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Dual system

I am not a professional plumber, although , have been exposed to a fair amount of plumbing work. I did plumb my house, but, certainly do not claim to be an expert! I have a Buderas Boiler with a logomatic control. Previous summers , after home heating season, I have heated my domestic hot water with an indirect off my boiler. I use about 250 gals of oil per summer for this purpose. I have solar panels on my roof. In an effort to move some hot water expense to electric, I have purchased an electric heat pump hot water heater. I am finding the heat pump electric heater cannot handle my demands affordably. I was considering letting the heat pump unit act as a "preheat" tank feeding the indirect 110 - 115 degree water instead of 47 degree well water. Will this save any appreciable amount off the 250 gals of fuel oil consumption. Since my boiler would still be my source , I assume it will still have to maintain temperature, thus burning oil. The question is ...will "topping off" the heat of the already hot water reduce the boiler fuel consumption?

Comments

  • hot_rod
    hot_rod Member Posts: 18,227
    Where abouts are you located? What size family and how many solar collectors do you have?

    Properly sized solar should, could handle most of your DHW loads in the summer.

    Usually solar is designed to a 40- 50% solar fraction SF. Meaning the solar would cover 40- 50% of the DHW needs.
    Winter moths it drops off, summer may see 90- 100% DHW.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • wayneschiller
    wayneschiller Member Posts: 5
    I do not have solar, hot water collectors. My solar is Photovoltaic. That's why , as I explained, I was trying to take advantage of this electric heat pump unit?
  • Jamie Hall
    Jamie Hall Member Posts: 20,477
    Your concept will work; how much oil it saves must be determined by experience. Probably the biggest problem -- depending on where you are located -- is that your hybrid heat pump water heater should run for a good fraction of the time, and your PV system may (in fact probably does) not have the power to do that -- unless it is either a rather large array or you have a big battery bank. Try to set your controls so that when the PV system can't handle the heat pump load that the backup electric resistance elements, if you have them in your unit, do not come on.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • wayneschiller
    wayneschiller Member Posts: 5
    Yes , I understand the heat pump will need to function alot. I am in Vermont. I have a 59 panel array and my produced electricity is either used in house or stored with my local utility for future use. That part will not be a problem. I am just not sure whether there will be enough savings to be worth doing this. I was hoping there was a more efficient way to keep the Buderas boiler on call since it may not need to go on that often. Every so often it fires up to keep itself ready at 140 degrees and I wonder if that is where the majority of the 250 gals of oil is used.
  • hot_rod
    hot_rod Member Posts: 18,227
    SunBandit and a few others dump pv directly to a resistance element in a tank.

    Resol builds a controller and a pump package to monitor and dump excess pv into a dhw tank. If the utility buy back is low for any excess you might generate, might as well use it in your home for DHW or heat.


    http://www.resol.de/index/produktdetail/kategorie/12/id/210/sprache/en
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • wayneschiller
    wayneschiller Member Posts: 5
    That's a good point, but, its handled a bit differently here. The utility stores the returned credits, accumulate them and I have 13 months to use them. It's not bad.
  • hot_rod
    hot_rod Member Posts: 18,227
    Same here but my utility credits me 2 cents, but charges 11, so if I had excess and could put it to better $$ use it would make sense.

    Germany and other Euro countries keep changing the FITS based on unexpected PV growth so so products like the Resol are being developed for PV owners to better use excess. But you need a good load to dump to if you have a lot of excess.

    Resistance elements are an inexpensive way to use any excess. Heat pump water heaters would have a higher efficiency but higher front end and repair costs. Also the exchange energy needs to come from somewhere in the building for the heat pump WH, they are not always the best DHW method, depends on your application.

    Tom up at Solartechnics has probably crunched the numbers for a climate and application like yours, he is up in Maine and deals with HPWHs. He has some refurbished HPWH on his site for a price maybe worth the gamble.

    www.solartechnics.com
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,146
    to answer your question if your well water is 47 deg and the heat pump makes 110 degrees. 110-47=63 degrees. a gallon of water is about 8.3 lbs so 8.3 x 63=523 btus

    so for every gallon of hot water you use you will take 523 btus off the boiler load. If the boiler is say 83% efficiency you will save 630btus/gallon of water heated.

    for every 1000 gallons of hot water used will save you 3.73 gallons of fuel oil.

    This assumes the heat pump has enough capacity to heat all your
    dhw from 47-110 deg and does not account for the energy used by the heat pump.
  • hot_rod
    hot_rod Member Posts: 18,227
    Any thoughts on how many gallons of hot water you use per month. 20 gallons per person per day is one rule of thumb, some suggest 20 for the first person 15 for additional.

    Once you get an idea on how much you use, then you couuld calculate fuel consumption and cost. it sounds like you have a fairly efficient combination for DHW, possibly a heating zone runs during DHW call?.

    I've heard 25- 30 gallons of oil per month as one rule of thumb, but that obviously varies from household to household.

    250 gallons for how many months?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • wayneschiller
    wayneschiller Member Posts: 5
    250 gals Covered me from mid May to mid October. The savings calculation by Ed does not sound like the preheat tank will save much. I have 5 people in my household.
  • newagedawn
    newagedawn Member Posts: 586
    something is wrong !!! if you have an indirect and use that much oil for hot water only, check the nozzle size or find out if someone takes 1 hr showers, i have people with the same system and only go through 150 gal of oil for 6 months of hot water here in new england,
    check the logimatic, i am not a fan of those had alot of problems with those over the yrs, put a l7224u on it and see the difference
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"