Solar Drainback Water Heating System

[Alphacarina]

I'd like some input from you guys who are familiar with drainback systems

Currently, I have an open loop system which I'm going to convert to a drainback system to make things 'automatic' so I won't have to worry about draining my panel on cold nights. There are other benefits too, but freeze protection is the major goal

As usual, before I make any changes I've been reading up on the subject and I find that I'm giving up some things I don't really want to (like efficiency) by converting away from open loop, so I'd like to minimise those losses. No matter what I do, it's going to use more electicity than my system does now because I'm going to need a whopping 1/8th HP pump for the new drainback feature in addition to my current 1/40th HP circulator. I'd like to increase efficiency and minimise that wasted electricity if I can

With an open loop system, the collector is always full of pressurized water which gets heated as the panel warms in the sunshine - Very efficient . . . . nothing else comes close. One small 1/40th HP circulator which uses very little electricity. It can even be solar powered if you like

With a standard drainback system, the panel is always empty when the pump isn't running, so the panel heats up much faster, since it's not heating any water. Once the cooler water from the drainback tank is introduced to the hot panel, it doesn't take long before the pump stops running and the water drains out of the panel again. You get lots of pump cycles on that high HP pump and it wastes lots of electricity to lift the water all the way up to the panel each time the pump cycles - About a 25 foot lift in my case. Lifting a column of water 25 feet every time the pump cycles on makes no good sense . . . . at least not to me

What I would like is for my panel to stay full of water until there is no more opportunity for solar gain . . . . so that anytime the sun is shining on my panel, it's heating WATER and not just an empty panel. Heating an empty panel and then filling it with water isn't good for the panel - You get lots of rapid expansion and contraction when you fill a hot panel with water suddenly a hundred times per day and that ultimately shortens the life of the panel

I would like the panel to be full all day and then a single drainback when the sun goes down and then only one refill for the day the next morning once the collector temp says there is energy to be harvested. Fewer pump starts and stops and only one 'big lift' each day . . . . less energy wasted, fewer cycles on my pump, no more filling a hot panel with cooler water a hundred times per day - What's not to like?

There are some very sophisticated, pretty complicated, quite expensive, patented commercial setups available which do exactly what I want . . . . keep the panel full all day and then drain it every night - But I'd prefer something much simpler and something I can do myself without spending a fortune. I have a complete open loop system which has been working very well for the past 3 years that I designed myself, built with readily available parts which didn't cost me a fortune - I probably have about $2500 in the whole thing. A single 40 sq foot collector, two very well insulated 55 gallon storage tanks, a timed switch to add energy if needed after a couple days of no sun, a single 1/40th HP circulation pump and a pretty sophisticated, fully adjustable differential controller - Actually I have 2 different controllers and I play with them all the time, tweaking things for best efficiency. My system provides nearly 100% of my needs for about 9 months per year and a pretty large percentage of what I need during the winter 3 months

Back to my proposed drainback system - What I would like to do is put a check valve just above my drainback pump so that the distilled water it pumps out of my drainback heat exchange tank up to the panel cannot drain back everytime the pump stops . . . . the feed pipe and the panel would remain full of distilled water. The rest of the system design would be similar to a standard drainback. Two differential contollers . . . . one to control the drainback pump and run it when the collector temps are hotter than the bottom of the drainback heat exchanger and a second one to circulate the domestic water through the drainback heat exchanger and into the storage tanks whenever the drainback tank temp is hotter than the bottom of the storage tank. A 3/4 inch pipe from the top of the collector would allow the heated water to fall back into the vented drainback tank whenever the pump is running. I have a vent at the top of the panel now which would also remain . . . . a second air source to make sure the panel drains completely . . . . when I want it to at the end of the day

