Advice on adding a LPG Gas Boiler on Central Heating System with a Hot Water Storage Tank heated with Solar Panels

Question

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

Background:

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.

Plan:

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.

Question:

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.

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