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Old 06-21-15, 06:55 PM   #11
jeff5may
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There are lots of ifs to consider before ruling out a vacuum breaker or pressure relief mechanism up high. Whether or not you can tolerate the sound of your drain pipe gurgling slowly is one decision only you can make. Even if you design the system so it should never stagnate, it still might. If there is no pressure relief installed up high, any steam generated will clear your drain for you quickly and violently.

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Old 06-21-15, 10:34 PM   #12
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In the late 70's, early 80's, drain back solar DHW systems went up all around my area. By 2000 there was not a single one still in use. I don't know all the details of every system or why they were removed eventually but in this area/climate they did not prove themselves(as I don't think anyone would give up cheap DHW). But the drain back design was a must for freeze protection. The newer isolated anti freeze systems are more reliable and has a longer use life but the added cost isn't swaying many people because we just don't get enough reliable sun on a daily basis to satisfy hot water spoiled people here. More convenient/relatively cheap ways to provide it are still available.

A lot of variables work into how mainstream "green" practices really become over time when things like mortgages, homeowners insurance proper maintenance and such get into the equation. Even energy conservation itself is becoming a "cost" in itself as we are told we need to be pay more because we are using less. Work that into a payback calculation.

I would think that a heat "dump" might be more applicable to your overheating concerns than "drain-back" ability.

I'm really following the progress of the hybrid PV/HDW systems being developed.
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Old 06-22-15, 04:11 AM   #13
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@Drake
I would be very interested in reasons why drainback systems are being abandoned in your reagion.
And yes mate, you are tackling some very important issues. "Green" and "eco friendly" are all very nice, but the whole thing should also be sustainable and financially benefiting.
So I do try to work everything into realistic payback calculation. That's why drain back systems look so tempting, they do seem cheaper both initially and from the perspective of the "cost of the ownership", maintenance and stuff like regular glycol changes (which drainback doesn't need, since it uses water).


I'm not sure how many sunny days you do get in Minnesota, but if this link is right - you do get quite a lot of sun (even if climate is quite cold, as I understand):
Sunshine & Daylight Hours in Minneapolis, Minnesota, Usa Sunlight, Cloud & Day length

Skopje, Macedonia also gets quite a lot of sun:
Total Annual Sunshine in European Cities - Current Results

In both our places this points to a conclusion that investing in solar technology, should result in realistically achieved (and shorter) payback time for the investment made.


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I would think that a heat "dump" might be more applicable to your overheating concerns than "drain-back" ability.
Seems simpler to just downsize a bit the collector power, and live with somewhat lower temps of the heated water.
But even with dedicated "heat dump" in the system, my worries about overheating are really about overheating of the glycol, not the water (DHW). Overheating of the glycol, when public electric power grid goes down on a sunny day.
Dunno how good is the public power grid in Minneapolis, Minnesota - but power shortages are bound to happen at least few times yearly, here in Macedonia. Those power shortages can go on for longer than any reasonably sized backup UPS can cover up. PV panels and DC pumps are option, albeit seemingly more expensive one.

Can somebody recommend dependable and cost effective PV panels and DC pumps?

Also, if doing the standard closed loop glycol solar system, I was considering placing the collector lower then than water tank/boiler and thus benefiting from thermosiphon movement of the glycol, when power shortage occurs and the pump stops. This option would require some additional stuff (like additional piping with electro ventil) to circumvent the halted pump blockage (not sure how understandable is all this, english is not my native language).


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There are lots of ifs to consider before ruling out a vacuum breaker or pressure relief mechanism up high. Whether or not you can tolerate the sound of your drain pipe gurgling slowly is one decision only you can make. Even if you design the system so it should never stagnate, it still might. If there is no pressure relief installed up high, any steam generated will clear your drain for you quickly and violently.
Yes, in previous post I was concerned about the steam (after a stagnant phase). So as you say - a pressure relief valve wouldn't hurt anything. Gurgling sounds in the drain pipe, is something I can live with - since it will be only be heard in bathroom/attic.
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Old 06-22-15, 05:41 PM   #14
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I thin it overall comes down to the willingness of adjusting to a little change in convenience to live long term with sustainability. Here in my fortunite affluent country few are willing to give up anything in convenience for a little extra cost. On demand DHW has been a expense most have been willing to pay for some time so it doesn't take much inconvenience with a "green" sustainable system for most to abandon them.

Yes, MN does has overall get a lot of solar exposure, but it is like our temps very erratic. Avg is just a concept when trying to survive comfortably the 100 degree F or -40 degree days. Three days without enough sun for hot solar water is something most don't want to live with and if a backup system is added to the cost of the renewable one it is even a harder sell.

The feedback I was able to get over the year for those with solar around me varied greatly from mechanical failures throughout system over the long run, issues with snow removal and difficulties with homes being bought or sold and banks acceptance of the systems. Overall it was the not consistent availability of DHW. as we are in a major city with relative affordable N Gas option which gives very on demand DHW proven over time as compare to much of the world where solar DHW is a proven standard.

I agree with your thoughts on the value of hot water conservation and some lifestyle adjustments as being very beneficial as well as just being 'green". This country is just waking up to the needs of clean water conservation whether it be hot or cold. Thought being from the Land of Ten Thousand Lakes - Minnesota it is still not in short supply.

