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The Solar Site Heat Dump

Solar Heat Dump and Radiator Dump

A personal record of my green initiatives for Solar Hot water and heating by evacuated tubes / panels - Dumping Excess Solar Energy













Solar Heat Dissipation

Not so much an issue on conventionally sized systems where they generally top up heat for hot water though as you can see my system is not an off the shelf design as it has been deliberately oversized to give solar radiator heating via standard wall hung radiators ( wish I could get under floor heating  in easily). I estimate at peak performance , time of day and time of year I am generating just short of 40k BTU the same output of a small gas fired boiler. It horrified me to literally dump the excess heat so went even further, installed more tubes to heat the house for the autumn through winter to spring seasons, supplemented by the gas boiler and only dump heat at the height of summer. (great for drying the washing too!)

Dumping excess heat on an oversized system needs to be considered to avoid damage to components through overheating. My system shown below covers an area of approx 15 Square meters - which allows me to absorb more energy during autumn, winter and spring than a conventionally sized system with the added benefit of top up heating via radiators. The downside is that at the height of summer I will be producing excess heat that I will need to either use or dump. In reality in summer I will heat my hot water system to a suitable level before the Resol controller diverts the excess to  radiators, either in the loft or the house depending on the time of year.

South Facing with large domestic solar panel of 90 tubes over 4 panels.

large domestic solar panel and radiator dump





The heat output of this system at peak performance with the sun perpendicular to the panels is approx 750 watts per square meter. therefore I will generate 11.2 Kw or 39300 BTU's of energy. As you will see not far of the output of a small domestic boiler though mainly for a few hours in mid afternoon.

I have three ways to use  or dispose of this excess energy/heat
1) One very large Copper  tank, twin coil tank 300 Litres (54 Gallons approx)
2) One Large Double Radiator (double fin) 1800mm X 600mm 10600 BTU in the loft plus pipe runs (deliberately not insulated ) estimated 200 BTU bringing this up to 10800 BTU - (This my only true or dedicated Heat dump as I try and use the heat output as much as possible)
3) Three  additional radiators in the house connected to the solar circuit normally used for top up heating from late Autumn to Spring ( or if my wife wants some emergency drying of washing !)
    2000mm X 600mm Double Radiator (single fin)  9200 BTU (in the lounge)
    900mm X 600mm Single Radiator (Single Fin)  3000 BTU (in the Dinning Room)
    1000mm X 600mm Single Radiator (Single Fin) 3200 BTU (in the Hall)

Therefore, excluding the hot water store I have a maximum of 26000 BTU  approx output at 50oC Plus the water store (which may or may not be up to temperature) On first glance it looks like I am short on Radiator dump output, but remember this is height of the day for a few short hours and I will be running the radiators much hotter than a normal domestic boiler and thus the heat loss increases proportionally to the higher temerature.

Note BTU figures are quoted by online radiators manufactures at a  DeltaT value of 50oC, I will be running the loft Radiator at whatever the solar panels can output at, which will be more than the normal DeltaT value, in practice it tops out at around 75oC using the large double in the loft with the window slightly open.

I also plan to install a second Copper tank to store and pre heat additional water as a buffer on exceptionally cloudy days where dense cloud continues over several days - remember the solar collector will still heat up without direct sunlight.

Radiator Dump in the Loft


Heat Dump Radiator Heat Dump Radiator
1800mm x 600mm double convector

This Radiator in the loft is used solely as a heat dump, approx 10600 btu output  (at normal domestic temperatures) though generally runs at 75oC at peak summer month and at peak times of day.

To maximise heat dissipation, the pipe runs which account for a approx 5-6 metres to and from the heat dump radiators are left bare with no insulation ( every little helps).
I also have a window in the Loft directly above the radiator which will also allow rising heat to escape and allow further heat dissipation particularly as this radiator will run very hot. Maximum the panels have got to was 75oC  whilst being cooled by this large double Radiator and I specifically installed a Solar rated separate pump for all Radiators. (its safe as not in an area where it will be touched.)

March 09

Running more or less self sufficient on Hot water only turning the boiler on when several consecutive days of very heavy cloud cover to top up the water temperature.

April 09

Turned the boiler off completely 2nd of April though even through March it was running only occasionally to take the chill of the cold nights - 

May 09

Now with some good strong sun we are starting to dump excess heat. with the tank fully up to temperature the flow to the large double radiator in the loft automatically kicks in and runs at about 64oC constant - shutting down again when the tank temperature drops 1/2 degree. This summer will be the first good test if my calculations are wrong, I will always have two more radiators to dump more heat and I just need to manually open the valves. I am very conscious of not overheating the panel, pumps and other components.

June 09

Its now the 1st of June and we have had some very warm sunny days around 25oC. I have turned off the radiators in the house otherwise I would have a sauna. The Large Double radiator in the loft is currently my only source being used to dump excess heat as the hot water tank reaches its maximum by midday depending on water usage. The double radiator in the loft peaks at around 71oC so I am still happy the system is performing well. Its a shame I have to dump heat, goes against some of my green principles but at least its not costing the planet other than a very small amount to run the pump. On the plus side I get hot water and some radiator heat in the winter with the oversized system.


Sept 09

With the summer behind us, I have been so pleased with the whole system its time to get a little more energy out of the panels.  have just purchased another single radiator, second hand  , but virtually new and  cheap sized at 1600mm by 520mm. this will be added to the existing radiators to give a very worthwhile background heat during the Autumn, Winter, Spring period. Experience has taught me that despite what you may think this additional radiator will not significantly degrade the heat output from the panels. I have found that the panels are very efficient at maintaining a temperature even when trying to drain the heat from them - the more heat you extract the harder they work to replenish the heat. Therefore even in winter I expect the now 3 radiators in the living area to provide low but useful heat during the day and on a good winters day the gas heating will not be required until evening. Importantly its keeping the fabric of the house warmer for longer and thus the gas  does not have to reheat a cold house.


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