Jacuzzi Tankless water heater sizing

Gas tankless water heaters... Choosing the correct size for your home

Navigating through all the hype about tankless water heaters is not easy. Endless hot water, and energy savings being the two most often touted benefits. I doubt many people need "endless hot water"! With the recent rebate programs being implemented, and rebates of 30% and up to $1, 500, saving energy and being green are probably achievable goals in many cases.

Water heater companies, both tankless and storage, recommend sizing water heaters for multiple fixtures. I don't feel this is necessary. If I am going to take a shower, my family members know enough not to start a load of laundry or turn on the dishwasher. I'm sure there are large families that would find it desirable to be able to have two showers going at once and maybe a sink too, during occasions where everyone was getting ready to go on a family outing or something. But I think most families don't need a water heater big enough to fill a hot tub, or to run three fixtures at once.

Sizing a tankless gas water heater

For most families I think running one shower would be sufficient for sizing a tankless water heater, or any other kind of water heater. I suggest that if you are considering the purchase of a tankless heater that you take a 1 gallon milk jug and measure the flow rate of the fixtures you use regularly. Like the shower head and the kitchen sink. The bath tub fixture will have the highest flow rate.

Measure your flow rate

Just measure how long it takes to fill the jug from the hot water faucet, and then divide the 1 gallon by the number of seconds it takes to fill it, and then multiply by 60 to get the gallons/minute. If you want to be able to use more than one fixture at a time, then just add the flow rates together. This should provide you with a very conservative figure, since you usually don't run just hot water for things like showers, it's a mixture of hot and cold.

Measure your hot and cold water temperatures

The next important detail you need to know is how much temperature rise you need. Measure the temperature of the cold water coming into your house. Do this by running cold water long enough to get water from the source, whether it's the water main or your well. Fill a cup and put a thermometer in it.

Unfortunately it's best to do this both in winter and in summer, in case there is a big difference in cold water temperature between the two seasons. In some areas there is and others there is not.

Measure the hot water in a similar fashion. I would let the hot water run for a couple of minutes to make sure you have reached a stable temperature.

If you want your water hot enough for dishwasher's recommendations, then subtract the cold water temperature from 140 to get your need temperature rise. Otherwise pick the highest temperature you would need from the water heater and use it.

Keep in mind that in many areas the water can be much colder in the winter than in the summer. Use the coldest temperature that you expect to be entering your water heater.