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.

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.

Here is a diagram I made of the system. Somebody asked me for this someplace...

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.

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.

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.

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

[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?

[QUOTE=WillyP;62913]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;

https://donkey32.proboards.com/threa...l-build-thread

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.

Solar hydronic floor
 

WilliP

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.

Randen

I am absolutely interested. To be clear, I have no intention of running oil through the slab. I was going to use oil to go through the evacuated tube heater and into coils, inside the water tanks. My main concern was freezing at night, which is why I wanted oil not water. But pretty much everybody agrees this would be a bad idea, so I am leaning towards water and antifreeze.

I would love to hear some first hand experience information. By "flat plate" are you talking about a box with tubes in it?

Sound good

Willie, keep it simple / just go with glycol / proven liquid / no freezing
You'll be fine.. :thumbup:

Ironically oil was my way of keeping it simple. It wont freeze, it lasts much longer than Glycol, and it is cheaper. Unfortunately I now know it also doesn't hold heat as long and would be very inefficient.

In ref to your idea about using air to create hot water..
I have 2 hot air collectors I built that have been in use now for over 7 years doing a great job.

They push 98 to 130 degree temps back into my house (when the sun is out)

I once thought about putting copper tubing inside those 2 units to create hot water which would be pumped into my basement to a holding tank for distribution to radiant floor type heating however after thinking it over I decided the 2 collectors really only cranked good heat about 4 hours a day when the sun was out.

I don't think the tiny bit of water contained inside the piping would be worth the venture so pulled the plug on the idea.

The piping probably would have only held like 2 gallons of water tops.
By the time you figure in the cost of the additional piping, supplies, pump, controller, holding tank, radiant floor installation.. I said forget it. My return on investment just wasn't there.

So I use the 2 hot air collectors for what they are on the days it's sunny.
They do a fantastic job / turn on / off automatically / and snow drops right off them if covered from over night snow within 5 minutes after the sun hits them in the am... so I can't complain.

If I had all those evacuated tube collectors you have I would be setting up a drain back system immediately.

If your collectors were setup on an angle for easy flow thru the headers they would drain back quickly into a small drain back tank which would drain down to a heat exchanger located in the bottom of a 80 gallon insulated storage tank.

Of course the larger the storage tank the more water you would have to distribute after the sun goes down.

You have 2 big insulated tanks now that I saw..
You could fill those babies up to about 6 inches from the top...
Pump water up to your collectors using one of the bottom fittings... then have the return water from collectors return to the top fitting on the same tank.

2 other fittings on that same tank could be used to pump water out and back from a radiant floor system or even a radiator located in your glass house.

Doing it this way allows you to use your storage tank as your drain back tank and distribution tank at the same time.

The only thing you would need to do is put a pressure relief valve on the top and setup your solar controller to shut down your pump at a desired temp that you felt was safe.

Those are nice tanks you have there, I would definetly put them to use for sure.:thumbup: Just some ideas

If those tanks could be situated in a place that never freezes you wouldn't even need glycol... :)

water heats quicker then glycol

Reading back through the thread, I gained some understanding of the system you are considering. This leads to more questions.

What space or spaces are you going to be heating with the rig?

Are you trying to get domestic hot water?

Why do you consider such a large amount of thermal water storage for such a small e-tube collector?

What about power outages?

The answers to these questions will lead to a more effective and/or efficient rig.

Jeff:
I am not using it for domestic hot water. I will be using an electric hot water heater, powered by the PV panels (grid tied system).
I will be heating the whole house.
The Evacuated tubes will heat about seventy gallons of water. That water will feed heat exchangers inside the underground tanks and the discharge tank. Each tank will have its own thermal switch, with its own pump. That way I can regulate the temperature of the water that feeds the radiant floor heater.
The heater itself will have its own PV panels with battery backup. It wont be attached to the main system for the house. That way it will work in a power outage.

Ok, so with your setup and goals in mind, I would recommend a smallish hot loop with glycol, non-drainback. Maybe 15 to 20 gallons max to go to and from the collector. Either a brazed plate heat exchanger or a custom coil in tank jobbie. Secondary loop going to your slab or whatever radiant heating setup through a mixing valve, both discharging to a warm water heat store. If you have enough sun power, the warm water tank could dump the extra heat out into a supermassive underground cavern.

Controlling it just depends on what you are comfortable with. Electronic stuff is cheap enough nowadays that it's easy to browse the web and find a suitable plug and play setup for cheap. That being said, if you're a professional engineer capable of conjuring code, have fun.

Willy, how is your system build coming along?
I've been sitting here since July tapping my foot waiting for your update.

Take about 2 million pictures ok?
I like looking at pictures with my morning coffee. :-)

Sorry man! I am in the design phase right now. I am asking a million questions and have already realized some serious errors in my plan. I wont begin construction on the house, until spring of next year. But I plan on taking pictures and videos as well.

I retire in April of 2022, once that happens I will begin building the house.

OK Sounds good.

Better hurry up before the dollar implodes. Your retirement account might need another zero on it before you can retire. Mr president is already on with the rhetoric: more stimulus, more spending, minimum wage hiking, and we NEED inflation. His plan sounds like your original plan: a hundred square feet of sunlight, to heat pipes, to oil, to glycol, to water, to (I think) slab.

Meanwhile, in my world, I gave away another window heat pump to my buddy. R290 txv heat, original cap tube cool. It was rated for 10kbtu and 10 eer as an air conditioner. It got down to 20 degF here last week, and dude said it kept his workshop as comfy as the propane heater he was feeding a BBQ tank a week to do the job. He keeps asking me how much he owes me for it. I told him happy birthday.

