I have not posted in a while, been really busy building our high efficiency home. Along the way, I thought of something that has proven to be phenomenal, and a search could not find it so might be unique. I apologize if it has been covered. What if I told you about a HPWH that uses no energy directly and lessens your water heating needs by as much as 33%? And costs $170.00. It is simply a water storage tank placed in the conditioned space. My example: Here in Central Oregon our water comes out of the well at 49 to 50 degrees. So if you set your water heater at 120 degrees, you obviously have a delta t of 70 degrees. However, if you place a water storage tank that comes close to your average daily use in the conditioned space and let it slowly warm to ambient, which would be 70 degrees, you have just dropped your delta t to 50 degrees. When you use water for bathing etc, you don't usually turn on all hot but a blend of hot and cold. Because the cold water is 20 degrees warmer, you will use less hot water. In essence you have created a low cost HPWH that uses no fuel. Obviously if you have a very efficient heating system such as a ASHP or GSHP it is even better. We placed ours in a mechanical room, next to the freezer, so they enhance the space for each other. We are using a marathon electric heater as we have no gas, and the room is too small for a HPWH. If using a regular HPWH I would make sure they are separated as far as practical. Although our temps may be lower than some, this will work in any area as ground temps are typically in the 50 to 55 degree range. If you also use this water supply for outside irrigation, that would be in the warmer months and the extra cooling brought in with the water is an advantage. We are currently keeping the building at about 61 to 65 degrees and the storage tank is staying right in that range with minimal use. If one of the contractors does use 30 or 40 gallons the bottom of the tank drops to about 60 to 61 but the top shows ambient. Two added benefits are the storage tank acts as a settling chamber should we draw any sediment, and I have 85 gallons of fresh water should the power go out.

A tempering tank is only a HPWH if the heating system is a heat pump. If the house is heated with a wood stove it's a wood fired water heater, if the house is heated by propane, it's a propane water, etc. In the summer it would be a solar hot water heater. Tempering tanks have been used to lower the direct hot water heating costs for many decades, but the cost in interior space and other issues have left it off to the side of the road where modern homes are concerned. Among those other concerns:

Tempering tanks in locations with higher humidity air than Central Oregon can encounter copious condensation issues with 50F entering water. The summertime dew points in New England average in the mid-60s, with comparable well water temperatures to yours. That can create moisture/mold/rot conditions under the tank, and sometimes a puddle around the tank.

Tempering tanks that are allowed to stagnate in summer to temps north of 75F become a breeding ground for pathogens, including legionella.

To think that a storage tank located in the conditioned living space will lessen your water heating needs is pure folly. Your building heating system is now heating this larger volume of water to the conditioned living space temperature. So the total energy required to heat your water will now be higher than if you just used a standard hot water tank...

I don't get how the water heating need would be higher than with a standard hot water heater? If you're heating the house with a condensing gas furnace and heating water with an EF 0.58 gas-burner you'll have modest efficiency boost during the heating season. If you're heating the house with a heat pump but heating the hot water with a standard electricity it'll be an even bigger boost.

But the heating season isn't 12 months out of the year. In a high-R house in a zone 5 climate its about 6 months out of the year. A tempering tank will reduce the cooling load modestly during the cooling season.

But is it worth it? Not really, especially in climates where you would have condenstation issues. Most people would have a better use for the 4-5 square feet of floor area too.

Dana, the “total heating energy” (i.e., BTUs) required for the water holding tank option would always be higher than a standard electric hot water heater because of the larger volume of water that would need to be heated from the sourced water temperature to the room temperature by the house heating system. The actual "total fuel energy" (e.g., kWh) used and associated actual “total heating cost” would either be higher or lower depending on the specific heating system coefficient of performance and specific heating fuel cost of the house heating system versus the DHW heating system that is actually used. Perpetual motion and other free energy schemes are not possible...

Yes, a house heat pump (HP) heating system and standard electric hot water heater would nearly always have an actual higher "total fuel energy" and associated actual higher "total heating cost" than the water holding tank option.

The water holding tank option would always reduce cooling load and always have an actual lower "total fuel energy" and associated actual lower "total heating cost" since the cooling water is sourced from outside the living space...sort of like a HP...

