Navien 240S On demand hot water...in principle

[steve agnew]

Just installed a Navien 240S, 199,900 Btu/s hr, enough for 3 simultaneous showers...or so it would seem. There is a lot more to on-demand hot water that the brochures do not mention at all, but the technology is great anyway.

First of all, if you put in a big unit like the 240s, make sure that you have enough natural gas line capacity. Turns out, in my bedroom wing, the line was 280' of 1-1/4" black pipe, which includes many elbows at 5' per elbow. Thus, this run only allows 150,000 btus/hr...by the table and I wish that the Navien brochure had said something about this. Here is the piping chart...

However, there are online calcs for pressure drop and those only showed about 1" drop for 200 kBTU/hr. I had to neck the flow down to 3.2 gpm and two showers by turning the supply valve down to the unit...seems silly, but it works...

A call to Joe at Navien technical support assured me that what I was measuring was impossible unless the gas inlet was plugged somehow...a brand new unit and a brand new 3/4" flexible line...a plug didn't seem likely, but Joe was right. The Ace Hardware 3/4" line came with fittings that had flow limits of 165 kBtu/hr, and I simply did not read the fine print. Replacing this stupid flow limit corrected the problem and now I have 120 F at 6.0 gpm and no sandwiching at all.

Finally, of course, the sandwich effect of cycling hot and cold, which is a common feature of all on demand hot water, even electric. So...there should also just be a parameter to set, right? To adjust the time constant right? This would normally be an easy thing, but instead, Navien not only does not inform you of the inevitablity of sandwiching with low gas pressure, but your contractor's solution means more money for recirculation with a tank or a loop. So much for efficiency...My suggestion is simply to go slow at first to warm the unit up and then let 'er rip...

Honestly, this is even sillier because all you need to do is to start the flow low, wait for it to get hot, increase the flow again, and then do your shower. In other words...all it takes is a few extra minutes of time and wasted water...which is a lot cheaper than gas. But if you do not have the gas pressure, these two effects pile on each other and it took me a week to figure them out. Also had to buy a manometer to measure the natural gas pressure at the street and at the Navien. It turns out that I really have plenty of gas pressure, 6-10 in.w.c and my 280' 1-1/4" pipe run only drops about 1" w.c. at 200,000 btus/hr.

Now I'm glad I bought the 240s, but there would still be the sandwich problem if the gas flow is drops...so far, there has not been a problem. I have two gas furnaces at 80 kBtus/hr each, a gas stove, and a pool heater at 300 kBtus/hr, but no issues so far...very pleased...

Once I removed the flow restrictor that I had inadvertently installed, it was necessary to adjust the MIN1 gas pressure for the minimum fire for a flow. The spec was -0.02 to +0.30 w.c., but my pressure always fluctuated between -0.02 and +0.10, so I put the manometer on average and set the gas pressure best I could. Until the MIN1 gas pressure was set correctly, there was some sandwiching that occurred as noted above. Now all is well...so sandwiching at all...

Also, the high gas pressure at the Navien regulator was still just 3.5" wc that dropped to 2.0" wc when full flame even though the line outside the regulator was 7.5" wc at the unit that drops to just 6.5" when unit is full on. There is no problem and the 240s runs great even though the spec sheet says it should not run at these gas pressures. It now fills an 80 gallon whirlpool bath in just 8 mins...

 

[Terry]

Tankless work on a temperature rise formula. Those in the South with warmer ground water get the advertised GPM of heated water. People like Steve and I in Washington get half of what someone in Florida would get. I sometimes feel bad wasting water as I wait for my tankless to finally feed me some warm water. I'm thinking of moving my tankless closer to my bathroom core to speed things up a bit.

 

[Sponsor]

 

[Bannerman]

Because water must be flowing before the burner will ignite, some cold water will exit the heat exchanger before the burner is ignited and a full flame is achieved. When hot water continues to be consumed, then the burner will continue heating the water and no cold-water sandwich condition will occur. A cold-water sandwich will occur when hot water is consumed, then consumption is temporarily stopped which will cause the WH flame to shut down. When hot water is soon again needed, the hot plumbing lines will already contain hot water, but before the burner is reignited for the 2nd flow, another slug of cold water will again exit the heat exchanger followed by additional hot water once the burner is again up to capacity and heating water.

