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Connecting HPWH to Tankless Coil

GeneinMASS
GeneinMASS Member Posts: 6
edited November 2021 in Domestic Hot Water
Hi All,
I live in Eastern MA, and my Mom's water tank needs to be replaced. Right now she has a WM boiler servicing 2 zones of baseboard heat with a tankless coil that dumps HW into a 40 gal 'lectric heater that the plumber wired to a pump to create an Aquabank storage system. The tank is not otherwise connected to power and the elements are dormant/vestigial. Kinda nifty and imaginative I think and she's never run out of HW. The fella that did it sold his company and retired to parts unknown. I am intrigued with HPWH and wondering if it would be a good option for a replacement. What I want to do is this: Install HPWH in the basement. The electrical panel is just on the other side of a dividing wall and would be a simple matter for my brother the electrician to run a 220 circuit to the mechanical room over the top through the ceiling of removable tiles. The basement is semi-finished and only used for storage and laundry a few times a week, And instead of wiring the back up heating elements to heat water during period of high demand, to wire the thermostat to pump water through the tankless to meet high demand periods, These demand periods would only be a few times a year when the kids and grand-kids visit, and almost never day to day...which may mean a cold start on the boiler during the summer once or twice a month. 
i'd like to get rid of the summer firings if it will save money and not damage the boiler.  Only two people live there, and aside from 2 showers a day, a load of dishes in the dishwasher a day and say 3-4 loads of laundry a weak, that's it for HW usage. Would there be a net savings of energy and therefore lower oil+electric bill? If I decide to go with a HPWH, should I just leave it be and let the elements provide backup when needed? If, even after the rebates and tax incentive, should I just replace like with like, which is tankless coil with 10 year tank as an aquabank? As an aside, based on previous experiences in other homes, everyone that lives there has developed a habit of spacing out HW usage to allow for recovery and are willing to continue to do so if it means saving dinero on the energy bill and help save the planet, and a reasonably priced tempering valve is also on the table. I am afraid I can't give any more specs at the moment, such as current prices per gallon or oil or KWH of 'lectricity or boiler efficiency. Advice solicited and welcomed with my thanks. My apologies if this been covered before and you are welcome to point me in that direction, with my appreciation. Gene

Comments

  • psb75
    psb75 Member Posts: 584
    Install the HPDHW unit (replacing the old electric DHW tank) and let it make all the hot water, all of the time--if you don't want to have oil/gas making your DHW. The HPDHW will allow you to realize up to 1/3 savings in hot water production per annum. The touch-screen interface on the HPDHW unit makes it much more user friendly for adjusting temperatures and modes of operation --i.e. when house has additional occupants.
    HVACNUT
  • HVACNUT
    HVACNUT Member Posts: 4,672
    Forget about the tankless. 
    You can leave it and repipe the coil and the heat pump in parallel with isolation valves. If the heat pump ever goes down, you have a backup. 
    What aquastat is on the boiler now? There's control options to make it cold start and still have condensate protection. 
    The dishwasher runs every day with only 2 people?
  • JakeCK
    JakeCK Member Posts: 897
    Go with the HPWH and keep the tankless coil as a back up, as HVACNUT said.

    Some other considerations to ask yourself is how cold does the incoming water get in the winter? If it gets really cold like it does for me consider using the coil in the winter to at least preheat it, another option is a drain water heat recovery unit on main stack below the showers if the plumbing allows.https://energy.gov/energysaver/drain-water-heat-recovery

    Another consideration is how cold does the area it is in get? In the winter your boiler will put heat into the house which the HPWH will steal. If both the boiler and tank are in the same area and is not intentionally heated(IE unheated basement or garage) that may be a win even in the winter, the tank will scavenge waste heat from the boiler before it escapes through the foundation. If you are intentionally heating it then the tank will lower the temperature by a few degrees when it is running and the boiler will have to burn fuel to replace that heat. In that scenario it might also be better to use the coil in the winter to heat the water and send it to the tank.

    In any case get the biggest tank you can fit down there. Seriously I bought the 50 gallon and I wish I could have gotten the 80. Also install a tempering valve and run it at 140f. These things in heat pump mode only use about as much energy as a dehumidifier. In the summer they actually dehumidify and cool the area too, so the longer run times are desirable then too.
    Kafox15
  • psb75
    psb75 Member Posts: 584
    The sizes for HPDHW units range from 40, 50, 65 and 80 gallons. Depends on brand. Rheem has all of those sizes.
  • GeneinMASS
    GeneinMASS Member Posts: 6
    edited November 2021
    So I called the Town Water Department and asked what the temp coming in off the street was in winter. The dude said they have no idea-they just make sure the water is clean, potable and is billed correctly. Your tax dollars at work, I guess. Judging by the number of water main bursts here every year-at least 1 to 2 a year-I’m going to guess the water is wicked cold in winter. The basement is unintentionally heated, is not used as a living space at all-just storage and laundry. My brother and I did have a photography darkroom down there at one time, but since film has mostly gone the way of the dodo bird that room is now defunct. As for wastewater heat recovery, I’d have her consider it if the manufacturer wants to guarantee materials and workmanship for at least as long as the expected payoff for two people. As for running the dishwasher everyday, the dishes in her house multiply like tribbles from Star Trek. Haven’t been able to figure out how…
    MikeAmann
  • Kafox15
    Kafox15 Member Posts: 92
    I thought you might live in my town until you said you get 1-2 water breaks a year. We get 1-2 a MONTH here! :#

