We just installed a new heat pump water heater, which comes with a scheduling feature. I'm trying to figure out a good way to schedule the heater that both (a) is energy efficient and (b) protects against legionella. The internet seems to be full of people who are trying to do either A or B, but NO ONE who ever gives thought to both somehow.
This thread from last week addressed this question sideways, but I'm hoping for input about a few more specific questions:
https://forum.mrmoneymustache.com/do-it-yourself-forum!/water-heater-recommendations-116869/msg2744337/#msg2744337Some background info:
- Current I have the thing set to 125.
- Energy efficient people recommend the heater stay at 120. Disease prevention people want it at 140. It was installed with a thermostatic mixing valve that should prevent scalding at the higher temp. Legionella grows like crazy under 110, needs to be over 130-something to kill it (slowly), higher to kill it fast.
- We are on a time of use energy plan - peak rate is from 5-8pm.
- Bath/shower time around here tends to be about 7pm, but we have lots of flexibility about laundry/dishwasher.
- I can see from the stats that it is cycling on every other hour overnight, through a period of about 8 hours when we're not using hot water. Seems like entirely a waste.
- Otherwise, we have a medium peak in the late morning (recovering from morning shower) and bigger peaks in the evening from about 7-11pm (recovering from evening shower, dinner dishes).
I'm thinking about doing something like this:
- Turning the heater off entirely overnight (say, from 10pm to 5am).
- Spiking the temp up to 140 for a couple hours in the morning to kill off the legionella, then dropping the temp to 120 until peak energy at 5.
- Leave it off during peak energy, 5-8, then let it recover during the cheaper time and seeing if we have enough water to do showers, etc.
But some question about all this:
- If I'm turning the heater down overnight, especially if it hasn't fully recovered, then perhaps I'm risking it sitting in the legionella danger zone for hours and hours?
- How fast does legionella grow? How often do I need to spike the temp to sterilize it? Every day? Once a week? Once a month? How does that change if I'm letting it drop into a danger zone daily?
- From energy efficiency standpoint, is it worse to let the tank get relatively cold and then heat it way up again than to maintain it at a higher temp?
- Also, because it's a heat pump, efficiency depends on ambient air temp. Maybe if I'm spiking the temp, I should be doing it when it's hottest in the afternoon? But I don't want to create a legionella-rich situation for morning shower.
Insights from obsessive mustachians welcome.
