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Community and Q&A

Indirect vs heat pump water heater

Bdgray | Posted in Energy Efficiency and Durability on

Hi all,

We have natural gas powered radiant heat. We also recently installed a 14 KWh array of solar panels on our house. We live in Illinois and the state (despite being broke) passed some pretty insane incentives. We wound up paying about $6K for solar panels that had a total sticker price in the mid $30s.

I’d like to replace our hot water heater and I’m looking for thoughts on which would be better & more efficient between an indirect powered by the boiler and a heat pump powered (in large part) by free and clean solar . I’m leaning toward a heat pump because a) it reduces our carbon footprint (I’m assuming) and b) I like the added benefit of it serving as a dehumidifier in the summer months. We have frigid cold well water (despite being 13 miles from downtown Chicago) so on hot, humid days the pipes sweat a bit even when the AC is on. I’m also mildly worried about an indirect competing for BTUs with our radiant on cold days. We hit -17 F last winter.

Any thoughts/feedback would be greatly appreciated.

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  1. Expert Member
    Peter Engle | | #1

    Much of the trade-off analysis will be based on the stats for your house and your boiler. How large is the house? Well insulated and air sealed or not? What kind of boiler: how many Btuh rating, what efficiency level, etc. How about solar sizing: did you size it to just meet your needs, or are you trying to sell back power?

    1. Bdgray | | #2

      The hot water heater is kind of the last project to greenify this house. Its a 50s ranch that we bought as the worst house in the best neighborhood. We slowly gutted and rehabbed. The roof deck was spray-foamed last year (with the new friendlier propellant) to code (~7 1/2 inches I think). The boiler is brand new and is 90 or 95% efficient (can't recall which). It feeds PEX that I installed between the floor joists. All the exterior walls were gutted from the inside and we added 2" of rigid foam insulation before we re-drywalled. Fireplace has an insert to tighten that up. The solar panels are as big as the state would allow 14 KWh or ~100% of our annual electrical usage. Once I swap out the hot water heater I can then seal the vent it uses through the chimney which I suspect is the house's only real achilles right now. From a tightness perspective, that DHW vent is the last part of the house that needs to be improved. I'm not actually super green but my wife sure is. Hence, everything we did in the reno was done with conservation in mind.

      We also have a ducted mini split so in theory we don't need the radiant (although we love it). We needed the mini split for AC and figured we might as well install one that can heat and cool. Point being, I could go nearly all electric if I really wanted to.

  2. Jon_R | | #3

    > the added benefit of it serving as a dehumidifier in the summer months.

    Don't put much stock in this unless you have numbers showing otherwise (eg pints/day under plausible conditions). You are likely to get an insignificant 2 pints/day.

  3. Expert Member
    Peter Engle | | #4

    I'm not sure your operating costs will be much different either way.

    If you go with the HPWH, it will be robbing heat from the basement during the winter. That heat is provided by the boiler, so in effect, you're using an indirect (very indirect) water heater during the winter anyhow. Slightly increased cost of HW because of the electricity used to run the HP. But, in the summer, the HPWH cools and dehumidifies the basement. Possibly some savings on electricity there, depending on how the basement is used.

    If you go with the boiler, you probably have better recovery time for your hot water. Plus, the boiler/indirect combo is much quieter than a HPWH. Your winter WH cost is lower, but you need to run the boiler during the summer to get hot water. The waste heat from the equipment warms the basement and may increase your electricity costs for A/C.

    So, likely a wash on operating costs. I'd go with the indirect water heater. More capacity, better recovery, quieter. Also probably longer life, with zero moving parts. However, if you see going all-electric in your future, that decision certainly changes.

    Final note: FWIW, you won't be powering the HPWH with "free and clean solar." Your panels are sized for 100% of your current demand, meaning the current demand is all used up. If you add a new, biggish load, it's going to be powered by the dirty, expensive grid. Unless your local grid is clean, of course.

    1. Bdgray | | #5

      Fair point on the sizing of the panel but I suspect we're actually oversized. For the past year the house has been attached to an unfinished and uninsulated addition. Today for example that addition will probably get to 120 degrees (it has been a back-burner project while we renovate the rest of the house). There is just a cheap hollow core door between it and the rest of the house. My gut tells me I'm overpaying on AC in the summer months right now because of all the heat that room creates. In winter it's more of a tax on our natural gas consumption since its colder than the rest of the house. Also, we've produced more power than we've consumed in every month since the panels were installed (January) so I'm not sure the calculations were accurate. We're not allowed to sell that back to our utility. They are like rollover minutes on a phone plan. Eventually we'll get a battery and then probably an EV to suck the overage up. Anyway, that was part of the equation in my mind. I think we're producing more than we're currently consuming, despite the original sizing at 100% of historic consumption.

