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Community and Q&A

To HPWH or not?

Ben Balcombe | Posted in Mechanicals on

Our house has a 40 gallon electric water heater, currently located at the wrong end of the basement. When I remodelled the first floor I ran new pex to all the new kitchen and half bath, and also ran new pex lines up to the full bath. The pex lines run back to a manifold in the basement, which is located below the bathrooms to minimise pipe runs.

With a family of 5 (the kids are 6, 4 and 2) I don’t think 40 gallons is going to cut it as the little people get larger, and we add a second full bath at some point in the future. As I look to install a larger tank (located next to the manifold) should I go simple electric resistance, or take advantage of rebates and tax credits (if they still exist in 2017) and install an HPWH for pretty much the same price?

We have a separate meter for our water heater, so I can see over the last 6 months of living in the house we average about $50/month for water heating. I’m not sure how much we might look to reduce that by with a HPWH?

Heating is a Vitorond 100 oil boiler supplying baseboards, so natural gas/propane aren’t options.

Any thoughts/recommendations?

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  1. Expert Member
    Dana Dorsett | | #1

    The (always oversized for the space heating load) oil burner can probably deliver more than enough BTU to serve up endless hot water type showers, buffered with an indirect tank, and the additional load will improve it's as-used efficiency.

    How many feet of baseboard, and which model Vitorond 100? The very smallest version ( VR1 ) has 91KBTU/hr of output, enough to keep my not-so-superinsulated house warm down to about -75F outdoors.

    If the house is divided up into multiple zones &/or there isn't sufficient baseboard to emit the full output of the burner and the burner has 50KBTU/hr of burner output or more beyond the design heat load it may be better to install a "reverse indirect", a buffer tank full of heating system water with an internal coil for the potable water. That makes the tank the only zone the boiler sees, taking advantage of it's high thermal mass to guarantee reasonable minimum burn times.

    It's a nice, well insulated boiler with low idling losses, but there's only so much you can cheat the duty cycle and ignition losses before it eats into as-used efficiency. AFUE testing is done at a 25-30% duty cycle presuming a 1.7x oversize factor from the design heat load but even the VR1 would have a seasonal average duty cycle well under 10% at my house without a domestic hot water load. YMMV.

  2. Ben Balcombe | | #2

    Hi Dana,

    I thought the issue with oil fired hot water is that it's great in the heating season, when the heat is 'free', but in the summer you're running the boiler just to supply hot water and suddenly it's not so efficient?

    It's a VR1-22 with a Riello 40 F3 burner fitted with a 0.5GPH nozzle, controlled by a HydroLevel 3250+ hydrostat. We've got ~60' for baseboard, and a Beacon Morris K84 kickspace heater. The 3250+ is set at 140/190 low/high limits and the purge setting activated. Two zones of heat, each controlled by an Ecobee 3 and we heat to 68F.

  3. Norman Bunn | | #3

    The Rheem HPWHs have energy factors of around 3.5, which means they will heat your water 3.5x more efficiently. Assuming you use the same amount of hot water, your monthly bill will be about $15 and you will have $35 a month more in your pocket with the HPWH.

    If you can place it near the boiler, it will convert the waste heat into hot water. In the summer, it will provide dehumidification and cooling.

  4. Expert Member
    Dana Dorsett | | #4

    Let's run the napkin math on this:

    With the 0.5gph nozzle it's taking ~70K in, 60K out.

    60,000/60' of baseboard= 1000 BTU/hr per foot. With typical ~2" fin-tube that balances only at temperature well above the operating temperature of the boiler. With 180F average water temp (190F out, 170F return) typical fin-tube is good for about 600 BTU/hr per running foot, so you're looking at emitting 60' x 600BTU/ft= 36,000 BTU/hr at it's maximum, which leaves the other 24,000 BTU/hr of "extra" that's going to be slewing the system temp higher. At 145F AWT (150 out, 140F back) at the beginning of a burn the fin-tube is emitting about 350 BTU/hr per running foot, or about 21,000 BTU/hr with about 39,000 BTU/hr of "extra".

