Question re: recycling rainwater and ciscterns

Not sure just where this goes, but...

I'm building my dream house in my head.  One thing I'd like to do is to put in a cistern to collect rain water and snow melt to use in watering plants.

When I was thinking about different heating/cooling systems, it occurred to me: If I used a ground-loop heat pump, what would happen if I ran one of the loops through the cistern?  Would that make the system more or less efficient?

It depends a bit on what kind of climate you live in.  One of the issues with collecting rainwater in winter is freezing.  If you have a big enough mass of water, and if you remember to drain all your piping before freeze-up, and if your winter climate is not too severe, then your cistern won't freeze too badly, and you might expect to come through a winter with no damage.

You are not clear about which loop you would run through the cistern.  Assuming you mean the outdoor loop, remember that, in winter, it is collecting heat from the environment to deposit in your home.  It is basically a giant refrigerator for the ground.  Running the outdoor loop through your cistern will refrigerate it (i.e. draw heat out of it), making it more likely to freeze.  Probably not a good idea.

If you meant that you will run the indoor loop through the cistern, then you are heating the water.  This is good for preventing freezing, but means that your system will require extra capacity to heat both the house and the water.

There are cisterns designed to be incorporated into building walls. This boosts the thermal mass of the house, and (usually) prevents freezing of the cistern. Obviously, there has to be some way of handling condensation when room air is warmer than the cistern...ideally, it could be collected, and added to the cistern, but I'm not sure that's been done.

Depending on the local climate, solar heating and/or geothermal cooling might eliminate the need for a heat pump.

If all you're interested in is getting enough water to plants, building field capacity might take you farther than building cisterns for a given amount of work and money, though of course they're complimentary methods. Plants that leave a lot of root mass in the depths, and good soil management in general, can give you a huge amount of storage within a year or two.

Thanks for the replies.  I live in the Midwest, on the border of zone 5 and zone 6, where it can get pretty cold in winter.  I'm interested in the cisterns mostly because we had major flooding here two years ago and I'm trying to think of ways to do my part to keep snow melt and rainwater from hitting the creeks and streams all at once.  I'm also trying to integrate systems: if in the summer, the water in the cistern will be cool, why not use that as part of the house AC?

Then again, I'm the kind of person who wonders why someone hasn't invented a fridge/freezer that uses a heat pump.  I mean, why spend the money to keep the fridge cold when it's -20F outside?

A fridge IS a heat pump.  And cold cellars have been around for a long time, though they fell out of fashion once fridges were invented.

It might be worthwhile calculating how many BTUs you plan on dumping into the water from your A/C system, and what effect that will have on the temperature of the water.  It won't stay cold for long if you are dumping all the heat from the house into it. 

Similarly for winter use, you should calculate the amount of heat you can safely draw from the water under various conditions.

At the very least, you would need to have a valve that can shut off the loop in the cistern if the water temperature gets too hot in summer or cold in winter.

Muzhik wrote:I'm interested in the cisterns mostly because we had major flooding here two years ago and I'm trying to think of ways to do my part to keep snow melt and rainwater from hitting the creeks and streams all at once.

In that case, I would highly recommend The Keyline Plan, by P. A. Yeomans. He's all about that sort of thing: he had been a mining engineer, then became a rancher and a soil-management luminary. I think his narrative of how development led to flooding in Australia might sound familiar as you read it, I think similar things have happened to the Midwest.

The book is freely available here.