      Had not thought about cooling effect of a heat pump in winter nor recovery time. Sounds like there are pros & cons to each and possibly no right answer.

      Appreciate the input from everyone.

      1. vap0rtranz | | #6

        14 KW of PV array is max capacity ? Wow. Maybe find out where that elec usage is coming from. Still, that is an awesome State incentive you got. Props to using it!

        We went with HPHW b/c it fits with our overall goals: we're going all electric (so no fossil fuels). Ours is a Bradford White Aerotherm (previously GE GeoSpring*) in a 700 sq ft unfinished basement directly below our only bathroom (to minimize hot loss in the plumbing run ... though the pipes are also insulated.)

        If you have a large enough space, then how big is this concern over a HPHW "stealing" heat from a basement? Maybe I'm crazy, but I think about these units that "move" heat energy around in general, including heat pumps, fridges, etc.. Take our freezer: its in the basement, and we have a fridge-only unit (or so-called garage fridge) in the kitchen. Why this "weird" configuration? Well, I can ask the fridge-freezer combos to a 72F to 32F temp drop, so from room temp to freezer temp; or to do a 50F to 32F drop, so basement temp to freezer temp. One looks obviously more efficient than the other. And we _tripled_ our capacity of refrigerated & frozen foods while dropping our energy usage in _half_. I checked with a simple Kill-o-Watt. Nobody talks about this! Yet folks keep buying these huge, energy inefficient combo units and put them in kitchens.

        If one is serious about the math, it'd be even more ideal if all fridges/freezers were in the basements, not kitchens! but we often compromise on convenience. And the fridge/freezer should be accused of "dumping" heat into the basement, no? You never heard about that. Yet I hear talk all the time about "stealing" heat by HPHWs in basements. Is the temp in an unfinished basement a huge concern for you?? Compared to the energy needed condition the space inside a fridge/freezer? If not, go for it.

        I'd like some consistency in recommendations about all these heat "moving" appliances if folks are serious about the implications.


        1. calum_wilde | | #8

          A fridge or freezer, for the most part, just moves heat around, as you described. Unless you cool something and take it out of the house, but that's trivial.

          A water heater doesn't just move heat around though. The vast majority of the heat energy ends up going down the drain with the water. In that regard, seeing a HPWH as removing heat from the home is accurate.

      2. Expert Member
        Peter Engle | | #7

        If your panels are oversized and you can't sell excess back to the utility, then the excess power is indeed free. If that's likely, go with the HPWH, hands down. And buy an EV, too.

        Does your utility let the meter spin backwards (effectively), but then zero you out at the end of the month, or do you just never get any credit when you have excess capacity? If you never get credit for excess production, a battery would be a great idea, so long as its software can be set to soak up excess production on a minute-by-minute basis.

        As for comments in #6, I agree that we should consider all heat "moving" equipment, as it does have an impact on loads. You are absolutely right that it's more efficient to run refrigeration equipment in an unconditioned basement in most houses. But most of us don't want to go to the basement every time we need a beer.

        We don't usually talk about this sort of equipment "heating" the basement because additional heat there is generally a plus, not a minus. OTOH, a HPWH will take a few degrees off of an already cool basement. If it's just used for storage, not a big problem. But if it's used for workspace or living space, that heat has to be replaced so there's a net increase in energy use. Not huge, but it needs to go in the spreadsheet.

  4. calum_wilde | | #9

    Given your climate, I don't think HPWH make sense. I'm in a very similar climate and after running the numbers several times I just can justify the costs. Indirect water heaters burning natural gas would likely be cheap, but the amount of heat they throw in the summer is staggering. At least the ones I've seen.

    Personally, I'd highly suggest you install the biggest drain water heat recovery you can fit. I installed mine as a DIY project, it's really not that bad of a project.

    Next up, go on Amazon and pick up a Niagara Earth shower head and give it a try. Worst comes to worst, you lose $8-$20. Best case scenario you use half as much water hot water for showers (which for most of us should be the majority of our hot water usage). My wife has thick, curly, long hair and is completely fine with the shower wand style (1.5gpm) one in her bathroom. I have the 1.25gpm shower head in mine and like it also. The lower flow rate will also increase the efficiency of the drain water heat recovery.

    Potentially, with just ~$700 worth of parts and a few minutes of your time, you'll have just cut your hot water costs by ~¾ and you'll have extended the number of minutes of shower on time - a much more useful measurement IMO - by the same ~¾.

