Hot water and Passivhaus Part 2

In my last water blog I wrote a short piece about the fact that an adolescent can require 2,989 kWh of energy a year to heat their shower, which is more than the space heating requirement of a 100m2 house retrofitted to the EnerPHit standard. I finished the piece ‘And that’s why Passivhaus design in the UK needs to look at hot water use more closely’.

In retrospect that was a slightly lazy piece of journalism but the upside was that I was tweeted by Nick Grant to point me to a paper he and Alan Clarke wrote back in 2010 entitled ‘The importance of hot water system design in the Passivhaus’. PHPP predicts 3.5kWh required per day for hot water but the authors calculate that for a Passivhaus in the UK the figure will be 4.6kWh/day. Over a year that is 17kWh/m2 for the hot water in a 100m2 dwelling. They modelled four different hot water system layouts and the heat losses from them. System losses are calculated to be 5-10 kWh/m2/a for good design and far higher for poorly designed and installed hot water systems, highlighting why the services design is so important.  The paper can be downloaded here.


Waterless traps

water 1Surely some mistake here. Isn’t it the water that creates the seal in a trap? Well, yes, usually. But about 20 years ago Hepworth came up with the HepvO valve to replace a standard trap. It uses a silicone membrane that collapses shut to prevent drain smells entering the building. The membrane seal is easily opened by the flow of water. One of the best aspects of the trap is that it can be installed horizontally as well as vertically. This technology has now been taken up by for use in wet rooms. The linear screed drain is just 67mm total height from the base of the trap to the top of the trough. I’ve also recently seen the HepvO valve installed in some waterless urinals. That was less successful as the uric sediment caused the valve to stick open, thus releasing drain smells into the building.


Smart Water Meters

Forty percent of the 27 million homes in the UK are now metered for their water use. But very few of those are with smart meter technology. Meanwhile, every home in the UK is metered for their electricity use (and gas where relevant) and by 2020 this sector is on line to achieve UK wide smart meter coverage. The roll out will require a centralised data and communications network and some water companies would like to see household water data integrated into this network. There are several advantages to smart water meters and some difficulties in incorporating them into the gas and electric smart meter roll out.

Smart water meters provide data in 15 minute blocks or in real-time and generate what is known as BIG data. BIG data can play a role in helping water companies understand when and how their customers use water and to deliver against a number of key objectives. These are summarised as: better customer service, improved environmental outcomes, enhanced asset performance and enhanced service performance. The key objectives cover a wide range of factors. The impact on the environment will be less if raw water abstraction and associated flows back to rivers from sewage works are reduced; an associated add on is that less energy is used to treat and pump the reduced water and sewage volumes. Estimated billing should be virtually eliminated so customers will pay for how much they have actually used; both underbilling and overbilling generate problems for customers if they continue over a period of time. It will be easier to identify where leaks are occurring, how big they are, whether increasing in severity, and to minimise operational outages when repairing them. The two that especially interest me are the ability to show the building user unexplained usage at night (or other times when the building is unoccupied), and allow for rising block tariffs to be used to financially incentivise lower use.

The two main difficulties are the cost and the fact that most water meters are installed underground at the boundary of the property. This makes communication between the data and the network, or the data and an In Home Display device less reliable. Some water companies already have a policy of installing meters internally to buildings. I would like to see all water companies doing this, whether with smart or dumb meters. It enables customers to easily read their own meters and shifts the repair of branch supply pipes onto the water company.


Wastewater heat recovery – the Heath Robinson way …

Years ago I saw an attempt at wastewater heat recovery. It was using a horizontal plastic waste pipe (all 1m of it) with some hose wrapped around it which carried the incoming cold water. Most of the waste pipe was outside and the water went straight to the hot tap on a kitchen sink. This was wrong on so many levels no wonder I was told it “didn’t work very well.” Now wastewater heat recovery systems are the go-to choice for most new homes built in the UK given their efficient performance and excellent SAP score, (explained in more detail in an earlier AECB water blog). I have also heard that they may get even more points in the next iteration of SAP, currently under review.


Savings from WCs in schools

When retrofitting devices that reduce WC flush volumes in schools, estimated water savings are calculated on the basis of three uses of the WC per day per pupil. To check anecdotal evidence heard time and time again, ech2o collected and analysed data from over 600 school pupils about their use of toilets and urinals when at school. This report details the findings which shows that the average use is 1.6 times in primary schools and 0.5 times for adolescent girls, thus resulting in an overestimation of potential savings (both of water and money) by a factor of two in primary schools and up to six times in secondary schools.

