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Old 12-01-2016, 09:12 AM   #1
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Tankless water heater question


Hello Everyone,

I am in the process of choosing a tankless water heater for our home. Our house is approximately 1800 sq. ft. My main question is what GPM rating I should select. I'm looking at tankless water heaters that are in the 7.5-9.8 range right now. Should the size of the house (square footage, home many floors there are, etc.) be a factor in deciding what gpm rating to go for or should I choose solely on how many appliances that use hot water I will be using simultaneously?

Thanks in advance for any assistance and guidance you can provide.


Best regards,

-Steph746
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Old 12-01-2016, 10:23 AM   #2
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Re: Tankless water heater question


Where are you, what is the incoming temperature of your water and what temperature do you want it heated to?

Is this a gas or electric heater and do you have the power to run it?
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Old 12-01-2016, 10:44 AM   #3
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Re: Tankless water heater question


Quote:
Originally Posted by stick\shift View Post
Where are you, what is the incoming temperature of your water and what temperature do you want it heated to?

Is this a gas or electric heater and do you have the power to run it?
Thanks for your reply. I live in Maryland (near DC). We are currently renovating our home and this will be a newly installed unit. I am looking at gas tankless heaters. Most gas tankless heaters also require power?
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Old 12-01-2016, 10:55 AM   #4
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Re: Tankless water heater question


They require a very large amount of gas to operate - I used the word 'power' in a general sense, yours would be powered by gas. So, have you checked that your gas service and lines will supply the needed gas to the unit?
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Old 12-01-2016, 11:14 AM   #5
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Re: Tankless water heater question


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Originally Posted by stick\shift View Post
They require a very large amount of gas to operate - I used the word 'power' in a general sense, yours would be powered by gas. So, have you checked that your gas service and lines will supply the needed gas to the unit?
I haven't checked yet, but I will pass along models to our contractor and they will look into that I assume. I thought though that tankless water heaters were supposed to be more economical than a tank heater and that gas bills should be lower with a tankless heater?
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Old 12-01-2016, 11:54 AM   #6
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Re: Tankless water heater question


Tankless water heaters are not always the more economical. It depends on your household usage pattern.

The reason the gas line sizing needs to be looked into is that your storage type heater heats a given quantity of water over time. Your tankless requires about the same amount of gas to heat that water, but in a much shorter time window.

So, delivering that gas in the short time window, especially with other gas appliances running, may exceed the capacity of your current piping. All it takes is one bottle neck point in the system and the tankless may be starving for gas.

I've never run the exact numbers on the gas heaters. But as an example, a 40-50 gallon tank type electric heater is usually on a 30 amp circuit. To get something equivelent in an electric tankless heater, you are looking at 120 amps or better. So the instantaneous demand is about 4 times greater with the tankless.
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Old 12-01-2016, 12:42 PM   #7
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Re: Tankless water heater question


Additionally, I asked about incoming and desired outgoing temperatures, as tankless heaters are rated for the rise in temperature they provide. If you live in a hot climate, the incoming water temp can be high enough that the tankless can provide enough rise to work for you. Up here, they might only be good for getting the water to 95 and that's not enough.
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Old 12-01-2016, 02:08 PM   #8
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Re: Tankless water heater question


So, my plumbing contractor should be able to tell me if the current setup can handle a tankless gas water heater?

If the setup is sufficient for the tankless water heater, are you still saying that it may not be economical? What is the best way for me to proceed with my decision so I know I am installing the most efficient device?

Thank you.
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Old 12-01-2016, 02:41 PM   #9
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Re: Tankless water heater question


Sorry but you're missing my point - I'm not talking about economics, I'm talking about whether this will work in the first place. If your gas supplier or gas lines will not provide enough gas, this won't work. If the difference between the incoming water temperature and what you need the heater to output is more than the rated rise on the heater, this won't work.

Am I making sense yet?
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Old 12-01-2016, 02:51 PM   #10
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Re: Tankless water heater question


yes. this is making more sense to me now. thanks for the clarification. my concern would be that this could be installed even though it would not efficient. whoever is doing the installation should tell me either yes it will work or no it won't based on the current gas line? if the current gas line will not handle the necessary load, is it pretty costly to upgrade it?
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Old 12-01-2016, 03:21 PM   #11
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Re: Tankless water heater question


Yes, your installer should be able to tell you whether you have sufficient gas available but I'm not comfortable assuming they know to look for this. Is it costly to upgrade? Now we're in more grey territory but the answer is that it can be.

Additionally, please find out what your incoming water temperature is and what the rated rise on the heater is as well. If your incoming water in the winter is 35 and the heater is good for a 60 rise, you would be left with 95 water coming out and that would not make you a happy person in the shower.

One final point is that tankless heaters sound good on paper but in the real world they turn out to be a good solution a whole lot less often.
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Old 12-01-2016, 03:36 PM   #12
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Re: Tankless water heater question


Thanks very much for your replies. I appreciate your time very much. I'm a new homeowner, so thanks for bearing with my inquiries.

I will check the rise specs as you said and if I as ask my plumber/contractor, they should be able to tell me the incoming water temp? They are doing work on the house now and water has been shut off...

If I get a model or two of tankless water heaters, may I send them to you for your thoughts?

Also, if I decide to get a new tank water heater, is there one that you recommend?
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Old 12-01-2016, 03:41 PM   #13
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Re: Tankless water heater question


I'm probably not your guy for deciding between makes or models but others here will certainly be able to respond so please go ahead and ask when you have them.
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Old 12-01-2016, 04:51 PM   #14
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Re: Tankless water heater question


If I go with a tank water heater, is there a particular type/size you recommend? I see there are also "hybrid" tank heaters? Just finding it a bit overwhelming on where to start...
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Old 12-01-2016, 05:17 PM   #15
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Re: Tankless water heater question


Been my experience that the break even point of a tankless versus a standard hwt is far after the tankless no longer works. So the question you need to answer is.

1. Size of family
2. total full baths in house
3. average amount of times people bath at same time of day.

A 40 gallon Hwt can supply a family of four with a 2.5 bath home with no issue as long as showers and baths are spaced an hour apart.

The rule of thumb for a hot water tank is 75% usage.
A 40 gallon can supply 30 gallons of hot water then it starts cooling off.

The average shower has a 2.5 GPM flow rate 1 head. that would allow you about 30 minute shower. At a 1/2 by 1/2 hot cold mix.

Home depot has some sizing info based on family size.
http://www.homedepot.com/b/Plumbing-...s/N-5yc1vZc1tz

Last edited by Ghostmaker; 12-01-2016 at 05:29 PM.
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