    I have this exact setup, and it works great for us. Even with a simple 40gal resistance electric water we never run out of hot water, and the bill is extremely reasonable. (I also have R10 on the sides of the water heater, R20 on the top and bottom, and R6 pipe insulation on the cold water in, how water out, and the t&p valve and pipe. After measuring, we use an average of about 3kWh/day for hot water. With the insulation I might have paid $900 CAD, to upgrade the efficiency of my water heater. I'm getting better efficiency than I would have if u had just bought a HPWH and even if I do have to replace the water heater I can still get the cheapest thing in the store and still have the same efficiency.

    If you've made it this far, I'd also suggest a powered anode to replace the sacrificial one if you get a traditional tank style water heater. It beats the hell of of replacing anode every other year.

  5. NEplumber | | #10


    Go with the option that has less wheels on the bus, the indirect.
    Cost of ownership will be less, you can run the tank hotter to prevent legionella bacteria without seeing a plummet of COP.

    My beef with heat pump water heaters are they dont seem to publish the COP based off of different set points.
    Its easy to achieve a 4.0COP (as an example) using 115f water. Take the set point to 140 to prevent bacteria growth or get more hot water out of the same small foot print and the COP goes down.
    This same idea happens with VRF inverter heat pumps. The COP is rated at 47f outdoor air. Many manufactures now rate the unit at multiple out door air temps so you can see how the unit will fair in your part of the country.

    Moe Hirsch

  6. Expert Member
    Dana Dorsett | | #11

    How much hot water do you need/use?

    At 50-70 gallons/day of hot water use a heat pump water heater is enough to take a bite out of basement dehumidifer use in my area (comparable wintertime design temps and summertime outdoor dew points), but isn't going to be a huge factor in lowering the latent load for the whole house at ASHRAE 62.2 ventilation rates.

    14KW of panel is a lot for a house that isn't heated by a heat pump. Was that sized to be able to cover a pair of Teslas in the driveway?

  7. AppliedBuildScience | | #12

    I agree with Dana on the PV sizing---doesn't make much sense. How big is the house?

    I have a 6200 SQFT home, with 2-48k BTU heatpumps (100k BTU total), the whole house is electric dependent(with 80 gallon HPWH from Rheem) and I sized my PV capacity just right with a 14.88KWH array. I average about 80KW of production per day and my homes consumption (on a given extreme hot or cold day) is 50-60KW. So I'd say a 14KW PV array without heatpumps being used is overkill, fortunately this means you have tons of free electricity available so use as much of it as possible and electrify everything!!! (especially if you are net-metered)

    1. Expert Member
      Dana Dorsett | | #13

      >"'I'd say a 14KW PV array without heatpumps being used is overkill, fortunately this means you have tons of free electricity available so use as much of it as possible and electrify everything!!! (especially if you are net-metered)."

      +1 !!

      If you're producing 2x more power than you're currently using, investing in heat pumps for space heating/cooling may pencil out favorably, unless you are being actively paid at the residential retail rate for the annual excess. (Most states allow the utility to zero-out the balance annually rather than writing you a check.)

  8. walta100 | | #14

    If we assume Brian has excess solar electricity in the summer and not in the winter. The most economical option may be an indirect WH for use in the heating season and a standard resistance WH for the rest of the year.


    1. Expert Member
      Akos | | #16

      You can get a sidearm connected to the outlet of the boiler installed on a heat pump/resistance water heater.

      If the boiler is running, the most of your hotwater will come from it, if not, the heat pump will kick in.

      Simpler to install and much cheaper than an indirect. The only issue is if you have low temperature heating instead of mixing valve at the manifold, the boiler output will not be hot enough to heat the water.

  9. Expert Member
    Peter Engle | | #15

    Or buy a Tesla and shower at the gym.....

  10. Expert Member
    Dana Dorsett | | #17

    >"If we assume Brian has excess solar electricity in the summer and not in the winter"

    Why would we assume that?

    It might be the case if the house was heated with heat pumps, or if all the PV was facing north. For most medium sized houses/households with reasonably efficient lighting & appliances (including heat pump water heater) 14kw of panel would be producing a surplus even in the winter at Chicago's insolation levels.

    At my local utility rates a heat pump water heater would be ho more expensive than running an indirect any season of the year. YMMV. Even if it were marginally more expensive in one season compared to another, the net difference would not be enough pay for an indirect or even a sidearm solution over a typical lifecycle.

  11. AppliedBuildScience | | #18

    14KW for a home without heat pumps as a main source of heat will be producing excess electricity all year round (unless home owners are running a server farm in the house). Free electricity means free heating of hot water via HPWH. It's a serious no-brainer.

    It's also eco-freindly. You are using (at least logically) Solar energy to heat water versus a fossil fuel. In this day and age if you have the option to move away from fossil fuels without any impact then it should always be the option.

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