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Heat pump hot water heater tank size

Family of 4, 2.5 baths, dishwasher and washing machine in our house. We have a 20+ year old RUUD electric hot water heater w/ a 50 gallon tank.

We are most likely going to upgrade to a heat pump hot water heater - we live in Virgina, have an unconditioned basement that could use dehumidification, and have no gas connected to our house.

My question is what size HW heater to get. My initial thought was just another 50 gallon one, because our current HW heater has seemed sufficient. But then I read this:

"All of the researchers I talked to emphasized the benefits of a large tank size, so don’t buy the G.E. unit, the Rheem unit, or the smaller AirGenerate unit. “Bigger and hotter tanks are better,” said Aldrich. “It’s counterintuitive.” - http://www.greenbuildingadvisor.com/blogs/dept/musings/heat-pump-water-h...

The 80 gallon AO Smith and Steibel Eltron models are at least $1000 more than the 50 gallon GE model. How important is it to upsize? - that's a huge price difference. Anyone with a similar set up to mine using a 50 gallon hybrid successfully or having buyers remorse?

Asked by austin jamison
Posted Fri, 01/17/2014 - 20:34

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13 Answers

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1.
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Quick showers equal smaller unit.

The only way to care about costs,,,, is to "care" about costs.

There is no free lunch, but you already know that right?

aj

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 01/17/2014 - 21:29

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I would go with the 50. Low flow shower heads, wash laundry with cold water, etc. We have a standard electric 40 in our house, no issue at all. I see way too many huge water heaters in my travels.

Answered by David Meiland
Posted Sat, 01/18/2014 - 01:54

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David,
With all due respect, your experience with an electric-resistance water heater is irrelevant to the question raised by Austin, which concerns heat-pump water heaters. (You wrote, "We have a standard electric 40 in our house, no issue at all.")

As I wrote in my article ("Heat-Pump Water Heaters Come of Age"), "While the electric-resistance elements in a typical water heater can heat 20 gallons per hour, a heat pump can only manage about 8 gallons per hour (or even less, if the ambient air temperature is below 68°F)."

A family that is satisfied by the performance of a 40-gallon electric-resistance water heater won't necessarily be satisfied by the performance of a 40-gallon heat-pump water heater. The only way that the heat-pump water heater is going to keep up with the performance of the electric-resistance model is by operating in (expensive) electric-resistance mode.

I stand by the advice I gave in my article: if you want a heat-pump water heater, choose one with a big tank.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 01/18/2014 - 07:02
Edited Sat, 01/18/2014 - 07:04.

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Martin is correct - efficiency and comfort are improved with larger tank sizes for HPWH's.

Having said that, you MIGHT be able to get away with a 50 gallon HPWH. I have a first-generation GE unit. I'm also Virginia, with 4 occupants (2 adults, 2 small children), 1.5 baths, efficient dishwasher and washing machine, compact house layout. Most clothes washing is with cold water. We run the HPWH in "heat pump only" mode (no electric backup) set at 120F, 1.75gpm showerhead. Our big hot water draws are generally well spaced apart...typically 2 showers in the morning and a bath every other night for the kids. Dish washing is mostly in the evening. It's very rare for us to have enough clustered hot water use to run low....but then again, our hot water draws are ideally suited to a HPWH as well.

Answered by John Semmelhack
Posted Sat, 01/18/2014 - 11:43

5.
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Martin, I'm sure you're right about the way these heaters work, and my advice might be off for the OP.

My comments are based on a couple of simple observations. As I mentioned, we have a standard 40, and when we were remodeling the house in 2012 we lived in our small studio space, which has a standard 20 and a low-flow shower head. There were no issues with two consecutive morning showers in that space.

In our market, the 50-gallon Geospring heaters are at very low price points due to rebates. I saw one in use last week that had cost a net of about $375. By comparison, a standard 50 would cost me about $650 wholesale right now. I know of several now (even though I don't think them particularly suited to this climate) and have heard no one complain. Of course I don't know which mode they are running them in, and that's an important question, but I would be hard pressed to spend the large amount of extra money for an 85. Water heaters leak and fail, often prematurely, and a complex mechanical water heater well over a thousand bucks is a questionable use of money IMO. It would have to last a very long time and save a lot of dough to be worth the up-front cost.

One more thing--in our house we have taken your advice and wash clothes in cold water now. No difference in how they come out, at all. So, IMO, laundry is seldom a hot water user. We run the dishwasher at times when no one is about to take a shower, and it heats the water to where it wants it anyway, so not affected by a lukewarm tank if that were the case.

So, I think that for a household that can manage their hot water draws, a cheap 50 HPWH could easily work out. In our house, we think about stuff like that. We heat with wood, which requires active management since there's no thermostat. In a house where everything is supposed to operate without thought or inconvenience, you might need other systems.

