Solar radiant heat project begins

[Raspy]

I am still very, very early in design, I have two really basic questions:

1. Why the drain back system in the collectors instead of antifreeze and just stopping circulation based on temp differential?
2. What do you do with the waste heat in the summer? I would think you could not use enough hot water to keep the collectors cool.

These are two of the best and earliest questions in your design! You are definitely on the right track!

You could go either way. Antifreeze or drainback. Either way you need a heat exchanger to move the energy to another fluid and both will be closed loop.

Antifreeze: You have to decide which antifreeze. Ethylene is out of the question for me because it's very toxic and that brings a bunch of design considerations. Silicone or oil are out of the question because they are hard to manage, viscous and have a low specific heat. Propylene Glycol is very good. But it's an additional cost, requires the proper ratio, a standby amount for maintenance and topping off, possible changing in a few years to prevent PH problems, more complicated plumbing to allow pump changes and air management, and it must endure stagnation temps. This may require a higher pressure that is maintained to prevent boiling. Don't get me wrong, I like it, but it is not without it's drawbacks to me. I did this on my fathers house in 1979. It is still running, but has had some maintenance issues.

Drainback. Simple single wall heat exchanger or submerged coil. No toxic fluids. No freeze danger. Very simple plumbing. No cost for the fluid, which means easier handling later and simpler plumbing because you can dump it anytime and replace it anytime without considerations or hassle. It also has a designed in amount of air, so no air vents, no expansion tank, easy pump changes, no PH testing and no freeze testing. The highest specific heat, so the lowest operating temp and highest efficiency with minimal pump wattage. No matter what failure you can reasonably imagine with this system, the collectors are protected from freezing. If there is a leak, it's a small amount of water and its over: nothing toxic and no flooding.

Waste heat in the summer is always a consideration. The ultimate is to have a spa or pool to dump it into. If not that, it boils (pun intended) down to either stagnation, covering a few panels or having a very large storage and flagrantly using hot water. I always worry about stagnation and have a number of stories about that, but I recently removed a set of 30 year old collectors that had sat unused for a number of summers. They were fine. No apparent outgassing, no coating failure. Fine. So I have forced myself to concede that stagnating is not as bad as it seems. They are designed for that. And sitting empty is probably better than sitting full. Hence, drainback. Another way of limiting the overall output is to put a timer in that controls the power to the differential controller. A simple lamp timer from the hardware store is fine. Limit the run time to an hour or two a day, in the morning, based on the differential. Then the power goes off and it sits until the next morning. Some management is required, but it works. It's fun to see very high storage temps, but it is unnecessary. As long as the storage is above your domestic water temp, in the summer months, by 20 degrees or so, it's all you need until the next day.

There are other freeze protection systems too. Like draindown and recirculation. Please don't consider recirculation unless you live at sea level in Hawaii. Draindown brings it's own set of unnecessary problems. Bottom line is, the collectors must either be empty, or full of something that won't freeze. Empty is probably better in most situations.

 

[IXLR8]

John,
Whose collectors did you use? Would evacuated tube collectors work in a drain back system? I would think they would as long as you put enough pitch in the plumbing to get water to drain back properly. I understand from a cost/performance point of view that the flat plate collectors are better suited to your design.

 

[Raspy]

Jim,

I don't like evacuated tube collectors for a couple of reasons. I'm sure they would work though. Of course, any type of collector has to be pitched properly to drain in a drainback setup. The drain process should be fast and thorough. With a closed loop drainback there is only one quantity of water. It cannot build up any sediment or hardness deposits and it backflushes the system every time it drains down.

If you look closely at my earlier pictures you can see the top set of three drains to the lower right, down into the lower set, that drains out the lower left and back to the tank. The system is two sets of three panels, that feed in series.

I used 4 x 10 collectors made by Solarskies. They are pretty much a generic design done well. They have a Terralite all copper absorber plate with a selective (not black chrome) surface, which is a design that has been around for a long time. A single piece of low iron, tempered glass, extruded aluminum box, and a heavy aluminum back panel. In theory, they could be taken apart for repair if needed as they are not glued together. They have a nice EPDM gasket at the pipe exits. There is a variety of mounting hardware for them, but I didn't like any of it, so I devised my own.

