Hot water tank under basement floor?
I have been planning on putting a water tank under the basement of a construction house in Zone 6B. The purpose is to give me a versatile way to store heat.
The heat could come from many sources including capturing the heat from waste water, active, or batch solar hot water, or a heat exchanger on the Masonry wood stove.
The heat could be used to temper the cold water before it enters the hot water heater, heat floors, or preheat EVR air.
All of this is for future use. I just think it makes sense to put it under the floor since it is doable.
I thought the tank could be installed in an insulated box and use sand to support if it doesn’t have flat sides.
I haven’t done my homework, we are close to breaking ground and I don’t have a tank. I thought I could just buy a plastic septic tank but I’m told they can’t take the temperatures I might have. It was suggested to me to get a used milk tank. This would be a great tank but they can be hard to find and expensive.
Would a precast cement tank make sense? Does any of this make sense? I know PV costs have gone down so much that active solar HW systems are a more expensive option.
Sorry for the long narration but I wanted to be as clear as possible.
Thanks in advance, I thought if anyone had knowledge on this subject it could be found here.
Jim
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Replies
Jim,
Q. "Does any of this make sense?"
A. No.
LOL- without a long explanation- why not? Just too expensive?
I have seen it done above the floor for active solar hot water with radiant floor heat.
Jim,
A few more reactions.
"I haven't done my homework." No kidding! It's time to design your system. We have no idea if you want to design a drainwater heat recovery system or a solar thermal system -- but suffice it to say that these are totally different systems. If you can't (a) define your engineering goals, or (b) design your system, you certainly can't specify your materials.
A good buried tank is expensive. If you have a fat checkbook, you can place an order with SWHIFT Systems, 246 McGhee Road, Sandpoint, ID 83864.
A 287-gallon stainless-steel tank (rated for 135 psi) will cost you $4,272.
But I don't advise you to do that. First, you have to design your system.
Thanks Martin,
Like I said "All of this is for future use. I just think it makes sense to put it under the floor since it is doable"
I don't think it's unreasonable to consider a thermal mass, unpressurized "pool" of water that could be used with SHW panels. That's the way the above grade tank I saw was used. A heat exchanger on the wood stove could have a pump and drain back into the tank as well.
Your the expert, I wouldn't have thought a hydronic floor system could be as complicated as I have learned it is, so maybe this is a more challenging idea that it appears to me. I did see a 425 Gallon milk tank for $800. I have met people that used these the way I am thinking. But just because some people do it, that doesn't mean it makes good sense.
Thanks Martin,
Jim
Jim,
Milk tanks aren't pressurized, so they are considerably cheaper than the pressurized tank I mentioned. You can't bury a milk tank.
There are a lot of issues here. The most obvious issue is that the type of tank you might specify will depend on what the tank is used for.
I've written a lot of articles about solar thermal storage tanks in basements, and the common theme is that they all eventually leak. The economics of these systems are always bad, even from day one, and even if the tanks don't leak. Since these tanks all leak, the economics quickly move from "bad" to "disastrous."
Thanks, that's why I asked.
Jim,
Sorry if I was brusque. I wish you the best of luck, and I hope you will continue to ask questions here.
For more on the economics of solar thermal systems, see Solar Thermal Is Really, Really Dead.
In a really long-term planning sense, I think it could make sense to bury a large, low-pressure tank, for some not-yet-defined thermal energy storage application. If we overall move towards heating houses with heat pumps rather than fossil fuel combustion, the extra load on the grid on cold evenings is going to increase. Similarly, if we move toward a greater percentage of the supply to the grid being PV, the availability of electricity will be lower on those same cold evenings. A tank that stores thermal energy on a 12 hour to one-week time scale could help with time shifting some of the energy consumption. Right now, there's not really a need for it and there's very rarely a way for the owner of such a tank to capture the value from it. Given the lessons learned from solar thermal systems, it's likely to be expensive to install and maintain the tank and any system that uses the tank. So there's absolutely no rationale for doing it based on today's situation. I think that in the long term, maybe in 15 years, there will be a need and use for tanks like that. It might be that they are only practical in commercial applications, and nobody ever bothers with them for residential. Or it might be that it becomes a standard way to avoid very high peak electric power costs.
