heating water in cistern

Stdyhand

I know very little about solar water heating, but I will have a 1000 gallon cistern painted black to reduce sunlight penetration but the black will also get warmer. Is it possible for this to warm the water quite a bit over the day? I have a propane on demand hot water heater I plan to use, but how much fuel is burned depends greatly on the incoming water temperature. So if the water is already fairly warm in the cistern then my propane bill will be even less.

I've never stored water in a cistern before, but where were moving it is the only choice so it got me to thinking about heating the water in the cistern since it will only be used for washing clothes and bathing. Drinking and cooking will be from bottled water.

Is it possible to preheat the water in the cistern besides just letting the black collect heat and warm the water? I wasn't sure how long it would take to warm 1000+ gallons of water over the course of a sunny day.

Cariboocoot

Re: heating water in cistern

The cistern itself will probably not do much for heating the water. It's a matter of surface area exposed to the heat source (sunlight) vs. mass. 1000 gallons is quite a large mass but can be contained inside a fairly small surface area compared to the volume.

It is not difficult to construct direct solar water heaters though. Great site for it is Build It Solar: http://www.builditsolar.com/

mike95490

Re: heating water in cistern

cisterns are underground, generally. Poly water tanks are above ground, and are usually dark green, to exclude light. And they don't warm up much.

But if you can elevate a 30 - 50 gallon tank somewhat, you could make a thermosiphon loop of 3/4 " black poly pipe, and that would warm it a bit. Poly is resistant to damage from freezing, but the cheap connectors are not.

Grinnin

Re: heating water in cistern

I've been using a 1,500 gallon cistern of rainwater for all household uses for 12 years. I try to keep my cistern water cold since there will be less biological activity in cold water than in warm water.

I collect water from a metal roof that goes through a roof washer (rejects the first X gallons) then the water is strained before going into the underground cistern. When pumping out, the water goes through a sediment filter, charcoal filter, and UV exposure. The water is plenty dead before I drink it, but I'd rather start with less-lively water instead of getting water from algae squeezings.

As others have pointed out there's the mass. Lots of mass. The water in a black cistern may warm to a tepid, but it will probably lose heat as quickly as it gains heat. At the end of the summer the water should be about the average temperature for your area (day plus night average). Similar math for winter but a colder average.

If the cistern is your only source for water, do you want your cold water coming out tepid?

You don't say where you are. Equator? Pole?

Which brings us back to your question. You'd probabaly have better performance, as others have said, with a separate, smaller tank that feeds your on-demand heater. It can be a simple batch tank heater but it will get hotter with lots of collector area and smaller water mass. It will stay warm overnight with more insulation.

EDIT: More on cisterns and water: I also use rain barrels for watering the garden. I'm less picky about what goes into the rain barrels and just a little pollen and a little warmth can make a goey mess. I try to fill my cistern in early spring and late fall to keep the amount of airborne pollen out of my drinking water. I really don't want pollen or warm water in my cistern.

Stdyhand

Re: heating water in cistern

This will be in Arizona and it is not our drinking water. I am not too concerned with the biological in the tank because we have to haul in our own water which means it will constantly be emptied and refilled. The water will not sit long. I am less than two miles from a water station, hence the reason I am not using anything larger than 1k gallons.

westbranch

Re: heating water in cistern

Have to ask, have you done a study of your current water use? Just like solar when we recommend defining your loads, so too should you assess just how much water you expect to use.

I would go for 2 tanks, fill both and then drain one before dipping into tank 2, that way your water will stay fresher and less biological build up.

I went to our local maker of concrete septic and water tanks, sells plastic one too, when roughing out what we might need. Don't know how many bodies you are but with 2 of us, he recommended a 2-300 gal tank for all household uses, and he said if we were average users that would last us ~ 5 days, more with a bit of conservation. Supply is not an issue for us, it is pump time.

hth

Stdyhand

Re: heating water in cistern

westbranch wrote: »

Have to ask, have you done a study of your current water use? Just like solar when we recommend defining your loads, so too should you assess just how much water you expect to use.