As you indicated, condensation issues would be a big concern with the water holding tank option. I am not aware of any reputable designers or builders advocating this option these days. I think the experiments of the 60s and 70s pretty much made this option extinct.

OK, I used the term HPWH just as an analogy that it is acting as one by transferring heat from the room air to the hot water tank. Obviously it isn't as efficient as a HPWH but the 4 square feet to get 1/3 of the way there is a lot easier to design in than the 100 plus square feet that a HPWH needs to operate at Peak. Because this tank is sized to approximately 85% of our daily water needs, the flow of water for events such as a shower only changes the temperature in the tank a couple of degrees.

Although we still have the building at minimum (about 61 to 65) I don't see that changing much. A draw of 10 gallons or so make minimal difference. Although it is obviously best suited to dry climates, I think properly sized it would stay below dew point in many locations. Once again proper sizing will eliminate summer stagnation especially if there is any outdoor irrigation.

This is not pure folly as incorrectly stated above, of that were the case than all HPWH are also PURE FOLLY. We have a passive solar designed home. Additional Heating is supplied by minisplits HSPF 13,5 / SEER 30.5 These are far more efficient than The Marathon electric heater we are using, and most water heating systems. There is no gas available nor desired, and space would not allow a HPWH.

The total Heating energy required For the storage tank over a conventional water heater is also incorrect. The total requirement is a product of the amount of water used, not the container size.

Because todays home heating systems, and designs have on the most part become much more efficient than water heating technologies, this now makes more sense than it did before, IF it is properly sized and located.

If you are heating the holding tank via the conditioned space using your marathon electric heater, your actual "total fuel energy" and associated "total heating cost" will be higher than if you had used a standard electric hot water heater for the reasons I explained previously. If you were heating the holding tank via the conditioned space using your minisplits, your actual "total fuel energy" and associated "total heating cost" would be lower than if you had used a standard electric hot water heater.

Real HPWHs are NOT folly, but they are expensive and complicated (i.e., more prone to failure) and the actual breakeven point is often beyond the life of the system. If you carefully measured the increased energy used by your marathon electric heater to heat the holding tank water, you would better understand this situation.

Efficiency ($/gallon of hot water) wise, the extra tank is a win when you are heating the house with a mini-split, it's a win in moderate weather when the windows are open and it's a win when using AC. You have to weigh the savings (perhaps $120/year) vs the up-front costs and other concerns (like a tray plus drain).

Also see how low you can go on showerhead GPM - I'm fine with 1 GPM. Or maybe drain water heat recovery. It's unfortunate that a desuperheater isn't an option.

Posted By sailawayrb on 13 Oct 2016 12:23 PM
Dana, the “total heating energy” (i.e., BTUs) required for the water holding tank option would always be higher than a standard electric hot water heater because of the larger volume of water that would need to be heated from the sourced water temperature to the room temperature by the house heating system.

Not so. The higher stored volume doesn't increase the number of BTUs it takes, and it does not increase the standby loss. The total amount of BTU only changes with the daily volume USED, not the amount stored.

Not so Dana? Well, lets run some numbers on an example and see what we can learn and conclude...

Lets assume we have cold water entering our building at 50F, our hot water output setting temperature is 110F, our holding tank volume is 500 pounds (i.e., approximately 60 gallons), and our conditioned space heating system maintains the conditioned space at 70F which tempers our holding tank water to this temperature. I will work this out in pounds as one BTU is defined as heat energy required to raise the temperature of one pound of water by one degree Fahrenheit.

Let our example be taking a shower for 10 minutes and use our hot water at the rate of 10 pounds/minute (i.e., approximately 1.2 gallons/minute). So we will only use 100 pounds total or 1/5 of our holding tank tempered water which should allow us to reasonably assume that the tempered water stays close to 70F for the entire duration of our shower.

If we only use a standalone tankless electric water heater to heat our 50F water to 110F, our tankless electric water heater will need to provide 6,000 BTUs (i.e., 10 pounds/minute x 60F delta temperature x 10 minutes).

