4g tankless water heater

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Old 12-10-18, 04:23 AM
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4g tankless water heater

I'm looking into a tankless water heater under the sink to get instant hot water.
the current water heater is in the basement on 3)4 PEX and or takes about 1 minute to get hot water.
Are the small tankless heaters efficient or costly?
Is 4g better than 2.5?
What happens when the 4g is used up, don't you get a surge of cold water again, day when running a bath?
 
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Old 12-10-18, 04:40 AM
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Efficient and costly? That depends on your definition of costly. For an under sink installation you will need water which is probably already there. You will also need a appropriate power supply which will probably require a new circuit and wiring run from the electrical panel. Then add on the cost of the heater itself. Tankless electric heaters are quite efficient from a scientific standpoint but you need to figure it's operating cost based on your electricity rate to see how it might compare to a gas fired heater.

The size 4 gallons or 2.5 gallons isn't how much water they hold. After all they are called tankless. The number generally refers to how much water per minute (flow rate) the heater can bring up to temperature. 2.5 is good for one fixture like the bathroom sink only. The 4 is more appropriate if you also want to supply the shower.
 
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Old 12-10-18, 06:33 AM
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I should mention that the hot water pipe to 2nd floor is 3/4 pex but the water heater inlet pipe is 1/2" galvanized as it has not yet been upgraded. Could this be the problem? It's like pushing halfh the water i imagine

Wouldn't a 4g just get cold again briefly when the cold water hits it when you're running a bath?
 

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Old 12-10-18, 08:02 AM
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The old 1/2" galvanized piping is probably the limiting factor for your hot water system. If it were replaced and you had better flow the far away faucet would get hot water sooner but you would still waste the same amount of water. It all comes down to how quickly you can flush the standing cold water out of the hot pipes. If the water flows faster you get hot water faster but the amount of water dumped down the drain stays the same.

Tankless water heaters and your "when the cold water hits it" phrase don't really go together. There is no "cold water hits it" . A tankless heater is always fed with cold water so it's always being hit with cold water. All it does is raise the temperature of the water according to it's spec like 60 degrees temperature rise at 4 gallons per minute for example. On some units I've been able to feel when the heater turns on and off. Really annoying in the shower to have the water constantly cycling up and down ten degrees, but I've used ones that worked really well and the water temperature is nice and consistent.

If your bath faucet allows 2.5 gallons per minute to flow you could run the faucet forever and the water will never go cold. If you then also open a sink faucet using 2.5 gpm hot water you likely would see a drop in both the tub and sink faucets. It won't go totally cold but the 5 gpm consumption is beyond the heaters capacity so it can no longer get the water fully up to temp. It might only provide a 40 degree temp increase.
 
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Old 12-10-18, 10:29 AM
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So I'll get almost instant hot water but the water in the pipes is still waterw in the sense that the hot water heater in the basement still needs to heat the same amount of replacement water?
assuming a 6kw tankless doesn't use that much electricity as it's in bursts and when the hot water from the tank hits it it is just passing it through and not consuming electricity?
 
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Old 12-10-18, 11:21 AM
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Put a pump on the hot water heater. And a check valve under the sink of farthust fixture. Youll have instant hot water...

https://www.homedepot.com/p/Watts-Ho...5800/100426993
 
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Old 12-10-18, 11:24 AM
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The problem I have is the heat radiating out of30' plus of pex. A recirculator will exacerbate that I think.
 
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Old 12-10-18, 01:44 PM
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You can look at it from a different direction. If that hot water pipe is inside the heated envelop of your home the heat it looses is not wasted. It goes to heating the home. A negative in summer but at least a break even in winter.

In the end you have to decide if a circulation system or a separate tankless heater is worth the 60 seconds waiting for hot water to arrive. When considering don't forget the added complexity with those systems. It's one more thing (pump, heater...) to break down. A traditional tank water heater is very simple, old technology and can be very reliable.
 
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Old 12-10-18, 03:11 PM
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It doesnt work as you think it does.It only pushes the hot water line water back through the cold line after it cools




Originally Posted by qwertyjjj View Post
The problem I have is the heat radiating out of30' plus of pex. A recirculator will exacerbate that I think.
The check valves thermistor is set @98F.

A pump with a built-in timer is installed on the
hot water line from the water heater (Fig. 1). A patented
sensor valve (Fig. 2) opens when the water on the hot
water side cools and pushes the cool water back to
the water heater. As the temperature in the hot water
line hits 98, the valve closes.
file:///C:/Users/lawro_000/Downloads/PF-IHWRS.pdf
 
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