at drying much of anything if it's already saturated or close to it - at least not until it's heated in some way.

    The OP can sure take or leave my or anyone's information as he sees fit, but IMO only, you are unaware o potential problems or even what I was writing about. I try to keep my yap shut and keep my fingers away from a keyboard if I think I'm ignorant in an area. Something you may want to consider.

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    • #17
      I simply said I have not experienced any condensation on the exterior of the duct that I have used in two installations. It was an observation, not a disagreement with your theory. You have not used a heat pump water heater but your theory may be useful to the OP depending on his location.He said he was going to duct hot air from the attic. As to how much if any ducting of the cold air he was doing to the garage, it might have better informed your response if you asked the question first about how long that duct was before entering into a long dissertation on the dew point of moisture on cold surfaces.
      Last edited by Ampster; 03-18-2019, 09:51 AM.

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      • #18
        Originally posted by J.P.M. View Post
        .................

        The humidity or moisture content of the air coming out of an operating unit (which, again, has nothing to do with the possible condensation issue I was writing about), has an absolute moisture content will either be the same as it went in - if the ambient air's dew point is < the cooled air's outlet temp., or, if the ambient air's dewpoint is > the air's outlet temp., the outlet air will have a vapor pressure that will have a sat. temp. somewhere between the coil temp. and the air outlet temp. meaning the cooled air will be pretty close to saturation or near ~ 100% rel. humidity. In such cases, the air can't hold any more moisture...........
        That is quite a sentence. You really like to get off into the weeds in the interest of completeness. I know that cooler air holds less moisture even though as you point out it may be close to saturation. I guess it depends on how you measure moisture by the ounces of water per cubic foot or by the percentage of saturation. For the casual observer of this conversation I can say that the heat pump water heater removes moisture from the air. Are we splitting hairs over my use of the term "very dry"? Would it make you feel better if I said you are correct? You are correct in theory but how does that help the OP make a decision about how much hot water storage capacity he wants?
        Last edited by Ampster; 03-18-2019, 09:56 AM.

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        • #19
          Originally posted by Steeler.Fan View Post
          .........
          Is my logic about this data correct? How much greater would I see if the electric heat kicked in rather than staying in hybrid mode?
          Hybrid mode uses the resistive element for fast recovery and heat pump for normal heating when temperature of the tank has not dropped significantly. You control the Mode and temperature from the Rheem software. You can choose heat pump only, Hybrid or resistive modes, although I don't know why anyone would buy a heat pump and run it in resistive mode other than temporarily. I think the software reports energy usage. i don't know anything about the refrigerant. I don't run mine higher than 130
          Last edited by Ampster; 03-18-2019, 09:57 AM.

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          • #20
            Originally posted by Steeler.Fan View Post
            LucMan and Ampster, I read your posts. Not sure what you meant by, "The refrigerant used in the HP (r410A) has condensing temperature around 110 degrees so depending on the ambient air entering the HP the heat of rejection may limit the the heat output in the HP mode requiring the electric resistance heater to make up the difference." The intake air temp may vary from 100-140F since it will be from my attic.
            I am going to install an Aoetec z-wave 40 amp heavy duty switch for each HPWH, which will allow me to remotely power on and off these units according to my occupancy calendar and # of occupants and will also give me energy consumption data. I hope to use that data to compare how much energy that I am using when I change the tank temps from 100->120F vs 120->140F. If my back up electrodes kick when I go from 120->140F, the increase in consumption should be greater than when I go from 100->120F.
            Is my logic about this data correct? How much greater would I see if the electric heat kicked in rather than staying in hybrid mode?
            Without going into a long drawn out discussion on the refrigeration principles I'll get to a quick point. Heat pumps generally ( not considering the new inverter mini splits) are designed to work in the range of 47- 95 degrees. Operation in temps outside this range will effect capacity and efficiency. In the case of water heaters with no defrost circuit (cooling mode) you are limited to 37 degrees. Rheem states that the high limit is 145 degrees, operating at that temp I would expect the compressor to last a year or 2 at most. The pressures and temperatures created at that high ambient temperature are going to overheat the refrigeration system. Experience has taught me that operation above 95 degrees has an effect on capacity , efficiency and longevity. Try to keep your ambient air to the HP between 50-95 for the best performance and reliability. The Rheem water heater has a 10 year tank warranty, don't know what the warranty on the refrigeration system is, check the fine print.
            At 95 degrees I would expect a max water temperature ( in HP mode only) of 120 degrees, with a COP of 3+. Above 95 I would expect the COP to begin to drop. The electric heaters operate at a COP of 1.

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            • #21
              For what it is worth, I called Rheem HPWH support today. I was told that I could run the tank at 145F for as long as I wanted and that as long as the air intake was less than 165F, there would be no problem.

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              • #22
                Scanning down through the posts, this looks more like a small commercial system than a residential system. As such it should be professionally designed so that someone has liability if it does not meet your fairly extensive requirements. With a plumber coming a few days it sounds to me like the design process is being started several months too late.

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                • #23
                  What we had was a "failure to communicate" between contractor, plumber and owner.

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                  • #24
                    Originally posted by Steeler.Fan View Post
                    For what it is worth, I called Rheem HPWH support today. I was told that I could run the tank at 145F for as long as I wanted and that as long as the air intake was less than 165F, there would be no problem.
                    WOW! That throws 40 years of experience out the window. Time to retire!

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                    • #25
                      Originally posted by LucMan View Post

                      WOW! That throws 40 years of experience out the window. Time to retire!
                      I too thought that was on the high side by a lot, but I'm already retired.

                      SteelerFan: Maybe a miscommunication ?

                      I'd also second Peakbagger's opinion and suggestions that some plumbing outfit get involved and put their imprimatur on it.

                      We can all sit here and pronounce based on a lot of experience, but none of us are on site and that's a real liability.

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                      • #26
                        Originally posted by peakbagger View Post
                        Scanning down through the posts, this looks more like a small commercial system than a residential system. As such it should be professionally designed so that someone has liability if it does not meet your fairly extensive requirements. With a plumber coming a few days it sounds to me like the design process is being started several months too late.
                        The Sanden CO2 system also looks to me to be a commercial system with a compressor separate from the storage tank. It is also priced that way. The Rheem HPWH is available on order from Home Depot for about $1200 and at energy rates in California or $0.20 per kWhr have a payback of 3 or 4 years. Your mileage may vary depending on where you live. My calculation used the EPA kWhr savings from tag on the HPWH and applying the above rate of $0.20 versus the assumed EPA rate of $0.12 per kWhr ised on the EPA tag.
                        Last edited by Ampster; 03-19-2019, 02:20 AM.

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                        • #27
                          Apologies for the thread jack. Does anyone know if a DIY install voids any of the warranties on hybrid / heat pump water heaters?

                          Have installed 3 ordinary electric water heaters in my life. 3rd of which has lasted for over 23 years. Neither an electrician or plumber
                          but have a good handle on both fields. Built my own house (literally!) and was an industrial maintenance tech for five years. With the added complexity and HVAC that an HPWH adds the likelihood of needing servicing is much greater.

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