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Old 04-08-2014, 01:47 PM   #1
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Plumbing Hot Water Heaters In Series

I have one tankless hot water heater heated by the webasto hydronic heating system and one electric/engine coolant heated tank. I have to go to the engine room and turn a selector valve to use one or the other. I am thinking about plumbing them in series with the tankless unit feeding hot water (when it is on) into the cold water inlet of the tank unit. My goal is to not have to open any valves and to use the tank unit to store hot water created by the tankless unit which would give me hot water at the tap faster than the tankless unit can.

Any reason why this wouldn't work?
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Old 04-08-2014, 01:59 PM   #2
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I'd say it was a good idea. I did the same on a house I had because the "instant" h/w heater was so far from the kitchen. The tank served as an insulated reservoir close to the point of use. It worked well.
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Old 04-08-2014, 02:03 PM   #3
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I have one tankless hot water heater heated by the webasto hydronic heating system and one electric/engine coolant heated tank. I have to go to the engine room and turn a selector valve to use one or the other. I am thinking about plumbing them in series with the tankless unit feeding hot water (when it is on) into the cold water inlet of the tank unit. My goal is to not have to open any valves and to use the tank unit to store hot water created by the tankless unit which would give me hot water at the tap faster than the tankless unit can.

Any reason why this wouldn't work?
I did it the opposite way, the tankless got hot water from the storage unit, then if we ran low we switched on the tankless.
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Old 04-09-2014, 07:18 AM   #4
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DC solenoids to turn on and off fresh water are common and cheap.
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Old 04-09-2014, 08:53 AM   #5
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I think Hollywood's got the better plan. The tank might be heated by the engine, or even just warmed slightly by being in the engine room. In either case, the tankless doesn't have to work as hard, but you always get hot water. No valves needed.

When you're at a dock with electric, or for any other reason, you can flip a switch and build up some hot water in the tank. The tankless won't use any fuel until the water in the tank cools down.
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Old 04-09-2014, 10:29 AM   #6
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I think Hollywood's got the better plan. The tank might be heated by the engine, or even just warmed slightly by being in the engine room. In either case, the tankless doesn't have to work as hard, but you always get hot water. No valves needed.

When you're at a dock with electric, or for any other reason, you can flip a switch and build up some hot water in the tank. The tankless won't use any fuel until the water in the tank cools down.
We do a similar setup in the large custom homes we build, we combine a 60 gal. heat pump water heater (primary) and add inline a standard electric 60 gal. on the backside of the heat pump. We set the thermostat point 10 degrees lower on the standard heater and use a loop to recirculate hot water to the furthest fixture through both tanks.
As we have relatively reasonable electric power rates in Washington, the effecient heat pump keeps 120 ga. of water hot and ready to go. The only time the second heater kicks on is when the heat pump supplied water temp drops because of demand.
I got the idea to do this in homes after it worked in the boat.
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Old 04-09-2014, 10:49 AM   #7
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Originally Posted by hollywood8118 View Post

We do a similar setup in the large custom homes we build, we combine a 60 gal. heat pump water heater (primary) and add inline a standard electric 60 gal. on the backside of the heat pump. We set the thermostat point 10 degrees lower on the standard heater and use a loop to recirculate hot water to the furthest fixture through both tanks.
As we have relatively reasonable electric power rates in Washington, the effecient heat pump keeps 120 ga. of water hot and ready to go. The only time the second heater kicks on is when the heat pump supplied water temp drops because of demand.
I got the idea to do this in homes after it worked in the boat.
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You should reconsider which tank you run your recirculation line to in the system you describe.
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Old 04-09-2014, 10:58 AM   #8
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I see how Hollywood's system works. Nice! Here in the Northeast we don't have the benefit of low electric rates.

Now, in the OP's situation, replace that secondary 60 gallon heater with an on-demand heater, and you have basically the same idea.

In fact, you can do that in the house, too. You'd still get unlimited hot water, mostly heated by the more efficient heat pump, but you'd be keeping 60 gallons less hot all the time, which would bring some savings.
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Old 04-09-2014, 01:58 PM   #9
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So most on-demand HWH are 240Vac. Do they have a 120Vac on-demand HWH? Most boats are wired for 120, not 240.
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Old 04-09-2014, 02:11 PM   #10
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So most on-demand HWH are 240Vac. Do they have a 120Vac on-demand HWH? Most boats are wired for 120, not 240.
Most homes do not have the electrical capactiy to run an electric whole house on demand water heater. So it is doubtful that a boat would have that much capacity either.
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