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Navien DHW Increase

Posts: 2
I am going to attempt to increase DHW GPM on Navien CH-240. Theory: Set DHW to higher temperature (140) then mix in tempering valve. Replies?

• Posts: 3,056
Impossible

The only way to increase the gpm output of the Navien is to decrease temp rise.

gpm = btu/hr divided by (Temp Rise x 500)

5.7gpm = 199,000/ (70 x 500)

4.4gpm. = 199,000/(90 x 500)

Used 50 degree incoming water temp, your climate worse case maybe different. Temp rise is the difference between the incoming cold water temp and the setpoint.
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
• Posts: 3,271
Won't work

That only works with a tank where the volume of water is stored. On an tankless, you're limited by the max btu's output divided by (Delta T x 500) = gpm.

The flow restrictor orifice and your water pressure are going to determine how many gpm are going through the heater. The max. firing rate is going to determine what the temp rise (Delta T) is.

Raising the setting on the aquastat is only going to increase the output temp when there is a partial draw and the gpm is reduced.
Bob Boan

You can choose to do what you want, but you cannot choose the consequences.
• Posts: 7,265
Old Frats:

Which is why a lot of we old frats think that Instant-Hot tank-less heaters are no better (if not worse) than a tank-less heater in an oil boiler.

Potable Hot Water use isn't 24 hours a day. It is always intermittent draw. If you want hot water, use a storage tank. In electrical generation, they call it "Peak Shaving". You have a base load, and when your loads go up, you have additional "nuts" to apply to the system.

You can't get blood out of a stone, and you can't get 10# of meadow muffins (Cow Flaps) out of a 5# bag.

That's what you are trying to do. I've been hearing more and more dissatisfaction with Tank less Heaters, being oversold for abilities they don't have. And installers don't understand the concepts of what they are trying to do and the limitations of the equipment. I, in no way, have the field experience of some here like ME, but I'll guarantee you that I have had to fix way to many applications where there wasn't enough hot water and the only solution was a storage tank. It's like giving it the gas when driving a loaded truck up and down hills. You give it the gas on the way down the hill and hope you have enough speed and horsepower to get to the top of the other side without having to down shift.

Thermostatic Mixers only spread out the load. If you don't have enough

nuts" to get over the hill, you will run out. Every time.
• Posts: 2
Navien

While I've been a disciple of Dan Holohan for almost 25 years, I'm new on here. Started my post to get ideas & who is paying attention. I do have a life outside of work but I am admitted plumbing & heating geek. I love what I do for a living. So much of my life has been touched by fellow contractors and clients. I agree on the formulas posted. The Navien in question is installed in my own home and that is where I can tinker without getting a service call at 2 a.m. When I can get away with it (and when I have the time) I always experiment. Guess there are times I simply want to prove the theorems correct in the field. I have debunked a few over the years trust me.

Sorry, can't look back to address posts. Believe it was "Ice" wanting to stay with storage heating tanks and dismissing on-demand units. I've installed over 50 of these Navien combination units here in the North East over the last 2 years and another 50 of Baxi combi units the 3-4 years before that. I did have two consistent problems (not on all units but a high enough percentage to seek out another manufacturer) with the Baxi units but it was due to poor material design and had nothing to do with supplying heat or DHW. I have not had a single issue with the Navien CH-240 combination boilers. I do, however, gets service calls on Naviens installed by unqualified installers who simply did not have an understanding of proper piping or simply following manufacturer's instructions.

Thanks for your post replies guys. I'll keep you posted on my Navien experiment.
• Posts: 3,056
Update?

How' the experiment working out?
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
• Posts: 220
It's not like tankless

The Navien combi CH unit has no water adjustment valve only a selectable and changeable water restriction orifice. Because of this the flow rate is only restricted and controlled by unit internal piping, system piping and inlet pressure. So setting for 140 and flowing more than its indirect heating via plate heat exchanger based on BTU model, will only drop in temp when input max is surpassed.

Nothing to gain there in testing anything.
• Posts: 134
Yes, pressure drops on a combi

This is still enough to run two showers event at a 90 degree delta. For many folks, on the coldest day of the year, they are ok with that limitation.
• Posts: 134
Try the well water setting

We set the well water setting on the control unit settings and it helped.

Also, the unit is made for 3/4 in and out of the unit. Maybe there are flow restrictions to 1/2 inch piping on the code supply or return heat side to the house?
• Posts: 14
DHW Pipe Size Increase

I have a queitside DPW 120B installed. The manufacutrer sugests 1/2" for inlet and out on domestic hot water, however, since the hot water loop was changed to 1/2", I get noticible pressure drop when you run 2 faucets, showers etc...

Just wondering, if I increase the pipe size to 3/4 on inlet and outlet for DHW with 3/4 loop for hot water and reduced to 1/2 to each fixture. Will that take care of the pressure drop? Or will it not make a difference since the unit is internally piped with 1/2". Thanks
• Posts: 7,356
proper pipe sizing

depends on two components -- flow rate and pipe length.

Near-boiler easily conveyed by 1/2" pipe may well need 3/4" or 1" in order to to reach the other end of a building.
• Posts: 991
edited May 2013
Watrat

You are not going to get extra hot water by raising the temp and mixing it , actually the only thing you are going to get is higher energy bills, not heating the water to cool it down is one of the benefits of a tankless water heater... not to mention drastically shorten the life of your unit

If you want or need more hot water from a tankless you have a couple options one is add another tankless {costly, but it works}, another is add storage {also works but again you are taking a couple of the benefits away}..

I have had a couple customers that I installed rinnai 9.8's for call back and say they need more hot water, when in fact they didn't, with a 75* rise I have tested them and they make a solid 6gpm, at peak the customers have 2 showers running and a washing machine so they are at the edge of 6gpm which should not be a problem.... First thing I do is offer to change their shower heads to something under 2gpm {high sierras are nice at 1.5}, next is a recirc system which tends to cure all the problems of a tankless water heater, even a small one where you dont go all the way up stairs, you wouldnt believe how much nicer they work with a simple 20 foot loop installed, with that hot line hot all the time you only store a gallon or so of water at -15 under your desired temp and it takes out the 95% sandwich effect and the 95% of the delay... Now if you can go all the way to the point of use it works even better... And the extra energy use is easily offset by the water savings and the comfort increase.... But using a timer and aquastat are a must, constant circ wouldnt be good...
• Posts: 14
Pipe size

Thanks for the input. So basically I can run 1/2 in and out of the boiler, as long as I increase the pipe diameter to get to the fixtures furthest away?
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