    I'm also installing a HPWH and agree with @JakeCK
  • GeneinMASS
    GeneinMASS Member Posts: 6
    Weymouth for me. You?
  • JakeCK
    JakeCK Member Posts: 897
    About cold water in the winter, I measured mine a few winters back in January. I was getting incoming water in the upper 30's. So it can get quite cold here. This will be my first winter with a hpwh so I will be able to give first hand experience of how it handles the cold water input and how it affects energy usage and recovery. And basement temperatures. I want to point out that even with the increased usage due to cold water it is still more efficient than other options. Regardless of water heating technology, it still has to contend with those colder winter water temps.
  • psb75
    psb75 Member Posts: 584
    The incoming water temperature is really only most significant for "on demand" of "instantaneous" DHW systems--like a boiler coil or an on-demand electric or gas wall-hung unit. Meaning, those that heat the water as it is drawn, and without any tank storage of already-heated water. For " flow-through, coil-type" devices the "temperature rise" is critical. So...the colder the incoming water is, the harder the unit must work to heat water--its output volume will drop and/or the DHW temps. Multiple users drawing from a coil at the same time is also problematic. Some folks are more able than others to adjust their DHW-using habits to accomodate the device that is making DHW. Caveat emptor. "Let the buyer beware."
  • JakeCK
    JakeCK Member Posts: 897
    edited November 2021
    psb75 said:
    The incoming water temperature is really only most significant for "on demand" of "instantaneous" DHW systems--like a boiler coil or an on-demand electric or gas wall-hung unit. Meaning, those that heat the water as it is drawn, and without any tank storage of already-heated water. For " flow-through, coil-type" devices the "temperature rise" is critical. So...the colder the incoming water is, the harder the unit must work to heat water--its output volume will drop and/or the DHW temps. Multiple users drawing from a coil at the same time is also problematic. Some folks are more able than others to adjust their DHW-using habits to accomodate the device that is making DHW. Caveat emptor. "Let the buyer beware."
    100% true. But reheating incoming water from 35-40f to 140 vs heating 55-60 to 140 is going to affect how long it takes a tank to recover and the amount of energy it uses to do so. Laws of conservation demand it.

    On a gas fired hot water tank it isn't noticeable to the end user but in a heat pump it might mean a difference of an hour or more.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,637
    Hi @GeneinMASS , I agree with what @JakeCK said about tank sizing. Get the biggest one. This way you'll have ability to meet all loads and it will stay in heat pump mode, which is a good and efficient thing! :)

    Yours, Larry

    ps. The thing about HPWHs is that people often don't clean the air filter and this can reduce the efficiency to less than than of electric resistance!
  • JakeCK
    JakeCK Member Posts: 897
    ps. The thing about HPWHs is that people often don't clean the air filter and this can reduce the efficiency to less than than of electric resistance!
    I'm debating installing the vent connector for the intake so I can pull air from the near boiler plumbing and install a second more robust filter. Cleaning the built-in filter won't be an issue for me but I can see those coils getting nasty just like a window ac. and be just as much of a pita to clean.
  • GeneinMASS
    GeneinMASS Member Posts: 6
    I’d imagine a prefilter in the vent kit before the air intake on the heater help with the nastiness. We can go down such a rabbit hole just thinking about the add ins and modifications….
  • GeneinMASS
    GeneinMASS Member Posts: 6
    Why should I upsize and get the biggest heater she can afford? If it’s to mitigate slow recovery and to not run out of water, that means either I somehow connect the HPHW to the coil like I originally wanted, or avoid heat pumps altogether. Running out of water is a deal breaker for her.
  • JakeCK
    JakeCK Member Posts: 897
    edited November 2021
    Because it's more efficient for it to heat larger amounts of water over longer periods than to keep up with an instantaneous water demand? It also allows for it to be set to a schedule to optimize power usage based on demand pricing. And the cost difference between a 50 and 80 is not much.

    In my case I simply couldn't fit much bigger of a tank where I put mine but if I could have I would have gotten the 80 over the 50. In my house we're a family of four with two of those being small kids. It seems we are running a load of laundry and the dish washer almost daily. I have my tank set to heat the water up to 140 when my solar panels start producing and to shut off entirely when they are not to maximize my savings even further. We have yet to run out of hot water. I admit this is a unique situation but it is only one example of how a larger tank affords more opportunity for future savings. 
  • GeneinMASS
    GeneinMASS Member Posts: 6
    Substitute “tankless coil” for “solar” and that’s the setup I want. How did you set the controls on your tank? By looking at the Rheem wiring schematic, I might be able to disconnect the upper element completely, and wire the circulator to the wires for the lower element. That way when the tank calls for the lower element during demand periods, it will instead be pumping water through the tankless. Gene
  • JakeCK
    JakeCK Member Posts: 897
    edited November 2021
    Solar pv, not solar hot water heating.

    I would strongly advise against messing with any of the elements on a heat pump water heater. It is not just a dumb electric tank. It is a computer controlled tank that does self diagnostics. It will more than likely give errors if you disconnect an element. And it ultimately has control over when and why the elements run. Not necessarily you

    You are way over thinking this. Remember K.I.S.S. 

    I suggest you post pictures of your current plumbing. 
  • psb75
    psb75 Member Posts: 584
    Listen to JakeCK....please. Do not mess with the Rheem HPDHW logic. If you think you are "able", then you can quit your day-job.
    JakeCK
  • JakeCK
    JakeCK Member Posts: 897
    Here is the operation flow chart that Rheem put out with the patent for theirs and a link to a discussion about these tanks on greenbuildingadvisor. It is obvious from that, that you cannot simply disconnect an element with out also disabling the tank altogether. 

    https://www.greenbuildingadvisor.com/question/rheem-hybrid-pre-warm-mode-while-in-heat-pump-mode