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WillyP 07-14-20 05:29 PM

Need help designing a solar water heater for radiant floor heat
Anybody who has ever owned (or operated) a well-designed greenhouse*1 knows how well heat sinks work. However anybody who has owned a poorly designed greenhouse knows (without a doubt) that heat sinks are nothing more than the fantasy of wild eyed preppers, who believe heat loss is a government conspiracy. The truth is someplace in the middle.
I believe, the main key to building an efficient heat sink is: you must have an insulated barrier that goes well below the frost line. Without that barrier you will be trying to heat frozen ground. There also needs to be an insulated barrier that goes all the way to the wall system and integrates with the insulation system in the walls. A good rule of thumb is; start with a foot below the frost line, then add another foot for every three feet the frost line goes down. So if your frost line if two feet down, you should build a four foot deep barrier. If the frost line is six feet deep the barrier should be at least nine feet deep. This can be shortened by adding “insulation wings”. The wings are basically a panel of foam insulation laid flat below the frost line. I have learned this through working in several greenhouses.
SO I am building a house in central Maine. The frost line is about six to seven feet down.
The way I plan to do it is: I pour a six inch concrete wall to a depth of eight feet (approximately two feet below the frost line) with two foot wings on the bottom. In the top four feet I’ll install pex tubing, just like I do in the slab. But this system comes on in the fall. It serves to heat the soil around the perimeter of the slab. Without heating the house above it. Although there will be some minimal heating of the living space as well. By doing this we get a jump on creating a warm area below the slab. So instead of fighting the soil temperature below the slab, the soil is helping to heat the slab. Of course I’m installing solar powered systems so this method doesn’t cost a lot of money to operate.
The heater:
For my hot water heat. I am building a lean to green house on the south side of the house. It will be 10X16 inside will be a system that uses evacuated tube (vacuum tubes) solar collector, to heat oil. The oil will live in two fifty gallon tanks. We will call them ot1 and ot2. They will be connected near the top with an over flow tube. SO each tank will hold up to approximately forty gallons of oil. But the system will only have fifty gallons of oil in it.
Ot1 will have the intake for the solar heat collector. Ot2 will hold the output of the heat collector. When ot2 fills to the overflow tube, it will run off into ot1. Over the course of a normal day the oil should reach a temperature over 150°F. I believe maintaining a temperature over 140°F will be fairly easy.
From ot2 oil will be drawn to heat the water tanks. The oil from the water tanks will drain into ot1, where it can be reheated by the solar heater.
The water tanks:
I plan on burying two 850 gallon water tanks below the green house. In the green house (above ground) will be a 1500 hundred gallon discharge tank. Each tank will have a coil inside that hot oil flows through (from the above system). It should be noted the manufacture of the tanks recommends never draining them below 80% of capacity, so a 1500 hundred gallon discharge tank will be more than adequate.
I will label them wt1 wt 2 dt1
Wt1 will be one of the underground tanks, it will serve as the main feed for the radiant floor heater system. Wt2 will be the other underground tank and will serve as a slave for wt1. Basically wt2 will just hold hot water until wt1 needs it and then wt2 will pump into wt1. Any helpful ideas for making that work efficiently without a bunch of electronics would be greatly appreciated.
Dt1 will hold the water after it has gone through the radiant floor heater. Being above ground (in a greenhouse) and heated by coils of hot oil. It should easily reheat the water back up to 95°F then a heat sensitive valve will open and it will fill the below ground tanks (wt1 and wt2).*3

I am really hoping for some feedback from anybody who has built a similar system, or any part of it.
The main question nobody is willing to answer is. How much water does a typical radiant floor heater use, for a two thousand square foot house with excellent insulation.
Is 95°F hot enough for the input water of a radiant floor heater? I am getting all sorts of conflicting temperature recommendations. The most reliable information I get says 90-95°F *2is more than adequate.
Has anybody ever done something like this, with a barrier outside the foundation that goes below the frost line? All the science seems to work, but the internet screams never put in a slab without insulation underneath it. But I hear sometimes the internet can be wrong…
Thanks for reading this. I am looking forward to any helpful ideas, or discouraging information that stops me from doing something really stupid that costs a ton of money.

