Tankless hot water heater related questions

3 years ago

I currently own a tankless HWH which I am replacing soon. I have a few questions related to the new unit.

1. My municipality provides 7 CW natural gas pressure for my residential line. If I buy a tankless HWH that is rated for 8 CW, will it run?

2. How often do I REALLY need to descale the tankless HWH with white vinegar? I have a 2-person household.

3. For how long do I run the descaling process? What will happen if I run the descaling process longer?

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Comments (4)

Jake The Wonderdog
3 years ago
Can you link to your proposed water heater or tell us the model?

"How often do I REALLY need to descale the tankless HWH with white vinegar?" - it depends on how much hot water you use and how hard that water is. Do you know how hard your water is?

Do you have a water softener?

How long to run descaling... This kinda depends. If you have a unit that is regularly cleaned without a lot of buildup, Sixty minutes is the typical time - but 40 minutes for a well maintained unit is usually fine.
formulaross20
3 years ago
7" water column pressure is very close to 8" water column pressure; the unit will run fine.
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You actually want to run them in parallel. Right now, there's nothing to control where the hot water flow comes from at a given fixture, besides the piping losses (length/size of pipe, fittings). If you have a bigger line to/from the bigger heater, and it's closer to the cold water entrance to the house, hot water will want to come through it preferentially. You can get some brands (I know you can for Noritz) where they communicate between 2 heaters in parallel. But I believe that's really intended for where they're sitting next to each other. With them on the opposite sides of the house, you'll probably need to do what we did. We have a tankless on one end of the house for 2 bathrooms. The older gas tank heater is in the garage on the opposite corner of the house. In the crawlspace, there's a closed valve on the hot water line in between the 2 bathrooms and the rest of the house. Basically, this zones the hot water system. If either heater is out, I can open the valve and feed the whole house. But the 2 heaters don't fight each other under normal circumstances. To get your smaller heater to work, you need a closed valve somewhere. It might be on the extension/connection you mentioned in your OP. But that depends on how much stuff is on the 2 systems. I may be wrong, but I think zl700's check-valve solution would still lead to zero flow through one unit most of the time. That'd be the smaller one, since it sounds like it's running the least now.
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I had a loop in my last house. I calculated the heat loss at 8 times a tank. It was an insulated loop. People really overestimate the standby losses of a tank. An important factor is fuel. An electric tank has very little heat loss while gas models have more because you can't insulate parts of it as well - think the exhaust. I think people relate experiences with old tanks but forget that modern tanks are insulated better. When I did the math, my 80 gallon electric tank lost 350 kwh a year. I threw an extra layer of insulation on it that probably saved 100 of those. You can do the calculations. 100 feet of 3/4 pex pipe matches an 80 gallon tank in surface area. Code specifies R3 for pipes and going above R4 is hard based on space. The reality is many are not insulated at all. A good new tank has an R value of 24. When I calculated the "8 times", I had a 200 ft loop (estimated) and I tried to get R6 or so. Now pipes and hot water heaters aren't in the same places - and a hot water heater in a cold garage would be worse. So not all situations are equal. And pipe insulation done in the field with twists and turns will never match the factory foam put around a tank. On demand systems or better design will win every time. We put a small tank under the kitchen sink and all the other fixtures are close enough to the tank so no pump needed. We did plumb in a return to the farthest bathroom but haven't found the need to pump it.
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For M1
Original Author
3 years ago
Generic Chinese HWH from EBAY. We don't have water softener. 2 person household. We have hard water.
Jake The Wonderdog
3 years ago
So, @For M1,

This sounds like a very bad plan. Don't go into a tankless replacement/install without some good planning. Just don't.

1. How hard is your water? Get a hach test kit and find out. Don't guess.

2. Do not buy a "Generic Chinese HWH from Ebay"

there's lots of ways that can go wrong. You have no way of knowing it's been certified by various safety agencies including the gas part, the electrical part, lead free, etc. Not to mention warranty or service.

3. You need to know the approximate temp of the cold water in the winter and how much hot water you would use at one time. For example, do two people get ready for work at the same time (two showers).

4. You need to know the BTU input for the water heater. Then you must determine if you have adequate gas supply to the heater - and to the house in total. Don't guess.

5. You must adhere to venting requirements. No creativity here. Otherwise you will go to sleep and not wake up.

This is not standard DIY stuff, and I get the impression that you are inclined to guess / take shortcuts / look for cheap hacks. This isn't the project to do that with.

Tankless hot water heater related questions

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Jake The Wonderdog

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