I was lucky enough to be invited to present the data at the WatEf Network conference in August 2015 in front of an informed audience, who thinks this sort of stuff is important. Following the presentation Severn Trent have said they will install flush counters in some schools they are monitoring, to see whether their research backs up our findings, which is excellent news.


WatEff Network conference papers

I really value the WatEf Network, bringing together as it does, the divergent worlds of academia and industry. With key note speakers and presentations from around the world, it was an excellent and informative two days. All the papers can be found here.


Greywater recycling

water2water3We have been using greywater in our garden for at least ten years. We don’t take baths and as with all greywater systems don’t collect from the kitchen sink, dishwasher or washing machine. Our shower is flow regulated to 7.6 litres/minute (thermostatically controlled shower fed from a combi boiler), and in total the two of us only spend about 6 minutes in the shower a day. Yet even so, every day the plants are guaranteed 46 litres of greywater from showering; plus a bonus when friends are staying. Add in wash basin use and that is 350 litres of greywater a week. It was easy for us to retrofit a simple greywater recycling system as our kitchen is connected directly to the drain via a yard gulley and we have a two pipe above ground drainage system so our WC runs directly to drain via a soil stack and the bath, shower and wash basin run to drain via a hopper head.

We pump the greywater to the back of the garden and that, of course, uses power. In the UK it takes on average 1.2kWh of electricity to supply 1m3 of water into our buildings and to take away the corresponding 1m3 of sewage for cleaning. Therefore, as long as we use less than 1.2 kWh to pump the greywater to the back of our garden, we are saving both water and power. (This assumes we would have used mains water in place of the greywater, which in our case we would have done.) The pump in our greywater recycling system (the Banbuster from Watermatic) is very efficient. We are pumping against a small head (1m or 0.1 bar maximum – we sometimes pump up into a chimney pot) and little frictional resistance (open ended half inch hose). I measured the power consumption and it takes 0.4 kWh to pump 1m3 of water to the back of our garden and up into the chimney pot so it is two-thirds more energy efficient than using mains water.

Greywater holds bit of skin, hair and soap residues in solution, so anaerobically decomposes after 24 hours, and ideally needs to be used as it is generated. The pump is controlled by a float switch and therefore the greywater is pumped onto the garden immediately. It is coarsely filtered before it enters the sump chamber and cleaning the accumulated gunk that collects in the filter has to be done regularly and is not pleasant. But that is a small price to pay for a regular supply of recycled water. In autumn and winter, when we don’t have a requirement for the water, we turn off the power supply and open a valve and the greywater runs directly to drain.


The mysterious case of the sinking flamingo

‘The mysterious case of the sinking flamingo’ is a brilliant illustrated children’s book about why crabs hate combined sewer overflows and why flamingoswater4 love four minute showers. It was written to provide an uncomplicated, entertaining and educational way to introduce 5-8 year olds to how the urban water cycle works and why it’s important to save mains water and to keep rainwater out of the sewer system. It’s also pretty useful for architects too! Coming to a bookstore near you soon. Read more about it here.


Urinal controls rock!

The next time you go into a urinal (male readers only I’m guessing) check whether it’s controlled. Has a PIR sensor identified you hovering around the urinals? If so great. It means that either you (or an earlier user) has activated a countdown mechanism (usually 30 minutes), at the end of which a solenoid valve opens briefly to fill the automatic flushing cistern. As its name suggests this flushes the urinals automatically. Then nothing else happens until the next person comes in for a wee and the cycle starts again. Once the building is shut, no-one is using the urinals, so no-one activates the PIR and the urinals are not flushed when they don’t need to be (apart from a 12 hour ‘hygiene’ flush). If you consider that uncontrolled urinals are legally flushed at a rate of 7.5 litres per hour, every hour, 365 days a year, that is 65,700 litres of water per urinal per year. Add controls into the mix and the savings are massive. For example a pub is only open 50% of the 8,760 hours in a year, a school 23% and offices (assuming not open at weekends) 35%. Urinal controls cost between £200-250 installed. You need one control per cistern (and most cisterns will be flushing two or more urinals). Water currently costs between £2.09 and £5.32 per m3 depending on where in the UK you live. Whichever way you do the maths, the savings are high and payback is fast. Businesses can also get enhanced capital allowances from controls on the water technology list.

(If you enjoyed this section and don’t get Green Building Magazine then read this article by me about total washroom control. Like urinal controls only on steroids.)

Posted on the AECB website September 2015

AECB Water blog 7