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jadnashua

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In the fine print, verify the temperature rise when your water is outside of the advertised values...they assume 50-degree inlet water and Michigan in a cold snap will NOT be anywhere near that warm. I'm at the same latitude, and I've seen a true 33-degree inlet water temperature at my place.

Your meter will likely have an indication of the maximum number of BTUs in burner it will support. That then also assumes you've got the right diameter of piping so you aren't restricting its flow along the way INSIDE the house.

You'd want to add up the maximum usage in the home, which could include a stove, dryer, furnace, and water heater. If you have something like a pool heater, gas fireplace, etc., you'd need to add them to the list. It may be unusual to have them all on at max at the same time, but it could happen. Two showers at the same time, you may not want someone to start the dishwasher or clothes washer! Or, maybe even wash their hands. With a tank, it's hot until you've exhausted what's in the tank. With a tankless, it's hot until you exceed its capabilities. Think passing your hand through the flame of a candle...do it fast, no problem, do it slow, it can get really hot. You want it hot, and it can't get there if you try to draw a lot through it. Some designs have a flow limiter to try to ensure you get the temperature rise, most just end up providing cooler water as you exceed the capacity.

Right now, probably the largest gas user in the home is the furnace. For most people, a typical tankless will be at least twice the size of their furnace's burner, so what might work for the furnace or a tank-type water heater, may be too small for the tankless.

Nothing lasts forever, and while pretty much any plumber can repair or replace a conventional WH, not everyone can replace or repair a tankless, or would you have the parts available, so you might be without hot water for a bit while the part was ordered, or you waited for the store to open.

5gpm equals 2504 pounds of water per hour (ratings on heaters are based on per hour). So, say your incoming water was 33-degrees and you wanted to get it to 120-degrees, that's 87 degree rise. A BTU will raise one pound of water one degree F, so 87*2504=217.,805 BTU/hour. Now, the burner isn't 100% efficient, so you have to be putting in more BTU to get the desired rise, so lets say it's 98% efficient (note, that will only be true when brand new and maybe again IF you do the annual descaling procedure - are you prepared for that annual maintenance, and if you don't do it yourself, at least a couple hours of labor?) so to get the needed 217+KBTU, you'd need to put in that /0.98 or 222, 249BTU. On it's best day, a 199KBTU device cannot do that...but, it would be close, and would still be hot, but not the max. Problem is then, though, that as the usage gets above the ability to maintain a certain temperature, the output would be changing. SOme people easily adapt and live with it, others will be exceedingly annoyed.

It can get messy, and you start to lose efficiency, if you want hot water recirculation. A tank stays hot, a tankless only turns on when you draw hot. So, you not only have to purge the potentially cold hot lines (true on any system that sat), there's a slight delay as the burner comes up to speed, and there's a minimum flow rate before it will even turn on. That tends to be about 0.5gpm. IF you don't reach that flow rate, it won't turn on at all. Want warm at the sink...may not happen since you haven't met the minimum flow rate to turn the tankless unit on.

Now, a 2.5gpm shower head won't be using 100% hot, so yes, it should handle that, but any additional hot water use will be dropping the temperature. Some dishwashers can heat the water if it's not hot enough, but not all. Your dishes may not get very clean if it can't get the hot water it needs. Most clothes washing is done with cold or warm water, but there may be times when you want hot there. Not everyone's use pattern is the same. Just know what you're getting yourself into.

I like the concept of endless hot water, but it has its limitations.
 
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In the fine print, verify the temperature rise when your water is outside of the advertised values...they assume 50-degree inlet water and Michigan in a cold snap will NOT be anywhere near that warm. I'm at the same latitude, and I've seen a true 33-degree inlet water temperature at my place.

Your meter will likely have an indication of the maximum number of BTUs in burner it will support. That then also assumes you've got the right diameter of piping so you aren't restricting its flow along the way INSIDE the house.

You'd want to add up the maximum usage in the home, which could include a stove, dryer, furnace, and water heater. If you have something like a pool heater, gas fireplace, etc., you'd need to add them to the list. It may be unusual to have them all on at max at the same time, but it could happen. Two showers at the same time, you may not want someone to start the dishwasher or clothes washer! Or, maybe even wash their hands. With a tank, it's hot until you've exhausted what's in the tank. With a tankless, it's hot until you exceed its capabilities. Think passing your hand through the flame of a candle...do it fast, no problem, do it slow, it can get really hot. You want it hot, and it can't get there if you try to draw a lot through it. Some designs have a flow limiter to try to ensure you get the temperature rise, most just end up providing cooler water as you exceed the capacity.

Right now, probably the largest gas user in the home is the furnace. For most people, a typical tankless will be at least twice the size of their furnace's burner, so what might work for the furnace or a tank-type water heater, may be too small for the tankless.

Nothing lasts forever, and while pretty much any plumber can repair or replace a conventional WH, not everyone can replace or repair a tankless, or would you have the parts available, so you might be without hot water for a bit while the part was ordered, or you waited for the store to open.