    The VR1 holds 9.2 gallons of water (which is higher mass than most oil boilers these days) the radiation and distribution plumbing another 2 gallons , 3 at most, so let's call it 12 gallons, or (x 8.34=) ~100lbs of water.

    At 39,000 BTU/hr (650 BTU/minute) the temperature is slewing (650/100lbs=) 6.5F per minute at the beginning of a burn.

    At 24,000 BTU/hr (400 BTU/minute) the temperature is slewing (400/100=) 4F per minute at the end of a burn.

    Rather than show the real math call it 5F/minute, slewing through (190F-140F=) 50F, for a burn time of 10 minutes. That's great if both zones are calling for heat at the same time, but those burn times get dramatically shorter when it's just one zone, since the emittance of any one zone at 145F AWT can't be more than (a 30' zone delivering) ~10,000 BTU/hr, with 50,000 BTU/hr ( 833 BTU/minute) of extra with a slew rate of over 8F/ minute as a best-case. The shortest zone will have less thermal mass and less heat emitting, and will likely be cycling a bit, averaging 5 minutes/burn or a bit less.

    This isn't the efficiency disaster that most oil boilers would see when under-radiated, helped out primarily by the higher water volume and 3" of insulation on the boiler (which is more than 2x the average.) You could go with a standard indirect and still be OK, but a reverse-indirect would cut the number of burn cycles by 3 or more.

    I left out the K84 to keep the napkin scribbles simpler, but if you'd like to include it in any of your own analysis, it puts out about 5K @140F entering water temp, ~9K @ 190F EWT.

    With a ~25 gallon reverse-indirect tank (eg Turbomax E23) set up properly the minimum burn times can triple, and the number of burns per hour (per month, per year) drop to 1/3 of what would be without the additional thermal mass. The insulation on that boiler is at least R10, could be R12 (3" of rock wool, most likely) and the standby losses pretty low compared to the typical, so the bulk of any summertime standby loss is really going to be from the indirect/buffer.

    With the HydroLevel 3250+ set up for purge control, using a standard indirect your system would most resemble system #3 in this set of bench tests, probably at an oversize factor of about 2 for the space heating (but it could be 3x- run a fuel use heat load calc to find out.) See Table 2 for summertime efficiency, and Table 3 annualized average performance:

    With a reverse indirect the summertime performance would be about the same, but annual efficiency improvement would be slightly better, it probably wouldn't be enough to make the upcharge financially rational.

    What you get out of an indirect (or reverse-indirect) is rapid recovery times and at least 100 first-hour gallons with that boiler, even with a 25 gallon indirect, more if you go with a 40 or 50 gallon indirect. It would take at least an 80 gallon HPWH to deliver anywhere near that much first-hour performance, and with two full-baths (potentially two tub-fills going on simultaneously or in rapid succession) you'll need at least 100 first hour gallons to fully cover the domestic hot water load.

    What you get out of a HPWH is basement dehumidification, and a slight reduction in sensible cooling load for the house, and a "harvesting" of some of the heating system's distribution losses to the basement (assuming it's in a basement, not a first floor mechanical room or a garage.) But it'll have to be a big 'un.

  5. Ben Balcombe | | #5

    Dana - you must have larger napkins than me! Give me a few days to digest all of that and I'll probably have some more questions!

  6. GBA Editor
    Martin Holladay | | #6

    Yes, Dana Dorsett special orders his napkins. He uses napkins that are at least 36" by 48".

  7. FrankFulton | | #7

    Dana's napkins come in handy in these parts.

    We just this morning took our first hot showers with our new Rheem 80 gallon HPWH. We've moved into a new home last week, and the 50 gal oil-fired HWH had burst, so this was our first step away from oil in the house. I took down a wall (perhaps a bit extreme) so the unit has plenty of space in the finished basement, adjacent to the 110k BTU oil furnace. It's COP is listed as 3.5, with 90 gallons first hour production, and 29 gallons/hour recovery after that. Delivered by HD, installed by local master plumber. FWIW, the Energy Star tag lists annual estimate operating costs at $150.

  8. Expert Member
    Dana Dorsett | | #8

    When I run out of space on the 36 x 48s I sometimes have to continue with some lipstick-on-mirror, but so far I've managed to avoid stooping to crayon-on-wall...

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