Reducing hot water demand in Passivhaus dwellings

Cath Hassell | ech2o 

The Passivhaus standard is recognised as a methodology that has a major role to play in reducing the 50% of CO2 emissions that come from the UK’s built environment. This is either in a new build scenario or as an EnerPHit to existing buildings - always assuming that Brexit doesn’t mean it will no longer have validity in the UK. But to my mind it has one major flaw, and that is the absence of a design standard for hot water use. 6% of CO2 emissions in the UK come from using hot water in homes, so why isn’t the Passivhaus standard doing more to reduce this demand? After all, as you improve the building envelope, domestic water use becomes an ever increasing proportion of the energy load, as shown by the following bit of number crunching.

It is considered that the ‘average’ house in the UK (a 100m2 semi with an occupancy figure of four) uses 20,000 kWh of energy for hot water and space heating a year; of which 16,000 kWh is for space heating and 4,000 kWh is for hot water heating, so a 75%-25% split. By building to the Passivhaus standard of 15kWh/m2/year we reduce the space heating demand by over 90% - an impressive result and one for which Passivhaus is rightly applauded. But in the absence of considering hot water consumption then we have a situation where the energy demand for hot water is almost three times that for the space heating and the overall heating and hot water demand has only reduced by 72.5%. Even if renovating to the EnerPHit standard with its requirement to reduce space heating to 25kWh/m2/year, or the AECB silver standard of 40kWh/m2/year, hot water use is still more than, or equal to, the space heating demand.

Most hot water use in homes is from bathing, with showers now more popular than baths. In the UK 25% of total household water use is for showers with the average shower lasting 7.5 minutes (1). In England and Wales the 2015 version of Part G of the Building Regulations addresses water efficiency and sets a maximum flow rate from showers of 10 litres/minute (hardly a water efficient flow rate) unless local planning laws require the lower level which is 8 litres/minute. Water efficient taps, shower heads, baths etc. are eclipsed by the opposite in the market place. And, of course Part G is only for new buildings so when renovating to the EnerPHit standard there is no need to reduce shower flow rates and the sky’s the limit - 33 litres/minute from your shower anyone?

The privatisation of the water industry in England (and Wales) in 1989 has led to a fractured situation with (up until now) little cross company collaboration to spread national (or even regional) messages about the importance of saving water. The water companies supported the setting of water efficiency standards under Part G, because they are acutely aware of the pressure on water supplies. But their messages are restricted to localised advertising within their specific areas, and rarely make the link between hot water use and CO2 emissions. In Wales, Scotland, Northern Ireland and the Republic of Ireland at least the water supply is still (mostly) centrally controlled but I have seen little national push about saving hot water. So we are pretty much stuck with the four minute shower challenge and desperately need a national campaign to target hot water use.

In the UK there is support for solar thermal under the Renewable Heat Initiative (RHI), but with a yearly return of £290 - £390 against an installation price of approx £4,000, it hasn’t kick-started the industry yet. Likewise, in Ireland, the 1,200 Euro grant hasn’t led to the mass uptake of solar thermal that is needed to reduce this figure of 6% of CO2 emissions.

Wastewater heat recovery mechanisms on the other hand are going into most new speculative build developments in the UK because volume house builders have realised the positive effect per £ spent on this technology has on the SAP rating of the dwelling. But I rarely read that one has been installed into a Passivhaus. Indeed I rarely read anything about the water specification in homes built to the Passivhaus standard. Maybe the occasional solar thermal system. In fact I probably more often read that the property has an immersion heater to heat the hot water, but with no corresponding information relating to the effect this would have on CO2 emissions or what type of water efficiency appliances have been specified.

Of course Passivhaus does consider hot water use. PHPP predicts 3.5 kWh is required per day for hot water and it is argued that for a Passivhaus dwelling in the UK the figure will be higher at 4.6 kWh/day (2). But I would argue that this enhanced figure is still too low. Taking a shower with a flow rate of 8 litres/minutes (3) for 7.5 minutes, once a day, requires 57 litres of hot water. With very litre of hot water for a shower requiring 0.039 kWh of energy (4), the shower requires 2.2 kWh of energy. Over a year that is 803kWh or 8 kWh/m2/year in our 100m2 property (5). And remember this is for one person for one shower. There is still the other hot water use to consider. And, although energy (but not CO2) requirements for an electric shower are less, it is still is 511kWh or 5 kWh/m2/year in our 100m2 property. (6)

The Passivhaus concept is sold on excellent comfort conditions as well reduced bills with the question often asked “who wouldn’t want to live in a Passivhaus?” But my question is, “who wouldn’t want to save the polar bears?” And by reducing hot water demand in our homes we can contribute to exactly this. Relying on just Part G or the water companies to limit hot water consumption is the wrong approach. Passivhaus designers should be specifying wastewater heat recovery or solar thermal as well as reduced flow rates from showers and taps into all their designs.

An amended version of this blog was first published in passivehouseplus magazine, issue 14, Spring 2016


(1) At Home with Water Energy Saving Trust

(2) ‘The importance of hot water system design in the Passivhaus’. Nick Grant and Alan Clarke, 2010

(3) 8 litres/minute meets the more rigorous water efficiency standard of Part G and most ‘water efficient’ shower heads for dwellings have a maximum flow rate of 7.6 litres/minute.

(4) Waterwise

(5) To heat this shower using gas from an A rated condensing boiler produces 0.47 kg of CO2.

(6) The ‘best performing’ electric showers have a flow rate of 4.8 litres/minute if the user wants a hot shower which is 36 litres of water and 1.4 kWh for the average shower. To heat this shower using UK produced electricity produces 0.74 kg of CO2.

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