5gpm equals 2504 pounds of water per hour (ratings on heaters are based on per hour). So, say your incoming water was 33-degrees and you wanted to get it to 120-degrees, that's 87 degree rise. A BTU will raise one pound of water one degree F, so 87*2504=217.,805 BTU/hour. Now, the burner isn't 100% efficient, so you have to be putting in more BTU to get the desired rise, so lets say it's 98% efficient (note, that will only be true when brand new and maybe again IF you do the annual descaling procedure - are you prepared for that annual maintenance, and if you don't do it yourself, at least a couple hours of labor?) so to get the needed 217+KBTU, you'd need to put in that /0.98 or 222, 249BTU. On it's best day, a 199KBTU device cannot do that...but, it would be close, and would still be hot, but not the max. Problem is then, though, that as the usage gets above the ability to maintain a certain temperature, the output would be changing. SOme people easily adapt and live with it, others will be exceedingly annoyed.

It can get messy, and you start to lose efficiency, if you want hot water recirculation. A tank stays hot, a tankless only turns on when you draw hot. So, you not only have to purge the potentially cold hot lines (true on any system that sat), there's a slight delay as the burner comes up to speed, and there's a minimum flow rate before it will even turn on. That tends to be about 0.5gpm. IF you don't reach that flow rate, it won't turn on at all. Want warm at the sink...may not happen since you haven't met the minimum flow rate to turn the tankless unit on.

Now, a 2.5gpm shower head won't be using 100% hot, so yes, it should handle that, but any additional hot water use will be dropping the temperature. Some dishwashers can heat the water if it's not hot enough, but not all. Your dishes may not get very clean if it can't get the hot water it needs. Most clothes washing is done with cold or warm water, but there may be times when you want hot there. Not everyone's use pattern is the same. Just know what you're getting yourself into.

I like the concept of endless hot water, but it has its limitations.
Excellent info, thanks! Well, the tankless is being installed as I right this. I guess I'll see how it works out, but since I ruled out downsizing to the 50 gallon I couldn't pass on the tankless for such a modest difference over the 75 gallon tank. I guess prices are going way up on those. Hopefully it works out for us, we'll see.
 

Reach4

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Excellent info, thanks! Well, the tankless is being installed as I right this. I guess I'll see how it works out, but since I ruled out downsizing to the 50 gallon I couldn't pass on the tankless for such a modest difference over the 75 gallon tank. I guess prices are going way up on those. Hopefully it works out for us, we'll see.
We will be eager to read your report. This should be good weather to challenge it a bit.

That unit, like all tankless WHs, will require periodic treatment to get rid of hard water deposits. So read what is needed. https://www.navieninc.com/series/npe-a/warranty There was at least one maker who required that the hardness of the water being heated did not exceed I think 7 grains.
 
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John Gayewski

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Tankless units do not do the job. A boiler would. Water quality is also a huge factor. 85% of the US does not have the water quality for a tankless heater.
 

WorthFlorida

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Going from a tank to tankless is like going from a gas engine to a battery electric vehicle. Each has its advantages.
 

myhouse

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I'm too late here, but I would have recommended an indirect-fired water heater, if the OP has a boiler for home heating.
 
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We will be eager to read your report. This should be good weather to challenge it a bit.

That unit, like all tankless WHs, will require periodic treatment to get rid of hard water deposits. So read what is needed. https://www.navieninc.com/series/npe-a/warranty There was at least one maker who required that the hardness of the water being heated did not exceed I think 7 grains.
I'll do a "stress test" on it later today, but so far it's doing great! Have it set to 130 and shower temps are perfect. Although I have to replace at least one cartridge in the single handle faucets that we have. I'm getting some cross mixing with cold but that's been going on for a while.
 

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Alright I put the Navien to the test today, and it did better than I expected after all that I read.

With three showers running simultaneously it was only running at 51% heating capacity. The inlet temp was 49°F and the outlet was 130°F, and this was after a 15 minute continuous run. I didn't see any decrease in the water pressure compared to using the tank for this situation. The temperature was nice and hot at the showers (except for one that's been needing a new cartridge).

So I wanted to get the heating capacity up to 100% to test the limits on a big draw. I removed the aerator on the jetted tub and straight piped a direct flow of hot water for another 15 mins. That got me to 100%, and the flow rate stayed at 5.4 GPM. So next I wanted to see what effect the other gas appliances had on the output and performance. I turned on the furnace, fireplace, stove, and dryer while running at 100%... It didn't budge! Still 5.4 GPM and 130° output!
It seems to do a good job at modulating the flow to prioritize the temp outputs. But even at that rate and our usage we probably won't see any of the performance limitations that I was concerned may be an issue.

Overall I'm impressed, especially considering we've been in a cold spell in a northern state.

The big ticket item is the descaling which they said should be done every year, but I'll probably do myself every 6 months. Our water goes through a softener, but I'd still rather be safe than sorry.
 
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I forgot to mention that I added a recirc line to get to the farthest run in the house, and that itself has been an amazing improvement. Used to take forever to get hot water on that side of the house, but now it's only a few seconds... awesome.
 
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