My 'secret weapon' would be a tee just above the check valve on the output of the pump which would lead down to an electrically operated valve, which when opened would allow the collector to drain through that valve around the pump and back into the drainback tank. I'd like for that valve to be a 12 volt DC valve which needs 12 volts to remain closed - You can probably guess where I'm going next . . . . a PV panel on the roof to generate the voltage to keep the valve closed. When the sun goes down, the valve opens and the collector drains

I have some 'fail safe' ideas to make sure the panel still drains in the unlikely event the 12 volt valve should fail in the closed position. I also have a plan which would prevent it draining the panel everytime a cloud hits the PV panel but those are just little 'tweaks' I can play with and perfect over time and they don't affect the basic premise of the system operation

Any discussion pro or con on my idea(s) is welcomed . . . . especially if you have any suggestions to make it better

Thanks,

Don

[Aaron Becker]

Hi Don,
Sounds like some Rube Goldberg engineering that you're planning. I have a drainback solar hot water system in operation for a couple of years, and I've installed many systems, including pressurized glycol & drainbacks. I think you are needlessly fretting over the additional energy consumption for the high head pump requirement of a drainback system. What you can do to minimize the pump size is to locate the drainback reservoir as high up in the thermal envelope of the house (insulated attic if you have, or just second floor ceiling if not) - this will reduce your pumping head height significantly. Then, you can install the smallest pump based on your head & flow requirement down in the basement near the storage tank or heat exchanger. Also, you should jsut keep the circulating pump between storage & heat exchanger on the same circuit as the collector pump - don't fret about the lost btus that fall back to drainback tank upon pump shut down. If you want to get obsessive about pump energy, then start by using a Wilo ECM pump in series with a higher head pump, and use a 5 minute delay off timer for the larger pump. So, on start up, the differential controller activates all 3 pumps, 5 minutes later, the high head pump shuts off, letting the free fall of water returning down the pipe work for you, requiring only the power of a small circulator. As for having the panel filled just once during the day, again, you're fretting over nothing. The solar panel can take the thermal shock of start-up & drain-down. Your differential controller should be set for 4 - 6 degrees F. shut -off and 16 degrees F differential start-up. Let it do its job.
Best of luck

[protech]

I say you are fretting and spending all kinds of money that you don't need to. There are thousands of active-direct systems installed in central Florida many of which have been operating fully automatically for 20-30 years. Wanna see some pictures? I have tons.

It's just not worth the extra installation costs and energy losses to run a drain back system this far south. All you need is a freeze valve in the right place and you'll be fine.

You know, I know a local solar contractor that has lots of experience that is in your area that could get your system setup right <hint> <hint>

[procooper]

Don,

I completely agree with your not wanting to get rid of the efficiency of the Open Loop system by adding an extra pump and Drain Back Tank and Heat Exchanger. Would it be impossible to develop a way to incorporate a Drain Back method into the standard Open Loop system? Any and all comments on this would be great.... I have searched high and low to find an Open Loop system with one pump and controller that drainsback for freeze protection. On the TACO website I even found a diagram for a Direct System with a "Dump" when the system is idle.... any help?

Thanks,
Bryan

[protech]

That system is known as a "drain down" system procooper. Not to be confused with a "drainback" system.

[Mikey]

Only comment I've got is that a PV cell controlling the drainback might not do what you really want. Dark is fine, but cold isn't. No point in draining the panel if it's 70° but dark. If I missed something in your description that addresses this, I'm sorry.

[frankflynn]

Indeed - I have just such a system. They were popular during the late 1970's early 80's. The only unique piece of this puzzle is the draindown valve which typically lives near the tank and pump. It opens and allows your domestic water to fill the the panels during the day and then at night it closes sealing off your domestic how water and allowing the water in the panel to drain out. Add an anti syphon or vacuum breaker valve to the top of the system to let air out in the morning and let air in at night and everything else is the same.

