Heliodyne Solar Thermal, open loop, possible thermosiphoning

[js080]

Hi,

I'm in charge of a Heliodyne controller and 4' x 8' solar thermal panels. In reviewing temperature data from the controller, I'm concerned that there's possible thermosiphoning once the sun goes down. There is a check valve in the system to prevent reverse flow, but since the Grundfos pump is an impeller, my understanding is that nothing would impede water flow in the forward direction even when the pump is off. No, there isn't a flow meter anywhere in the system. Suggestions?

J

[J.P.M.]

Without a working check valve, the way most solar thermal collector systems are designed and plumbed, they need a working check valve or some way to prevent reverse thermosiphoning.
Often, and unless sophisticated (read expensive) equipment is used, thermosiphon type flow is low enough that most common type flow meters won't detect it. That low flow does not mean it won't rob the system of a significant amount of its performance.
The impeller of a centrifugal pump will not, in and of itself, prevent reverse flow. It may prevent reverse flow if it's a jet pump, but for residential of small solar thermal systems it's probably not a jet pump.
Most all pumped residential solar thermal systems' pumps are plain old centrifugal (have impellers).
Check valves, like most other components, need service.
In general, most check valves in residential solar thermal systems service need inspection and service more often than other components in the system.
That's because most check valves for residential service and applications are cheap and are mostly not fit for the service. They are usually an afterthought if considered at all.
They usually last about 2 years or less.
That includes check valves built into the pump housing.
It's more expensive but more reliable to use a motorized ball valve that opens on pump motor start and fails open if power goes out. Just make sure what actuates the pump is working as it should.
Most check valves in use with most any residential solar thermal system are either leaking (allowing bypass) or totally inoperative because of lack of service and/or inspection. That's most often caused by user/owner ignorance. The nature of the failure is most often either from fouling (crudding up of the mating surfaces) or deterioration/disintegration of the sealing material (usually an "O" ring made of buna "N" whose upper service temp. is ~ 140 F or so).
If you can't replace the check valve with a motorized ball valve, at least replace the entire check valve with one that uses temperature appropriate "O" ring material (like "Viton").
But do not use different "O" ring material in the same (original) valve.
Reason: A "check" type valve is designed for a specific sealing material "stiffness" range. Too soft an "O" ring material and it'll get crushed and leak. Too hard a material and it'll not seal properly (and leak).
A sealing material outside a specific and limited range (a "Shore" durometer number) will not work as well, if at all, on a check valve designed for a different sealing material.
Viton and Buna N material have different durometer numbers.

[js080]

Thank you for your reply.

There is a RED-WHITE 1-1/4 125 check valve in the system, but it looks like original equipment that dates back to the late 1980s. It's also soldered into the system so not easily replaced.

The pump is a Grundfos UP26-64F, this is a multifamily application so there's (12) 4' x 8' panels on the roof.

So do you not think that forward siphoning could also be a problem?

Cheers,
J

[J.P.M.]

Thank you for your reply.


So do you not think that forward siphoning could also be a problem?

Cheers,
J

You're welcome.

I'm fairly confident that even with regular maintenance, 30+ years is a long time for any component in a solar thermal system, particularly a check valve that's soldered in place and so likely never inspected or serviced, at least likely not on a regular basis.

So, the answer to your question is that I do think thermosiphoning (reverse flow) could be and likely is a problem in your system.

I'd have the entire system checked out by qualified personnel.
Also, you may want to consider that the system may be reaching the end of its useful service life, but without some ( pressure gages, thermometers, flow meter(s), etc.) to estimate performance or its deterioration, that will not be easy to determine.

Regards,

J.P.M.

[js080]

Hi,

I've replaced the controller and several of the solar thermal panels. Being in the east bay means that I'm only a few miles from Heliodyne's main office so I can pick up new panels myself. I've also replaced the thermistors and re-insulated the entire system. So far the storage tanks are holding up and I'll replace individually as needed.

The Grundfos pump was replaced maybe 3 years ago. I guess there's a chance that a check valve has also been inserted into the flange of the pump?

https://www.supplyhouse.com/Grundfos-525530-Check-Valve-Assembly-Kit-for-UPS26

Thanks,
J

[J.P.M.]

Hi,

I've replaced the controller and several of the solar thermal panels. Being in the east bay means that I'm only a few miles from Heliodyne's main office so I can pick up new panels myself. I've also replaced the thermistors and re-insulated the entire system. So far the storage tanks are holding up and I'll replace individually as needed.

