Posted By jonr on 25 Aug 2012 09:01 AM

In summary, half the heat comes from your grid, at a cost/BTU the same as a conventional tank.

You are adding to the confusion. For example, the above is not true when comparing a HPWH to a conventional nat gas water heater (a common case).

Not trying to confuse.  Ok, you can read my note as 'conventional **electric** tank'.  The point is that compared to a conventional electric tank, a HPWH will be cheaper to run in all 12 months, in a heating climate or a cooling climate (unless your space heating BTUs cost more than electric resistance BTUs, in which case you have other problems.).  When I was researching my purchase of one, I read a ton of stuff that suggested otherwise....not unlike some comments in this thread.

I'm pretty sure we're on the same page- it will NOT be cheaper to run in the winter in a heating dominated climate in a resistance-electric heated home. But it'll be cheaper to run when there's a cooling load.

But whether it has a financial rationale within the lifecycle of the equipment in locations with 8-9 months of heating-load and only very modest cooling loads and short cooling seasons is a dubious proposition, requiring more in-situ data from carefully & separately metered the cooling, heating, and HW heating loads than has been published to date. If the heating season is 8 months out of the year it's going to be a hard financial argument to make with resistance-electric as the space heating source.

Adding a gas HW heater and gas heating to the mix confuses the situation, but rare is the cool-climate market where the HPWH would be cheaper to run than a gas HW heater in a house with decent-efficiency natural gas heating + central AC. In climates/markets where the annual AC costs equal or exceed the gas heating costs it might be close, but the better first-hour gallons of a gas-fired tank might still tip the balance from a "What should I buy, a gas WH or a HPWH?" perspective.

In some markets resistance electric BTUs are now cheaper than propane or oil BTUs at recent fossil-fuel prices (which is indeed a problem!) But in those situations you's get more bang per buck plowing the HPWH + electric baseboard money into ductless air source heat pumps to substantially offset propane/oil use.

Without a flame, it should be possible to make these heaters with plastic (eg, PEX or PERT) lined tanks. If the heat pump is also well made, then these could last as long as tankless heaters.

Posted By Dana1 on 27 Aug 2012 03:51 PM
In some markets resistance electric BTUs are now cheaper than propane or oil BTUs at recent fossil-fuel prices (which is indeed a problem!) But in those situations you's get more bang per buck plowing the HPWH + electric baseboard money into ductless air source heat pumps to substantially offset propane/oil use.

How about when the climate is mixed, with both cooling and heating loads. So for the Chino Valley, AZ area, do you think a Hybrid Tank like this is worth it in the long run?

Ok, Dana.

We def agree that a house with NG should burn gas both for space heat, and then either a standalone or indirect fired gas DHW tank would be the way to go. That is indeed a very common situation in the US.

However, we often find places that don't have NG, and it seems that an emerging theme is that well-designed modern ASHPs or mini-splits are a pretty ideal solution for many of those homes. Specifically, in many places they look better economically than geos.

So, I am thinking about that case...if you are using a HP for heating (ASHP or geo) does it make economic sense to buy a HPWH?? Even if you live in a heating dominated climate?? Let's go.

Hypothetically, say I get an SCOP = 2.5 on my space heating, and run a HPWH with a nominal EF=2.2 in my conditioned space. The latter includes tank losses, so its average COP (esp running in a warm conditioned space) is probably also ~2.5.

During cooling season, my elec costs 1/2.5 or 40% of an electric tank (my other goto option without NG). That is, my DHW BTUs cost 0.40 of the other option. Whether I get useful cooling or not is hard to say....I probably don't circulate the air, and their big fans limits temperature drop and dehumidifcation. And my central system has a higher COP in cooling (>4).

Now, during heating season, the unit is making 40% of its BTUs from grid power and stealing 60% from the space heat. The stolen BTUs are made by my ASHP at COP=2.5 (averaged over the season). So, a DHW BTU costs me:

0.40*1 + 0.60*0.4 = 0.64 relative to a electric tank BTU.

So, in a summer month, the HPWH costs 60% less than an electric tank.
in a winter month, the HPWH costs 1-0.64=36% less.

If we heat for 6 mos/year, then our seasonal average savings are still 1 - (0.52) = 48% cheaper than an electric tank, vs 60% cheaper than in a place with no heating (Key West?). Simple payback is only 25% longer than in a no-heating climate.