Now why aren't these more popular? Because the valves can fail and if they do and it freezes you will have a big leak on your roof. This is the voice of experience talking. The valve I have is a "Sunspool" made by Heliotrope General (now out of business - because if a fire not a bad product) - it's a marvel of engineering and not difficult to maintain but most folks want maintenance free. Just google "Sunspool fail" - it's not that they fail often, mine has failed once in 30 years, but when they go it can be a big deal especially if it freezes hard where you live.

The issue with them failing is you don't notice it - if they do not close completely the water will leak into the panels which are sealed anyway so you won't see it until it has frozen and ruptured something and is pressurized again. Now every fall I check my drain down bucket, a 5 gallon bucket that should be 1/2 full - any fuller and the system is leaking.

Still, it is efficient. Where I live I can get 100% of my DHW from solar even in winter if it is sunny.

[seanmc]

If you're looking for a simpler approach to the standard drainback and standard pressurized glycol systems, check out the Simple Drainback system. They've basically boiled down the solar drainback system to one tank, one controller and one pump. It's that simple. As with all drainback systems their freeze protection is attained through gravity. When the solar collectors are hot enough to heat the tank, the differential controller turns the pump on and water flows from the tank and back, heating the tank. When the collector temperature drops to the point where the tank will no longer benefit from the collector temperature, the differential controller turns the pump off, and the solar fluid drains completely out of the collectors and returns to the storage tank.

Check it out: www.simpledrainback.com

Good luck,
Sean

[Alphacarina]

Now why aren't these more popular? Because the valves can fail and if they do and it freezes you will have a big leak on your roof. This is the voice of experience talking.

The issue with them failing is you don't notice it - if they do not close completely the water will leak into the panels which are sealed anyway so you won't see it until it has frozen and ruptured something and is pressurized again.

Still, it is efficient. Where I live I can get 100% of my DHW from solar even in winter if it is sunny.

More than a 'big leak on the roof' for me, Frank - A new $1200 collector would be needed to get up and running again!!

Thanks for the replies guys

Yes, 'failure' isn't an option, especially when you have a $1200 panel up on the roof which can easily freeze even in 34 or 35 degree temps

I've simplified things a bit. First, so long as there's not a power failure, my panel can't freeze - The differential controller begins circulating some of my stored hot water through the panel whenever it starts to get really cold . . . . I just don't like wasting the hot water and I don't want to depend on that because of the possibility of the aforementioned power failure - Even a teeny, tiny chance of losing my panel is just too great

Since last winter, I've simplified my drain procedure - I used to have to go down to the basement to close two manual valves, disconnect the pump from the controller (so the pump doesn't run all night when the controller tells it that it's COLD up on the roof) and then make a trip outside and open two manual valves to drain the water on the ground, wait for it all to drain and then close the valves so the system is ready to pressurize again the next morning - The whole thing was so complicated my wife needed step by step telephone directions to do it if I was away from home

I bought two 110V electric valves and soldered them on the ends of my drain pipes outside, ran those lines into one and through the basement wall, where I can see the water drain through a clear tube into my sewer system. The drain switch is mounted in a box with the pump disconnect switch right next to the two valves which must be closed so now it's simple enough now that my wife can do it in her sleep

We drain it anytime we're expecting temps below 35 degrees . . . . and anytime we're headed out of town during the winter. What used to take 10 minutes and a trip out in the cold and dark now takes 15 seconds down in the warm basement. This has worked so well over the past year that I've forgotten all about drainback systems . . . . and I can sleep comfortably at night knowing there's nothing to go wrong

Don

[flipper69]

FYI

the sunspool valve can be replace with 2 erie 3/4" sweat, 3 way valves(VT3313)

two erie pop top actuator-(2 position normally closed 110v 6.5watts) AG13B020

these valves can handle 200F temperatures if the temperature is not high enough you upgrade to the VS valves and acutators which can handle 250F

i use a C30 differential control to control the pump and the actuators

these valves switch very quickly and if you are concerned about hammering when the valve closes you can add a hammer arrestor to the supply side before the valve

flipper69