Temperatures are cooler at night and warmer during the day, so when using an air source heat pump, it it more efficient to run it at night for cooling or during the day for heating. A large water tank (underground or in the basement) allows this heat/cold to be stored. Cheaper electric rates at night can also make a huge cost difference (ranges widely 0-50% depending on location) for cooling or when geo thermal is used. I know systems like this are out there, but I haven't heard much about them. Is a tank such an obstacle that these systems aren't used? I've seen people spend far more to save less % on operating costs.

large hydronic systems often have 100s of gallons of storage which can take modest advantage of this (particularly in heating season).
Cooling as you describe (chills stored water long before you need it) may suffer more standby loss as most of the heat in a home comes may not infiltrate home 'til late in the day.
Why don't you see it? Who wants to market it?
"Yes Mr customer I want to put a XXX gallon storage tank in your basement wrapped up in insulation so it doesn't sweat on the floor. Yes it will be thousands of dollars but you'll get a tax credit and it'll save you a couple hundred/year. The bigger the better as we want to take advantage of your 2.5 cent discount during off peak."

Tough sell.

You will see it where we frequently see things like this: on the DIY site where a motivated handyman puts aside the value of square feet in his home to heat and cool it with less KWHs. Of course the tank can be buried outside as well which simply adds to the installation cost.

The problem with this better mouse trap is ROI for any but the DIY.
j

A recent ASHRAE journal piece discussed this at length for big buildings. I don't see it ever being practical on a residential scale in all but the most unlikely circumstances - It might be worthwhile in a desert climate - scorching hot days, cool nights AND if substantial time-of-day rate breaks were in place. The volume of water needed is so high so as to be cost prohibitive. Some systems make ice at night for daytime cooling. Tradeoffs there include design issues unique to ice as well as loss of COP to achieve the lower evaporator temperatures (20 vs 40s) needed to freeze water.

A more common method of applying thermal storage in a single family house is via the structure itself - passive solar, high mass walls, slabs, fireplaces, chimneys, and floors.

The thermal flywheel of my ICF house goes a long way toward allowing its 3400 SF under air to be heated and cooled with 2 tons.

Ice is interesting in that it needs little space. Cooling to 20F vs 40F is about a wash COP wise because of the day/night temperature differential (assuming air source, even in non-desert climates), so the payback would have to come from reduced electricity rates at night.

Some people pay quite a bit to get the minimal (often zero once the reduced gains from thermostat setback is accounted for) gains from passive thermal mass.

Basement space situations vary, but I know quite a few people who use it for nothing more than storing junk that they would have thrown out if they had any better use for some of the space (say 10x10).

i have ceramic storage system now . it is an 800 gal tank. worked great but alas now there is not a discount for buying off peak when i got the discount it was only 100.00 a month for a 2500 sq ft house . that was my total elec bill .

Here is a company selling offpeak storage. http://stsscoinc.com/Products_OffPeak.aspx dunno if I'd want to bet the farm on offpeak breaks lasting in perpetuity, as rikmeister illustrates, but once you have the tank you can heat it in multiple ways. In my view, an energy strategy should not be a bigger hammer, be it GSHP or olympic pool heat storage. Rather you want a tool chest that offers a better solution for the conditions on any given day.

Jonr, my AAC walls cost about $7/sf as semi DIY (I was helper to a moonlighting mason), plus $4.53 for stucco. I would not characterize those costs as "quite a bit" extra. In fact, toss in the savings on slab on grade vs basement and my thermal mass surely cost less than conventional construction. Nor would I count my goal -- $0 for heating and cooling* -- as "minimal gains."

* I see a house sized dehumidifier in my future. As I write at 3:15, it is 93 outside and 80 degrees inside, but decidedly sticky at 68 percent humidity. I suspect Mr. Carrier would disapprove.

Dehus swap latent for sensiible enthalpy and add heat iequivalent to their power consumption.

Operating a dehu in that environment would result in something like 85-90 degrees dry bulb and 50-55% RH. You'd be less comfortable, not more.

Wondered about that -- thanks for your input, Curt. I'll wait for a year to see how the heating season goes. Could be a heat pump water heater coupled to 225 gallons of storage will turn the trick. IIf not a mini split cooling the vertical chase that moves air between floors would be an easy fix. Might need heat in the shoulder seasons as well. Passive solar won't work that well until the leaves fall in early nov. The problem with passive is that "average" isn't reality.

a hydronic fan coil (pump and dump water) might remove humidity without aid of compressor (if its 80 in yur house). you also might consider an exterior space with whole house dehumi ducted into house (i.e. garage).....of course if you do that it is little different than a window air conditioner.
good luck,
j

Well, this one isn't as simple as it seems either. A Santa Fe Compact dehumidifier is rated (highly) at 2.4 liters/kwh drawing 5.1 amps or 580 watts. That translates to about 2000 btu/hr, or kitchen appliance output. Even in 24/7 operation 2kbtu/hr is a blip in a 1600 sf home surrounded by 100 tons of concrete. So the question becomes: wouldn't you want chilled air besides? And the answer is I don't know, or more precisely, yes in July but no in October, how ever that sorts out in a very tight house. An added complication: my hrv should dehumidify quite effectively in cold months.

