Tankless Water heater Ground Water Temp

[Alan Kish]

I want to install a natural gas tankless water heater, converting from a standard tank type. My house is 2 story 3200 square feet with 2 occupants. My ground water temp is 42F. I live in a cold climate in North West Montana, I am anxious to get the right system, any recommendations or concerns? I was looking at Rinnai tankless systems.

 

[Bannerman]

Consider a Navien NPE-240A2

Gas input anywhere between 13,000 - 199,000 BTU/hr @ 96% efficiency.

Stainless Steel primary and secondary heat exchangers.

Built-in buffer tank and recirculation pump.

120F - 43F = 78 degree temp rise. Rated at 5.2 GPM @ 75F rise and 4.9 GPM @ 80F rise

https://www.navieninc.com/products/npe-240a2

 

[Sponsor]

 

[Jadnashua]

First thing is to determine the maximum volume of hot water you will want at the same time, and then, at what outlet temperature. You probably don't want to be going to the unit or a remote and changing the settings frequently. Say washing dishes, uou want it fairly warm. You may have occasion to do a load of wash with hot, and unless your dishwasher has a heater in it (and if it does, when it needs to heat the water, that extends the cycle, maybe by a lot of time), your dishes won't come out clean unless it's near max (nominally 120-degrees in a home unless you can plumb directlyu the DW with a hotter branch).

Water weighs about 8.4#/gallon. Take your GPM max you want to support, and the temperature difference between the incoming water and your desired set point. Multiply the gpm * 8.4 then * 60 minutes (heaters are rated at BTU/hour), then multiply that by the temperature rise you want, and (without taking into efficiency) that will give you the BTU requirements. My guess is that will be more than the largest one you can buy. The end result will be cooler water coming out, or it will throttle the volume, neither one is great.

The ratings are done with the average incoming water temp of 50-degrees...your max temp at those same volumes will be less by how far below that your incoming water is.

FWIW, when I thought about going tankless, I measured my incoming water after a cold spell, and it was barely above freezing...while still liquid, it read 32-degrees on my thermometer. So, consider that in your assessment. Southern NH isn't generally considered all that cold...Wyoming can be colder.

 

[Tim Ross]

Running a tankless on its own loop taking water from a holding tank and circulating it back in (controlled by an aqua stat and external pump) would seem, on the face of it to solve many of these problems, might have to mix some cold water in with the water coming from the tank to keep the tankless in its sweet spot. I am contemplating this for my own setup and should allow the tank to recover extremely fast and provide both dump capacity and fast recovery for a big shower etc.

my thought as I stated in another thread I just resurrected, is to use a thermostatic valve to balance the water coming from the tank and mix some cold into it so that as tank temp ramps up to set temp, the cold water mixes in more and more so the tankless stays in something more closely resembling a sweet spot that should keep the tankless condensing.

The idea here being to keep the tankless on a loop that will not impact flow to appliances or showers as the hot water comes from the holding tank and never directly from the tankless. No idea if I’m on to something here. ‍♂️

 

[Bannerman]

Here is a link to a page from an older Navien NR-240A manual which shows the plumbing configuration when using an external hot water storage tank.

https://www.manualslib.com/manual/937053/Navien-Nr-180-A.html?page=48#manual

An alternate consideration:

Cascade 2 identical tankless units so each will supply 1/2 of the hot water requirement. Because the flow rate will be 50% through each, each tankless unit will be capable of greater temperature rise.

https://www.manualslib.com/manual/937053/Navien-Nr-180-A.html?page=45#manual

 

[Jadnashua]

Some tankless systems are designed to be able to gang them. This requires some communications between them, that not all are capable of. Also keep in mind that you gas supply line was probably not installed with the thought of maybe 400K BTU of burner! This can require some major reworking of your supply from the street to the meter and beyond. It certainly can work. Some places charge a demand charge, and while your average use won't be high, the peak demand can be extreme if say you're doing laundry, the gas dryer is running, the furnace is on, and you're cooking Thanksgiving dinner with the oven and all of the burners cranked up...

