I have a modest 1300 sq ft single-story home. Water supply enters the building at the south end, and currently we have a conventional tank water heater at the north end of the building. We have to wait a long time to get hot water in the bathroom located at the south end of the building, so I installed a circulator pump atop the hot water heater. The circulator kit comes with a valve that I installed under the sink in the bathroom at the south end. And there's a timer. This works ok, but I think the valve is failing as we seem to have more hot water in the cold lines than we used to. Plus I think our water heater is near the end of its life.
I'm thinking of eliminating the circulator pump kit and installing a small tankless heater to serve the south bathroom, locating it in the attic just above that bathroom. It's a small unit. Google "3.0 GPM Indoor Natural Gas Powered Tankless Water Heater Model #i12-NG" from Home Depot.
Is this unit large enough to serve that bathroom? It has a sink and a shower. I don't want to encounter any performance issues at that shower as it's the one that my wife uses and ... well, it's the one she uses!
Can I connect this tankless unit in a "parallel" configuration?
Later, can I simply replace the tank heater with a tankless unit? Probably one larger as it serves both the second bathroom, the kitchen, and the laundry room; which are all located at or near the north end of the building.
It seem to me that having two heaters connected in parallel at opposite ends of the house will improve performance throughout the house as hot water can be drawn from both heaters at the same time, but I have a feeling I may be overlooking something important.
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The way to determine if the system is adequate is to add up the flow rate of all the fixtures. You may never run the faucet and shower at the same time but this is how you figure your GPM. Faucet is say .50-.75 and shower, if it is a low flow, around 2.6 would give you about 3.1 - 3.2 GPM flow rate.
Second thing you need to do is calculate your temp rise. How much above cold water temp you need to raise. If you assume 50 degrees f for a cold water temp and you want the typical temperature of 105 degrees f at the shower you need a system that can produce a 55 degree f rise in temp and flow at least 3.1 gallons per minute.
According to the description, that unit you mentioned does a 35-40 degree f rise at a flow rate of 4 GPM and a 77 degree f rise at 1.5 GPM. 50 degree at 3 GPM should be obtainable. It's max rating is 4.8 GPM.
The closer you can mount it to the bathroom the better and obviously it needs to be vented and it needs 110v power. Code will determine where you can mount it. If applicable. Here, we can't mount it in the attic and we have to pull a permit. Though these installs are hard to regulate and enforce. YMMV.
As for parallel as long as you are not feeding the hot water back into the hot line you can have as many demand heaters tapping off the 'hot' line as you have water pressure to support. (demand heaters can't be pre-heated.)
Replacing the north end will be expensive. That will need to be a larger unit. With the addition of the demand heater at the south end you could install a smaller tank at the north end. Say 35-40 gallons. Just a thought.
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Three of my friends have upgraded to tankless gas water heaters. Each had to go back and install larger gas supply lines, with one having to increase the supply line to the home. I realize that this isn't your question but I wanted to throw it out here for your consideration.