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Community and Q&A

Solar hot water system with oil heat, one hot water tank or two?

theaudone | Posted in Mechanicals on


I’m wondering if someone could help me. I am very new to the site so I apologize if I have missed the answer to this question in the archives.

We have a solar hot water system (geothermal, installed 2007, has already paid for itself – I have read the articles on photovoltaic systems) in a house that has oil heat. [Editor’s note: Audrey later clarified these remarks by noting that the reference to a “geothermal” system is erroneous; the intended reference was to a solar thermal domestic hot water system.]

Gas heat is not an option on our street. Our hot water tank that is connected to our oil tank has finally hit the end of its life and we are unsure if we should just replace this one tank, OR, replace both of our hot water tanks with just one combination tank. It seems to us that we should keep the solar tank and the oil tank separate, but we also don’t feel qualified to make this decision.

Right now the water coming in from the street at 55 degrees is heated by our solar system in the solar hot water tank, then that warmed water feeds the main hot water tank and if necessary, is heated the rest of the way using oil. Apparently we could replace both tanks with a combined tank with two coils inside; one for solar and one for oil. My question is: which will fire first for heating, my solar or my oil? Won’t the boiler just kick on and the solar might not get a chance to work? Whereas with the two tank system we have now, the solar always gets a turn first, then the oil just fills in the rest of the way. Am I missing something? Why would I want to go to a one-hot-water-tank system? Any input would be greatly appreciated! Thank you!

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  1. theaudone | | #1

    Yes you are correct, We have a solar thermal domestic hot water system, no geothermal (sorry about that) and it feeds an indirect water heater which is connected to our boiler.

    I'm confused by your response where you say the indirect system is efficient but then you mention the electric or heat-pump water heater. I probably need to read more on the site but I wonder if the electricity bill to run that would be high too?

    Sounds like you like our idea though of keeping a 2-tank system it's just looking at what type of water heater now to buy...

    I really appreciate the help!

  2. Tim C | | #2

    Charlie made a typo; he meant inefficient (the boiler itself starts off cold, and absorbs a bunch of heat while warming up the water; after it's done, that heat will warm up your house, making more work for your AC, wasting oil and electricity).

    Do you have any measurements on how much of your water is provided by the solar thermal heater? That could help guide your decision.

    In your position, the main options I would consider are a tankless electric heater (if the solar thermal provides the considerable majority of your hot water and you don't have any very high flow rate demands, like a garden tub), or a heat pump water heater. The heat pump water heater will almost certainly cost you less to run in electricity than the oil does currently. It's entirely possible that a standard electric water heater would too (tanked or tankless).

  3. theaudone | | #3

    OK. Thank you. We'll do some reading and come back if we have any more questions.

  4. theaudone | | #4


    Just circling back around on this after some thought. I checked on some numbers: our solar hot water system is sized to provide about 75% of our hot water, of course that depends on weather. Last winter the panels were buried in snow for awhile. Our boiler is heating the house from mid November through mid March here in New England. There are no high flow demands, typical shower, dishwasher etc.

    Does this information still lead you to the same thoughts that we should look at a tankless electric heater or a heat pump water heater?

    Thanks for your time on this!

  5. charlie_sullivan | | #5

    It's a little hard to be sure what you have from the description, but if I'm understanding right, then your description of the advantage of the two-tank system is right on--you want to use the solar for as much as you can first, and the two tanks allow it to work that way.

    A single-tank system can sort of work that way if it's not well mixed, but is stratified, with the hottest water at the top (heated by the oil) and the not-quite-as-hot water in the bottom, heated by the solar. But at this point, it would be cheaper to buy a replacement tank than to buy a tank with two heat exchangers at different levels to implement that plan.

    But buying a new tank to be heated by oil might not be a good investment. Oil's not cheap or low carbon, and the indirect system is a very efficient [edit: INefficient--thanks Tim] way to heat, especially in the summer when you aren't otherwise needing to fire up the oil boiler anyway. So you might want to consider an electric or heat-pump water heater instead. You can combine with the solar tank the same way.

    My confusion stems from the word geothermal in the first paragraph. I think you just mean a solar thermal domestic hot water system, with no geothermal or other heat pump and no PV.

  6. charlie_sullivan | | #6

    If your prices are similar to these:
    it's pretty close to the same cost for oil vs. heat pump (assuming COP = 2 for the heat pump, so it's half the cost compared to straight electric), if your overall oil efficiency is 75%. That's reasonable for winter, but for summer, the oil efficiency is much worse than that, because even if you only need a little tiny bit of extra heat with the solar doing most of the work, you still need to fire up the whole system. So I think your costs will be higher with the indirect than with the heat pump.

    If you had measured oil consumption from May-Sept. when the heat is really off, you'd know how bad that was, and be able to make a more informed decision.

    If you could buy a tank with an indirect heat exchanger and an electric heating element, you could use the electric in the summer to avoid firing up the indirect, and use the indirect in the winter. Your electric costs would be small because you are not having to add much heat, with most being done by the solar. But I'm not sure that's available.

    So it seems like a heat pump is likely to be a good choice and would mildly reduce your water heating costs some. Depending on what electricity and oil prices do in the future, or how your costs compare to the NH averages as of yesterday, the balance could tip.

    If you want to consider carbon impacts, the New England electric grid is pretty low carbon, and should get better with time, so that would make the heat pump a clear winner. So if the cost is similar, lower carbon sounds to me like a good idea.

  7. GBA Editor
    Martin Holladay | | #7

    There are two possible answers to your question: one based on system performance, and one based on cost-effectiveness.

    When it comes to system performance, your definitely want two tanks: a solar tank and a tank heated by your backup fuel. The only exception is if you take Tim's advice and use a tankless water heater as your backup heater. (If you do that, you just need a solar tank.) For more information on designing a solar hot water system, see Solar Hot Water.

    The cost-effectiveness analysis is much more complicated. Many people with a solar hot water system face a tricky dilemma when a major component of the system fails. Unfortunately, investments is solar thermal equipment don't have a good payback, so you are stuck with the difficult decision of how many hundreds of dollars you want to invest to save just $100 or $130 per year. Here is the brutal economic fact: the average American family spends only $267 per year to produce domestic hot water, so there is a limit to the savings potential for any scheme that tries to save energy for water heating. Expensive equipment has a very long payback period -- and for many schemes, the payback date is "never."

    For more information on the cost-effectiveness question, see:

    Solar Hot Water System Maintenance Costs

    Solar Thermal Is Really, Really Dead

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