*1 By “green house” I am referring to a glass room, not a house with a “green rating”.
*2 90-95°F is a temperature I got off the internet. I am open to adjusting that up or down. That is one of the reasons I am posting here, and on other forums like this one. Any advice from people with experience, monitoring water temperature, in a concrete slab with pex pipes would be wonderful.
*3 I am using a separate discharge tank, so cooled water from the slab won’t mix with already heated water, until it has been reheated. The discharge tank will have a temperature sensitive drain on it. When the water is reheated to the proper temperature, it will refill the two underground tanks.

gadget 07-22-20 01:46 AM

That temp is about the ball park for radiant. It doesn't take as high a temp since your heat exchanger area is large. Slabs to have lag affect and can take a bit to warm up and cool down.

Your idea sounds interesting but I would skip it go just insulate the foundation and stick with tried and tested. I would also put pex into the slab for radiant.

Keep it simple and insulate first before other strategies is always a safe and cheaper bet long term.

WillyP 07-30-20 05:28 PM

1 Attachment(s)
Here is a diagram I made of the system. Somebody asked me for this someplace...

philb 07-30-20 06:08 PM

Why do you want to use oil? Why not water? It's more efficient and cheaper to buy. You could possibly use a drain back system on your collectors.

Around here, the building contractors are putting pex into the sand layer under the slab. Under the 1 to 2 ft. layer of sand is board insulation. The zone rotameters and feed lines originate in the garage and must be turned on in August or Sept to have heat in October or November. A 50 gallon hot water heater fuels the whole system.

The same board insulation is also used to create insulation for the concrete.

WillyP 07-30-20 06:42 PM

my problem with water is; at the heat the evacuated tube heaters run, the tanks would have to be vented to be safe. There will be a lot of evaporation in a vented tank with 185° water. I think this will fog up the glass in the greenhouse. fogged glass will greatly reduce the efficiency of the whole system.

as far as what goes under the slab. At the forty forth parallel we have a frost line of six to seven feet. That is why everybody has basements.

jeff5may 07-31-20 07:49 PM

Sounds like you will be pumping a lot of hot oil. Make sure to insulate it well, as oil has less than half of the heat capacity of water. A hot pipe doesn't care what's inside, it lets the heat out the same. You're going to need that heat to make it to the water tanks.

gadget 08-02-20 10:50 AM

The other problem with oil is messy leaks. There will always be a leak at some point.

I ran an open tank hot water setup in my greenhouse a few years back. Water temp was 100F. The opening was pretty large since it was basically a blue barrel with no lid. I did not have to much water moisture issue in the air. Of course, every situation is different and your water temps are much higher. I would consider redesigning the system for lower temps since it is much more efficient. Hard water is more of an issue and oxygen if you have an ferrous metals in the system, especially if its vented to atmosphere. At those temps, you really should have a sealed system.

My frost line is about 19" and almost every house around here has a basement. Mine is not insulated. It was built in 1972

WillyP 08-02-20 11:45 AM

[QUOTE=gadget;62912]The other problem with oil is messy leaks. There will always be a leak at some point.

I ran an open tank hot water setup in my greenhouse a few years back. Water temp was 100F.

So in you system how did you heat the water? Did it just reach 100°F by being in the greenhouse? Or was there some sort of heater?

gadget 08-04-20 01:04 AM


Originally Posted by gadget (Post 62912)
The other problem with oil is messy leaks. There will always be a leak at some point.

I ran an open tank hot water setup in my greenhouse a few years back. Water temp was 100F.

So in you system how did you heat the water? Did it just reach 100°F by being in the greenhouse? Or was there some sort of heater?

I was heating with wood with an experimental wood heater. Its an interesting heater though it destroyed itself from to much internal heat. I have a build thread but I have not yet updated it with the tear down.

Its posted here if you want to check it out. I post on that site as gadget;

I'm building a new design this fall thats not as intense but its not for the greenhouse. I ended up switching over my greenhouse to a geo heater using 52F well water to keep it just above freezing. To much heat loss to do much more then that.

randen 08-04-20 08:37 AM

Solar hydronic floor

Yes we have done the hydronic heated floors powered with 280 sq ft of flat plate. We live in Canada were the winter weather is harsh. Depending on the sun we have more than 50% of our space heating for the season.

I caution the use of oil for heat storage as it lends itself to environmental hazard and the system you are describing is full of net losses.

Water is hugely better at storage and heat transfer. Much safer with the addition of propylene glycol for antifreeze. (food grade). you can drink it!! if it leaks into the environment no harm!!!!

If you are interested I can provide you with first hand experience to my system that has proved itself.


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