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Old 03-31-2014, 05:18 PM   #1
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Modulating tankless water heaters. I am shopping


I have learned a few things about tankless after buying a Paloma unit. Seems most of the budget units out there do NOT modulate the gas or BTU output of the burners to match the flowrate. The cheap ones basically restrict the hot water out get a different water TEMP, but the burner is either on or off.

I am looking for one only around 2.5-4.0gpm but I want gas modulating. I find this specification hard to find and you have to make a bunch of calls to people that don't get it.

I am looking for maybe an older, used, or hold over new unit, NOT the latest thing on the market today. Can anyone point me in the right direction?

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Old 03-31-2014, 06:13 PM   #2
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Modulating tankless water heaters. I am shopping


Just a bit confused here, yeah, it happens easily. never seen a water heater, pool heater, or boiler that adjusted flame rate to flow rate.
Could be something new, never said I know it all, but if you have a web page that shows this adjustable burner. Could you post it please?

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Old 03-31-2014, 06:20 PM   #3
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Modulating tankless water heaters. I am shopping


http://www.bosch-climate.us/products...rm-330-pn.html

Here is just one of them. I believe all Rinnai, Bosch, Takagi, etc do this. Otherwise you never get a variable flowrate and just does not make sense to even make! I mean, your dishwasher might pull .5PGM during filling. Your shower might be 1.2gpm, etc.

The one I have now is very irritating. You can turn the "heat" valve even with the burner off and see that all it does is change the flowrate of the water. Another glitch of this system is mixing valves. When you turn the cold on and there is any restriction, this reduces the pressure drop on the hot side (that is what turns the gas valve on), and the heater turns off!

In the middle of a shower, you get a little too much cold in the mixer and boom, you get a cold shower. I want to drive over this POS with my truck.

I called a company that is selling on ebay and they too confirm their gas valve is on or off and just adjust the water flowrate.
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Old 03-31-2014, 06:35 PM   #4
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Modulating tankless water heaters. I am shopping


that has two adjusters.
one controls the flow rate, the other controls the flame. at least judging by the small pictures. does not adjust flame to flow, except for manually.
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Old 03-31-2014, 09:35 PM   #5
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Modulating tankless water heaters. I am shopping


Huh, I'll admit I did not look to much at the Bosch stuff but it seems I might need to make some calls! I was told that the "modulating valve" was in fact a variable valve that the system used to vary gas flow for demand. Obviously if the adjustment is manual, that would make no sense.

I am already "modulating" the gas flow on mine now....with the shut off valve since it gets scalding hot in the summer and if you turn the flow rate up, there is not enough pressure drop to even turn it on.

So how do the "good" units handle all the demand differences? I keep reading claims of electronic controls to monitor and precisely output the right temp BUT if a burner is either on or off, simply physics is breaking down here. If the flow rate of water goes down, the deltaT will go UP. Even if they bypass the burner partially, there is wasted energy there.

I have to be missing something here. If a unit turns on at .5pgm but a max flow of 5-6gpm, both at 100*F water temp, those flow rates require different heating inputs.

I heard before about multi-staged burners. Is that what is happening here?
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Old 03-31-2014, 09:41 PM   #6
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Modulating tankless water heaters. I am shopping


viper, to be honest, i do not know enough about demand heaters. what i tested on my apartment (maint super) did not impress me so i canceled the contract. that was without discussing it with the management company.

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Old 03-31-2014, 10:08 PM   #7
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Modulating tankless water heaters. I am shopping


I have a Takagi Jr, with an optional temp control that you don't have to open the unit to adjuat. Works great.
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Old 04-01-2014, 12:57 AM   #8
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Modulating tankless water heaters. I am shopping


Google "tempering valve".
Also called a anti scald or balancing valve.
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Old 04-01-2014, 04:42 AM   #9
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Navien, and Rannia, have the type your looking for.
http://www.navienamerica.com/Product...age1/Details/7
http://www.rinnai.us/water-heater/products
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Old 04-01-2014, 06:16 AM   #10
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Navien is a nice unit----
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Old 04-01-2014, 05:16 PM   #11
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Modulating tankless water heaters. I am shopping


Quote:
Originally Posted by oh'mike View Post
Navien is a nice unit----

Older model Naviens are not. Have a 10 degree temperature swing on a shower. Actually most on demand have this problem. Gets worse if a second person in the house wants hot water.
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Old 04-01-2014, 05:55 PM   #12
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Modulating tankless water heaters. I am shopping


Quote:
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Navien is a nice unit----
Or if you have one where all the O-rings on their fittings turned hard and brittle as a rock after a year.
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Old 10-27-2014, 06:23 PM   #13
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Modulating tankless water heaters. I am shopping


I have a Noritz NR98 DVC. It has the modulating gas valve commercial grade heat exchanger. No change in shower temp. Needs 1" gas supply reduced to 3/4" at unit. Also Stainless 2 way pipe.
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Old 10-27-2014, 09:50 PM   #14
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Modulating tankless water heaters. I am shopping


I dont think you are going to get the answer you are looking for as far as a cheap alternative to what you have. I install and service Rinnai, Navien, and have worked on Rheem Commercial tankless. All have self modulating gas controls to account for volume useage. But they all also come at a price. Most of the time you get what you pay for.
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Old 10-28-2014, 10:11 AM   #15
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Modulating tankless water heaters. I am shopping


You need to look at your flow rate with the temp rise. That one was 35 and 3.3 GPM.

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