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Community and Q&A

Thought experiment: 30k cistern as thermal mass in passive home

mjezzi | Posted in PassivHaus on

Hi, we have a mountain lot in Colorado at 7500 ft elevation with lots of south facing solar heat gain, and we’re planning on building a passive home. We’re required to install a 30k gallon cistern for fire protection. A 30k gallon buried cistern seems to be extremely expensive, while a 30k gallon bladder type cistern is very cheap and could be installed inside the home’s envelope in a crawlspace. Assuming the passive home is well balanced for on average for a year, could the huge amount of thermal mass help moderate an even temperate, or would it become a problem?

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  1. GBA Editor
    Martin Holladay | | #1

    My first concerns about your idea are the following:

    1. What is the lifespan of a bladder cistern? Eventually it will leak, so the crawl space will need to be detailed to handle a major water leak. Good drainage and water management details are essential.

    2. When the bladder cistern reaches the end of its natural life, how easy will it be to remove the leaky cistern and install a replacement?

    Once these two issues are addressed -- assuming they can be -- my opinion is that it wouldn't be particularly difficult to make sure that your crawl space is inside your home's thermal envelope. I doubt if the presence of all that water will make much of a difference in the home's thermal performance, however, unless you are interested in installing a very large active solar thermal system. The cost of the hardware for a large active solar thermal system is much too high in relation to any conceivable thermal benefits, though, so I don't recommend this path.

  2. Expert Member


    From a practical perspective what use is a cistern located in a house for fire protection? Our local department frequently requires developers to install cisterns, but they have to be located 75 feet from the dwelling, which is the optimal distance to park a pump-truck and fight a structure fire.

  3. STEPHEN SHEEHY | | #3

    To follow up on Malcolm's question, what's the rationale for such an enormous quantity of water? Where we live, fire hydrants, installed adjacent to a pond, are few and far between. Our fire departments use pumper trucks that hold 1-1500 gallons. A thirty thousand gallon cistern must be pretty expensive. Would a sprinkler system be cheaper?

  4. mjezzi | | #4


    1) Agreed, we would need some sort of drainage system and probably a sump pump.
    2) The bladder would have to fold or roll up into a smaller size and be removed through a crawlspace access. That's one of the advantages of the bladder systems. It's fairly easy and cheap to replace. A 30k gallon bladder costs about $6k. Compare that cost to replacing a failed underground cistern.

    If the water doesn't make a difference, then that's ok, at a minimum I just don't want it to become a problem related to the performance of the passive home. Would it be possible that it could help temper overheating days? I really don't want to invest in complicated systems, but it could be possible to run radiant tubing in the concrete floor and exchange water in the cistern.


    That's a fair question. My thought was to create a line to a hydrant that's 50 ft away from the structure (50 ft is our requirement).


    The rationale that I received from the fire marshall is the access is something only a small truck can get down to. I still don't understand the full rational to be honest. I think it's way overkill myself and would rather not do it. Our home will have a sprinkle system and all siding and roofing will be non-combustable materials and there are no trees within several hundred feet from us, so it doesn't make much sense to me, but I don't get to make up the rules.

  5. onslow | | #5

    Mike, I am writing this from my mountain lot in Colorado at 7800 feet and also had the option of putting in a cistern for "fire points". Your locality may actually demand the tank, but first be sure you have not misunderstood or been misinformed about the fire mitigation rules for your area.

    A 30000 gallon tank requires 4000 cubic feet which would mean a 20x20x10 foot tank with a lid. Given the potential frost depth that makes it difficult to bury sufficiently to keep it ice free. Unlike a septic tank there is no flow of warm water or biological activity to keep things fluid. Aside from that, the chances that you could dig 12 feet without hitting rock seems unlikely.

    The engineering that goes into making water tight tanks of concrete is costly, so I also considered the bladder idea while wrestling with the fire issues. A bladder in the basement became untenable for the same reasons Martin points out, as well as I would lose my shop space entirely. Bladders don't get tall they get long and wide. The floor would need engineering despite the bladder's even footprint and the hook up details as required by the fire department made the whole idea sink. As Malcolm notes, driving up to a burning house to attach a hose in not very practical. The value as a heat bank/sink doesn't pencil out well either, as Martin also noted. I went through a whole year of trying to make the numbers work and costs always outweighed the benefit.

    Ultimately, I gained my fire points by opening up the fire suppression areas around the site, removing vegetation by the house, widening the driveway, using non combustible building materials, adding two exterior water taps 25' from the house and oddly, household fire extinguishers. I avoided sprinklers due to cost and fear of failures. I have a relative who deals with damage mitigation after sprinkler failures and they are very busy all year. Besides, if a fire comes through we are essentially on our own anyway.

    In response to Steve, fire hydrants are also infrequent in rural areas like ours and are really there to fill the pumper trucks. 1500 gallons weigh 13,000 pounds so hauling in 30,000 gallons takes a lot of effort or trucks. The reality of mountain fire fighting is lives first, structures second or third if that. Public water supply in the mountainous areas (if available) is apt to fail if the pumps lifting the water up elevation cut out during a forest fire. We have nine pumps between us and the main line in the valley. Sprinklers can work in individual home fires that start inside, but are useless against brush or forest fires. Not very comforting words to be sure, but still love living here.