I beg to differ. Adding 2kbtuh to a small tight house 24/7 will have a material effect on interior temperature regardless of how many tons of concrete are present. The concrete (thermal mass) helps even out daily temperature swings but won't effect a steady state load.

More importantly, if the Compact is removing (condensing) its rated 3 pints of water per hour, that's an additional 3kbtuh sensible heat added to the indoor air.

You are probably right, depending on how many hours the unit actually runs. It doesn't look good in the extreme case. At .35 ACH, 80 degrees and 80 percent humidity would introduce about 10 cups of water per hour into the house if my math is right. That would keep the Santa Fe unit humming . It is sized for 1600 sf of basement or crawl space. I need to check with thermastor as whether it can handle space vented by an hrv.
That said, 80/80 over 24 hours is supremely oppressive even for the midatlantic. There have been three nights so far this year that air conditioning would have been required, working on four tonight, and probably seven before this heat wave breaks. (Your tolerance will vary. I just came off a roof that is 120 degrees in full sun.) But this is with the house wide open 24/7 and the AAC blocks wetted to humidity levels in the high 60s. AAC is unbelievably effective at sucking up moisture, which is a good thing if the blocks are dry when the heat rolls in.
In short, dunno still. I'll know better when the stucco is done and I set up dehumidifiers to dry out the walls. I also need to see how much dehumidifying is necessary during the shoulder seasons and estimate the net gain/loss in comfort over 365 days. But thanks for challenging my world view, which is wrong as often as it is right.

Structure infiltration is only one component of total latent load - here are more:

1) Operation of exterior doors

2) Respiration by occupants, including pets

3) Houseplants

4) Standing water (toilet bowls, aquariums)

5) Operation of bath fans and range hoods

6) Cooking

7) bathing

8) buildiing assemblies drying inward.

The Thermastor / HRV question seems moot - dehu doesn't care about source of mositure.

A dehu operating alone is suited for environments which are both cool and damp.

It would be interesting to see numbers on the other sources of moisture. I suspect that over a 24 hour period in a typical house, all are small in relation to 10 cups/hour due to air infiltration.

the perfect answer is to move to maui where it is always 70 to 80 degrees.

Turns out that ASHP manufacturers are adapting Joe's idea of keeping dehumidifyitng heat outside the envelope. http://www.daikinac.com/residential/productsUnits20-humidity.asp?sec=products&page=55
I am rejiggering my ductwork to use the ERV during the summer rather than the hrv. The erv vents the bathrooms and kitchen via demand switches and power open dampers. A fourth damper in the vertical chase would allow it to exchange air as well.

Commercial Cool (Haier) makes a 1 ton double hose portable heat pump that blows condensation heat outside in dehumidify mode. http://www.epinions.com/reviews/Haier_HEATER_CPN12XH9_Air_Conditioner The last heat wave (102 in the shade) convinced me that $0 for cooling is not doable. The good news is that 1 ton is plenty. 93 outside today. 75 inside with the portable set on dehumidify. The humidity is going down slowly because of the moisture in the AAC block.

I believe I have the Danby equiivalent to that product. 2 hoses are key. On my last project I used it to make a master suite livable while we worked through an evap coil failure.

Now it is at another client's house, parked in a soaking tub for condensate drainage, running only to dehumidify the master suite to test weather we can combat a dust mite allergy problem via humidity reduction.

Curt if you are playing with indoor air quality, check out the Dylos particle counter.
j

I hadn't heard of that but will check it out.

humidity is a theme of this thread, so I'll continue along that vein

Client has a beach house with several issues -

1) Near constant seabreeze results in higher infiltration (not yet proven, but a suspect)

2) Too much cooling capacity - 8 tons in 4000 SF

3) Installer left DIP switches in default modes, fail to take advantage of dehu mode

Client had constant headaches, visited an allergist and dust mites are to blame. Client had been to Greece for several weeks (low humidity, they tell me and marble floors) and headaches vanished.

I found RH in the low sixties and took immediate steps to reduce that. They are planning to remove mastersuite carpet and replace with hardwood.