Water heaters in series
by Jude
(Missouri, US)
Can a tankless hot water heater be hooked up in series with a conventional hot water heater to boost the temperature higher?
Answer from Green Energy Efficient Homes
One of the main problems with tankless water heaters is that in cold climates, where the water coming in is particularly cold, the tankless water heater can't supply fully heated water at full flow rates, because it can't pump enough heat into the water to fully warm it at full flow. The heater typically compensates for this by slowing the flow rate down, or by not fully heating the water, depending on the tankless water heater design.
The cold water intake can be as low as 4C or 39F, for example if water is being drawn in from the bottom of a lake in winter. There's a lot of heat needs to be applied to raise water from 39F to the comfort level of 120F. For this reason, some tankless water heater manufacturers recommend installing two tankless water heaters in series for such cold water situations, so that the first tank can warm the very cold water partway, and the second tank can complete the process.
There's no reason you couldn't apply a similar approach but combine a tank water heater and a tankless water heater. But I'm not sure which one you'd want to put first. On the one hand, I can see the efficiency argument of putting the tank water heater first: the lower the temperature differential between the tank and the surrounding air, the less heat loss there is through the tank walls. (Even a well insulated tank will leak heat through its walls.) If you prewarm the hot water in the tank, and then fully heat it in the tankless water heater, you won't lose as much energy from the tank as if you use the tank for the finishing stage of heating.
However, there may be health risks in putting the tank first. The Canadian Office of Energy Efficiency warns against setting your hot water temperature too low, because lukewarm water can be a prime breeding ground for Legionnaire's disease. The disease is killed at 120F and above so a tank in the second stage would kill this. But I'm not sure flash heating the water from a lukewarm tank in the few seconds it is in or past the tankless heater (before it comes out the tap) would be enough to kill the disease.
You could take a page from the solar water heater book. ENERGY STAR rated solar water heaters consist of a two tank system: preheat the water using solar panels in a first tank, which then feeds a second tank where the heat is boosted up to comfort temperature by natural gas, propane or electricity. You get the bulk of the heat from solar energy (assuming it's daylight for a reasonable part of your day!) and you supplement with gas or electricity.
Based on my experience with tankless water heaters I would tend to go for the solar tank + gas/electric tank solution rather than involve a tankless water heater. But maybe I (and my sister and my best friend) have all had bad luck with our exploding or otherwise failing tankless water heaters. Good luck figuring this out yourself - hopefully I've at least given you some things to think about!
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