Getting Ourselves into Hot Water
We are planning to install a tankless hot water system in our house. This has not been an easy decision as we have heard many comments and horror stories about how these systems don’t work and that we should avoid them. This of course made us wonder, why don’t they work in Mérida? After six months of pondering and researching over the topic, we are now ready to share our thoughts.
In general, the problem seems to be getting the tankless hot water systems to stay on. They turn on and run for a bit, then for no apparent reason, turn off again. Frustratingly, the water is hot, then cold, then hot again as if the unit can’t make up its mind.
The issue is really one of achieving adequate water flow. Tankless water heaters require a minimum flow rate of water to start up and stay on - around 1 gallon per minute (gpm) is typical. Achieving this much flow in Mérida is totally possible, but not always desirable.
For most of the year, you really want a fairly cool shower - which means the amount of hot water needed is far less than what is required in colder climates. To complicate things even further, in spring and summer the “cold” water is already pretty warm, so you need even less hot water to get your shower to a comfortable temperature.
In terms of numbers, here’s how it works:
1. For a strong shower with good water pressure, you need a total flow rate of about 2 gpm. Some of this comes from the cold side and some from the hot;
2. At peak annual temperatures in Mérida, where the city hits a balmy 100+ degrees, the temperature of cold water sits around 90° F. That is only about 12 degrees cooler than what a comfortable hot shower needs to be (assuming we actually wanted one at such times); and
3. In our summer scenario, 2gpm of water translates to 1.2 gallons of cold water and only 0.8 gallons of hot water. 0.8 is less than the 1 gpm minimum, and thus the tankless hot water heater never turns on. This results in a cold shower.
How to resolve this? One obvious way is to increase the amount of hot water flowing to your shower. Turn the shower mixer towards “Caliente” and your water heater kicks into gear. The problem is, your shower is now scorching hot.
A second solution is to turn on other hot taps in other bathrooms to keep the flow of hot water up without having it directed to your shower. This is an effective solution except for the fact that you are now wasting a lot of water (and gas).
The preferable solution is to ensure that you install a tankless water heater that has a low minimum flow, one rated at 0.5gpm (or even less if you can find one). Although these units tend to be a bit more expensive, they use significantly less energy in the long term and so are worth the initial investment.
A low-minimum flow water heater is the solution for us. We are choosing between two models, either a 0.4gpm model from Rheem, or a 0.1gpm model from Navien. The second unit achieves an incredibly low flow rate by employing a small 0.5 gallon buffer tank that’s built into the unit. Although not technically tankless, water is still heated on demand, so it achieves our overall objective.
