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New to solar water heating. I just hooked up a solar drainback system. I have 2 flat plate solar collectors, both sloped properly, about 20 feet of 3/4" copper going into an Eagle Sun 10 gallon drainback tank. A Taco Solar Pumping Station is below the drainback tank. The panels should drain , but when the pump is shut off only a small amount drains back into the tank. The water level on the sight tube only goes up only about 3/4", and if I open the drain plug on the panels a lot of hot water comes out of the panels. The panels are mounted on a roof that is about 5 feet above the top of the drainback tank. All piping has a good slope towards the tank. What am I doing wrong?
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There is your clue. I am an EE with little experience solar with Thermal Drain Back systems so my experience is mostly academic and limited as such. Like a Teacher, or Politician I am really do not what the heck I am talking because I have not actually done much of it if anything
Sound likes you have a relief valve blocked or stuck. Hopefully JPM will chime in. He is the ME of the group with considerable experience.Last edited by Sunking; 04-18-2018, 05:03 PM.MSEE, PE -
My understanding is that a relief valve is not needed, that the water drained out of the panels is replaced by the air in the drainback tank. But it does seems that there is a vacuum caused by the water draining out of the panels that stops the complete draining. I am at a loss.Comment
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You may have an internal check valve in your pump, or inline in the pump pack preventing the water from draining back. You should be using a Taco 009 BRONZE pump with a drainback system.
The Taco pump packs usually are used in the pressurized evacuated tube systems.Last edited by LucMan; 04-18-2018, 10:25 PM.Comment
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The Taco Solar Pumping Station has a Taco 008-VTF6 pump with integral variable speed control. Inside the pump control box there are 5 DIP switches There are several setting for drainback systems, and the specs for the system say " supports drain back applications "Comment
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If the system is new, you did it yourself and it seems to be non drainable from the git-go, I'm with Lucman on first checking to see if the Taco pump is one of those with a built-in check valve. If so, it will have a model number ending in "IFC" on the pump nameplate. The cut sheet on the "SPS-PC" pump station seems to indicate it has check valves in it. If so, the check valve is in the body of the pump at the inlet.
I've got a taco pump with a built in check valve and it's a rather easy matter to remove/replace it, but you'll need to break at least the pump inlet flange to get at it, and from the looks of the pump station, maybe both flanges because you may not be able to get a pair of needle nose pliers into the right position to get at the check valve without removing the pump entirely from the pump station. Twist/rotate the valve gently with the pliers until free and pull straight out.
The pump will operate just fine either with or without the check valve. Getting rid of the check valve will give you a bit more flowrate - not much more - but always a good thing if for no other reason than more operational flexibility.
All the Taco "00" circulators, both with and without check valves use the same bodies to save inventory. So, even if the model number does not end in "IFC", it may be worth breaking a flange or two to see if a check valve got installed in error. Stuff happens. Because the check valve bodies are so flimsy, and use buna "O" rings with a max. service temp. of a rather low 140-160 F or so, I change out the check valve on my Taco 008 1X/yr. So, I know it's an easy process to remove as described above. If you have isolation valves on either side of the pump station, and can get at/remove the pump, it's a 10 min. job. Do it when the system is cool.
A friendly reminder, as with any flange break/reassembly, never reuse gaskets, and retighten these flanges opposite sides a bit at a time. Recheck for leaks after the system reaches operating temp., and again during and after 1st cool down, and again after 2d restart to operating temp. Thermal expansion can do funny things to flanged joints. See the net for new gaskets. FWIW, I'd suggest 2 sets as spares.
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Thanks J.P.M. My first thought was that there was a check valve somewhere in the system. All the literature that came with the Taco pumping station says that it can be used with a drain back system. There are several pages in the instructions that have the setup for drain back, and no mention of a check valve. Yes, I suppose it is possible that one got put in by mistake. I had looked at information on the IFC, and watched videos of replacing the cartridge. It's easy access the pump on my installation. I hate to have to take it apart just to see if they accidentally installed an IFC in my pump. Perhaps there is a way to check. The system will drain back some, very slowly, but does not seem to completely empty the panels. I put a little over 1/4" per foot slope in the panels,
and well over that in the rest of the pipe. The water in the sight tube only rises 1 1/4", and that takes well over an hour to happen. I need to do some calculations to see how many gallons that is in a 10 gallon drain back tank.Comment
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On a properly operating drainback system you should be able to hear the water draining back into the tank. Usually no more than 10 -15 minutes for all the water to drain.Comment
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You are most welcome.
