Two small hot water tanks more efficient?
by Ornella
(Ucluelet, BC, Canada)
Hi, if you have a small suite in your house, would it be more efficient to have two smaller hot water tanks, one for the house and one for the suite, or one large one for everyone?
Answer from Green Energy Efficient Homes
The answer depends on whether you want to motivate the occupant of the suite to save energy by charging them for their own hot water. If you install a separate tank for them and charge them part or all the expense of the electricity or gas to heat their hot water, they are much more likely to conserve energy than if you are paying the whole bill and factoring some of the cost into their energy use. (Some people will economize with hot water for environmental reasons, but many need the price hint to cut back.)
If your tenant naturally conserves hot water, then it is probably more efficient to buy one larger hot water heater for both sets of occupants. I would recommend an ENERGY STAR condensing hot water heater. These are almost as efficient as on-demand hot water heaters, but don't have some of the on-demand hot water heater problems (such as poor suitability to northern climates where input water is cold in winter, and poor reliability). They can provide virtually continuous hot water because they heat up so quickly, and so you don't need as much capacity to deal with multiple suites using the same source.
Whether you and your tenant can agree on an acceptable tank temperature is also an issue. Ideally, to save energy you should lower your hot water tank temperature to 49C or 120F. But some people will want it hotter (the factory default is typically 60C or 140F) and the higher the temperature, the more you pay to keep it that hot (due to heat losses through the tank walls etc.). If you can agree with your tenant on a lower tank temperature setting, share a tank. If not, you either have to live with the temperature they want, or have separate tanks.
The other issue to consider is whether a single tank can deliver hot water to both your own part of the house and the rental suite at the same time. If your tenant decides to take a shower at the same time you do (and you're running the dishwasher too!) will you have enough hot water? Having the two suites on different tanks will address this issue better than having a single tank. But if you both use low flow shower heads you may not need to worry about this.
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