Solar Thermal notes

Notes whilst gearing up to install a solar thermal system in Summer 2011:

Solar Notes

  • Sizing tank and panel: allow 5.5 - 8.5 Litres of water per solar tube (30tubes for 175-260L).  May want 40 tubes given non-optimal position and daily baths for GJ.  40 tubes at the recommended 5L per 47mm tube = 200L.  “210L is over size for two of us” (but with GJ we’ll use a lot more water).  But Ken reasons that a 40tube panel can deliver 18kWh in peak sunshine in June, which will raise 250L by 62 degrees C.
  • Shouldn’t need planning permission but a good idea to get it
  • 20 tubes = about 50 kg
  • need to double-check that all the pipes can be accessed from one side
  • don’t use standard pipe insulation as it will melt about 100 degrees C
  • any standard CH pump will do, start it at low speed
  • Thermostatic mixing valve to prevent scalding (valve compliant with TMV2 good). Some valves can’t supply much pressure.  DN20 (3/4”) TMV complying with BSEN1287 for bath fill applications, e.g. HeatstatT2 (available from Navitron)
  • 47mm tubes sound good for us
  • Navitron do courses every month and an 'EASYMCS Scheme' - allowing to to subcontract under our MCS certification, so your installations will be MCS-approved for government incentives. (Microgeneration Certification Scheme)
  • divert excess heat to radiator in airing cupboard???
  • measure total energy produced
  • RHI
  • 30 tubes = about 750W
  • Ivan’s next Solar Thermal installation course is 24th June, in Oakham (2.5hrs drive north of london). Solar Thermal course is £175.  I’m pretty sure the 1-day course contains no practical component.  There is a 4-day, £2995 course where you install your own system on your own house and the kit is free (!).  This may be cost-competitive
  • It's also worth bearing in mind that you can get convection in horizontal pipes as well. So every pipe coming out of the tank needs a dip near the tank to prevent this unless they fall anyway.
  • Alternative heat stores:
  • Alternative panels:
  • Suppliers:
  • Boilers:
  • Controls:
    • opinions on wireless room thermostats:
  • Some talk of thermal stores “dying very quickly in hard water areas”.  We are in a hard water area (266.0 mgl CoCO3 ppm, 18.6 Degrees Clarke, 14.9 Degrees German (DH), 26.6 Degrees Franch).  <100ppm = soft, 100-200ppm= medium, >200ppm = hard.  “We also live in a very hard water area. I use a simple phosphate water softener and have had no furring problems anywhere in over 20 years. In fact when we first fitted it, it cleared lots of the scale that was already there. (link)” (a specific recommendation for a water softener)  Scale can be cleared with acid flushing.  Some talk of replaceable DHW coils, although I think they may be for really large (2000L) tanks.
  • Tallness and insulation at the top are important for stratification
  • “Heat Bank” (basically a normal, cheap vented cylinder which feeds a heat exchanger.)  (diagram)The big advantage is the the heat exchanger is easily replaced (whilst the DHW coil can’t be replaced so the whole heat store has to be replaced, I think), which is important given our hard water.  Plus I think flow rate is higher (not sure though). Wookey: “The point about this set-up is that hot always goes in at the top from any source, and is extracted from the top so you can run the system stratified and without heating up a load of water unless you mean to. I intend to never heat more than the top 50l or so by boiler-power.”  Some talk of using “diffusers” on the inputs to ensure inputs don’t break the tank’s stratification.  Wookey: “I found this paper last night: which refers to another paper (sadly in Danish) which tested stratification with different input baffle shapes, and found that a flat-circular nozzle is the best diffuser for stratification, with T-pieces next then drilled-pipe diffusers, then direct holes, which seems a bit disapointing as drilled-pipe seems to be standard. I am quite surprised by this finding. No doubt Newark aet al will fit T-pieces if one asks.” Stratification good because you want a low temp (at bottom) to get max power from solar, but high temp at top.
  • Sounds like the controller should be set to heat the tank if the top of the tank drops below 55 degrees C.
  • Apparently individuals usually need 40-80L of DHW per day.  Tap temp = 45 degrees.
  • Diagrams:
  • One problem with thermal stores / heat banks is that the condensing boiler will often receive a high return temp and so wont condense (not sure what to do about this though)
  • Perhaps consider running the bathroom UFH from the hot water tank.  One possibility would be to use the UFH circuit as the heat dump.
  • “The pump you require is a bronze bodied type (rather than the cast iron standard).  Look at  and put "bronze pump" in the search box. They offer the DAB VA55/150 but they are significantly more expensive than CI pumps.” (link)
  • There are three types of water conditioner.
    • There are the electric/magnetic type. The theory is that calcium ions have to be set at 60o to each other to bind. The magnetic field lines them up so they can't bind to each others. Other people think they use witchcraft.
    • The second type are the phosphate ion conditioners. A small amount of a phosphate salt is added to the water. This binds preferentially to the calcium ions. With this type of conditioner, the calcium stays in the water and so you get staining from evaporation. Unlike normal scale, this is easily wiped off.
    • The third type is the ion-exchange conditioner which removes the calcium from the water.
    • Ion-exchange is definitely the highest quality solution but it also the most expensive (by some distance), uses lots of water for the flushing cycle and you have to keep filling it up with salt.
    • For us, we didn't think the advantages of ion-exchange over phosphate salt were sufficient enough for us to stump up the extra £hundreds.

