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What about using a pool for water source HPs

Posted by david_cary (My Page) on
Tue, Mar 2, 10 at 7:08

Ok, new house here with HE HPs. Live in NC where heat costs about 3x a/c - hard to believe but energy star thinks its true. It is at least 2x just to give an idea.

Thinking about putting in a pool - imagine 20,000 gallons (nice even number).

Now - could you use a water based outdoor unit to use the more constant temp of the water as your heat source? In the summer, I wouldn't expect much of a difference except on hot days but it would have the side effect of heating the pool.

In the winter, the water with a solar cover would probably average 50 degrees as opposed to the air which might be 35 degrees. That probably represents a 20% improvement in efficiency assuming that water and air are equal. Obviously water needs to get pumped and also has a lot more heat density but I don't know what these differences would do to the efficiency of the system. The winter water temp could be supplemented with solar if it made sense but it is pretty sunny and I suspect a solar cover will heat it up a decent amount during the day.

On a really cold night, I calced a 5 degree drop in water temp from the heat pump alone. In a cloudy and cold situation, the water temp could get down to 32 degrees. People don't drain their pools around here. Certainly if you were driving the water temp to near freezing, you could switch back to air source.

In case you are wondering, no real room for geothermal except vertical loops and there is a real problem getting drills onto the property.


Follow-Up Postings:

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RE: What about using a pool for water source HPs

Lindeburg publishes books on studying for the PE exam in several fields, including mechanical engineering. Your local library might have this book, and almost certainly there will be a chapter covering this type of thing.

I recommend running the numbers using their formulas to see if the continuing energy savings is worth the inconvenience and costs associated with this method. I can help with this.


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RE: What about using a pool for water source HPs

Gary - I thought I ran the numbers right. It was an extreme 24 hours of using 10 therms. I don't know the actual usage since I have NG and HP. My worst winter bill was $70 or about 60 therms. But throw in some electicity use and it might be 180 therms total for the month. So I figured a cold 24 hour period might be 10 therms. Now I figure most of that is at night or early in the AM.

It is a fairly large house with lots of windows and our average temps this winter were just about 10 degrees below average. So all in all, I expect 4 therms to be an average daily usage in the coldest month, with the heat loss overnight to be about 2 therms or just under 1 degree drop.

Someone on another board actually suggested not insulating the sides or bottom. Because 60 degrees in the summer is great (probably no shortage of heat) and 60 degrees in the winter is probably great also. Now - in the shoulder months, you might get the water to 70 degrees and need some heat in the am. But the delta T is small with the earth and either way, the COP is high.

Good point about the solar cover not being that efficient in the winter. That may be a big deal. I can always use an underground storage tank for solar heated water. There would be less heat loss. That has to be balanced with the heat gain of the solar cover. And then the efficiency of the panels would be better because the pool water is colder. That is a hard one.

I was ideally planning a solar cover and then a night time insulating cover. Might be a bit much....


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RE: What about using a pool for water source HPs

I did think this is loooking like a good idea.

Going to nonchlorine cleaning means you should be able to
water with the stored water.

If you're serious look up 'water reeds' for water treatment.
It's a simple way that reeds put air into the water which
then lets dirty water become cleaner with the proper
microbial action. A 3 or 4 section starting with , I think,
sand filters, and a reed bed, and then the outlets is a
viable treatment for water.

In the UK, where they're becoming very good at conservation,
a video Grand Designs, showed the use of a reed bed for a
4 family house. The size for 4 was about 10 x 10 feet or
less. (I remember it was 10 x something and it was rather
small).

Good luck. If you can find other threads on managing grey
water post a link to them and then this thread can be a part of a good education.

Andre


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RE: What about using a pool for water source HPs

The million was an extreme situation and a 24 hour one. If you think about NG and its cheap price, it is only $300 a month. And it was definitely not an average but a worst case scenario. It would involve the aux heat running the entire 24 hours - I suspect a better design number would be 200,000 btus.

The insulation question is a good one. The soil here is wet and I suspect pretty warm just a few feet below surface. It really is a balance of solar gain and household use to determine if water is above ground temp. Either way, the delta T will never be much. We never freeze below a few inches. No doubt the insulation would block the cooling in the summer.

So much to think about. I've decided against PV for now so the roof space is not much of an issue. I've got some serious efficiency things to work on. I'd like my electric bill under $100 a month - I think my hot water electric strips might be running. I have the velux 80 gallon 2 panel system on a south roof so the gain is fine but the recirc pump is causing some losses. I keep inching the thermostat down but I've pissed off the wife before with too low of a setting.

It shouldn't be hard to get under $100 a month, we are all CFL/LED inc LED TV. Primary computer is a laptop. All energy star appliances. But there are 2 fridges and heated tile floor in the bathroom. I wish I had the budget for radiant heat in the bathroom.....


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