    Meantime, to Mike, re-check the rules and maybe consider whether a chain of pre-cast septic tanks can be strung together to meet the requirement. At least they come water tight and predictable in digging requirements.

  6. user-6184358 | | #6

    Solar heated sand layer see this site for the Bob Ramlow sand bed. It won't leak like water.

  7. GBA Editor
    Martin Holladay | | #7

    The "sand bed storage" idea just won't die, in spite of the number of wooden stakes that have been driven through the idea's heart.

    It doesn't work. It's a waste of money. No researcher has verified any benefit.

    More here: Using Sand to Store Solar Energy.

  8. ohioandy | | #8

    Reminding Tim of Mike's original purpose here (the heat storage was a secondary hypothetical), it's true that fires can be doused with sand, although seems like it would be harder to pump out of your basement.

  9. user-6184358 | | #9

    Ah, Stuff on the internet lives for ever!

  10. user-2890856 | | #10

    Mark this date everyone . Martin and I agree . Sand is the dumbest thing ever .

  11. GBA Editor
    Martin Holladay | | #11

    Agreement is good!

  12. mjezzi | | #12

    CAROLINE, thanks for the response. I’m not sure if a system like that could work in a crawlspace. It would need to be water tight to avoid excess moisture. And we can’t put the cistern outside above ground because it would freeze. Am I misunderstanding your idea?

  13. tommay | | #13

    To bad it is others who decide what you have to do. If you want your house to burn down that should be your choice. If it were my concern and could do it my way, I would create a small fish pond, that would provide food and an emergency water supply in case of fire.

  14. CASUDI | | #14


    This maybe be a bit off the wall but perhaps worth researching.

    We use the corrugated metal tanks with liners for our rain catchment systems. 30k is not unusual, In our case the liner would be for potable water but you can get liners for non potable use.

    About 2 years ago I researched the feasibility of Geothermal heating using a 20K metal/liner water storage tank, with coiled tubing in the tank instead of trenched the usual way. I found a company (south of Seattle?) that said they could do this, it was feasible. I’ll dig up the information if you are interested?

  15. Expert Member
    MALCOLM TAYLOR | | #15


    A large part of any building code concerns the safety of occupants, be it hand-railing heights, stair dimensions, proximity of exits, structural minimums, fire-stopping, most of the electrical and much of the plumbing code, separations between buildings, etc.

    There are a few obvious problems with leaving it to the owners to decide how houses would be built. They won't live in the house forever, and while they are there they aren't the only occupants. The houses also can affect the safety of their neighbours.

    In light of the catastrophic damage recent fires have caused, the requirement that there be a source of water for firefighting purposes seems fairly logical.

  16. Robert Opaluch | | #16

    You probably already figured this out, but...30,000 gallon tank, requiring 4,000 cubic feet of water, given your building's footprint, could be about 22'x60'x3'. So putting it inside your crawl space could fit (forget your exact house's footprint). 3' height would reduce pressure of the water at the bottom edges, likely adding to longevity and leak-resistance.

    In the crawlspace, the cistern water would appear to be well-insulated from freezing and more affordable, with the house above, and minimal edge insulation area. Connecting to a hydrant 50' away from the house, it seems to meet the requirement. If you are required to have that 4,000 gallons, crawl space has that cost and freeze protection advantage, even without the thermal mass advantage. What's the alternative??

    Alternative ideas: A cistern removed from the house could be designed with south-facing glazing (e.g., below the high edge of a shed roof) to heat the water in the tank. So the water would serve as thermal mass, but just to keep itself above freezing temps. Or PV could be used to directly heat the cistern water. Or for once, a thermal solar system might make sense economically, being directly integrated with the cistern.

    I used two manufactured passive solar hot water pre-heat tanks (in series) and it worked well for providing hot water throughout the year in town in Boulder, CO, given the sunny winter climate. Even during the coldest winter period, water was only lukewarm in AM (and hot by afternoon). That's nowhere near freezing, and a boost from the water supply temperature. Passive solar hot water tanks disappeared after incentive rebates expired.

    Would be interested to know about your final design and experience with the cistern.

  17. CASUDI | | #17


    I would not consider the water storage inside/under the house.

    My idea was to use the 30,000 galleons to provide the geothermal mass for heat exchange for a radiant floor (concrete with pipes installed) ~ the tank could be a corrugated 30,000 galleon tank which is approximately 20' in diameter and 12’ high. You might recess the tank 3-4’ in the ground with a space between the tank and the surrounding earth if you are able to. I checked with Ken Blair at Rainbank and he said that in a climate like yours installers insulate between the tank and the liner with hard insulation. He thought (as I did) that the 30,000 gallons mass of water, the insulated tank with the water flowing/ moving through the coils in the tank, the heat exchanger and then circulating through the pipes in the floor would be sufficient to mitigate the risk of freezing. I am no expert but if you are considering radiant floor heat you might pursue this further. Sorry it took me so long to get back here. Good luck to you.

  18. Jon_R | | #18

    Large thermal storage using water makes more sense when combined with an active solar heating system (eg, PV + to water heat pump).

  19. CASUDI | | #19

    Good point Jon R ~ I'm going to check that out for future reference and compare costs.

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