The Building Conservation Directory 2020

148 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 C AT H E D R A L C O MM U N I C AT I O N S HEAT RECOVERY The heat demand of hot water systems can be considered in three main areas: usage (the energy in the water from the tap), storage losses (energy losses from cylinders and while boilers are on stand-by) and distribution (from pipe work, generally considered for these purposes in terms of primary and secondary circulation loops and dead-legs). In many systems, the storage and distribution losses far outweigh the useful heat demand, meaning that significant amounts of energy is wasted. This is mainly a summer time problem, when the waste heat doesn’t offset space heating and can be a major contributor to the risk of overheating. The AECB also promote energy efficient design of hot water systems. Various design strategies can have a big impact, such as minimising primary and secondary circuits and insulating them properly, use of radial pipework and careful selection and location of the primary plant. Sometimes such strategies can conflict with regulations and strategies for mitigating legionella risk, which naturally takes precedence. However, in most cases, the conflict arises from other constraints such as the long distances from storage to point of use. In new build, there is no excuse for such design errors, but when retrofitting, particularly in the constraints of heritage buildings, it can be more challenging to design out such issues. Often, the best strategy is to simplify systems to the greatest extent possible. This sometimes means challenging beliefs around what is considered good practise. For example, a gas-fired system with poor efficiency might lose two-thirds of the input heat through storage and distribution, making it no less carbon and intensive or costly to run than a direct-electrical system. In such cases, it might be more sustainable, and less invasive and expensive to fit point of use heaters. However, every building should be considered on its merits which means a proper set of calculations to quantify the implications of different strategies. Radial pipework is one strategy to address losses from circulation loops, and can be particularly interesting to those involved with heritage buildings because the pipe is typically narrower with flexible plastic, so installing it can be less disruptive than a conventional system. However, the layout of the building sometimes precludes this strategy because the length of each pipe is limited to between 12m and 15m. WWHR: MAKING A FULL RECOVERY Another water technology that has gradually risen to prominence over the last 10 years or so is Waste Water Heat Recovery (WWHR). The water leaving our showers and baths is quite warm and has a useful amount of energy, which can be used to heat incoming water from the mains. It is feasible to recover some of this heat, and there are various products commercially available to achieve this. However, the simplest possible way to harness the heat from your used bath water is simply to wait until it is cool before you pull the plug. As it cools down, its heat is transferred to the air in the building. If using technology to make life easier, showers are the best candidate because typical heat exchangers depend on transferring energy from one stream to another in real time. The delay between running and draining a bath means simple heat exchangers won’t really work. WWHR devices usually either replace a section of waste pipe or SVP, or are built into a special shower tray. The in-line types consist of a section of waste pipe with a coil or jacket around it which carries the cold fresh water on its way to the shower mixer or DHW cylinder. They are often copper, a very good conductor, to maximise heat transfer. They must be mounted vertically to allow the warm waste water to spread around and maximise contact area inside the pipe. This can make finding a suitable location tricky, in which case the in-tray devices may be more appropriate, although they tend to be less efficient. Typical heat recovery efficiencies range quite widely from 40 per cent to 70 per cent depending on the device, flow rate and the way it is plumbed in. For many modern households, showers are the main use of hot water, so a significant proportion of hot water energy can be offset. The technology is free of moving parts, and is at no more risk of blocking than a normal drain, so is as close to maintenance free as is possible. Like the water efficiency strategies, reducing the hot water energy demand has several benefits – reducing carbon and operational costs, but also potentially reducing the size and cost of the primary plant. In some cases, the cost of the WWHR system can be wholly offset by the savings on the primary plant. To conclude, in some cases it may be appropriate to install rain or grey water recycling systems in heritage buildings, however as in new build the environmental benefits can easily be outweighed by the costs. A careful assessment might show that a well-designed, installed and maintained system does yield ecological and economic benefits, but current evidence suggest that such cases are the exception and not the rule. Water conservation is much more likely to yield net benefits to energy, carbon and cost, because it can be implemented inexpensively, even in the case of a light-touch refurbishment. Waste Water Heat Recovery is increasingly considered in new build and refurbishments alike; although the best systems require some careful integration at both design and installation stages. As with the water recycling systems, the benefits can sometimes be negated in sub-optimal examples, so a proper analysis is essential. References The Consumer Council for Water, Water, water everywhere: delivering a resilient water system (2016/17), December 2017, Environment Agency, Water stressed areas – the final classification , July 2013, Environment Agency, Energy and carbon implications of rainwater harvesting and greywater recycling , 2010, rainwater-and-grey-water-recycling The Association for Environment Conscious Building, AECB Water Standards , 2009, TOBY CAMBRAY is a building physicist and services engineer. He co-founded Greengauge ( where he now focusses on moisture risk analysis. A simple waste water heat recovery system for a shower