We could argue that the cooling effect is economically relevant, but on the other side, stealing heat from an unfinished basement might not boost the space heating demand on a 1:1 basis, etc.

My argument is that in a HP heated home in a climate that gets real winter, the HPWH still saves ~50% on a seasonal-average basis, NOT counting the cooling benefit at all, but scoring the heat stealing at 100%. For a family of 4 with typical electric costs, an electric tank might cost $500/yr, and the HPWH would save $250/yr. IN a cooling dominated climate they would save $300/yr under the same (conservative) assumptions. If the incremental cost of the HPWH is ~$1000, this is a simple payback of 3-4 years on units that have a (limited) 10 yr warranty (or faster payback if you get rebates or score the cooling benefit).

IOW, I see the logic of the rebate program in MA. Their customers will reduce their demand significantly, esp during the summer peaks (reheating the tanks midday after morning showers).  By my reckoning, those oil-burners in MA are still getting their BTUs for ~60% of the cost of electric resistance heat....and the HPWH will still save those customers money in January (just about 25% relative to an electric tank).

In those parts of the US where readers might see HPs as an optimal space heating solution, HPWHs are an excellent, money saving and energy efficient addition to those homes.  Suggestions that heat stealing destroys the economics or energy savings in most US climates are not correct.

I agree with that. Personally, the choice between a HPWH and a tankless nat gas water heater would come down to issues other than operational cost. As the stickers say, it's close. I would not choose propane or electric.

Nobody has made the argument that houses heated & cooled with heat pumps wouldn't see a net benefit with a HPWH, quite the contrary (though for GSHP with desuperheater HW capacity the numbers would work differently.)

The net benefit to MA customers during the summer still don't add up to the retail price of the unit over it's expected lifecycle, which is why it NEEDS to be subsidised. The primary financial benefit is to the utility and to the entire rate-payer base benefitting from the lowered peak power, not to the owner of the HPWH. What's surprising to me isn't that it's subsidized, but the AMOUNT to which it is subisdized (which is nearly the full purchase price of the unit in some areas, making the installed cost cheaper than a standard tank), and a larger cash rebate than for installing a high-efficiency mini-split heat pump.

The reckoning on oil-boiler heating hot water vs. a standard electric tank needs adjustments to reflect typical efficiencies & heating oil/electric rate prices. The HW heating efficiency of an oil boiler is pretty abyssmal when there is no space heating load, unless the boiler is outfitted with an indirect tank (rather than an embedded tankless coil) and heat-purging boiler controls (recommended), and is a "cold start" type that can be regularly fired up cold without damage to it's heat exchangers.

A "typical" system in MA runs at between 35-40% efficiency in HW-only mode, even with an indirect. See Table 2:

http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf

During the heating season at the typical 3x oversizing it's more like 70-75% (see table 3 in that document.) If you assumed an annualized HW heating efficiency average of only 60% (which would be on the high side), at $4/gallon you're looking at ~$48/MMBTU delivered to the water.

If you're looking at the cost of the space heat extracted by a HPWH use 75% rather than 60%, that's still $38/MMBTU- in, but you still have to account for the tank's standby loss. At typical insulation levels for a 0.90 EF tank or HPWH that would make it ($38/0.9=) $43/MMBTU net.

One MMBTU=293kwh, but in straight-ahead 0.90 EF electric tank it takes (293/.9=) 325kwh. At 15 cent big-utility rates from say (about the combined residential power + delivery cost through National Grid) that's about $49/MMBTU, for the electric portion of the heat going into the water.

So yes, at $4 oil the heat extracted from the heating system from an HPWH comes at a slight discount, at $4.50 (like many I know were paying this past season), not so much.

Predicting oil pricing over the lifecycle of a hot water heater is something of a fool's errand, but the 20 year or decade-trend hasn't been encouraging, more than quadrupling since the mid-'90s low:

http://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W_EPD2F_PRS_SMA_DPG&f=W

If you're on one of the local municipal-systems with ~12 cent electricity (rather than NStar or Nat'l Grid) you're better off NOT stealing the heat from an oil-fired heating system- it would be cheaper to heat that water with resistance electricity in winter. With an HPWH you'd get maybe a 50% discount for 3-4 months, but it would cost more during the other 8 months.

