Can a large thermal mass tank replace a gas boiler ? Opinions needed

Hi

My name is Jose, I live in Bath, Somerset. I have decent plumbing skills having installed several bathrooms and kitchens over the past 10 years. I have a question about a system that I have been thinking it might have the potential to replace my gas boiler and bring me closer to be completly off gas and somewhat mains eletric independent and would like to hear your opinions.

I am having a big solar PV system of about 30 x 400 W panels and 2 x 12 kW battery installed on my 3 bed 100sq meter house, this will allow me to be off the grid and actually export excess all year around. I have looked at Air source heat pumps and Ground source heat pumps and since I know people that have installed one and on cold winters days they can struggle to extract enough heat into the house I dont think they are what I need at the moment.
My idea is to dig and build an underground 5000L concrete tank completely lined with 200mm PIR insulation in my basement, this thermal storage tank will act like a very large battery kept at over 60 Celsius with minimal thermal losses capable of storing over 200KWh of thermal energy that would be used to feed on demand the central heating system completely eliminating the need for a gas boiler or a heat pump. Since even on winter overcast days my solar system can direct 5-8 KW per day to immersion heaters to top up this thermal storage tank my calculations tell me that the tank temperatures would stay over 50 C even if I got 2 weeks with no sun. My house is well insulated with recent doors and windows, on a cold winters day I require about 8 kW of heat to keep the house over 18C.

There are some pratical difficulties with this install not just digging a 5000 L hole and building the tank which will certainly take a very large amount of man hours but also how would I connect this tank to the current sealed pressured central heating system. I would be interested in hearing your thought and criticism.

Thank you

100m2 isn't too large a property and your heat loss is 8KW?
The equivalent of two floors of 10x5m so area of an average semi so I'd say it isn't that well insulated.
Well insulated for that floor area would be 5kw or less.

Heat pump tech is actually very well proven despite the scare stories and you're guestimating your winter solar output before it's even been installed? Brave, cold boy. Realistically there is around an 80% loss compared to summer output.

Lots of similar products that run off electricity and can make use of battery storage and solar or plumb in a log/pellet burner with a back boiler/store.

With no heat loss it would take approx 2 weeks to raise the temp of 5000L of water from 30 to 60 degrees using the probable 2kw for 6hrs a day you'd get from your array during the winter. To reduce the temp of store from 60 to 30 degrees (no heat loss) using 8kwh would take about 21hrs.