I would go for 2 tanks, fill both and then drain one before dipping into tank 2, that way your water will stay fresher and less biological build up.

I went to our local maker of concrete septic and water tanks, sells plastic one too, when roughing out what we might need. Don't know how many bodies you are but with 2 of us, he recommended a 2-300 gal tank for all household uses, and he said if we were average users that would last us ~ 5 days, more with a bit of conservation. Supply is not an issue for us, it is pump time.

hth

Yes, we use about 4k gallons a month where were at now. However, that will be less at our new location because we have a HE washer, 1.28 gallon flush toilet, etc. Were were at now we are stuck using a standard washer, 2.5 gallon flush toilet, etc. We rent right now so we can't make any changes for conservation. Our new location is owned though so we'll be able to get by with far less water plus we intend to use greywater for flushing the toilet. Again, our current location does not permit greywater usage.

Stdyhand

Re: heating water in cistern

I have another question about pumping the water. I plan to use a separate 12v system to power two 12v water pumps. I was going to use a cheaper PWM charge controller. How many 12v batteries do you think I would need? One pump is to move fresh water to the house and the second is to pump grey water to the toilet and for irrigation.

It is not likely both pumps would ever run at the same time or for very long. They will likely run only for the amount of time it takes for the washer to fill up, so a few mins at the most. Would two 12v batteries do the trick or would that even be too much?

Cariboocoot

Re: heating water in cistern

Stdyhand wrote: »

I have another question about pumping the water. I plan to use a separate 12v system to power two 12v water pumps. I was going to use a cheaper PWM charge controller. How many 12v batteries do you think I would need? One pump is to move fresh water to the house and the second is to pump grey water to the toilet and for irrigation.

It is not likely both pumps would ever run at the same time or for very long. They will likely run only for the amount of time it takes for the washer to fill up, so a few mins at the most. Would two 12v batteries do the trick or would that even be too much?

What you need to know here is how much each pump draws and how long it will be on for. That will determine the battery capacity. You don't want a lot of batteries (quantity), but you may need a lot of battery (Amp hour capacity).

westbranch

Re: heating water in cistern

If you have a pump or 2 in mind , it would help to know what it is. so it is not a theoretical discussion. Choose one otherwise that you think will work and we can help assess it.

Stdyhand

Re: heating water in cistern

The pump I'm looking at is 10 amps @ 12v. At the most it might run for a load of laundry, dishes, couple of showers, and when the toilet gets flushed. It's hard to imagine this being more than an hour or two of the water pump needing to run.

Cariboocoot

Re: heating water in cistern

10 Amps for one hour is 10 Amp hours. For two it's 20 Amp hours. That difference is significant. If that is all the battery will run it will be either (at 25% DOD) a 40 Amp hour battery or an 80 Amp hour battery. Or you could 'blanket cover it' with a 100 Amp hour battery. After that you calculate the panel & charge controller size.

In that capacity range it would be simple: a single 140 Watt panel & 10 Amp controller.

If the numbers are wrong, it doesn't work and the battery goes dead sooner rather than later.

westbranch

Re: heating water in cistern

Showers..hmmm.... do you desire a pressurized system too? or are you prepared for the noise of a small pump while you shower? they have maintenance issues too.

Cariboocoot

Re: heating water in cistern

westbranch wrote: »

Showers..hmmm.... do you desire a pressurized system too? or are you prepared for the noise of a small pump while you shower? they have maintenance issues too.

No worries; the singing will drown out the noise of the pump.

Stdyhand

Re: heating water in cistern

westbranch wrote: »

Showers..hmmm.... do you desire a pressurized system too? or are you prepared for the noise of a small pump while you shower? they have maintenance issues too.

The pump will be located in a small shed which will also contain the 12v battery, CC, my generator, and the panel mounted on a pole next to it. I don't expect to hear much from it. Eventually I will install a pressure tank, but this is to get us going right away.

Stdyhand

Re: heating water in cistern

Cariboocoot wrote: »

No worries; the singing will drown out the noise of the pump.

I plead the 5th