Hot water diversion load on propane tank?

TenMile
TenMile Solar Expert Posts: 62 ✭✭
Hey all,

Is it possible to combine a DC heating element as a diversion load into a hot water tank that uses a propane burner as it's primary heating element?

Comments

  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Hot water diversion load on propane tank?

    I've never seen an Electric heating element bung on a Gas water heater. I've been told that some mobile home (modular ?) gas water heateres have a bung for an electric element. I've not looked at these, so I dunno.

    Regarding an electric heating element on an existing gas WH, think that it would be difficult to get enough contact area. Perhaps you could strip off the metal shell and insulation of an existing gas WH, and kludge some kind of pad heater onto the shell of the tank, and place the insulation/shell back on. I would NOT try this, as a fire could result. You might just consider an electric water heater as a pre-heater for the gas one.

    I have considered just such a configuration, running on my 106 Volt Vmp from the panels. Would require a DC Solid State Relay to switch the DC PV in, and you should not uise the Original Equipment thermostatic switches to switch DC.

    Depending on your PV array's Vmp, you might also need to change the heating elements, and so on. Maverick Solar had replacement elements, the last time I looked ... Off to the mtns, Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Hot water diversion load on propane tank?

    Home Depot carries a range of small GE hot water heaters that you could use as a pre/post heater or bypass the gas with ball valves. I have no clue about what you think you have to dump.

    http://www.homedepot.com/Plumbing-Water-Heaters-Point-Of-Use/h_d1/N-5yc1vZbqo7/h_d2/Navigation?langId=-1&storeId=10051&catalogId=10053&Nu=P_PARENT_ID&storeId=10051&catalogId=10053&langId=-1

    I just got finished figuring my gas for the past year. Heating hot water was $220.00 and heating the house all winter was $273.00. I intend to put a small electric hot water heater (20 Gal ) in and by pass mine to dump extra KW's that my Electric company will not pay me for.

    I can't see where the thermostat on a heater would care if it AC or DC. It's just a bimetal analog disconnect.
  • TenMile
    TenMile Solar Expert Posts: 62 ✭✭
    Re: Hot water diversion load on propane tank?

    Well, our HW heater at the cabin is a traditional 50-60 gallon tank that uses propane as its heating source. When we initially put it in place, propane was relatively cheap, but that is no longer the case -- it's costing us >$1000 per year now for HW, fridge, stove, lights and BBQ. We're moving the lighting to solar and adding in a TV and PC that we could not use here before.

    The solar system that we are installing at the cabin is likely going to produce more than we are going to consume for lights and small appliances (large appliances like the fridge and stove are propane today). Was thinking that I could divert extra load to pre-heat the water in the tank.

    I like your idea of using a small tank to pre-heat water and plumb it into the larger tank. I'm not a plumbing specialist but will see if I can read about how to move the heated water from one tank to another -- I am sure there is a way.
  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    Re: Hot water diversion load on propane tank?
    Vic wrote: »
    Perhaps you could strip off the metal shell and insulation of an existing gas WH, and kludge some kind of pad heater onto the shell of the tank, and place the insulation/shell back on.

    Along this line of thinking.

    http://www.mcmaster.com/#flexible-heaters/=bnfy22

    We use this type at work.
  • TenMile
    TenMile Solar Expert Posts: 62 ✭✭
    Re: Hot water diversion load on propane tank?

    Actually think I found the solution. Take an old electric hot water tank, swap out the AC heating element with a DC element. Hook up the new DC diversion tank's hot water output, to the cold water input of the propane HW tank. Divert any extra DC power to the element in the new tank and pre-heat the water entering the propane HW tank.

    In theory, it would mean that the propane tank would require less energy to heat the water to the required temperature as it would draw it from the pre-heated water from the DC powered tank. Doesn't eliminate the propane from water heating, but could cut it down substantially.

    Found a guy in the local buy and sell getting rid of commercial tanks with no elements for $100 -- and you can pick up the DC diversion load elements for $120. A couple of bucks for plumbing stuff and I think it can be done.
  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    Re: Hot water diversion load on propane tank?

    That how I am doing it right now with the old 40 Gal tank.

    I have an in-direct DHW. So I need to burn X LP just to get the boiler/lines up to the same temp. as the DHW tank. Looked at many options, but the cost of parts work be more than any savings.

    Winter is not so bad. Summer, the system burns LP for 15-20 each night.
  • TenMile
    TenMile Solar Expert Posts: 62 ✭✭
    Re: Hot water diversion load on propane tank?

    Thanks N3qik,

    Not completely following your description. If I read it right, are you saying that the water in the diversion tank actually gets hotter than the water in the propane tank?

    Wonder if that's something that could be adjusted with some manual tees and valves so that you could take advantage of using the diversion tank as your primary HW source?

    This whole diversion hot water discussion got me thinking. Two years ago when we renovated our home, we considered putting in a solar hot water system using vacuum tubes. However, at a quoted cost between $8500-12000 for the system, it didn't make any sort of financial sense. When I now look at the cost of installing some PV panels, a controller, inverter and some batteries -- I could build myself a pretty good backup power system that diverts excess load to a pre-heat HW tank and do pretty well on the energy savings. Our home HW tank is electric today -- thinking I could take half the money I didn't spend on residential solar HW heating, run maybe my home media server (which runs 24x7), plus get the benefit from HW heating savings and likely come out ahead. Will have to run the math.
  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    Re: Hot water diversion load on propane tank?

    TenMile,

    I babel at times, so lets try this again.

    With a propane DHW, it is going to take X amount of propane to warm up the heat exchange. Then after that point, you will start to put Btu's into the water. By preheating the water in a separate derision load tank. One may get to a point where more gas is used to preheat the exchange rather than the water.

    This is what I am seeing with the in-direct DHW. This might not be a issue with a gas fired DHW tank. The last gas fired DHW tank I had was 13 years ago.
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hot water diversion load on propane tank?
    I can't see where the thermostat on a heater would care if it AC or DC. It's just a bimetal analog disconnect.

    Arcing. DC circuit breakers/switches/fuses have to deal with breaking an arc that does not like to be broken. Since AC by definition turns itself off many times a second its far easier to kill an arc. The AC water heater thermostat would most likely arc and self-destruct if it was dealing with DC.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Hot water diversion load on propane tank?
    TenMile wrote: »
    I like your idea of using a small tank to pre-heat water and plumb it into the larger tank. I'm not a plumbing specialist but will see if I can read about how to move the heated water from one tank to another -- I am sure there is a way.

    No need to. Leave it in the electric-heated tank until it gets drawn into the gas-heated tank through normal use.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is