If we then feed 70F tempered water from our holding tank to our tankless electric water heater, our tankless electric water heater will only need to provide 4,000 BTUs (i.e., 10 pounds/minute x 40F delta temperature x 10 minutes). Wow, that’s a 33% reduction in water heating!

Oh, but wait...we still need to account for the BTUs that our conditioned space heating system will also need to provide in order to create and maintain this wonderful source of 70F tempered water. Initially for startup, the total holding tank volume of 500 pounds x 20F delta temperature equals 10,000 BTUs. Subsequently, the actual volume of water used of 100 pounds x 20F delta temperature equals 2,000 BTUs. Wow, I just lost my 33% reduction in water heating...and initially even more depending on the volume of my holding tank!

So for this example and in summary, the holding tank option initially requires 10,000 BTUs for startup and subsequently requires 6,000 BTUs (i.e., 4,000 + 2,000) for the actual shower. The standalone tankless electric water heater option only requires 6,000 BTUs period.

In terms of “total heating energy” (i.e., BTUs), the holding tank option can never be better than a standard water heater...the recurring BTUs will always be identical to a standard water heater and there is also an initial BTU penalty for startup that increases with the holding tank volume. The only way to make the holding tank option a winner (i.e., only in terms of operational cost...because it can never be a winner in terms of “total heating energy”) is to have a conditioned living space heating system that has a higher COP and/or uses a lower cost fuel than the water heater system. I don’t believe that is the case here. My principal objection was with the OP claim of a 33% decrease in water heating that neglected to account for the additional 33% increase in water heating that must be provided by the conditioned space heating system.

So where exactly is my thinking, physics and math flawed?

Here is the simple way I see it:

Heating makeup water with an electric resistance heating element = COP of 1.0

Heating the room and effectively the makeup water with a high efficiency electric heat pump = COP of 3.0 (would be less in very cold weather)

This implies the room heating via heat pump uses 1/3 less energy to heat that water than if it had been heated via the electric resistance heating elements in the water heater.

And in the summer, heating the makeup water from 50 to 70F is free and actually reduces load on the heat pump in air conditioning mode.

That's precisely how I see it too. However, the OP indicated he is heating the holding tank with an electric resistance type of heater (i.e., a COP of 1). So there's zero benefit in the "total fuel energy" and "total fuel cost" during the building heating season. There would be a benefit during the building cooling season.

That's crazy. I said the holding tank was being warmed by ambient air. A practice you called pure folly.
Why would anyone heat a storage tank with a resistance heater? As I said the building is being heated by a combination of passive solar and a Hyper heat Minisplits which some advertising is claiming a COP of nearly 4.0
The passive solar has a COP of infinity
This warmed water is then being introduced to the Electric water heater at a Delta t of 50 Degrees instead of 70 degrees.

and as noted above, that is at the most inefficient time of the year, the heating season. during summer and shoulder months it is either greatly reduced or as noted a net positive. If you consider all the facts including seasons and utilizing heat from the adjacent freezer, I suspect the savings is more than 33%

The reference to startup just muddies the waters as startup was before any heat was on in the building.

Well, that clarifies that portion for me. I was under the impression that you had the holding tank in another room that was being heated with an electric resistance type of heater. In fact, you are heating the holding tank with your mini splits and passive solar. It is your water heater downstream of the holding tank that is an electric resistance type of heater. In this case, you will indeed have a significant benefit in the "total fuel energy" and "total fuel cost" during the building heating season.

Yes, if you initially heat the holding tank without using your conditioned living space heating system (mini splits and passive solar), you can indeed disregard the startup BTU penalty. So in this case, in terms of “total heating energy” (i.e., BTUs), the holding tank option can never be better than a standard water heater...the recurring BTUs will always be identical to a standard water heater. When you stated "that uses no energy directly and lessens your water heating needs by as much as 33%", that implied to me that you were talking about “total heating energy” (i.e., your "heating" BTU "needs"). In fact, you were really only talking about "total fuel energy" and "total fuel cost". I would suggest that had you instead stated "lessens your electricity cost by as much as 33%", this would have been much clearer...as least to me.