  • “re cheap electrically-activated valves, the best value by far is the Honeywell HR20 (aka rondostat). They are cheaply available in Germany at about €15-20 each (search Free Software is available to make them do more stuff and use a wireless interface if desired. They will drive any valve with a standard 5mm-travel plunger, as found on TRVs. I have not yet found a cheap electric mixer valve, although Belimo seem the least extortionate candidate.” (link)
  • Some talk of vented heat stores being considered a pressured system if using a coil to heat mains-pressure water; hence it may be necessary to use a PHE to avoid expensive yearly checks.(link)  “re: pressurised coil in the cylinder. no this doesn't need to be serviced every year - there's a minimum volume requirement of such a pressurised system from the nanny state to qualify, so cyinder coils fall under that. Or so I'm led to believe.” (link)
  • “Our coil-in-tank is simpler and supplies two showers with ease. “ (link)  Link to the scale reduction system Richard Owen loves.  Richard also loves the Danfoss wirelss room stats / controllers from BES with set-back
  • Lots of local installers:

our kitchen tap delivers 5L in 25 seconds = 1L in 5S = 12L/second

Heat the tank directly from the boiler (rather than via a boiler coil)

Deal breaker: our boiler system will be pressurised, hence we’d need a pressurised tank (bad idea) - NEED TO CHECK THIS.  (But it must be true - the CH circuit can get up to 3 bar).

Asked on Navitron if it’s worth installing an unpressurised CH circuit.  Understanding of pros and cons of direct connection of boiler to heat store / heat bank:

  • faster response (although perhaps this is a little academic.  An 18kW boiler coil could raise the top half of a 172L tank from 10 to 60 degress C in 18 minutes)
  • more likely to keep the boiler in condensing mode for longer because the boiler can pull cold water from the bottom of the tank
  • can heat the tank “top down” hence only heat as much as you need


  • Have to be careful to preserve stratification
  • Need to have a vented CH circuit, and the boiler may want to be installed downstairs (check minimum pressure required!)

Forum threads:

Heat Store versus Heat Bank

Pros of heat bank:

  • Heat exchanger is external and hence replaceable if it gets filled with gunk
  • Can extract more heat from the tank
  • The entire system is a bit cheaper (because the tank is cheaper)

Cons of heat bank

Pros of heat store:

  • Simpler
  • Hot water even when no electricity

Forum threads:

Feed and Expansion

“Make sure that you size the F&E tank properly.  Most are designed to cope with just the heating circuit.  If you add a 200 or so litre thermal store, the potential for expansion increases substantially.

If you are going thermal solar as well, the F&E tank needs to cope with higher temperatures than usual.  E.g. 95 dC instead of 65.  Check that it can cope.  Perhaps there will be a market for decent size galvanised tanks again.

Remember to install a couple of MagnaClean or equivalent filters.  One on the boiler circuit and one on the radiator return to the store so as to maximise protection.” (ref)


Add new comment