Solar hot water rebuild

My house has a solar hot water system, and it’s been very sad for quite a while because the (terrible) chinese solar hot water controller (OC, original controller) has been pretty unreliable.

I also have a bunch of extra parts to integrate into the system. So I’m hoping to document and discuss that project here.


Theres an array of evacuated solar tubes (SH1) on the roof with a thermocouple (SH1T1), and a pump (SH1P) at the bottom. The solar hot water controller is meant to detect the water at the heat exchanger is >8c than the water in the tank, run the pump until it drops to the same temp, and repeat.

My hot water tank (T1) has a thermocouple in a bottom port (T1TB) and one sort of nestled against the anode at the top (T1TT) (not ideal). The hot water tank has a heater (T1H) in it like most units. When we refer to the ‘temperature of the tank’, by default its referring to the temp at the top. (T1T)

The house has 2 metered supplies, the main load supply (MLS), and the controlled load supply (CLS). Controlled load can be turned off at whim by the electricity company, but its cheaper. One of the early issues was that the OC was on the CLS, and the internal battery had failed, and so was having its configuration reset all the time. Another issue is that the OC is pretty dumb, it basically has time zones where it is ‘allowed’ to turn on the T1H, in order to boost. Since you generate hot water during the day and use it at night, this can be a bit tricky if you’re putting load on the system to get configured in a way that is not counterproductive. The T1H is also very slow and power hungry, so once you’ve used up hot water this will not rebound fast at all.

Hot water tanks should not spend appreciable time below 60ºc for health reasons, the growth of bacteria etc.

Free Parts!

I’ve got a bunch of free extras for the system to be integrated.

  • A second tank (T2)
  • A number of solar hot water panels (different to the tubes, less efficient but way way more durable) (SH2)
  • A heat pump designed for hot water systems (and a second one marked ‘not working’ but not sure why) (HP1)

Other Relevant Tech/Ideas

This gets a bit navel gazy, its not super critical, feel free to skip to next section.

In your house plumbing, there are two main factors, flow rate and pressure. Urban Utilities says water pressure is delivered at a minimum 30psi, but around the world pressures range 30-80psi, most regard over 60psi as a problem and a lot seem to idealise around 50psi. I’m not sure what our pressure is here but it feels a touch low. I’m going to test it soon with a meter to get some real numbers.

If the water pressure is too low, you can buy and install residential water pressure boost pumps, $500-$2k. Hopefully wont/dont need this. If you have high pressure, you need to have water pressure regulation of some kind to not overpressure the system. If you have regulation, you need to also have an expansion tank because as you heat water it expands and you can easily go 50-250psi if your system is designed incorrectly.

The house is almost universally plumbed with 1/2" copper to my knowledge. This gives a cap on possible flow rate. Bar some of the external hot water line and solar loop, none of this is insulated. Its hard to get good numbers on the efficiency loss of not having insulated pipes, but you can guess. if 1m of 1/2" pipe is approx 110ml, to raise 1ml water 1ºc takes 0.00133333wh. so 110*(60-18)*0.00133 = 6.14wh. at $0.2/kwh, thats $0.001228. if you assume 5m of pipe to each hot fitting, thats $0.00614. Call it half a cent, each time you turn on a hot water fitting and let water cool in the pipe to ambient. 6 fittings in the house, do it a few times a day, realise that depending how you heat that water you can multiply that number due to efficiency losses, plus the fact that you’re not just moving hot water to the fitting, you’re also heating the pipes to temperature until they’re equal and your supply is a stable temp, plus losses to air, and yeah, lack of insulation might cost you $1-2/day. But, how much insulating those pipes helps is tricky to dead rekon.

Some systems recirculate hot water, where hot water is pumped to taps etc and returned to the cold flow of the hot water. Convenience factor aside, this is designed for where you want to minimise water usage and power is cheap, ie tank water + solar power. I’m the opposite, i want to minimise my energy cost because no solar panels. So we don’t have this system.

Theres some very cool tech in Grey Water Heat Reclamation. A DIY of this would just be copper pipe wound around your shower drain pipe where you supply cold water, have it heated by draining shower water, and combined with the hot water supply to reduce demand on fresh hot water. Theres commercial products for this, they claim 30-40% efficiency, which is super cool. However, gotta build that into your house from the start, so no go here. Still, wild to think you pay all this money to heat water and hop in the shower and 95% of that money goes down the drain. These things dont help for sinks, basins or baths, because you dont have that long period waste flow.

