Need help designing a solar water heater for radiant floor heat

[WillyP]

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.

[WillyP]

Originally Posted by randen

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

[AzurePower]

Sound good

[solarhotairpanels]

Originally Posted by WillyP

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.

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

[WillyP]

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.

[solarhotairpanels]

Originally Posted by WillyP

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. 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

[jeff5may]

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.

[WillyP]

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.

[jeff5may]

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.

[solarhotairpanels]

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. :-)