I was wondering how others have solved this problem.  I purchased an 119 gallon
WaterFurnace geothermal storage tank.  It comes with only 2” (R-16) insulation.
My water to water E-series heat pump heats the water up to 119 degrees, but
when I go to bed and then wake up in the morning, the temperature in the tank
is now at 109 degrees.  So over the span of about 10 hours, the tank loses
ten degrees!!  To me, that is a lot of lost BTUs of heat.  I sat the tank on a one
inch thick hard rubber pad, and the pipes to and from the storage tank are
insulated in one inch thick Armacel closed cell rubber insulation.  So my next
step was to completely wrap the tank in AstroFoil, but it still loses about one
degree of temperature per hour when sitting in my unheated basement. :(

http://www.astrofoil.net/astrofoil.insulation.tips1.html

What is ambient air temp?

Also, where are you measuring water temp? If measurement location is low in the tank you may not be losing as much as you think. I'm in the process of evaluating a Geyser air source heat pump water heater (HPWH) and in the course of that I have learned that overnight, stratification within the tank increases, with the tank bottom losing much more than the tank top. The tank top may lose 2 degrees but the bottom (measured at the shank of the drain valve) will drop 10-15 degrees.

I'm also a bit suspicious as to tank bottom insulation - may not have the advertised 2" on the bottom of my tanks. I wedged the probe of a thermometer a couple inches under the tank and measured a 3 degree rise above adjacent slab temp.

My 85 gallon GE tank loses 2-3 degrees per day at the top. A couple of times we've left the house for 5-7 days and I de-energized the water heater and measured the HW temp before departing and upon return.

I have data, since I measure everything, continuously.  My temperature sensor probe is at the
bottom of the storage tank.  There is no water activity during the night, since I measure the
water flow rate, as well.  The water meter needle is at the same spot when I wake up as when
I went to bed.  Clearly, there is a need for better insulation in these storage tanks.  The ambient
temperature in my basement is around 70 degrees.

You are absolutely right about the bottom of the tank, although I think heat rises, and the top of
the tank needs more insulation!  I made the mistake of putting my 120 gallon solar water storage
tank on the concrete floor.  All the heat just flowed out of the tank, and warmed up my concrete
floor.  It was nice to walk around in bare feet, but my wife complained about the cool shower
temperature.  That is why I stuck a very thick horse stall mat under the Geo storage tank.

I also understand what you mean by stratification.  I am smarter than my wife, so I jump in
the shower first.  She doesn't want to because she thinks it takes a long time for the hot
water to get from the storage tank, up to the shower.  So I get that nice hot layer of water
off the top of the tank.  When I get out, and she gets in, she complains it is too cool for her!

The other thing which I observed, is that when my solar storage tank heats up to 160 or 170
degrees by 4 pm on a summer afternoon, it loses heat faster in the evening, than when it heats up
to only 120 to 140 degrees in the winter.  I think an Engineer would be able to explain that to a
layman like me.

Hard rubber is not a good insulator - I'd use foam, even if it crushes some. Also, try closing all the valves some night and see if it holds the heat better.

A hotter tank loses heat faster owing to the greater temperature differential across its walls. if the differential between the tanks contents and the ambient air is doubled, then the rate of heat loss will double.

So in response to the problem, I installed heat P-traps in the plumbing, and that helped cut down on
the temperature loss.  The heat was traveling up the pipes, even though there was no water flowing through
the pipes.

Thanks to "engineer" for explaining what I am observing.  So instead of allowing the sun to heat my
119 gallon storage tank up to 160 to 170 degrees each day, I added a second 50 gallon storage
tank, and wired in a circulation pump, such that when the main storage tank starts to get too hot
(i.e. greater than 140 degrees), the little circulating pump exchanges water with the smaller 50
gallon storage tank.  Now this way I have two tanks to lose heat from, instead of just one.

First, heat doesn't rise- it flows from hot to cold. But at the temps we're talking warmer water is less dense than cooler water, so over time it will tend to stratify when there is no flow. (Those forces are surprising small in a not-so-tall tank though- doesn't take much flow  or momentum left over from a flow to suppress stratification.)

Second, have you considered drainwater heat pipe recovery on the shower?  Any decent-sized one will reduce the heat drawn during showers by half or more, leaving your wife a little less to complain about- what's $500-700 compared to couples-therapy counseling, eh? :-) 

(Oh yeah, and it saves half the energy too...)