Another great tech thats not around much in au that i can spot is AC Heat Recovery, which are units plumbed into your AC heat exchangers between compressor and condenser. You plumb cold water through them, and if you are in a hot climate where you run AC a lot, you can reclaim a lot of that energy into your hot water tank, almost to the extent of not needing any other boost in some cases. I have 3 AC’s in this house, and I would love to do this, but the extra plumbing would be pretty gnarly. Put that one on the maybe later pile. If you switch your ac to heat you’d want to have something that isolates this system so it doesnt cool your tank.

Interestingly, HX hot water systems obviously output cool air, and this is considered a problem if the unit is internal and you’re in a cold climate. I’m tempted to try and utilise this somehow. This kind of stuff makes me wonder if theres a good way to design a house where you have a smart heat exchanging system that shuffles cold where you want it, hot where you want it, rather than what we do atm which is make cold things cold by venting hot air to atmosphere, and elsewhere making hot things hot by venting cold air to atmosphere. seems dumb.

Another minor efficiency tweak, letting the tempering valve sit on the hotter side of things. If you have no kids in the house, this lets you put more energy out at the outlet which means you use less volume for the same temperature.

There are even products for you to be able to insulate your PTV. If you ever touch this on your tank, its typically hot af, so who knows how much money this little thing burns.

For those that have PV solar panels, a common tactic with excess generation is to dump it into the hot water system. You cant easily reclaim it (like say a battery pack) but it will lower your energy reqs to heat water. Theres a cap to how much you can do this though, and that leads to an ‘issue’ with solar hot water:

What can you usefully do with overgeneration? On a good day, a solar hw system can boost the tank, but its hard to find what a safe limit is on tank temp. Every resource says 65 absolute max, but in australia we use tempering valves, so theoretically you could push this a bit. Even then, you wouldnt want to go too crazy. All the ideas i have here are kind of (actually) crackpot:

  • Dump it into a hx for your spa or pool
  • Redesign your hot loop so every outlet has a tempering valve and run a loop at full tank temp (lol why)
  • Remove your overpressure valve, let it go to steam, run that through a turbine to generate power

If people have good ideas what to do with excess hot water i’m def curious.

New system

So how to mod my current system to improve it.

The design thinking at this stage is to do 2 tanks in series, where it goes cold supply > T2 > T1 > house. Have the HX unit setup to boost T1, and plumb SH1 to T2. This way, T2 is a preheat tank for T1, most of the time solar covers our hw needs, if we need to boost it, the HX1 is efficient at doing this.

I’m considering putting 1-2 solar hot water panels on the roof in series with the evacuated tubes. I don’t really need to but after having the tubes destroyed by a hail storm some years ago and I’m still finding glass from it, I think this feels like a first step to removing them. But in the meantime, it will definitely overproduce if i do this.

Controller wise, i’m moving to a DIY system using ESPHome because every controller on the market is terrible and expensive. I’ve got a decent sized enclosure and some din rail and not afraid to use it.

New Controller (NC)

Design goals:

  • Switching for SH1P, T1H, T2H, HX1
  • Temp sensing for SH1T, T1TT, T1TB, T2TT, T2TB, and perhaps at one or two key points in the piping, like municipal supply cold input etc.
  • Pressure sensing for the water line on cold/hot.
  • Switching for CLS/MLS. The controller should run on MLS, and the system should run on CLS, but it would be good to have HX1 run on MLS if needbe for boost in certain scenarios. IE, lots of guests over or something.
  • Behaviour based on time but not a slave to it. Its job is to keep the water hot.
  • Current sensing for all powered components so we can calculate load, cost, etc
  • Data logging so we can have a nice ui to show us the interactions of the system between temps in the system and power load

So this means, at this stage, an ESP32 8 Relay board, a circuitsetup 6 channel power monitor board, a bit of protoboard to do voltage dividers for analog sensors like thermocouples and pressure transducers.

Plumbing wise, i’m going to swap the temp sensors to be on T joints at the inlet/outlet of each tank to try and get more accurate and stable temp sensing.

I might need to swap the anode on T1, its probably extremely worn.