My input was that here the drain back design function of the systems was for freeze protection. It may really be worth thinking about if a system can be drained back to protect against low temps it might also do the same to protect against high temps if controls are modified.
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Old 06-22-15, 06:24 PM   #15
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And indeed it does also protect against high temps, the controller simply shuts of the pumps when set high temp is reached.

Is there any consensus about efficiency comparison on drainback vs closed loop, solar systems?
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Old 06-22-15, 06:37 PM   #16
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One of the main benefits of a drainback system is that there is no need for a heat dump. When the heat store reaches its high limit setpoint, the pump shuts off, and the collector drains back. You would think that there would be a problem if the system started back up when the collector was super hot after being drained, but the thermal mass of the collector is relatively small compared to the amount of heat it would take to immediately boil the incoming water. Unless the collector is super huge, it stands no chance of raising the water from 150 degF or so to boiling in one pass unless is is cherry red hot. Sizing the collector is actually easier for a drainback system due to this shutdown at max thermal input from the sun.

Recirculating systems are another story. They are designed to always have liquid in them. On those hot, sunny days, you have to have a heat dump active to prevent the brine solution from boiling. Just a few degrees of delta T adds up on multiple passes in a hurry when there is no load. All recirculating systems are required to have a blowoff valve in them (in America) to prevent a catastrophe in case the power goes out. Many have redundant backup pumps that only operate when the power is out. Due to the heat dump and its fan, recirculating systems are less efficient than drainback systems. The extra hardware (heat dump exchanger and plumbing, assorted valves, sensors, controls) adds a lot of expense to the system as a whole.
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Old 06-23-15, 06:02 AM   #17
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Sorry guys but lots of odd and old info in this thread.

First, I have put in lots of all types of solar thermal systems and really the only drawbacks to a drainback system is pumping power and a bigger pipe size/ need for slope on the pipe.

You only need one pump. With your "flat plate" HX at the bottom of the storage tank, and the one pump pumping up to the panels, you can have a good system. The heat will thermosyphon to the top of the tank and draw from the bottom.

You need a 20-30L tank (placed as high as possible) as a water reservoir for 6m2 of flat panels (do not use tubes), DO NOT use a vacuum breaker or auto air vent on the solar side (this is 70s thought) and they always fail after some time. A good pump will push the air/steam/water back through the system until the hot water reaches the HX then it will flow well.

Use a RESOL BS2 controller (good German brand) but also put a secondary sensor (it can just be any old switch that only turns on above 20C) on the panel to prevent the controller from switching on at night. They seem to have this issue often enough and at -20C it is a big problem. It is a problem with glycol systems too. I have had 4 failures in the last 2 winters because of this.

And (sorry Jeff), recirc systems are not less efficient than drain back and if you look at the SRCC docs, the top systems are glycol based. A properly designed glycol system will "steam out" when the panels reach stratification. In other words the heat will drive the glycol out of the panel and into the expansion tank and all that is left in the panel is water steam, which does not degrade. Of course an improperly designed system will allow glycol to be captured in the panels and will degrade.

All systems, everywhere, need a pressure relief and this is usually set at 6-7bar (some are 10bar) and are there to protect piping etc. If you look at PAW pump stations for example the pressure relief, check valves and exp tank are designed to allow the glycol to flow back to the exp tank. This is actually German law.
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Old 06-23-15, 06:26 AM   #18
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Quote:
Originally Posted by Mikesolar View Post
Use a RESOL BS2 controller (good German brand) but also put a secondary sensor (it can just be any old switch that only turns on above 20C) on the panel to prevent the controller from switching on at night. They seem to have this issue often enough and at -20C it is a big problem. It is a problem with glycol systems too. I have had 4 failures in the last 2 winters because of this.
Can you write more about that? I use a Resol DeltaSol BS Pro controller and have occasionally noticed it running the pump when I wouldn't have expected it to. I thought it was something to do with what the manual calls OTC Tube Collector Special Function, where a 2-degree rise in collector temperature causes a 30-second running of the pump, but now you have put a doubt in my mind.
When you say to only turn on above 20C do you mean 20C tube temperature, not 20C ambient?
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Old 06-23-15, 04:21 PM   #19
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Can you write more about that? I use a Resol DeltaSol BS Pro controller and have occasionally noticed it running the pump when I wouldn't have expected it to. I thought it was something to do with what the manual calls OTC Tube Collector Special Function, where a 2-degree rise in collector temperature causes a 30-second running of the pump, but now you have put a doubt in my mind.
When you say to only turn on above 20C do you mean 20C tube temperature, not 20C ambient?
The problem is, I have not found a reason for this to happen. A 30 sec run of the pump won't hurt anything. It has to be long enough at -20C to freeze out the water in the other side of the HX and 30 sec won't do it. RESOL has no explanation for it either because the rest of the programming is fine. It is as though the relay sticks and then unsticks later on.

The only way to do it have something in line with the pump circuit that prevents flow if the temp is below 20C (for example). I have heard of other controllers with the same issues going right back to the mid 70s (the first ones I've worked on).
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Old 06-24-15, 07:58 AM   #20
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Yay, finally someone who knows what they are talking about, with experience. I was running out of straws.

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