Hot water eclipses space heat.

[Nick Grant]

Just a taster from some work Alan Clarke, Judith Thornton and I doing on hot water and carbon for EST. Not published yet but this graph from Alan's spreadsheet illustrates what we have known for a while. i.e. that as we approach Passivhaus levels of energy efficiency, domestic hot water use starts to dominate.

Nick

[Mark Siddall]

Hi Nick,
Very interesting. What is "Space heat demand net of gains"? Uncertain how to interpret this.

Cheers,
Mark

[Nick Grant]

Sorry Mark the report will explain all that!

Delivered hot water is what comes out of the tap
Useful hot water gains contributing to space heating are losses from the hot water system that directly reduce the need for space heating.
Space heat demand net of gains is what the heating system has to put in to space heating.
Hot water system losses are wasted losses from hot water system, eg outside heating season or when building already warm enough from other gains.
Utility company is the CO2 to supply mains water and treat the resultant sewage.

As the building requires less space heating relative to the gains, the percentage of the gains that can be usefully used decreases (utilisation factor).

HTH

Nick

[Mark Siddall]

As the building requires less space heating relative to the gains, the percentage of the gains that can be usefully used decreases (utilisation factor).

Utilisation factors always pickle my head. Is the decrease in utilisation factor due to the fact that the heating season is shorter? .... in which case the gains are critical  in determining the space heating requirement (assuming the fabric remains the same.) On this basis utilisation factors always seem to be miss leading somehow.

Space heat damenad net of gains: If this is the active heating (in a PH the 15kWh/m2.a) then I'm suprised at the imbalance the graph shows. In Frieberg, Germany the measured energy use has been about 17 kWh/m2.a which is a little over the PH standard but nothing like Alan's graph would appear to suggest (If I'm reading it right.... and I'm probably not!).

Mark

[Nick Grant]

Mark

First graph is CO2 not kWh although we could have plotted that.

Utilisation factor is typically then multiplied by heating length/year length (eg 200 days/365 days) so can have different utilisation factor but same heating season. That is as far as I can go without Alan chipping in. Spreadsheet will be in the public domain from EST but for now available to anyone who will peer review it but not circulate widely in case of errors we have not spotted yet.

We were not too surprised as had demonstrated the same sort of thing within PHPP.

Nick

[ALEC MORROW]

Nick this isn't only for passive haus standards..

Modern condensing boilers installed in older housing stock with the right control options all point to the same thing...that boilers dont actually need to be that big.


Indeed I can point to several houses where boiler sizing is reduced by some 30%  by conventional standards  with increased comfort and probably lower gas bills.

Its also why modern boilers that distinguish between demands for hot water and heating (required in other European countries as part of tougher energy efficiency requirements) almost all have the facility to enable the heating to be down rated...


after all, if you are using temperature profiling as opposed to intermittent heating you dont need so many kws to raise the temperature to a  habitable temperature. Indeed with temperature profiling, you can burn gas at higher efficiencies because you are not overcoming so much thermal inertia...