 

[Tim Ross]

Great info folks, I’ll check the info for sure, ya I’d like to avoid 400k BTU gas use. I’m not heavy on gas other than a 90k BTU fire table and a furnace. All the same it will have to pass code so that’s a consideration. Thanks so much.

 

[John Gayewski]

Also people never consider that a shower valve mixes cold water in. The water heater numbers don't quite add up to everything people expect, without figuring in 10 to 20 percent cold water mixed in.

I've read many posts about people wandering why they don't get endless hot water. Endless warm water, yes.


The line between a boiler and a water heater is 200kbtu. Get a boiler. Then you can add some radiant heat or something nice like that. Endless hot water for a water heater is hard. Places that don't get cold have an easier time, but many places (85 percent) don't have the right water for tankless.

 

[Fitter30]

Need to know how many gpm and temperature you need including if you have a muilti head shower or a garden tub. A single heater with a storage tank with a plate heat exchanger might give.you.what you want.

 

[Tim Ross]

Need to know how many gpm and temperature you need including if you have a muilti head shower or a garden tub. A single heater with a storage tank with a plate heat exchanger might give.you.what you want.

Master Shower is 9GPM measured, my current setup has been awesome and has had no issue feeding it, but it was an original AO smith vertex 100 which was closer to a commercial unit than current models as I’ve been told, a replacement would be stupid expensive locally. My tank temp was 160deg, this gave dump capacity and let me lean on a slightly more even mix of hot and cold than traditional tanks for at least the first few minutes of the shower, this helped with providing really solid pressure when the shower was at at full flight.

I’ve been digging in and realize now the recommended setups for the Navien S2 199k BTU unit is MAX 4gpm external circulation pump to run it with an external storage tank. Personally I would want to run the water fairly hot in the storage tank, I’m being pragmatic here, I understand people’s resistance to higher temps, but this is how we roll. The hope of course is that 4GPM will be enough to provide a solid first hour performance, not just because of a needlessly powerful master shower, but because of the 4 of us all trying to get ready in the morning at the same time.

the more I type the more I feel like saying screw it and going with the Polaris 130

dare to dream.

 

[Tim Ross]

Also people never consider that a shower valve mixes cold water in. The water heater numbers don't quite add up to everything people expect, without figuring in 10 to 20 percent cold water mixed in.

I've read many posts about people wandering why they don't get endless hot water. Endless warm water, yes.


The line between a boiler and a water heater is 200kbtu. Get a boiler. Then you can add some radiant heat or something nice like that. Endless hot water for a water heater is hard. Places that don't get cold have an easier time, but many places (85 percent) don't have the right water for tankless.

I couldn’t agree more, I was very cognizant of that when originally designing my shower, my AO smith vertex 100 was at 160deg operating temp so that I could enjoy a more favourable split between hot and cold water and enjoy the performance we got from it. Of course running high flow Thermostatic and Flow control valves is absolutely key too.

‘We are on to version 2.0 of the shower and I’m keeping all of the hardware the same (though buying new) except I ran 3/4” water lines to the shower to feed a 3/4” Master Shower thermostatic valve. From there the single 3/4” line out of the thermostatic valve simply forms a manifold with 3/4” to 1/2” takeoffs to feed the body spray pressure balancing loop and shower heads, I was happy with my purely 1/2” setup before and I actually don’t expect much extra performance out of it other than the design is good science and should…SHOULD offer that extra percentile of performance when all of the things are on. Before it was great, now I just need to wring a little more out of it.

It’s all academic until we fire it up, in the meantime my failing Vertex tank which has served us well, needs replacement so I’m hoping to elevate the experience with MOAR hot water for longer. This part won’t be as easy as I thought unless I’m willing to open the wallet.

 

[Fitter30]

Here is a hot to cold water calculator
https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing