BPN’s readers ask the magazine’s resident expert, Dick Clarke, to help them out of a design quandary.
Q: We have a building with two sinks roughly 20m apart which both need to be plumbed with hot water. Current practice is to install an instantaneous electric water heater underneath each sink, but electric water heaters are very greenhouse gas intensive.
We’re trying to demonstrate that a single, instantaneous gas water heater installed in the ceiling above one sink, flued through the roof and plumbed to both would be cost-effective, but unsure how to. The water demand at each sink is impossible to determine, but it’s been estimated as being infrequent and low, so the savings in running costs through using gas are unlikely to impact the calculations.
Any help you can give would be much appreciated.
A: My guess is that the activity area will have the more frequent demand, if not the highest volume. This means it will have the highest rate of standing losses and wasted water — neither of which are good. This is the sink above which the single booster should be located. Short frequent draw-off will waste the least water and energy there. If the cleaner needs hot water, it is likely to be once or twice per day in larger volumes, thus reducing wastage per draw-off.
However, it is important that the pipework be really well insulated — meaning thick foam and foil wrap, not the usual thin green excuse. But even with good insulation there will be losses, and it is also my guess that the cleaner will want the hottest water.
In my experience, temperature drops of 10°C are not uncommon, and hundreds of litres can be wasted while people vainly hope for the water to get hotter. So, it may be worthwhile not having the tempering valve — the device that remixes cold water into the hot to limit its temperature to 50°C — located in the usual place and on the outlet of the booster. Rather, have a tempering valve at each sink, so that 50°C water is available where it's needed.
Dick Clarke is principal of Envirotecture, which provides design and consulting services. He is an accredited building designer with 30 years experience, focusing exclusively on ecologically sustainable and culturally appropriate buildings.