WRT Power supply, i think if i run SH1P from MLS, and then T1H, T2H, HX i switch on two relays each where R1 is on/off and R2 is M/C LS, defaulting to CLS. Takes 6 relays but means i have ability to swap each part over. I anticipate that i will probably never use T1H or T2H, but want them available as escape hatches / redundancy for failures. We also have to be aware that the CLS circuit to supply this system is breakered at 20A, so we cant run say T1H and T2H simultaneously. The MLS circuit is 16A, so we have to be more careful if swapping load to MLS. As above, I think the SH1P and HX are the loads that are candidates for MLS 99.99% of the time. I may put a breaker or two in the box itself so it trips internally rather than at the upstream circuit.

The NC should work regardless of externalities. So it cant rely on Home Assistant, Node-Red etc for core/basic functionality. So logic should be on chip.

In terms of 240v wiring etc, I’ll get sparky friends to sort it out. Down the line, I might swap to something like the shelley din units (shoutout to tlm lol) so its all proper and approved.

At this stage, this is just a big info dump. Pictures of things to appear soon! I think first up is getting the controller box roughed out. Just din rails, where and how the power supply and outlet are. I think MLS and CLS will come in from the back of the enclosure via cable glands. The wiring for all powered units are probably also going to be cable glanded. Potentially i might just shroud all the load lines in 20mm conduit, which means i can bring in the thermocouples in that conduit as well. I did consider putting on weatherproof/captive switched 15a GPO’s but I dont really see a need. I’ll put din switches internally if i need to isolate a load, and theres no need to disconnect the loads easily with a plug.

I have considered relocating the SH1P into the enclosure but I dont think theres a need and it complicates things a fair bit.

New Parts

The controller is probably the next item to get going, but I’ll also need to put in the tank and the HX and do the plumbing for that, or rough it out enough that i can plumb them in when i’m switching over.

Some obvious things

This is an effort to save money, however lots of these things are expensive and theres always the issue of time to reclaim cost. I probably wouldnt be messing with a lot of this if the large expensive parts were not free. My costs are largely cheap electronics, wire, copper pipe etc. I also don’t have an existing solar PV system or battery bank that could be argued as a better alternative for expenditure if it was one or the other. For most, this isn’t a super interesting build, but it might be if you’re planning to build a home and do some of these things from the start. If I were building a home I’d do a lot of this, plus solar pv, plus lots of ‘dumb’ optimisations like sinking pipes in the earth as insulation (gotta be careful re movement creating leaks, but unbeatable insulation), running partially or fully on tank water, choosing different sizes of HW tanks, so on so forth.

The big win in this for me is not only cost savings, but the ability to log and automate the data of the system. The lack of telemetry on off the shelf systems is a huge pain because you cant look at the data and make decisions. How much power is the system using? how efficient? who knows! Is it managing temps correctly? who knows!

Down the line, maybe I can do tricksy rabbit automations like tracking showers using temp/humidity sensors in the bathroom, temp sensors on hot water supply line to showers, get typical times and durations and do predictive preheating or otherwise more smartly manage the tanks. With pressure and temp sensing on the HW line we can detect load on the system from outlets and appliances like washing machines and dishwashers.


Ok, next post soon hopefully as we try and eat this elephant.

This to note for you. You may be violating your agreement with your supplier being able to
switch a device from control load to main supply. If you are on “origin standing” you are,
but your legal situation there may vary. Basically they want to be able to switch you off
in high demand times. They don’t just want you to be able to give them the middle finger
and switch over to the other power point.

Also the temp a stored pressure tank like yours can reach is slightly over 100 degrees. So it is a good idea to have a tempering valve on whole house and not just the legally required bathroom areas. Also there are specific “solar compatible” tempering valves you need for the higher inlet temp. I think from memory about double the price ($120 vs $60)

My system is quite different to yours. My entire house was replumbed and specifically
designed/routed to accommodate the free solar stuff we got from the generous seven driver.
As well, when Saxon went bankrupt, I bought very cheap at auction a brand new 400L heat
exchanger hot water system.

I have
-30 evacuated glass tubes
-A Saxon copperflow so do not have to be concerned about legionella (or pressure as much)
-Two tempering valves (hygiene area 50 celsius and kitchen 70)
-All my pipes are insulated as far as I could reach them
-Although a don’t 100% need a auto pressure value on the roof like you, I still have one

What I would like added/changed to my system

New controller that
-Only boosts if you make it to 3 hours before sunset and not enough hotness
-Will not try to boost after showertime (9pm) unless critically low temp
-Has a (maybe wireless) boost button where you can manually override logic above
-Has a (maybe wireless) display in the kitchen to show current info

Put up a small 100W PV solar panel on roof to run the circulation pump (and inverter etc)
If there is no sun for the PV panel there is not sun for the heat panel. This will actually
save me much more money than any other feature.

