- Pain threshold for a human is 106F 
- If water is 60F year round from a supply - does 39F rise (see Tankless Electric Water Heater) suffice?
- 60+39= 99F.
- Average shower temp is 105F 
- This article says that 99F is better, so you potentially avoid eczema
- It appears that 120F water is a relic of long piping - the hotter the temp the quicker your water will get warm. But for on-demand, lower temperatures appear to be better.
- Thus - minimize piping length to point of use. [[Seed Eco-Home specification is keeping heater distance to within 4' of point of use.
- Tap water is 40-70F if coming from a well - depending on USA location - . Varies for surface water.
- Ground temperature varies significantly with seasons, unless you are 30' deep. 
- Earth temperature is constant 30' below surface - this is called the mean earth temp. .
- Earth temp varies significantly with seasons - to full seasonal variation - at surface. , Thus, water pipes, at 3' or so below, show large variation of seasonal temperature.
- Human body is 98-100F.
Anything higher than body temp feels warm.. No, it's about the temperature of the skin - which is around 32C (90F). Thus, the absolute minimum shower temp must be 90F for it not to be uncomfortable. . From 
- If 90 is minimum acceptable, then minimum ground temperature of water must be 51F.
- this can be alleviated with a tank kept at room temperature inside the house.
- Design around Bricor 0.625 gpm shower head. 
- Assume house temp is 68-78F 
- Assuming that house temperature is 68F and up all year - use this ambient temperature to preheat water by leaving a water tank exposed to this temperature. This can effectively almost halve the heating energy use for water.
- 0.32 gpm flow from a 1.8 kw heater which produces 39F temp rise - gets us from 68 to 107F. If we dilute this half way - then we have 19.5 rise from 68 - or 87.5F. Marginal for comfort, we could use more heat.