When you are looking at collectors and hearing the hype about "better", you have to decide for yourself what is better for your application. For instance, a black chrome absorber surface might give a higher stagnation temp, but that is not what we are looking for. In fact it could be a problem in the summer. Evacuated tube collectors might reach a higher temp for a smaller amount of water, but the overall BTUs collected will be less, and the units are fragile. Aluminum absorber plates are cheaper than copper, but they corrode near the ocean and become useless. Tracking systems sound appealing, but why spend a lot on extra hardware that produces no energy and may have questionable reliability? Sometimes it makes sense, but not always.

 

[cycle_gator]

John:

Thank you, great info. I like the HX + aux. source plan. Are you running H2O in the radiant system? I am thinking antifreeze there. Thoughts? And Thank you, again.

 

[cartod]

John:

Thank you, great info. I like the HX + aux. source plan. Are you running H2O in the radiant system? I am thinking antifreeze there. Thoughts? And Thank you, again.

My thoughts: anti freeze $5.00 a gallon x 300.

 

[Raspy]

John:

Thank you, great info. I like the HX + aux. source plan. Are you running H2O in the radiant system? I am thinking antifreeze there. Thoughts? And Thank you, again.

I am running straight water in my radiant system. I toyed with the idea of running antifreeze and will admit there is a slight risk of running only water. Your particular climate and use pattern definitely factor in.

In my case, I wanted to get it going, wanted to test for leaks, just wanted an initial run and was willing to keep an eye on it. So I put in straight water. I also wanted to measure exactly how much fluid volume it actually had, so I added the water, gallon by gallon, at the top, by pouring it in. This way I would know how much antifreeze to get when the time came. I don't like the 50/50 premixed stuff. If I knew how much the total volume was I could decide on the ratio to water and add the right amount of 100% before topping it off with water. It took 58 gallons to fill it. I also want to flush it out after a few weeks of running to get any contaminants out.

Then I had to drain it down to modify my piping a bit, and again I need to to that to move a valve, so water is very practical right now. When everything is proven and ready to be closed for good, I'll probably drain it completely, add about 40% propylene glycol (approx 23 gallons) and top off with straight water. This won't be until next winter or later. 40% gives good freeze protection and good corrosion resistance while using less material than 50%.

I have valves on each side of both pumps, a valve on the expansion tank line, valves on the heat exchanger and on the manifold supply and return pipes. So even if it has propylene in it I can change parts with minimal hassle. It seems crazy, but I've had to drill eleven holes in the slab since I got it going and I plan to drill at least six to eight more. Being able to isolate the loops and manifolds could save a bigger problem if I hit something.

My system has an iron boiler and a steel expansion tank. It has an air volume at the top for expansion, not a bladder tank. It has iron pumps and flanges. So even though it is a closed system, it has trapped air that contains enough oxygen to allow some corrosion. I have addressed this issue, until I make up my mind about propylene glycol, by adding a non toxic corrosion inhibitor.

 

[Welderbryan]

thats a nice house

 

[AaronS]

Great info here. I'm in the VERY early stages of designing our home and am very interested in installing radiant heat in both the home and shop. It will be a long pole barn with living area on one end and basement under that living area.

My question is: can cold water be circulated through the pex in the summer time to help cool the house?

 

[rmorey]

Great info here. I'm in the VERY early stages of designing our home and am very interested in installing radiant heat in both the home and shop. It will be a long pole barn with living area on one end and basement under that living area.

My question is: can cold water be circulated through the pex in the summer time to help cool the house?

Cold water can be circulated thru the pipes, but you may have a condensation issue to resolve. The colder the water, the more it will condense. Figure on summertime 80 - 90 degree air temp and well water at 50 degrees, there may be wet spots on the floor.

 

[AaronS]

Cold water can be circulated thru the pipes, but you may have a condensation issue to resolve. The colder the water, the more it will condense. Figure on summertime 80 - 90 degree air temp and well water at 50 degrees, there may be wet spots on the floor.

Makes sense, thanks.

Now to see if I can find out how much electricity a tankless boiler uses and see if solar can keep up with it. Lots of homework to do.