I am from Kathmandu, Nepal, and presently trying to install a central heating system in my house using Solar Water Heater.


Winter days in Kathmandu lasts about 130 days a year. Minimum temperature during the night is 0°C and during the day time is 18°C. Out of 130 winter days, 120 days are sunny days (6 to 7 hours every day).

Solar hot water heating systems for domestic hot water are very popular and have been successfully used for more than 2 decades already.


I am going to use the similar system to heat the water and store it in a storage tank to circulate it through radiators throughout the winter. This new system will be used solely for heating purpose and NOT for domestic hot water.

After careful calculation of heat requirements and radiator planning for every room, I have decided to have a 1000 liter hot water storage tank and 5 solar panels to heat the water on my roof top (I have ample of space on my roof for this).

Water temperature is easily heated to 70-80°C solely by solar panels. This water will circulate in the radiators and back to storage tank. In my calculation the 1000 liter tank will keep my house warm until approximately 3 am, then the water temperature will go down, as there is no sun during the night and also during cloudy days. From that point, I wanted to install a LPG Gas Boiler to heat the water.


I have some confusion on where to install the Boiler:

  1. Shall I install it bypassing the storage tank thus boiler needs to heat only the volume circulation in the radiators and not the whole 1000 liter tank?
  2. Shall I install it to heat the water inside the storage tank and heat the whole water?
  3. Water from storage tank flows through boiler then to radiators. The boiler is controlled by the room thermostat. But I'm not sure if the water can still flow when the boiler is not running. (Diagram Attached)

I have 2 thermostats in the room as a reference of temperature. One thermostat to stop the circulating pump if room temperature is more than 24°C. The other thermostat is to start the gas boiler if the room temperature falls below 16°C.

The problem I am facing if I choose Option 1: the Boiler will start when temperature drops below 16°. Then I do not need to run the circulating pump - how do I turn it off. And vise versa?

I am attaching a diagram for the reference on what I am planning to do. Any help and if anyone had an experience with similar installation would be a really great help.

Presently during winter days, we are using fireplace in the sitting room and everyone gathers around it in the evening, while the remaining house is gets as low as 9°C.

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  • 1
    This was a well written and well researched question. Too bad the OP never came back to update us with what he ended up doing. Writing up an answer would be a help to the whole community.
    – FreeMan
    Mar 23, 2021 at 12:08

1 Answer 1


I have no experience working with your type heating system but worked in the boiler business, heating and process applications, for 40+ years. My recommendation would be something like this; install the boiler into the hot water system using 3 way zone valves for connection and isolation, or 2 way zone valves for isolation, and a primary/secondary loop arrangement, and a 2 stage thermostat or 2 thermostats if you prefer. The first stage would control the solar system and the second stage would open the zone valves, and power the boiler's circulating pump through a relay, allowing the boiler to add heat to the circulating water as needed. The boiler would only add as much temperature to the water as needed, while still allowing the storage water to produce some of the heating. An outdoor reset control could be utilized to keep the boiler from overheating the water in the loop. Control of the whole system would be of your own choosing. Hope this helps

  • Thank you so much for the tips. Let me digest it for couple of days as the info you have provided is new to me and might be the right solution. Will work on it and get back with diagrams for your expert opinion. Thank you once again for taking time to read and reply. Warm regards. May 2, 2018 at 14:46
  • If I understood you correctly and with little modification of my own I need a second loop for the Gas Boiler: I will insert a Tee on the hot water side of Solar tank after the pump and before the manifold distribution to radiators and connect to a Boiler (hot Out part of the Boiler). Then in the returning pipe before entering the Solar Tank I will again add a Tee and connect to Boiler (cold In part). Then when room temperature goes down, shall turn on the Gas Boiler (with inbuilt pump) and also turn off the Solar circulator. When a temperature rises - the vise versa. May 3, 2018 at 13:13
  • Instead of using just tees you could use 3 way electric valves with the common port to the heating system and the other on/off ports to the boiler and solar supplies. Changing ports would use one system while isolating the other system. These valves are made by Asco, Taco, Honeywell and others. .A Honeywell model is V8044A
    – d.george
    May 3, 2018 at 21:10
  • Do I need to add a relay in this case? May 7, 2018 at 1:15

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