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heat transfer with 70 degree water.

Posted by rustico_2009 (My Page) on
Sat, Feb 22, 14 at 12:45

I know there are similar threads to what I am about to ask but it usually involves using hotter water.

I have three 5,000 gallon tanks of water , one black and two dark green. Last night with a low of 47 the water stayed 68 dropping 4 degrees from 72 yesterday afternoon(green tank).

I am thinking about using the radiant heating method of burying tubing and recirculating water from these tanks to try to keep my soil warmer for some tomato beds. A few times a year we get well below 40 and even around freezing but the average is 44 for low. On those cold nights I think the water stays in the high 50's low 60's depending on what the day weather has been like. Much warmer than the soil in the wee hours of a frosty night ...that would be around 48 left to nature.

It seem logical that with cooler water I would have to maybe double the normal amounts of piping per square foot to get decent transfer and that would do it?

I know this is a pretty mild climate for considering greenhouse heating but it does average 44 at night for 4 months straight and there is plenty of sunshine in the day time almost always.

Does that seem right about just doubling the piping?


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RE: heat transfer with 70 degree water.

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Sat, Feb 22, 14 at 21:24

I'd use the sun to heat your soil directly. Put down black plastic or weed barrier and over that put clear greenhouse poly. This will heat your soil much warmer than what you are talking about. The advantage of water is that it would transfer some heat captured on sunny days during cloudy weather that follows.

But to really do a good job you need to heat your water up to 80-90F. Even higher to carry thru a week of clouds.That could be accomplished with some solar pool heaters. Think of it this way, your water tanks can only collect so much heat and that's dependent on the area of tanks exposed to the sun. If the area of tanks greatly exceeds the area of your tomato bed then you can possibly transfer a useful amount of heat from tank to bed. If not you can't gain much.


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RE: heat transfer with 70 degree water.

Thanks for you reply fruitnut,

I am heating some beds with plastic and they have row cover over hoops too. You are right that is super effective. I will have to shade the set-up on really hot days.

The solar pool heaters are a good idea. My problem isn't very big once the plastic mulch is installed so I was trying to keep the solution small. WIth a bigger garden in a large high tunnel, that's an awesome idea. I have also thought about putting a high tunnel over the tanks and letting them control the climate in there , including circulating the water. The water temps would go up if the tanks were in a tunnel...and they might pull excess heat during the day.

I am at 2000 feet in Southern California. We have a ton of Sun here, but it gets fairly cold at night and even the soil under the plastic mulch has gotten under 50F. It is nothing to have 85F during the day and then that day or a day or two later have a frost. That pattern goes on for 4 months(averages out to 60-44) and nights still take a long time to get above 50 once spring comes. What I would do is put the recirc pump on a soil thermostat for around 60-65F...I tend think that there is far in excess of enough water for this a few hours during the occasional cold night....

If plastic works though, that is way preferable. I put some water bottles with black water in ...so far tomatoes are doing well since valentines day, but the low has only been 38f.


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RE: heat transfer with 70 degree water.

I'm certainly no expert, but why not use excess hot air that accumulates on sunny days and pump that through your beds? You could collect the air up high, circulate with a fan through the beds and then exit out low, keeping the air in your greenhouse circulating. The soil in your beds would then become the heat sink to help modulate the fluctuating temperatures from day to night. Just a thought.


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RE: heat transfer with 70 degree water.

That's a neat idea, not really looking for that kind of project yet..but there are interesting threads on it around here.

It would be easier for me to use hot water either recirculated or passive. Around here the heat is only used at night and to avoid over heating you have to let most of the air escape and even use shade in winter to keep pots from getting too hot in a greenhouse.

So far in low tunnels with just plastic mulch and covers of row cover on 24-7 are doing pretty good with tomatoes and zucchini planted on Valentines day. I have started throwing milk jugs with black food colored water in there to....looks like spring has sprung.


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RE: heat transfer with 70 degree water.

If I had 3- 5000 gal water tanks I think I would insulate them and look for an external heating system, solar or woodfired, to warm the water as Fruitnut mentioned to well above the current temps. You could set up circulation loops, one to heat the water, another to circulate into your greenhouse. My preference would be to have a heat exchanger (fan blowing thru a radiator) within the greenhouse to warm air at night rather than installing underground pipe network. You may be able to push your season limits more with such a setup.


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