Posted By: GaryBHmmm, I think I need to clarify why I believe the OP's proposal to be a sound and robust strategy for the particular dwelling.

Most of the energy used for water heating in the UK doesn't heat the water used by the occupants - it heats the internal space or outside (technically it all ends up outside). The lowest energy form of heating, ignoring solar or heat pump generation, is via electric immersion heating.

The primary pipework is just an additional source of loss - the water and pipe has to be heated every cycle, the pipework loses heat continuously when the primary circuit is activated and there are continuous additional standing losses by conduction from the cylinder.

Some example figures from SAP2009 for a 3 bed semi of 85 m2 floor area & a 200 litre cylinder for comparison:

- Nett hot water use: 1,498 kWh pa (no standing losses)
- Electrical immersion heating: 1,987 kWh pa (based on the 200 litre cylinder heat loss factor permitted by DECC)
- Gas combi boiler, 90% efficiency: 2,497 kWh pa
- Gas system boiler, 90% efficiency, insulated & timed primary circuit: 2,791 kWh pa
- Gas system boiler, 90% efficiency, insulated but UN-timed primary circuit: 2,852 kWh pa
- Gas system boiler, 90% efficiency, UN-insulated but timed primary circuit: 3,139 kWh pa
- Gas system boiler, 90% efficiency, UN-insulated & UN-timed primary circuit: 4,118 kWh pa
- Gas system boiler, 90% efficiency, 25mm (or 50mm cylinder jacket with gaps): 6,457 kWh pa

Even connecting up the primary circuit pipes increases the standing losses by 208 kWh pa (assuming pipework is 10mm insulated).
Obviously if the boiler efficiency is lower or the primary pipework is routed outside the thermal envelope then the losses are much worse and do not contribute to space heating in the winter.

Posted By: GaryBIf by 'decent' insulation you mean 20mm then the pipe losses drop from 21 W/m to 13 W/m (Engineering Toolbox data) and the losses are still 129 kWh pa. For comparison uninsulated pipe is 56 W/m.

DECC permitted standing losses for a 200 litre cylinder are 2.26 kWh/day = 825 kWh pa (if the cylinder were maintained at a constant 60 deg C). SAP calcs suggest this is actually around 490 kWh pa, to allow for reduction in loss as the cylinder cools.

The standing losses due to the primary pipe connections are therefore 26% of the 'base' cylinder standing losses. A similar loss occurs in the DHW flow and cold feed pipe, so total standing losses will be:

20mm pipe insulation: 129 + 129 + 490 = 748 kWh
10mm pipe insulation: 208 + 208 + 490 = 906 kWh
No pipe insulation: 555 + 555 + 490 = 1,600 kWh

That is why mine are insulated with the equivalent of 50mm insulation...

For a solar cylinder the primary loop is in the top of the cylinder so wouldn't reduce due to stratification. In the winter most of the standing cylinder losses should indeed contribute to space heating, but are a complete loss in summer. For a rambling Victorian semi the primary circuit is likely to be routed outside of the thermal envelope for most of its length and primary losses will be losses.

The point is that use of a primary circuit increases overall energy consumption and losses.