Is it ok to disconnect and drain flat solar hot water panels when we're gone?

[upgrading]

After re-roofing I'm putting our hot water system back together, but with some changes to hopefully improve it. We used to have the tank on the roof, or a thermosiphon system. Unfortunately, the heavy storage tank on the roof in time created leaks in the roof. Furthermore in past years, when the system would overheat, the hot water flowing out of the top mounted pressure-temperature safety release valve damaged the asphalt shingled roofing. (Tip: I fixed this 2nd occasional overflow problem with a 'T' off of the output of the pressure-temperature safety release valve. I added one pipe from this valve's output to the ground, and a 2nd pipe about 16" tall sticking straight up and open to the sky. This 2nd open pathway to the sky is for safety just in case the overflow pipe to the ground ever got accidentally blocked. Normally if the pressure-temperature safety release valve opens, the excess hot water drains out the pipe to the ground.)

Now that the roof has been replaced, I've purchased a new solar tank to be mounted below and inside. Also I have a small circulation pump and differential controller to run the pump when the collector is hotter than the tank, but only when the tank is below a certain maximum temperature.

My questions is this: what is the best approach for when we're gone (i.e. away from the house, like on vacation) for an extended time in summer?

I'm hoping to 1) disconnect the circulation lines to the hot water panels (with 2 valves at the tank), and then 2) with a third dump valve, entirely drain the water from the hot water panels. BTW, I will also be opening up the relief valve at the top to let air in, so the water really does fully drain out of the already sloped panels.

I was thinking that even though the dry panels would get hot, since there was no water in them that they would not be damaged. They are copper panels inside an aluminum frame with a glass top, with some high temperature sealant to seal the glass.

What do you think? Is there any chance that the panels could get damaged over and above normal usage just sitting on the roof dry?

[J.P.M.]

To confirm: It sounds (reads) like this is a direct system with potable H2O circulating from the tank to collectors and back, right ?
Also, there is a check valve in the system that prevents reverse thermisiphoning ? And there are air bleeds and a pressure/temp. relief valves in the system ?

When I am absent for more than a couple of days, summer or winter, I cover my flat plate thermal collectors (2 of them) with corrugated fiberglass roofing panels from big box. I attach them to the collectors with bungie cords at the bottom, middle and top of each of 2 , 4' X 8" collectors plumbed in series and in portrait orientation, one above the other. Highest measured wind gust at the collectors has been 11.6 m/sec. The roofing panels have stayed put.

I do not do not drain the collectors. I do not close any valves. I do shut the pump off if absent in the summer. Covered panels will not overheat. They get somewhere close to or slightly above the amb. air temp. on the roof. They may freeze in the winter, but that contingency is handled by the controller. The pump stays plugged in during winter absences.

Dry, uncovered collectors panels will get hotter than collectors with stagnant fluid in them. Dry collectors will see something close to stagnation temps. That's the temp. at which panel energy thermal losses, mostly from the heat transfer mechanisms of thermal convection and thermal radiation from a hot collector will just equal and balance the energy input from the sun. as a first approximation, on a sunny day with an angle of incidence close to normal, that temp. may be roughly approximated something like ~~ 300 - 350 F. or so above the ambient roof temp.Such temps. may be OK 1X/awhile, but should be avoided.

If flooded but with no fluid circulation. if/until the fluid boils off, the collectors will probably be at ~ the saturation temp. of the fluid in the collectors. If that fluid is water at say 40 PSIG, the temp. of the collector will probably be ~ 280 F. until boil off is complete. Then it will be dry stagnation temp. The tubes and the absorber will probably be OK unless the tube to absorber solder bond fails due to differential thermal expansion - which failure BTW is usually hard to see. If you have any buna or non viton or cheap silicon O-rings or cheap gasketing at the piping, those materials may fail due to temps. beyond their limits - sooner or later.

I avoid the possibility of overheating altogether by covering the collectors when I feel it's necessary, such as during absences of more than a few days. Draining collectors is probably not a good idea anyway, particularly if you aren't going to cover them as it might tend to dry things like gaskets/o rings etc. out and generally change things that don't need to be changed like oxidation forming on relief valve surfaces for example. Stagnating a dry collector is an invitation to trouble and a short(er) service life.

If I'm away in winter, the freeze protection circulation feature of the controller will operate the pump. That's worked fine for me for 13 + yrs. or so.

Just a comment on your relief valve scheme. If a sharp AHJ sees it, it might get flagged. Long lines running away from the outlet of a PRV may change (increase) the lift pressure and also increase the response time of a PRV. For those reasons, putting extensions on a PRV is, technically, in violation of the ASME Boiler and Pressure vessel code as it pertains to relieving devices. Besides all that, it's just a poor idea.

Also, if the valve lifts (actuates) from overpressure (an uncovered and stagnant collector being a good example of how such an event can and does happen, BTW) rather than a (small ?) weep, the parallel extension will take a lot of the flow, probably > half of it and you'll be back to roof damage. PRV's are safety devices, not routine overpressure valves. Handle routine overpressure with good design. FWIW, I'd put a properly sized expansion tank on the system to handle weeping from the PRV which is probably due to fluid thermal expansion and blow the relief valve effluent straight up.

The way I learned to design boilers, never restrict the outlet to a PRV, and never place it where its outlet can cause damage or risk personal injury. I know PRV extensions and poor placements happen. I've seen lots of examples. Just hope your design never fails or you never get caught by your insurance carrier. Just sayin'.

Take what you want of the above. Scrap the rest.

[azdave]

Is there any chance that the panels could get damaged over and above normal usage just sitting on the roof dry?

In my own experience, that situation caused no issues. I had a home in Phoenix for 20 years with an open loop hot water system. It sat dry in the desert sun for 5-6 years before I bought the home. I replaced the failed storage tank and put it back into service. I never saw any issues with the panels after that long idle period but I did have to replace the safety valve/air bleed up top. No telling if your panels can take the same abuse mine did. My panels were USA manufactured with the typical copper plate and aluminum frame construction. I still see them on the roof of my old home and assume they are functioning. They have to be 40-45 years old by now.

[LucMan]

I have a drainback system on my roof with 3 flat plate panels connected in series. When I go away I just unplug the controller until i come back. The panels are dry and have never had any damage. Just drain your system and leave a valve open to atmosphere to purge any steam created from any residual water in the panels, don't forget to close the valve when you refill the system.

[peakbagger]

I have a length of slant fin baseboard in my basement that is tied to the hot line from my collectors as a heat dump. I have never used it for summer time issues with my flat plates as they at best only have a 80 degreee temp rise over ambient and in my location its pretty rare to go over 80F. I have used it in the past when I am gone in the fall or spring to put a bit of heat in my basement under an area where there are a lot of pipes.