NOTE: Make sure you read the first three posts (in order!) before tackling the rest, or it could be confusing: Post 1 is Designing the future, Post 2 is Setting up the problem, and Post 3 is Estimating basic requirements.

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Monday, October 23, 2006

Water Requirements Throwdown

Once you’ve reasonably developed the high-level requirements for your design, it’s time to whip out as many requirements as you can. As one of the major concerns following Peak Oil is adequate water supplies, we can start by focusing on water requirements. We’ll also explore a few examples of effective requirements writing.

Let’s start with the following test requirement:
The water system shall provide potable tap water at a temperature suitable for drinking.

This requirement has the right idea, but how do we define what temperature is suitable for drinking? We could say it will provide “cool” water, but there’s no solid definition for this. According to the Association of Home Appliance Manufacturers cold tap water (for appliance purposes) is less than 86 ° F. There aren’t many solid references on “comfortable” cold tap water, so for now let’s choose (by intuition) 50 ° F for the lower end of cold tap water temperature range.

Similarly, we need a requirement for hot tap water. We can use guidance from the reference above:
The water system shall provide potable tap water at 112 ° F – 145 ° F.

Since one of our driving objectives is that the homestead will be safe for children, we need to ensure that the hot tap water isn’t too hot. However, there is a risk of Legionnaire’s Disease if they are allowed to incubate in hot water.
So we need a balance between water that reduces the risk of Legionnaire’s Disease yet doesn’t scald children. You can weight the competing concerns yourself, but remember that you don’t have to have a hot water heater – you could use a tankless heater or go old fashioned and boil water on the stove yourself. So there are design solutions that could meet the competing requirements.

With an eye toward maintainability, we can write:
The water system shall provide a source shut-off mechanism.
This function will enable practical servicing of the water system.

The water system shall remain above 40 ° F at all points.
This is to ensure the pipes won’t freeze in winter. It’s unlikely you would need a similar requirement on the high end – but worth considering if there’s a chance you could approach boiling (maybe a design-specific requirement when the time comes).

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