I looked a bit further and it seems that if you have pump control station model # SPS-PC, there may be other check valves in the module besides or instead of (or in addition to) a check valve in pump body at the pump inlet, although it's noted on the cut sheets in what I'd consider a rather cryptic fashion. Without getting real involved in it, Id suggest those valves may have some purpose in preventing reverse flow from the 10 gal. capture tank to the main DHW storage.
Just a shot and unlikely: No vac. breakers and/or air bleed valves at system high point(s), right ?
Do you have pressure gages in the system ? A gage at the system low point will show the greatest pressure difference between full and drained conditions, or maybe help est. system level.
Not that your system is identical to mine, but FWIW, my system volume with 2 ea. 4 X 8 Sunearth collectors and ~~ 150 ft. of 3/4" supply and return lines, some instrumentation, a few loops etc, is ~ 4.65 gal. total system volume. If your system vol. is similar, that may be useful to judge how much of the sys. vol. is draining back. The coll. vol. is 1.0 gal./collector so 2. gal. The rest of the vol. is in the piping system.Comment
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Yes, it is the model #SPS-PC. And no air bleed valves.
I shut the system off yesterday when the sun was off the panels. I am running the system in manual mode with well water until I get everything working properly, them will put in distilled water.
Initially there was a little drain back, about 3/4" on the sight tube. I calculated that 1" on the sight tube equals just under 3/4 gallon, so in about an hour after shutting the pump off a little over 1/2 gallon had drained back.
This morning I checked the sight tube and it showed that about 1 3/4 gallons had drained back, after being off for 14 hours.
By my calculations the solar panels hold just over 1/2 gallon and the pipe going to and from the panels hold about 1 1/4 gallons.
So it seems that eventually all the water did drain back. I checked the lowest plug on the panels and they definitely were empty.
So, the system is draining back, but it is taking several hours to complete the process. That doesn't seem right.
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The air has to bubble up, and the water trickle down. If there are any airlocks, it could take a long time. And don't use straight distilled water, you meed some corrosion inhibitor usually.
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Without a piping diagram and a look at the system, it's difficult to analyze what's happening. IMO, you probably have either an air blockage somewhere in the system brought on by the piping layout/design, or you have a check valve you don't know about, probably in the pump station that's keeping things from draining back. Read through the pump station manual and note/look for any reference to check valves that can be opened or bypassed for drainback operaton. A reasonably well designed 2 collector drainback system ought to drain in about 5 minutes or less.
A call to TACO may be in order.Comment
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If you look closely at the cut sheet you will see there is an internal air bleed in the pump pack. That is a dead giveaway that the system is designed for a pressurized system.
There is a work around to your issue. You can install a bypass from the discharge of the pump pack back to the water inlet with a normally open zone valve or a solenoid valve. Wire the valve to the pump or pump relay to close when the pump is powered on.
You may also have to disable the variable flow feature of the pump as drain back systems need full flow at start up until the water starts to return to the draIn back tank. The pressurized systems are designed to start at a minimal flow and speed up as the panels return warmer water to maintain a set delta T.Comment
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Yes there is a manual air bleed valve, but I don't think it would inhibit the flow in any way if it is not used.. The system is designed for either pressure or drain back. There are a series of DIP switches on the computer board in the pump. According to the instruction there are two different settings for drain back. One setting ( switch 1, 2 & 3 on) starts a second booster pump first then after 3 minutes turns on the variable speed pump. The other setting ( switch 3 on only ) starts the variable speed pump at full speed for 3 minutes, then goes into variable speed mode. I do not need a booster pump since the Taco pump has a head range of 0-16ft, and the top of my panels are about 7 ft above the pump.
My first thought was that the pump itself was blocking the return flow, and thought about putting in a bypass loop with a one way valve. But everything is draining, it's just taking several hours to completely drain the panel.
Maybe that is not a problem, It never gets cold enough here to freeze the water in the panels in that amount of time, and it seems if the temperature in the storage tank reaches the max that it is set at, and the panels start to drain, that the expansion of the warming fluid will force it out quicker. ?????
It seems that if an IFC accidently installed in the system that it wouldn't drain back at all.
Hard to do a drawing of the system, but I will take some photos and post them if I can.
In the meantime, I am running the system manually, as I haven't had a chance to check out the sensors to see if they are working properly. It is kicking out a good amount of hot water given the limited amount of full sun we have been having.
The tank temperature has been getting up to as much as 105 F. So my on demand hot water only needs to raise it another 15 or so degrees, instead of up from 48 degrees.
So eventually I will see a huge drop in my electricity bill. We have a 5.3KW grid tie system on the roof, and bank up enough through the summer to not get an electrical bill until December.
Aiming for net-zero.
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