Two Tankless Water Heaters - Does linking them make sense?


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Old 12-08-12, 08:00 PM
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Two Tankless Water Heaters - Does linking them make sense?

Hello,

My brother and I own and live in a 3-plex home, in a cold state. We have two tankless water heaters (Richmond 95s, 9.5 gpm).

The two water heaters have the ability to be "linked" with a cable, essentially setting them up in series. I'm wondering what the advantages/disadvantages are to using this cable?

Is the ability to link essentially there to prevent the need for a major overhaul of your plumbing system? Our house is plumbed with pex, and plumbing each water heater to handle two of the living units in our house would be no problem.

We want to set these up to maximize the availability of hot water and energy efficiency.

Thanks.

Bob
 
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Old 12-08-12, 08:18 PM
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Do you have and issue with hot water? How long have these been installed?

Yes its common for large homes to use two units together. But if you have separate living areas, and are piped to separate homes it may not make sense.

Again what is the issue? Its not just for the sake of doing it because it says so, right?
 
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Old 12-08-12, 10:44 PM
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We had a 50 gallon and frequently ran out of water, so we bought these to replace.

They haven't been fully installed yet. I'm in the process of putting them in right now, and was just wondering what the purpose of the cable is. The instructions list linking them as an option, but don't explain why someone would choose to do it. Make sense?
 
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Old 12-09-12, 05:34 AM
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Tankless have a limited temperature rise. The first would raise the temperature to warm enough that the second could raise the temperature enough to give you water hot enough to satisfy your needs. Are these electric? This is often done on cheaper electric tankless heater that use only a single 60 amp feed. Edit: I see that they seem to be gas not electric but the reasoning would probably be the same.

Not asked but it is unlikely if you were unsatisfied with a 50 gallon storage heater you will be happy with a tankless.
 
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Old 12-09-12, 05:48 AM
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It appears the cable is only used if the water heaters are plumbed together.

They would then act in tandem as one heater in high demand applications.

I would slightly modify the plumbing shown below in the 3rd image.

It would be better to "tee" off the incoming water and split the runs evenly to each unit.

This would ensure an equal flow to both.....and do the same for the output.

As it says.....best to contact the factory for all the details.....


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.
 
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Old 12-09-12, 06:00 AM
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Hmmm, I would of assumed they would be hooked up in series.... How will parallel increase demand. You will have the same temp water entering both units. They will only raise the water temp so much.
 
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Old 12-09-12, 06:24 AM
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It puzzled me as well but after some thought I surmised the following.

Let's say for argument purposes.....the max. inlet piping to the heater is 1/2"

Feed the 2 units in parallel with 3/4" split to 1/2"......you increase the volume.....same temperature rise.

If you're maxed out at 1/2" flow and you connect in series.....then yes....hotter water.

But....the flow rate is still limited and it may not be sufficient enough to feed 3 bathrooms.

And I understand if you have hotter water you will then need less volume.

But there is a limit to how hot you can go before it becomes dangerous.



.
 
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Old 12-09-12, 06:35 AM
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It would work out that two heaters in series would give you an increase in outlet temperature at the flow rate of one heater.
Connected in parallel would give you double the flow rate at the outlet temperature of one heater.

As far as temperature rise goes higher outlet temperature is not always the concern.
Some places have a very low water supply temperature that requires a very large amount of heat to raise the temp to normal hot water levels.
This all has to be taken into account when selecting tank less with some cases requiring a series connection to get decent outlet temps.
 
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Old 12-09-12, 08:18 AM
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Well Minnesota in the winter should have a really cold water supply.

Or is it Main. (Stupid two letter state codes instead of the three letters we use to have.) Either way colder then a brass monkey's ....
 
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Old 12-09-12, 08:48 AM
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I have 400 gallons of indoor storage......essentially my water is pre-warmed.

Temperature rise is not an issue in my case.....so parallel would be my only option.



.
 
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Old 12-09-12, 08:52 AM
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How many units is it? Is it all one residence or three separate?
 
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Old 12-09-12, 09:00 AM
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MN is Minnesota
ME is Maine
MI is Michigan
MS is Mississippi
MA is Massachusetts

Ground water temp is the same in MN as other states ground water temp as it is well below the frost line. (my well is 128') IIRC it is about 50 degrees. This may change due to geothermal temps.

In MN I would think you would want to run them in series as well.
 
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Old 12-09-12, 09:14 AM
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Yes and at a 70 f temp rise you only get 3 gpm. One fixture. Good for 1 bath homes.
 
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Old 12-09-12, 09:21 AM
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If the diagram you show in post #5 is from the instructions for your heater the method to connect two tanks in parallel is not correct.

The difference in length of tubing in both the supply and tank outlet after the tee will cause uneven flow between the two tanks.
The tee needs to be exactly in the middle of the tanks with the tubing the same length on both sides so one tank does not draw more than the other.
 
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Old 12-09-12, 01:32 PM
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The difference in length of tubing in both the supply and tank outlet after the tee will cause uneven flow between the two tanks.


Not sure of that Greg... This units have flow type controls to even the temperature?

With them linked it will adjust from my understanding.

I say this because on a Noritz I did there was an issue with one of these flow restrictors. Its would reduce flow to like 1 gpm, while still producing hot water. We had to replace that faulty part.

But I may be totally wrong..Just going from what I remember from servicing a unit 7 years ago.
 
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Old 12-09-12, 02:40 PM
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"If the diagram you show in post #5 is from the instructions for your heater the method to connect two tanks in parallel is not correct"



ahhh.....<cough>.....in the same post I qualified it by clearly explaining this exact issue.....



.
 
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Old 12-09-12, 02:45 PM
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It is entirely possible there is a flow restrictor.
An issue I could see though is if one unit slowed the flow due to imbalanced piping it would reduce the maximum flow of the unit when in a restricted state.
I have more experience with large commercial tanks connected in parallel where in some cases bad piping could cause one tank to seldom operate.

Pretty sure those demand tanks should have balanced piping on both sides to maximize efficiency.
 
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Old 12-10-12, 12:21 PM
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Thanks for all the replies. Just to clarify a few things:

We live in Minnesota, and our groundwater is colder than in some other states. The temperature "delta" that we're looking for is pretty high, so yes, we will probably be looking at getting about 3 gpm from each water heater unit.

The 3-plex in question is three seperate living spaces, with shared laundry. There is only 1 dishwasher, and all of the bathtubs and showers have low flow heads. My point is, I've done the calculations, erred well on the side of caution, and am confident that we can get enough hot water where we need it.

The problem we had with our tank water heater was not the inability to get warm enough water, it was that if we did laundry, dishes, shower or two, we would run out of water. With the tankless, we just need to ensure we aren't surpassing 3 gpm, per unit.

So, if I'm reading the responses correctly, linking the two units would be a good option if I didn't want to rework a lot of my existing hot water plumbing?

Due to the construction and layout of the house, I can essentially "attach" whatever faucet or device in the house I choose, to either water unit.

Seems like thats the route I should take. Interesting discussion.
 

Last edited by ray2047; 12-10-12 at 01:01 PM. Reason: Remove unrecognized special characters.
 

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