The Navien NPE-240A is very similar to the 'S' model you purchased, but 'A' models are equipped with an internal buffer tank and internal circulator pump.

The buffer tank within the 'A' models will continually contain a small volume of hot water so that initial slug of cold water exiting the heat exchanger will mix with the hot water already within the tank thereby buffering or moderating the temperature of the initial flow to faucets even when there are multiple ON/OFF/ON burner cycles.

The 'A' series internal circulation pump will periodically circulate the water from the buffer tank through the heat exchanger so the buffer tank will be maintained at the appropriate temperature.

The internal circulation pump may also be configured for circulating hot water throughout the home. While a dedicated return line is preferable, the cold water plumbing may be utilized as a hybrid return line for homes not equipped with a dedicated return line.

 

[Plumber69]

you could upsize your regulator at the meter, that would mean adding a reg to each other gas appliance

 

[Fitter30]

Don't think gas pressure can be raised in residential. 5.5" -7"
https://www.google.com/url?q=https:...YQFnoECAsQAg&usg=AOvVaw1lRUgBrC1jjb6DM_N1d1SU

 

[Bannerman]

Fitter30, are you referring to page 1 of your link where it says 'a typical residential meter buildup'?

"A typical residential meter build-up has a regulator that reduces main pressure down to 5.5 to 7 inches water column (approximately 1/4 of one psi) ..."

 

[Reach4]

Don't think gas pressure can be raised in residential. 5.5" -7"

It can be in at least some places, and not others.

This would be really nice when wanting to add an on-demand WH or a whole house generator.

https://www.socalgas.com/for-your-business/builder-services/elevated-pressure

https://www.texasgasservice.com/media/TGS/Forms/OGSElevatedGasPressureRequestForm.pdf
https://www.wisconsinpublicservice.com/partners/builders/pdf/builders_forms_159-0310.pdf

 

[steve agnew]

Once I got rid of the stupid safety flow limiters, the unit worked well and my 280' run of 1-1/4" pipe only dropped about 1" wc, even when the furnace is also on at 80 kBtus/hr.

 

[Fitter30]

Fitter30, are you referring to page 1 of your link where it says 'a typical residential meter buildup'?

"A typical residential meter build-up has a regulator that reduces main pressure down to 5.5 to 7 inches water column (approximately 1/4 of one psi) ..."

Yes

 

[Fitter30]

Once I got rid of the stupid safety flow limiters, the unit worked well and my 280' run of 1-1/4" pipe only dropped about 1" wc, even when the furnace is also on at 80 kBtus/hr.

I'm not familiar with safety flow limiter except for a seismic valve or a excessive flow valve between street and meter. Have brand and model of this valve.

 

[steve agnew]

Because water must be flowing before the burner will ignite, some cold water will exit the heat exchanger before the burner is ignited and a full flame is achieved. When hot water continues to be consumed, then the burner will continue heating the water and no cold-water sandwich condition will occur. A cold-water sandwich will occur when hot water is consumed, then consumption is temporarily stopped which will cause the WH flame to shut down. When hot water is soon again needed, the hot plumbing lines will already contain hot water, but before the burner is reignited for the 2nd flow, another slug of cold water will again exit the heat exchanger followed by additional hot water once the burner is again up to capacity and heating water.

The Navien NPE-240A is very similar to the 'S' model you purchased, but 'A' models are equipped with an internal buffer tank and internal circulator pump.

The buffer tank within the 'A' models will continually contain a small volume of hot water so that initial slug of cold water exiting the heat exchanger will mix with the hot water already within the tank thereby buffering or moderating the temperature of the initial flow to faucets even when there are multiple ON/OFF/ON burner cycles.

The 'A' series internal circulation pump will periodically circulate the water from the buffer tank through the heat exchanger so the buffer tank will be maintained at the appropriate temperature.

The internal circulation pump may also be configured for circulating hot water throughout the home. While a dedicated return line is preferable, the cold water plumbing may be utilized as a hybrid return line for homes not equipped with a dedicated return line.

Yes...but if you don't need a buffer tank or recirculating line, it is really more efficient...