What is the cold water sandwich effect?
The ‘cold Water Sandwich’ is a phenomenon that is common to most tankless water heater systems. According to this, when you turn on the heater, you receive a warm gush of water initially, then a small amount of cold water, and then hot water again. This happens because the traditional water heater takes some time to heat off the heat exchanger. However, during this time the small amount of water that gets stored inside the exchanger already makes its way through the water outlet and it comes out as cold water. But initially the user still gets some hot water as a small amount of hot water is already present at the mouth of the faucet. So, the sequence runs as this—hot water from the mouth of the faucet> cold water that runs down from the exchanger> the flow of freshly heated water. This ultimately causes the cold water sandwich effect.
In case of short draws, for example when you turn the faucet on to wash your hands or for some such minor use, the effect may not be too noticeable or won’t give rise to any great inconvenience. On the other hand, if you are under a shower, the sandwich effect can be really annoying. There are, however, a few ways to correct this situation.
The most common remedy is to install a recirculating system. This can be done by adding a return loop back to the heater with the help of two flow check valves. This works fine enough, the only problem with the system being that you need a pump with high head capacity and this means the system will consume more energy to operate and ratchet up your utility bill amount. In addition, since in this system, the heater is made to ‘work’ more, it tends to wear off sooner, so that the longevity of the system is compromised.
A more sophisticated alternative to this is to add a small storage tank heater that is plugged directly into the water outlet. This is a small capacity (5-6 gallon, 110V) heater which requires very little energy to heat up the small amount of water stored in the tank. Now, the recirculation loop is connected from this small heater to the tankless heater, thereby saving the latter the heating overwork and keeping its energy efficiency intact. Rinnai manufactures this type of system and you can request a diagram of how the system works through its website.
For further clarification on this or for any other HVAC and plumbing needs for your home, do get in touch with us and we will get back to you at the earliest.