See:

http://www.renewability.com/ 

http://www.renewability.com/uploads/documents/en/home_retrofit.pdf

http://www.gfxtechnology.com/VGFX.html

independent testing on select models:

http://www.gfxtechnology.com/NRCAN-6_29_07.pdf

http://www.cmhc.ca/odpub/pdf/65680.pdf

http://www.regie-energie.qc.ca/audiences/3637-07_2/DDR3637_2/RepDDR/B-12-GI-23Doc1-2_RepDDRSE-AQLPA_3637-2_28sept07.pdf

Basically, as long as you've got a 3"x 60" or 4" x 48" version (from any vendor) the heat drawn from the tank drops by more than 50%.  I you don't have the headroom, fatter drainpipe allows you to go shorter, since you get more heat-exchange surface area per foot of length.  Even a 4" x 36" version has significant return. 

Since it's a counter-flow heat exchanger the water flow has to happen at the same time as the drain flow to get anything like full benefit, which is why putting it on the drain from the main shower is key- it does nothing for tub-filling capacity.  But for showering families these suckers are better than sliced bread, no matter what the hot water source:

With solar it boosts your solar fraction...

...it doubles (or more) the continuous/successive shower time in electric or solar tanks...

... it gives you higher effective-flow with a tankless-  you can now run the laundry & dishwasher & run a bath without fear of freezing the person in the shower...

...a typical 30-35kbtu/h tank HW heater now becomes a continuous flow shower heater, (pretty much as-good-as a low-mid output  tankless for that purpose.)

So, it's up to you- pay for the drainwater heat exchanger now, or the marriage counseling later, eh?

Actually, my neighbor has one of those gizmos and he is totally against doing anything green.  I guess the
architect installed it when they designed the house.  I am not sure if he can tell if it is working, or not.
I like to see some sort of gauge on a device to show that it is making a difference!!

I probably would install one, but my drainpipe goes right down the inside of a wall, and is at the
other end of the house from the utility room and water pipes.

Thanks to a Private Message from one of the old-timers on this forum, I think I now have the problem
solved.  I got my wife to join me in the shower each morning, and things have straightened right up.
Much better performance, and now family bliss.

Water of a uniform temperature will not stratify if you leave it alone. Ref: entropy.

> The water meter needle is at the same spot when I wake up as when I went to bed.

I doubt that it can measure the very low flows that convective circulation causes.

jonr,


According to the Second Law of Thermodynamics(entropy), a system is closed when no energy is being added to or removed from it. Since we know there is standby heat loss, A.K.A. energy, in a storage tank, water stratification will occur. Convective flow can happen inside the system piping, further removing heat from the system.

A gas fired water heater will loose about seven degrees an hour.
An electric water heater will loose two to three degrees an hour.
A super insulated buffer tank will loose about one half a degree an hour.
This does not include convective losses. 

Bergy

Bergy: If there is daily agitation of the water from actual flows (or convective looping in the whole system that you point out) the forces that create stratification are "in the noise". Stratification-tank designs typically need to use heat exchanger coils for heat insertion/extraction to minimize agitation to induce and maintain the thermocline. Displacing fluid flows into/out of tanks are orders of magnitude larger than the vibe induced by flows within the heat exchangers so the mechanics of the fluid in the tank remain mostly unperturbed. But even fairly low flows will swamp the natural stratification effects at the delta-Ts we're talking. Leave it alone (zero flow) for a few days and the stratification will be measurable.

Induced thermoclines in water heaters & buffer tanks using dip tubes (like the Water Furnace tanks) do not persist when the flow stops- it's not a true slow-steady-state type of stratification. But it can take awhile (typically on the order of hours- luckily for eisensms' wife :-) ) for the thermocline to fully dissipate as the turbulence subsides, and only later will other stratification forces dominate. (Water is fairly thermally-conductive stuff in it's liquid state.)

Did you replace the hard rubber pad underneath?

You could switch to plastic pipe. Or add more insulation. Maybe use an infrared thermometer to see where you have loss.

Let's see if I can tell you approximately what is possible without a mistake. Say a 2.5' cube tank to make things easier. 37.5 sq ft of surface area. Say you want .2 degree per hour, k = .17, 70F delta T, ...

I get 2.3" of foam insulation. A non cube tank shape and pipes will increase that. Maybe make it 4" thick.