If you rely on AFUE efficiency numbers for making the per-BTU cost analyis for oil it would be a mistake. Unless the boiler is a latest-technology smallest-in-class versions, the Brookhaven Nat'l Labs test data indicate true efficiency numbers well below any AFUE-test, and that's for boilers tuned up in a test lab, not the typical residential system that hasn't seen a burner-tech since it was installed (or since the last time it backfired or wouldn't fire up.) Even the smallest oil boilers out there have output 1.5x or more output than the design condition heat loads of the houses they are heating (which would be small enough to hit their AFUE numbers- AFUE testing assumes 1.6x) but energy-audit surveys in central MA indicate 3x oversizing is more typical, and that's consistent with my (albeit limited) direct experience.

The picture for $/MMBTU for propane (even at 95% condensing efficiency in a right-sized mod-con) isn't as rosy as oil, at recent prices.

I've said my peace about the merits of the these units in a HP-heated house w/o NG. I'm glad we all agree.

On the oil thing, I was reckoning from $3.50/gal (averaged over the last few years), 18 kWh elec, 80% space heating eff and not counting the tank losses from the boiler as losses (during the heating season).

Until I had it torn out, I had a 1990s vintage 1.2 gph oil boiler, and it was burning through ~0.9-1.0 gallon/day during the summer just to fire a tankless coil, at least 70% of which was standby losses or other inefficiencies. Thus, during a 150 day summer season, I could easily burn through 140 gallons, and make a bunch of heat to pump out with my AC. At $4/gal, that $560 for oil plus another ~$100 for the additional AC, for a summer DHW cost of $660 ! Installing a conventional electric tank (for summer use only) would make my DHW for ~$200, saving me $460/yr. OF course, shutting down the boiler for the summer is a bit dicey, but not as rough as trying to run it cold start on a recurring basis. (And a lot of folks shut down their boilers for extended periods). A HPWH running during the summer should have a COP ~2.5, so it could run all summer for $80, and maybe save a little AC on top of that, so I would save say $540-560 per year just running it in the summer relative to oil.

So, in the age of $4 oil, it makes sense to have some sort of summer backup and to shut down the boiler. An electric tank has a pretty nice ROI ($460/yr), and a HPWH will save an additional $1200-1500 over its 10 year warrantied life (and might last longer only running in the summer), relative to straight electric. This number doesn't seem too bad relative to the incremental cost relative to a 10 yr conventional tank.

And if the home owner elects to insall some minisplits to heat the house during the shoulder seasons (in a future period of even more expensive oil), then I can save even more with my HPWH by running it more months.

In the age of $4 oil all SORTS of things become economic if it cuts down on oil use! (And tankless coils just plain SUCK, both on flow/output and efficiency.)

A 1.2gph oil burner is about 165KBTU/hr-in/140KBTU/hr-out. Most pre-1980 mid-sized homes in MA that have had ANY sort of air-sealing or insulation upgrades since they were built still come in at under 50KBTU/hr, and often under 40KBTU/hr. With that type of oversizing, even if the AFUE or DOE efficiency of a freshly tuned burner might be in the mid-80s, the as-used AFUE at the fractional load is in the low to mid 70s, not more.

Check out the regression curves in the appendices of that Brookhaven Labs document- without heat purge and cold-start it's real part-load CLIFF. With 40K of space heating load at design condition and 140K of burner output it's never operating at more than the 30% point, and averaging near the 10% point on the curve! With an indirect HW heater and a heat-purging economizer (eg. Intellicon 3250 HW+) you can move the knee of the efficiency curve a bit higher and to the left, but it won't move too far unless the boiler is cold-start tolerant. http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf

In a house in MA with a 40K design condition heat load a 2.5-ton mini-split/multi-split is capable of reducing oil use by about 70% or more. Even though the oil-burner maybe necessary to support the design-condition load, the mean January load is only ~65% of that load, and the rest of the time it's lower still. At $4 oil and 15 cent electricity and an annual COP of about 2.7 (some do a bit better, some will do a bit worse), simple payback on the ~$6-7K upfront cost of the ductless is usually under 5 years.

Subsidies for HPWH are usually only extended to those already heating hot water with electricity, even though the financial argument even just for summertime-use is compelling for those heating water with embedded coils in oversized oil boilers. (To the grid as a whole it's still an additional load, albeit a lower load than a standard electric tank.) But applying the money toward a ductless & air sealing is still going to be a better bang/buck in most (but not all) of those houses.