My apologies, I tend to get very precise about this stuff...like when someone states that a heat pump system is "more efficient" than an electric boiler. Efficiency should be reserved for describing how well a device converts energy from one form to another. An electric boiler has an efficiency of 100%. There is nothing more efficient than an electric boiler at converting electricity fuel into heat BTUs. Heat pumps don't convert energy. Heat pumps steal heat energy from the environment and than convey it into a building using very little electricity. So one shouldn't talk about efficiency when one talks about heat pumps. One should instead talk about COP and use a COP of 1 for the electric boiler when comparing it to a heat pump and calculating "total fuel energy" and "total fuel cost". Anyhow, I think we are now on the same page...

Yes, passive solar is hard to beat when done right...which means properly accounting for the building heat loss, clear sky irradiance heat gain, and expected climatic irradiance heat gain BTUs to maximize passive solar heat gain and avoid over-heating any building rooms. We have been designing and building passive solar buildings (often integrated with hydronic radiant heated floors...and occasionally with onsite generated hydropower...hence our logo) for many years. We are now based in southern OR, but we were previously based in the Seattle area. You might find our suite of passive solar design software located on our website useful.

Sailaway-You initially stated:

"Dana, the “total heating energy” (i.e., BTUs) required for the water holding tank option would always be higher than a standard electric hot water heater because of the larger volume of water that would need to be heated from the sourced water temperature to the room temperature by the house heating system."

Which is different from what you later stated:

"the recurring BTUs will always be identical to a standard water heater and there is also an initial BTU penalty for startup that increases with the holding tank volume."

The "...always be higher...." now becomes "...always be identical...", which was exactly my point, and it didn't take any math. In a closed system the BTUs in are always equal to the BTUs out, per the first law of thermodynamics. The initial BTU input hit is only incurred once, and thereafter it's identical.

The source of the energy going into the tempering tank varies (see my initial response), and it could indeed be a heat pump at some COP. But in practice, the solar gain make up for that initial hit MANY times over in any given year, so it's always somewhat better from a purchased energy point of view (but still not worth it.)

My original statement is correct if the holding tank is initially heated by the conditioned living space heating system. However, the OP indicated his holding tank is initially heated by the environment and NOT by the conditioned living space heating system...so my modified statement is correct for this specific OP situation. To say the initial BTU penalty only occurs once may be typical, but it may also occur seasonally or much more frequently in a vacation home, etc. So my original statement is technically correct and more precise. I did the math to prove the point and clearly illustrate the situation. I fully concur that it's not worth it.

Still not correct.

In the original post he described it as:

"However, if you place a water storage tank that comes close to your average daily use in the conditioned space and let it slowly warm to ambient, which would be 70 degrees, you have just dropped your delta t to 50 degrees. "

"...in the conditioned space..." means it's the space heating system (and not "...the environment...") is providing the additional BTUs.

Whether the drawn water enters the tempering tank or whether it enters the hot water heater, it's the same daily volume at the same total delta-T.

The notion that the initial BTU hit for the additional volume is taken more than once is just silly, unless the house is allowed to drop down to the well-water temperature (or lower). Adding thermal mass that communicates with the interior of a house also tends to lower it's overall heating energy use, even if the standby temp while away from the vacation house were 50F, it's a net improvement. If the vacation house is used in the summer and the house is allowed to stagnate at 85F+ it's an even bigger net improvement.

I fully agree with you...the notion that the initial BTU hit for the additional volume occurs more than once is indeed unlikely given that it is unlikely that the house would often be allowed to drop down to the well-water temperature (or lower). However, unlikely is not the same as impossible or never, and this does not invalidate my original statement...even though I also fully agree that the initial BTU hit also becomes increasingly insignificant as the length of time that the holding tank is actually used increases and given the other opportunities to potentially achieve an even bigger net improvement. As I wrote five days ago...my apologies, I tend to get very precise about this stuff...

I question the assumption that a buffer / pre-heat tank located in 70*F air temperature space heats the water within it to 70*F. I'm willing to bet that since the buffer / preheat tank is emptied 1-2x times per day it doesn't have enough time to soak up enough heat to raise the water within it from 50 to 70*F

Although time will tell, as I mentioned the tank was sized to approximately our average daily use. Spread over 24 hours I am guessing it will come close as Our main use is early and late in the day.