So overall, I agree with Martin that on balance it does not make sense to install a tank. But you are not crazy to consider it, and if you want to maximize the potential for your house to work well with a different energy system in the future, it might even be worth putting in.
If that tank is used with a heat pump, the temperature capability of a polyethylene septic system tank is plenty high enough--it's certainly good for 140 F and that is higher than you'd want to use. And it would make sense to run it unpressurized.
Thanks Martin, I did feel a little verbally abused. LOL
I would rather have the straight story than have someone candy coat the answer.
All good
Jon,
I've lived off-grid for over 40 years, and I keep up with off-grid news. Very few off-grid homeowners have air conditioning, and I have never heard an an off-grid air-conditioning system using water storage. It's an interesting idea, but no one does it, because the capital cost of the equipment is too high.
There are cheaper options -- basically, installing a window-mounted air conditioner or a small ductless minisplit -- but even these cheaper options are so expensive (because of the number of PV panels needed to make them work) that they are very rare.
Whenever thermal storage comes up, I feel like it's worth a moment to google "eisspeicher". Not because it's something you're likely to implement yourself, but because it's super interesting! It uses a giant tank of water that is frozen and thawed over the course of a year to store energy in one season and use it in the other to take advantage of the super high latent heat of fusion of water.
Most of the info is in German. Here's a paper in English: "HEAT PUMP, SOLAR ENERGY AND ICE STORAGE SYSTEMS - MODELLING AND SEASONAL PERFORMANCE Christian Winteler, et al". My link is triggering the spam filter, but you can download the paper if you google it.
Last time I was rambling about this, Dana posted a project in Laramie Wyoming using a thermal battery: http://www.thermalbatterysystems.com/featured-systems/laramie-wyoming-thermal-battery-system-example/#.WRMujsm1sUE
Not quite relevant to the original question, but fascinating stuff!
A low pressure tank with internal heat exchangers could arguably be useful placed under the slab (though it would make service/repair more difficult), but without even a napkin-math design for how it would be used there's no way to specify the tank size, let alone the heat exchangers.
I can think of many more entertaining ways to blow a few grand though...
Regarding off-grid AC, there are some 1-ton 48VCD mini-splits out there which can work, provided your off-grid battery array is 48V, one web-priced at under $2K:
https://websolarsupplies.com/product/hotspot-energy-solar-dc-off-grid-air-conditioner-dc4812vrf/
The amount of panel (and battery) needed to run it varies with the anticiapted duty cycle, but even 1 kw of additional panel is enough to make it useful for cooling, in modest cooling degree-day climates, or even some auxilliary heating during shoulder seasons. Using it after dark would require additional battery, and could be abusive of the battery system.
http://www.eventhorizonsolar.com/Off%20Grid_Air_Conditioning.html
Hacking an off-grid water storage system with a water-to-air heat pump would be enormously more expensive than buying more panel and battery to cover the power used a bigger cooling mode duty cycle. Buying enough battery to allow it to run for several hours after dark could be daunting, but even that would simpler (and probably more reliable) than a custom designed kludge for water to air with water storage.
Jim, I had done a little looking into water storage tanks. Above ground plastic tanks are available new around $1/gallon. Repurposed agricultural sprayer tanks etc. may be cheaper. In either case, shipping is a killer.
Burying a tank would bring with it a whole pile of complications. Given the thermal density of water, a lot of energy can be stored in a relatively small space.
100+ gallon electric water heater style tanks are an attractive option, but costly. Rheem's polymer tanks are guaranteed not to leak forever in residential applications but are about $12/gallon IIRC.
Capturing the heat from an air conditioner would be pretty impressive and could provide seriously preheated water to your water heater, but in a heating dominated climate that would only last for 3 months or so.
As has been suggested, financially justifying a system like that would be tough. Adding solar collectors for winter use would broaden its appeal at a considerable additional cost.
I expect there are current, viable cases for residential thermal storage using water tanks. For example, off-grid AC with PV and a heat pump (perhaps $10K for equipment, can deliver 24 hour cooling/dehumidification and some Winter heat for the next 20 years). Some larger applications find it viable even for time shifting use of utility power.
http://www.chiltrix.com/documents/Chiller-with-solar.pdf
I haven't seen plastic tanks leak.