When you take a bath the waste water goes down the plug hole. Obviously this waste water is quite warm. Waste water heat recovery means recapturing the heat from this water. A copper pipe is coiled around the pipe that the waste water travels down. The water in the copper pipe will now be heated by the waste water. Two things could happen to the warm water in the copper pipe. It could travel straight to the mixer system in a shower and be added to the cold water side. This means that when you next take a shower you will find that the cold water is slightly warm and you will use less hot water. This is an instantaneous system. The warm water could go into a hot water tank where it could stay until someone next takes a bath or a shower. The hot water tank is larger than the mixer in a shower and insulated so more of the warm water can be kept warm for longer. This is a storage system.

How effective are these systems? Instantaneous systems probably work better if the bath and the showers are quite close together. In our house we have a shower over the bath in one bathroom and a stand alone shower in the bathroom below. I can’t really see how a very simple instantaneous system could work unless the bathrooms were this close. Around the rest of the house there are, in total, 4 bathrooms. 2 have baths and 2 have stand alone showers. A storage system would be better if the warm water had to travel a long way. There are problems with transmitting hot or warm water over long distances and residents often complain about the temperature of radiators and hot water on the upper floors of shared houses. Storing the warm water in a tank would give it some chance of arriving at the first floor and of reaching the other shower in the annex across the yard. The real problem in our house is that all the equipment is in the basement, not making distribution of hot water easy. The problem is that hot water tanks require space. We would need somewhere to put a tank that might be about 80 litres. That is the other limitation of the storage system – having the available space. So it would go in the basement, where it might still work, but not as well as it would if it was somewhere else.

How much money would this save? According to my SAP software the instantaneous system reduces the fuel bill by 1%. The storage system reduces it by 2.5%. The real figures could be more than this. A family or a group of people living in a shared house could arrange to take a shower shortly after someone else had had a bath. The figures on the SAP software are averages. Intelligent use of the system could achieve worthwhile results. To me, this will be another device a SAP assessor uses when they are trying to get a house through SAP 2012 on a budget. An instantaneous system costs a few hundred pounds and gives you an extra 1%. Worth knowing about if government building standards are high and customers are strapped for cash.




Waste Water Heat Recovery
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