Jadnashua
Retired Defense Industry Engineer xxx
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.
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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