The Grundfos pump was replaced maybe 3 years ago. I guess there's a chance that a check valve has also been inserted into the flange of the pump?

https://www.supplyhouse.com/Grundfos-525530-Check-Valve-Assembly-Kit-for-UPS26

Thanks,
J

Probably not. Looks to me like the pump housing never had provision for a check valve.
Now maybe Grundfos has a separate housing that will accommodate a check but I didn't see one on any of their parts lists or pump literature.
The entire pump is also listed as unavailable (in the U.S. at least).
Maybe available in Great Britian, the E.U. or the rest of Europe ?
Anyway, to inspect the current check valve, sounds like it'll need to be cut out of the line.
I'd suggest replacing the current valve with a check valve that's flanged and also plumb a valved bypass around it so as to be able to change out the check valve and not shut down the whole system.
Or, as I did, replace the whole check valve system with a motorized ball valve as previously described.

Good luck

[js080]

For the motorized ball valve, what make and model did you use?

Thanks,
J

[LucMan]

Thank you for your reply.

There is a RED-WHITE 1-1/4 125 check valve in the system, but it looks like original equipment that dates back to the late 1980s. It's also soldered into the system so not easily replaced.

The pump is a Grundfos UP26-64F, this is a multifamily application so there's (12) 4' x 8' panels on the roof.

So do you not think that forward siphoning could also be a problem?

Cheers,
J

The horizontal Red/White check valves can be disassembled by unscrewing the hex cap and removing the internals for inspection or replacement.
If you are going to use a zone valve select one that opens fully ( ball valve type) not the Honeywell style that only partially opens and has a higher resistance. Check the Taco Zone Sentry Line, but I think they max out at 1".
Maybe the original pump that was replaced had an internal check valve?

[J.P.M.]

The horizontal Red/White check valves can be disassembled by unscrewing the hex cap and removing the internals for inspection or replacement.
If you are going to use a zone valve select one that opens fully ( ball valve type) not the Honeywell style that only partially opens and has a higher resistance. Check the Taco Zone Sentry Line, but I think they max out at 1".
Maybe the original pump that was replaced had an internal check valve?

Lucman:

Since the OP wrote that the pump was replaced about 3 years ago but seems to be listed as discontinued in 2020. Seems confusing to me. Maybe the OP is outside N. America and the pump is still available elsewhere.
I don't think the Grundfos pump the OP listed has an internal check valve, at least that option was not listed in the literature of their cut sheets but maybe I missed something.

Opinions vary, but I'm not a big fan of internal check valves on pumps as I've found those checks to be for the most part not as robust as necessary and not much better or the same as other stuff one usually finds on SDWSs and so they needed frequent service. They are also a PITA when it comes to access/frequent changeout, at least much more so than a dual check system or simply a check valve bypass which itself can be easier to service but with a cost (including more valving) that starts to approach the motorized ball valve route.
My Grundfos pump came with an internal check and I simply took it out the first time I inspected it after about a year's service and found most of the "O" ring gone. That was when I said to hell with that and conjured up the other valve system including a bypass for easier inspection. The entire hot water system's been up since 2007 with no problems - just routine maint. for the whole system including some new 10K sensors and their surrounding insulation as well as a switch on the differential controller and some aluminum tape on the outside closed cell foam insulation that birds peck at.

To the OP: On your question about what I used to prevent thermosiphoning: My valve setup is kind of customized and a bit funky. When working, among other duties I spec'd valves and other control equip. for power and process systems as part of my engineering function. For my pretty much self designed SDHW system at home I conjured up a Magnatrol valve with an actuator and motor from Foxboro which was one of the divisions of the outfit I worked for before moving to CA. FWIW, all the piping was 3/4".

As far as what you might use, there are lots of choices, but I'd agree with Lucman on his suggestions.
Question: What size is your piping ?

[js080]

Hi,

The system is in Oakland, California and my pipe is 1-1/4". The Grundfos pump is installed with flanges but the red-white check valve is soldered in place. I could post photos, but this website gives an error when I try to upload.

Cheers,
J

[LucMan]

The internal check valves that come in pumps are plastic junk but they serve the sole purpose of not having to install a flow control valve in hydronic systems. Cheap way out! Standard competitive check valves also fail frequently. The Bell and Gosset flow control valves (https://www.supplyhouse.com/Bell-Gos...e-Flow-Control) are extremely reliable and can be opened manually if required. One problem though they are cast iron! so not usable in an open solar system such as a drain back. I'm also not sure if they close tight enough to prevent thermosyphon as I have never used one in that application. Another option is to pipe 2 zone valves in parallel to meet your flow requirements as 1 1/4" z valves are pretty rare and probably costly.