Tankless water heater output
Posted by Mike Holm on June 26, 2009
I am constantly finding companies (contractors, not manufacturers) who advertise that the tankless water heater they install will give you 7-8 gpm of hot water when heating from 10C to 50C (122F). Sorry, but this should not be believed. The amount of gas needed to give this amount of hot water is twice the input rating of most tankless heaters and is more than the gas meter outside the house will allow.
It is a simple fact that a tankless water heater with a 200 mbtu input could give 3.5-4gpm of hot water. Most tankless heaters have a lower gas input than 200 mbtu.
People who come from countries where tankless heaters are common understand intuitively how they work. They do not expect the units to supply every tap in the house at the same time. For some reason, however, we in North America seem to feel that our water heater should be able to supply all our demands at once. It is a wasteful attitude some of us have, and I applaud those who rise above it.
The amount of flow you can get is determined in large part by the gas supply and the temperature rise you desire. If you would like water that is heated only to 40C you will get a higher flow. Alternatively, if you preheat the water with solar so that it is only being raised from 30 -50C, you also will get a higher flow.
Tankless heaters also have the reputation for taking longer to get the hot water to your tap. This can be true for a lot of units. One of the problems with a normal tank (and it is partly why they are so wasteful) is that the heat from the tank rises through the output piping towards your taps even if you do not use the water. It therefore takes less time until you feel warmth coming to the tap than with a tankless heater. We sacrifice a small amount of time for gas efficiency.
The other truth about most tankless heaters is that a minimum water flow is needed to trigger the gas valve. Therefore, when you turn on the tap for only a few seconds the unit tries to come on but does not complete the process as you have finished using the water. If you need a small amount of water for hand washing, use soap and cold water. The soap will still work fine. When shaving, fill the sink rather than turning the water on and off for a few seconds at a time. The heater will last a lot longer and you will have a more satisfying shave.
Another issue is the desired temperature of the heater. Just like the tank type water, we should keep the set temperature as low as possible. This means that when a tap is turned on, there will be a higher flow through the unit and it will operate as desired sooner. Having a high temperature from the water heater forces you to mix more cold water with it at the tap and the resulting flow from the heater can be too low, causing it to turn on and off rapidly.
A few simple rules will help us get the most out of our tankless heaters.