Conventional resistance electric hot water heater .. or .. heat pump hot water heater ?

Both use electricity .. can be powered by the solar / batteries.

But the heat pump option have 3 benefits over 'old school' resistance electric heaters.

#1> Efficiency
Less wh of electricity input per wh of heat output.

#2> Temperature gradient
Extract wh of heat out of the solar thermal sun room collector .. even when it's cold at the collector .

#3> Reversible
Can provide both heating and cooling

- - - -
The expensive (but easier) route is to buy the heat pump unit as an off the shelf from OEM plug and play unit .. the less expensive (more effort) .. is to convert one of the mass produced heat pumps to do the job.

The window air conditioning unit .. is a common mass produced conversion .. for people wanting temperature regulation for things like gaming CPUs , brewing (beer or wine) , pet enclosures (reptile or aquarium) .. commonly used for the the cooling (aka chiller) side .. but both heating and cooling are options .. do a web or youtube search for DIY 'chiller' and you'll see at least a few DIY conversions.

IamIan,
Take a gander on this discussion group. Geothermal forum. Probably half a dozen build threads where members have rolled their own. My favorites are Randy and Jake (randen, memphis). Randy used a smallish window AC unit, Jake built a DX ground loop from scratch. Jake amped his up by adding an indoor dehumidifier loop. Both run off 120 volts and save energy.

https://ecorenovator.org/forum/showthread.php?t=4413

https://ecorenovator.org/forum/showthread.php?t=3247

Heating a slab for solar space heating
 

The utilization of solar heated hot water for heating a concrete slab for space heating is Nirvana!! On all accounts its a home run for: saving money, complexity, and comfort.

Here in Canada our winter weather is a little harsh. But I have once described the comfort like this. "On a cold windy morning standing infront of a window with a coffee in hand watching the wind swirl the snow around with my sock feet on a porcelain HEATED floor is the most comfortable feeling know to man!!!"

I have 220 sqft of flat plate solar collector on a south facing wall. with a full day of sunshine from 10:00am til 4:00pm will warm the concrete slab to 28 Deg. C which will store enough heat for 24 hrs. and hopefully the next day more sun!!

Complexity well start with planning!! The concrete slab has to be well insulated a minimum of 4" HD foam insulation under. the slab an 2" around the edge to mitigate any heat losses
Tubing in the floor. There are a few manufactures of this product designed for this application Wirsbo comes to mind. They have a complete system of manifolds valves etc. This tubing is tied to the weld mesh thats in the concrete at a foot apart. There is a lot of information now on lengths required in the loops.
DO NOT over control these loops!! IT is NOT necessary to sense and control the temperature of the zones!!! The liquid heating of the slab is very uniform and thats what you want. KEEP IT SIMPLE!!! Choose a central location in the house and place a thermistor (install a little well for the Thermistor it maybe necessary to replace) in the concrete between the loops to control the slabs temperature/Circ. pump.

DO NOT consider storage tanks, buffer tanks, mass storage (tank with boulders) Phase change materials ETC.! I know I probably made some enemies there. But hear me out: You cannot store enough water and keep it insulated and manage the pumping, heat transfer and the cost for all this is overwhelming. KEEP IT SIMPLE.!!

THE CONCRETE SLAB IS YOUR HEAT STORAGE.

We have experienced power outage (no power for circ. pumps) in a winter storm. The concrete floor maintained enough heat during that event for THREE DAYS.


To heat the floor with solar make a DEDICATED hot water loop in the floor. Hot Water/Glycol (propylene Glycol for freeze protection) Employing a small circulation pump, small expansion tank and Mixing valve on its own manifold. Having a dedicated solar heating loop raises the efficiency doing away with heat exchangers ETC.

KEEP in mind your goal of heating with the sun cost nothing going forward.!! Depending on the amount of sun in the winter SOLAR has provided HALF our homes heat!!

Install a second loop offset the tubes in the concrete in the middle of the dedicated solar loop. You will also need a manifold and small circ. pump and expansion tank. This will be your supplemental heat if you don't receive any sun.!! Your heat source here is your choice Propane boiler Demand heater Wood boiler or the King.... HEAT PUMP

Dedicated loops just saved you costs in both efficiencies and extra costs related to heat exchangers and valving. Plastic tubing is cheap by comparison.

Space heating is the largest energy cost for the home. Solar hot water heating is direct and no energy conversions like using Solar PV. and inverters ETC.

It works, we've being living with solar space heating since 2008 saving energy saving money and very comfortable

Randen

Thanks for your input. It sounds like you have a system that works for you. I am curious about two things. How do you keep the slab from cracking? If you use deep footers, are they insulted?

concrete cracks. put reinforcing in it to prevent displacement.
I like this idea, but I also like conventional construction with a crawlspace for various reasons. I am *so* on the fence about slab on grade vs frame flooring.

I grew up in Manitoba Canada. The ground freezes solid there to 8 feet deep. If you do not have a basement you have a slap on grade. A floor on a raised bed of aggregate so that it will not stay wet. Properly done this is more than enough to support a house. You do have to use Re-bar. If it is a larger building you can thicken the edges of the slab. As was mentioned the slab has to be insulated from the Ground. When it gets that cold it is a good idea to insulate anyway!

Oh, and if you are wondering, this frozen ground thaws in the spring and you can dig like normal. It's not permafrost. Where I live now near Ottawa Ontario the maximum frost depth is 5 feet. Under snow cover it's more like 18 to 24 inches. Same goes for Manitoba you get 5 or 6 feet if you have good snow cover. Roads are where you will get maximum penetration. Or windswept areas.