Answered by David Meiland
Posted Sat, 01/18/2014 - 12:55

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Martin, the posted question is in regard to saving money. All the responses here make sense as to saving money except yours.

To save money one needs to use less hot water smartly as David posts.

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 01/18/2014 - 13:15

7.
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Extra money would be much better spent on drain water heat recovery, in my opinion. It will boost capacity greatly and reduce energy use even further.

I have a family of four (though the kids are young and not taking long daily showers like teenagers) and the GeoSpring is adequate for us at 120° in heat pump only mode. If you need more capacity, you can turn up the temp or use one of the two "hybrid" modes, while you mull over your DWHR purchase.

Answered by Nick Welch
Posted Sat, 01/18/2014 - 18:47

8.
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Drainwater heat recovery extends capacity only for extended simultaneous flows, not batch draws like tub or washer fills, or short draws of a few gallons or less (hand washing, etc.) It's definitely worthwhile in households who prefer showering to tub baths. Back-to-back tub fills would usually require a significant recovery period in heat-pump-only mode with a 50 gallon tank, but with drainwater heat recovery back-to-back showers (of reasonable duration, not "endless shower" teenage bathers :-) ) aren't a problem.

Answered by Dana Dorsett
Posted Mon, 01/20/2014 - 15:02

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The insight has been great. It's basically coming down between a 60 gallon AO Smith Voltex or Nyle Geyser hooked up to an 80 gallon electric tank. I don't have the clearance in my basement for any of the 80 gallon integrated units.

Any way to compare the efficiencies of these two setups? I haven't seen a lot online comparing the Nyle unit with the all-in-one HPHWs. I think the 60 gallon Voltex would about $300-$500 less than the Nyle with a 12-year warrantied Whirpool electric 80 gallon tank.

Answered by austin jamison
Posted Wed, 01/22/2014 - 10:56

10.
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Drainwater heat recovery only works if the waste water pipes dump into a central vertical pipe. If they drop down from several different points upstairs and join under the basement slab... you're toast. I'm a victim of that practice done some builders in the mid 60's --- Seattle area.

An 80 gallon tank is pretty large... I guess the first question I have -- have they ever run out of hot water with the current 50 gallon tank?

The bigger tanks probably aren't 80 gallons of hot water -- but a larger hot layer of water in the upper portion of the tank -- correct? If so they are counting on the (volume) water stratification that would be lost by the smaller tank.

If your utility is offering rebates -- they may only be valid for integrated units. They may exclude the Nyle unit. :-(

Answered by Dennis Heidner
Posted Thu, 01/23/2014 - 01:07
Edited Thu, 01/23/2014 - 01:15.

11.
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Hi Austin,

I'm in Virginia too. I have the Airgenerate ATI66 (66 gallon model). It serves an extended family of three adults, two teenagers, and two children. Like John Semmelhack and Nick Welch, we operate ours in "heat pump only" mode, year round. We don't have any problems with capacity.

Having said that, we are fairly conservative with our water use. We use low-flow shower heads (1.5 GPM) and Energy Star rated appliances.

Most units have several modes of operation, which have an effect on capacity. If you are planning on putting the unit in an unconditioned basement, you could always change modes according to the season (use HPO mode in summer, hybrid mode in winter). These units definitely run longer in colder temperatures, when the "lift" is greater.

You ought to read Marc Rosenbaum's blogs on HPWHs. You will find some great information on several topics, including integrated vs. add-on units, effect on basement temperatures, and links to other studies. Here is one from GBA:

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/getting-hot-w...

Answered by Daniel Ernst
Posted Thu, 01/23/2014 - 11:04

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Dennis: "Drainwater heat recovery only works if the waste water pipes dump into a central vertical pipe. If they drop down from several different points upstairs and join under the basement slab... you're toast."

Huh? They work with ANY vertical segment downstream of the shower drain. If you have multiple showers that only join below the slab, that doesn't mean you can't have multiple drainwater heat exchangers (but the plumbing can get a bit complicated if they're on opposite ends of the house.)

Answered by Dana Dorsett
Posted Thu, 01/23/2014 - 12:30
Edited Thu, 01/23/2014 - 12:31.

13.
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Thanks for the Rosenbaum links. At the end of one of them he says he would go with an integrated until over a Geyser and separate tank, but I didn't quite follow why. Also, his comparison was with a Steibel, which is out of my price range.

One thing I should have added, is that both of my kids young (so will be teenagers down the road) and our family might not be done growing, so 2 small children, 2 adults might be different than 3 teenagers and 2 adults.

The questions for me are:

1) Will the Geyser/80 gallon set up be worth in terms of being able to stay in heat pump mode longer compared to the 60 gallon AO smith or will its standby losses to keep 20 more gallons of water heated override that?

2) Life expectancy of an integrated unit like the AO Smith versus being able to separately replace the tank when it fails in the Geyser/80gallon setup.

Answered by austin jamison
Posted Fri, 01/24/2014 - 15:02

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