After my PV ad battery upgrades last year, I went a couple of months without running the generator at all. Sounds good - but since the waste heat from the generator is what provides our domestic hot water, I was in the silly position of having to light a fire once a week just so we had hot water!
My cure to the problem was to spend up and get a cheap evacuated tube solar water heater. These tubes are made of borosilicate glass. Their construction is much ilke a thermos flask - 1.8m long by 57mm dia (about 6' x 2.5" dia). They're two tubes, one inside the other. The inner tube is coated with a "selective surface" that absorbs as much solar energy as it can, while reflecting and re-radiating as little as it can. The space between the two tubes is a (near) vacuum to prevent heat escape by conduction and convection.
deleted- get a life
A central manifold connects (and seals) the open end of all the tubes. The way I'm using it doesn't require a header tank. It would thermosyphon nicely, but I don't have a tank above it. I really should raise the assembly a bit to catch a few more watt-hours during winter but this will do for now.
This thing will boil water in a few minutes, so I've got a circulating pump to keep it under control. It heats my inside "storage tanks" which are 2x2000 litre (around 1000 us gallons total), so they hold a fair bit of heat. Primary purpose is for domestic hot water, but surplus heat is pumped through the concrete floor slab to provide heating.
My original layout used a simple breather tube to let steam out, and ensure the system couldn't pressurise much, but the water flow rate I needed to keep the water from boiling in midday sun made the riser pipe impractically high, so another option was required.
I tried to get a commercial TPR (temperature/pressure relief) valve, but the lowest pressure anyone had was 800kPa (about 100psi - or about 90psi more than the tubes are rated for). I found some low-pressure ones, but no way could I justify $594.53 on one!! So... the rest of this post is about a "DIY low-pressure relief valve"
I wandered around the plumbing supplies section of the hardware store and collected the bits I thought I could use, and headed home about $20 poorer.
Everything here is stock standard bits - the spring is from the junkbox.
The whole thing was tested and works great. Variable from virtually zero to about 20psi with the spring I used. Now, what do I buy with the $574 odd that I saved?