The Brookhaven numbers were interesting....there is so much effort put into rejiggering controls and indirect tanks for these old-tech oil hydronic systems, and frankly, I think its all lipstick on a pig. The basic tech of a segmented, cast-iron, non-condensing oil-fired boiler....blech. Grew up with one + tankless, owned one myself (w/tankless) recently for several years.

I bought a v poorly airsealed 2300 sq ft house near Philly in 2005, with oil hydronic/tankless, and burned ~1400 gallons/yr to be uncomfortable in the winter, and to take showers that 'never runs out of hot water', but which feature wildly swinging temps and you better pray no one runs a faucet (let alone a second shower) at the same time.

After a lot of airsealing, I now have the typical January demand down to ~24 kBTU/h, maybe 40k max, and space heat with a 4 ton conventional split system HP. DHW from a AO Smith HPWH in an attached, semi-conditioned garage. Heating and DHW are currently ~9000 kWh/yr, with further room for some improvement.

And the oil boiler has gone to the scrap yard.

We couldn't agree more- even the latest condensing lowest-mass triple pass oil boilers aren't worth owning if you live in a climate where you can take advantage of heat pump technology (which includes vast majority of the lower 48 of the US now that mini-splits can run at -25C/-13F.) There's no prospect for the decade-average price of heating oil to fall much going forward (barring a worldwide economic depression that would make the 1930s look like a picnic), and there will continue to be jarring price volatilty no matter how fast the poke holes and pump.

The worldwide oil production rates will not keep up with the increased demand from developing-country transportation fuel needs, which will continue to drive the price, even as developed countries cut back. A sustained price of $100/bbl has not increased pumping by more than a few percent over the past 4 years. It went from ~84mbbl/day in 2008 to maybe 88bbl/day now mostly from increased Saudi production, as much as people like to cheer about North Dakota & Brazil coming on line.

I fully expect to see $3 heating oil again, (if only briefly) but it would take $2 oil (or cheaper) to be competitive with heat pumps in all US markets except diesel-fired generation on isolated island-grids. The scrap yard is exactly where most of those boilers belong.

Our current oil policies are like a guy in the desert with only one bottle of water - drink it fast, it will be gone soon. The result will be a hastened, abrupt and traumatic withdrawal.

... or a major bloody fight over the last 3 swigs.

Agreed - the fighting has already started.

I've looked around quite a bit but can't seem to find any split HPWH, other than the Daikin Altherma solutions. I do not want to install the WH outside, so a split unit make sense but all that I can find are self contained.

And all seem to be for water only, not space heating. I'd like to see if there are less expensive alternatives than the Daikin solutions, although I may just acquiesce and budget for the Daikin.

Daikin has several split water heaters for the Asian market (including the EcoCute consortium in Japan, using CO2-refrigerant in split hot water heaters.) There's some discussion of releasing an R410A refrigerant split system in Australia this year, but if there is plans for the US market it's in stealth mode.

Since the topic came up again- Does anyone make a reasonable, reliable heat pump water heater? I almost bought a GE unit which was half price at Home Depot ($650) until I did some research on the reliability. It is dismal on all of the lower priced units i could find. People wound up having the unit repaired or replaced repeatedly until giving up and going back to resistance heating. My ICF basement is ideal, ranging from, 55 to 75 degrees year-round. I'll probably go old-school for the time being, as I don't want the aggravation of a failing WH.

Not speaking from personal experience, but A.O. Smith apparently makes a good one. Perhaps others can post their personal experiences or anecdotes on the AO Smith units? The GE units have a massive amount of bad reviews out there (Amazon alone)

I wonder when the Fujitsu Waterstage air to water heat pumps will be available in the US. I suppose if you really want one, you just order from the UK. Or a Panasonic Aquarea T-CAP.

Posted By jonr on 17 May 2013 11:20 AM
I wonder when the Fujitsu Waterstage air to water heat pumps will be available in the US. I suppose if you really want one, you just order from the UK.

I'd be really wary about trying to hook up a high-power device designed for 230VAC/50Hz into a US power grid without prior sign off from the manufacturer.

I'm not surprised that the other Japanese mini-split builders are getting into the European hydronic-output ASHP market though.