Do a grey water heat recovery unit. My HWS was installed directly below show to facilitate this

Big pie in the sky idea - if I have too much hotness in summer then shed the excess into a
desiccant wheel dehumidifier. If it is hot and steamy which is usual for brisbane rather than hot
and dry - the dehumidifying is most of the work your air conditioner is doing.

My similar setup has similar issues, such as a cheap, non configurable controller etc.

The main features of the Arduino and Android app I made are:

  • Don’t boost until about 2pm. i.e. let the evacuated tubes do their work first
  • When boosting, only do so to 65 degrees. Again, so the evac tubes can do their stuff the following day.
  • other obvious features in the app like manual boost, adjustable run time for boosting, adjustable target temperature etc.

Yeah i have to look into / confirm this. It’s not really a normal function, more like an oh fuck button if for some reason it really needs to boost. But will have to check the legals. Esp since external hx boost is kind of a different animal, similar to an aircon.

I cant find any good numbers on the whole, can you go to 100 in the tank thing. every google hit is ‘60 for legionairre’ which is useless. My tempering valve is whole house, not sure if its a ‘solar’ one.

Yeah, if I could i would aggressively insulate all the hot pipes. at this stage, thats a very big very not fun job. Would have to cut out a lot of drywall, reroute some stuff etc. Extra loops for diff temp valves would be interesting. I dont know if its a product, but are you aware of any pipe insulation / lagging that is better than the foam stuff? I’d like something that can a) go on after joining, and b) has a more rigid and uv durable outer shell. I’ve seen you can get some products that are rockwool and foil outer, not sure how much better or more expensive that is. seems like it would cope with uv much better. I know the ‘fibreglass’ stuff they use on aircons all perishes in quick fashion. Have considered getting some black ag pipe or similar, cutting down the length of it and popping it over the top just to hide the stuff from the sun.

I just realised for context i should add my hot water units are outside.

A solar pv panel just to run the pump is interesting, i would not have thought that would be a very high cost but i guess we will find out when i get some data.

I could do grey water recovery on one shower, the upstairs one, but the one in master is on slab, you’d have to dig that up, which is very much in fuck that territory. If i ever build a house one of the rules will def be do not build directly on/into slab. such a pain.

Interesting, more info please? how would this work?

The only boost to 65 is a good idea, i’m sure i would have figured that eventually but good to get it early.

I am wondering as well if its an idea to try and sense ambient temp and perhaps even lux for cloud cover or pull in optional weather data for smarter decision making. ie, if low temp, low lux/cloud cover, and rate of heating linear regression angle too low, supplement earlier with boost etc. If you had a rain sensor could add that in also. Bad day for solar might mean diff decisions on how to maintain temp. It will be a suck it and see thing, but on bad solar days the smart move might be switching some valves and isolating the preheat tank so the boost isnt working to heat the preheat as well via the heat backflow. or that could be totally overthinking it.

For the solar loop I used Sekisui Thermobreak. It has a very thick foil outer on it (intended for external/UV) and a fibrous inner shell that copes with the higher heat of the solar loop. It is NOT easy to cut and retrofit.

Regular EDPM plumbing insulation is pretty easy to slice to fit over already installed pipes. It is also able to be glued with regular contact adhesive. Example of my messy gluing attached.

The prices I could find any Polyiso or Rockwool insulation materials in Australia made it seem like buying bulk iPhone 13s and then piercing the battery and using the resulting fire to heat your hot water would work out a cheaper alternative overall.

With my setup that has 30 evac tubes and a 400L heat exchange Saxon we very rarely boost. So although the pump running the solar loop is not much (<100W) it is the only viable vector to save money. Also for obvious reasons the 100W pump is also only running when our solar PV is producing. This is not costing us 28c/kwh but is taking away from out 44c feed in. So the value proposition for me to do the pump on a PV panel is even higher.

Watch Big Clives video on desiccant wheels Desiccant drum dehumidifier teardown - YouTube.

Those wheels exist up to human hamster wheel sizes. I am not sure yet if they are easy to buy though.