In the basement we could easily house a large thermal store (say 2000 litres) which could be heated overnight by cheap off peak electricity at 12 - 15kw with a target range of 95C when stored and 50C when fully depleted. (NB: 1.16 kWh raises 1000 litres 1deg C). This would give quick response for a ch system - almost instant heat. It could be operated as a direct system, or an indirect system via a coil or plate heat exchanger. The latter would require a second pump, but is more responsive.

Advice please.

all these and other alternatives have been considered. Note that before long the surge in windfarms will mean that most off-peak elec will come from low-carbon sources

funcrusher, one possibiltiy is to work out the heat load for the house over the non-E7 period (ie. 7am to midnight or whatever). Then back that kWh number into the m.cp.dt calc. That will give you your required volume of water, to provide the heating during the day. During the E7 period, you'll be re-charging the TS and heating directly, so that would lessen the volume of water required. Say you got down to 1000 litre requirement, you might be better to have 2 or 3 tanks, so you could reduce the volume from 3 in winter, to 2 spring/autumn, to 1 summer. I do really like solar thermal, which would give you 95% of your summer DHW into the 300 litre store, but the sums may not work out for you?

This application is in SW Ireland, not Uk. The off-peak elec is 9 hours not 7. The climate is v mild in winter - like S Devon, but wetter and less sun. All the calcs and evaluation of other heating options have been done. You need to have a high water temp for efficient ch rads - otherwise you have to feed UFH which is not cost-effective unless you want heat 24/7 like an nursing home. Unvented high pressure systems operating above 100C are common outside Uk - which is notorious for nervous out-dated Victorian regulations (unvented ch heating systems, direct mains fed DHW, and air admittance valves on soil pipes were all prohibited until about 25 years ago)

Options of twin tanks (or twin off takes on a single large tank) remain under consideration.

A large off-peak electrically heated water store in the basement is efficient - any heat loss still heats the house. You get almost instant ch response - that's a huge advantage usualyl ignored in costings/convenience . Most occupiers only NEED heat intermittently, but the slow response of heating systems mean they run it far longer than necessary.Maintenance/service costs are minimal. There are no flue/air supply/air tightness issues.

"Unvented high pressure systems operating above 100C are common outside Uk" - Are we talking environmental heating here? Not quite sure what your comment is saying, or what the point of 100C + water would be exactly, when most renewables are sloping down.

"You need to have a high water temp for efficient ch rads" - Erm, no, just oversize rads to the required level of the temperature you have.

"otherwise you have to feed UFH which is not cost-effective unless you want heat 24/7 like an nursing home" Not my systems, yours perhaps.

When you say all the calcs and evaluations have been done, if this is so, you are reaching some fairly 'interesting' conclusions IMO. As above.

A great error of heating/insulation analysts is to make the utterly false assumption of 'steady state'. Effects stemming from changes in climatic conditions, thermal mass, social occupation and social behaviour combine to make reality a complex dynamic problem. UFH system for example usually represents a low temperature storage heater with extremely low responsiveness and much waste of heat unless the house is in constant occupation.

A system comprises a heat generator and a heat distribution system. These must be selected and designed to match the requirements, including the budget. The history of all technological development is that as technology develops, economies of scale prevail. Central generation of electricity is, for most of the population, going to be more cost-effective and greener than micro-generation - because large scale (whether 'renewable' or not) provides opportunities for sophisticated management and control systems. Using that electricity in the home produces no additional greenhouse gases.

A ch rad which is 'over-sized' is inefficient. What Crusoe means is that he needs larger radiators because the system is inefficient. Radiators are just that - they supply a significant output be radiation as well as convection. Radiation output increases with the fourth power of the absolute temperature. Lower temperature water means not just larger rads to produce the same heat, but a profound loss of radiation output made good by increase in convection. That is a crucial change because it is long -established that mammals gain greater comfort from radiation than convection. With convection alone you have to heat the entire room to a higher temperature to achieve the same comfort than with a convection/radiation mix. Take an extreme case: we can be bask in winter sunshine on a freezing day, or be as warm as toast round a bonfire on a freezing Nov 5th.

In domestic situations, convection heating or UFH instead of hot ch rads require a room temp about 1C higher for human comfort. They also have slower response time a owing to thermal mass inertia effects and higher pumping requirements owing to thermal mass flow.

I fear the whole subject is too complicated to address on a forum such as this.

fc: Any rad is, by that definiton, inefficient (avoiding definitions as that will take us seriously off-topic and has anyway been attempted before). A lower temp rad, oversized, is MORE effective than a standard sized rad as it provides a more even distribution of heat - nearer the agreed ideal of radiation - than a small, hot rad. Unless you happen to be the lucky one sitting near the rad. I have seen this on thermal imaging equipment and it is a valid pitch for oversize rads.

It also lowers the hysteresis threshold by stopping massive overshoot after controls have switched off, simply because the room takes longer to get hot, doing so more slowly and evenly than small, hot, quick-fixers.

It also allows use of lower temperature sources at their best efficiency - heatpump and solar among others.

So no, crusoe doesn't mean that. And Stefan's law covered black, not white