What Happened to My Hot Water?

CC BY 2.0. Lloyd Alter: Thermostatic mixing valve on top of water tank

Part of an ongoing series on downsizing and renovating a hundred year year old house in Toronto.

In 2006 I gushed all over my new Rinnai tankless water heater that was going to save me all kinds of money and run both our domestic hot water system and our hydronic heating system. It was a small unit that would not be able to cope with our current renovation, where we are tripling the population density in the house and building a new bathroom in the lower level. It never really worked properly and for the last few years, you never actually knew if you were going to get a hot shower out of it. So out it went, and in came an even more efficient gas fired boiler and a big, well insulated tank that has the capacity to serve everyone in the house.

Being a good TreeHugger, I built my new bathroom with 1/2 pipe and a water-saving shower head and knew that it would not be the same experience as before, where I had 3/4" lines coming to an uncontrolled gusher of a shower head. But I didn't know how different it would be, because there is another factor I didn't account for. And that's the temperature of the hot water.

In the United States, everyone from the Department of Energy to right here on TreeHugger advises people to save energy by turning down the temperature of their water heater. According to the DOE:

Savings resulting from turning down your water heater temperature are based on two components: reduced standby losses (heat lost from water heater into surrounding basement area); and consumption (from water demand or use in your home). Set too high, or at 140oF, your water heater can waste anywhere from $36 to $61 annually in standby heat losses and more than $400 in demand losses.

A few years ago I wrote that this is a really bad idea, because of the possibility of your water tank becoming a petri dish for Legionnaires Disease. In Canada, turning down the water heater was not recommended because of the risk. At the time, the American websites didn't even acknowledge the risk of Legionnaires; interestingly, now the DOE site concludes:

And while there is a very slight risk of promoting legionellae bacteria when hot water tanks are maintained at 120oF, this level is still considered safe for the majority of the population. If you have a suppressed immune system or chronic respiratory disease, you may consider keeping your hot water tank at 140oF. However, this high temperature significantly increases the risk of scalding. To minimize this risk, you can install mixing valves or other temperature-regulating devices on any taps used for washing or bathing.
watts mixing valve

© Watts Thermostatic Mixing Valve

Scalding is indeed an issue, and in Canada, they don't give you a choice in the matter. That thing on the top of my water tank is a thermostatic mixing valve, because the hot water tank MUST be set to at least 140 to prevent legionellae bacteria from having a party in the hot tub, while the hot water is then mixed with cold water to reduce the temperature to 120 degrees. So Canadians are paying a whole lot more to heat their hot water, just to have to dilute it down to what feels almost tepid to me.

It's a tough call. In Canada, millions are spent heating water just to cool it down again, in the name of safety. In the States, you are on your own. I am all for the nanny state but I want my old hot shower back.