Solar Panel installation for domestic hot water:-

RM Solar:-
250 Litre copper twin coil vented tank
18 litre expansion tank
Double pump station
2 x S Class Collectors
In roof mounting kit

Solastat-Plus2 differential Controller

Roof/Panel Orientation - South, South East at 30 degrees

Commissioned 22 July 2009

  • Requirement
  • Roof Orientation
  • Panel Type
  • Cylinder
  • Internal pipe work
  • Tempering Valve
  • Controller


My requirement was to

1) Reduce my domestic hot water heating costs

2) Retain the look of the property
This is one of the main reasons for installing an in roof flat panel

3) Increase the domestic hot water capacity from 120 litres to 200 litres
I recently upgraded my 25 year old oil boiler with a condensing oil boiler from Grant Thornton, as condensing boilers operate at lower temperatures we were running out of hot water.
There are some solutions which retain the existing cylinder by installing a heat exchanger in place of the immersion heater but these are not as good as a purpose built twin coil cylinder and I was replacing my cylinder anyway, I went for a 250 litre cylinder as I read somewhere that you should not count the volume of water taken up by the solar coil when sizing the cylinder.

4) Increase the flow rate to the upstairs bath rooms and downstairs hot water taps
The water pressure I have is about 2 bar (pressure isn't everything) however the flow rate was very poor and has been since we moved in, installing a water softener did not help either. The solution for me was to retain the cold water tank in the loft and install a whole house pump from Salamander. This decision affected my choice of domestic hot water cylinder which was a 250 litre twin coil vented copper cylinder; as I was not moving to a fully pressurized system there was no point in having a stainless steel cylinder other benefits of copper over stainless steel is that legionella does not like copper and it is considerably cheaper.


Roof Orientation

Below is some of the data I researched prior to proceeding; For maximum solar gain the panel should be installed on a south facing roof at an angle of 35 degrees.

Even though my panel is installed at 30 degrees and South South East facing this still gives me a 96-97% efficiency.

So far the bottom line is that on a sunny day in october I can get a 250 litre tank up to 50 degrees C and my boiler kicks in for a short time to take it up to 55 degrees C


Tilt and Orientation
UK Eire map irradiation on flat  surface Variationof Light Energy with Month




Panel Type

The choice I had was evacuated tubes or flat panel; flat panels can installed in the roof so that they are less obtrusive and generally come with a longer guarantee, evacuated tubes are more efficient under certain conditions, as I wanted less obtrusive and a long guarantee I went for flat panels. RM Solar S Class panels came highly recommended, with a 20 year guarantee so I went for these.

Cylinder Type - Capacity - Twin Coil

When working out the capacity it is recommended that the volume taken up by the solar coil in the bottom should not be counted so if you have a 250 litre cylinder and the solar coil in the bottom takes up 50 litre, this gives a usable capacity of 200 litres.

RM Solar will also custom build the cylinder connection wise to your requirements.


Several choices:-

Stainless steel; this is used in installations where the domestic hot water is connected directly to the water main coming in, this requires that the mains pressure is sufficient and has adequate flow.

Copper - Vented; this is used where you have a cold water tank in the loft and there is poor mains pressure or adequate flow

Internal pipe work

The recommendation from RM Solar was to use 22mm for the pipe work in the solar heating circuit also that compression joints or Hard soldered (Silver soldered) copper fittings be used.

The reason for the 22mm pipe work was less friction loss for the pump to overcome when circulating the glycol mixture around the solar circuit. The RM Solar S Class Flat panel uses 12mm copper pipe, I calculated the friction loss between using 22mm and 15mm copper piping was insignificant in the short run I had from the panel to the cylinder when compared with the about 40 metres of 12mm pipe work in the two panels I intended to install, also the 12 mm panel connections use an insert collar which further reduces the aperture to about 8mm.

I would strongly advise adhering to RMsolar's recommendations unless there are very good reasons not to.

Another point worth noting is that you cannot use ordinary pipe insulation purchased from the local DIY store; the temperatures that the solar circuit can get get up to will melt this type of insulation, you need to use high temperature insulation. I used Armaflex HT-13x015 and again due to the short run I considered the heatloss negligable; for longer runs I would install thicker insulation. Should I be proved wrong I can easily beef up the insulation.

Tempering Valve

This is a safety device TMV (Thermostatic Mixing Valve) which mixes cold water with the hot water to prevent scalding hot water being supplied to the taps. I would strongly recommend installing this safety device, I used a tmv2 valve from Reliance and set it at 50 degrees C. there are Solar Tempering valves on the market but this one matched my requirements.

In the short time that the panel has been installed I have seen the temperature of the hot water be 65 degrees C which would give an Adult 3rd degrees burns in 2 seconds and a child 3rd degree burns in half a second (see the chart below).

Don't skimp on this, buy a TMV and fit it!

Burn Chart

Differential Temperature Controller

The brains that control the operation,

I am informed that the Solastat Plus 2 has the best algorithms to maximize the transfer of heat from the Solar Panels to the cylinder, it has tunable parameters to cater for any site specific changes that needs to be made. Out of the box, it works well with the default settings.

The Solastat Plus 2 model has the ability to control the Boiler/Immersion Heater and under certain conditions to hold off calling for this if the solar pump has been in use; I use this 240V output to switch a relay with two sets of contacts, one set switches the demand for heat from the boiler which I have set to come on at 16:30 to ensure I have the cylinder hot enough for showers etc, in July and August this rarely happened as the panels gave enough heat. As a backup to the boiler the other set of contacts in the relay I use to switch the immersion heater which is on a time switch, to make use of the economy 7 tariff.

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