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Tankless Water Heater

8K views 56 replies 11 participants last post by  Alan 
#1 ·
When I built my house I decided to try a tankless water heater. I'm not having any problem with this other then when hot water is first requested. It is a gas unit and the burner lights as soon as water is flowing but it takes about 10 to 15 seconds for the water to get hot at the outlet of the water heater. For someone to get hot water requires running water until the outlet temperature rises and then for this to reach a faucet. Richmond's help desk said I should have hot water immediately but I know better than this and it will take some time for the heat exchanger and the cold water that is in the heater along with what will be passing through, to get up to temperature. I will be calling back to the desk again but I'd like to hear what others have experienced along this line.
There is a second problem too. If hot water is being used and the faucet is turned off, the burner will shut down, but if the faucet is turned back on, there will be hot water from what is in the pipes but cold water will be drawn to heater and it will take the 10 seconds or so to get hot so after the pipes are purged you get a blast of cold water.
I'm thinking about some type of recirculating pump that could be on a timer that would move water through the heater before water is drawn.
Any comments on tankless water heaters and a recirculation pump?
 
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#2 ·
I also have a tankless heater, and love it. But with any kind of hot water heater, unless you have some sort of recirculation system there will always be a delay before hot water reaches a fixture. How much depends on how far from the heater the fixture is. Your second issue is what's known as a "cold water sandwich."

I don't know that a recirc system would work with a tankless heater. Unless the heater runs all the time, there's no hot water to recirculate.
 
#3 ·
My idea would be to have a recirculating pump that would be connected from the outlet back to the inlet. I would have to find a timer that would be mounted in the bathroom which could be used to turn on the recirculating pump prior to drawing water. This would cause the water heater to start up and pre-heat the water.
 
#4 ·
How far away from the fixture is the water heater? This makes a difference, especially if you have water-saver fixtures like 2.5gpm shower heads, and .8gpm sink aerators. There's going to be a slug of cold water in the pipe between the heater and the fixture that you will get until the hot water makes it to the fixture.

Also worth mentioning, if you are using these low-flow devices (especially the sink aerator) there may not be enough flow to keep the heater lit continually. It could be cycling on and off, especially if you are tempering the hot water with some cold (you will then be flowing half or less hot water, depending on where you have the valves set). The solution to this problem is to set the thermostat on the water heater to the most comfortable temperature you want WITHOUT needing to temper the water with cold. IE so you are using "pure" hot water. This, of course is the exact opposite that you would do with a tank style water heater (where you set it really hot, and then add cold water at the fixture to reduce the amount of hot you are using). For example, set your tankless water heater for about 105-110 degrees for a shower, and then just turn on the hot water valve and enjoy. THis may be too cold for a dishwasher, so turn on your DW's water heating function to make up the diff.

A recirculation pump may not work very good with a tankless. The water heater could stay on almost continually due to there being flow through the heater almost continually!
 
#5 ·
Thanks for the replies but I think the point was missed. I'm trying to overcome the delay in getting hot water out of the tankless unit. Maybe electric units are faster but in my case it takes about 10 to 15 seconds for the water exiting the unit, not a faucet, to be close to temperature. This means water has to be drawn for 10 to 15 second and then the additional time for it to travel the pipe to the requesting faucet.
When a recirculating pump is used with a conventional tanked water heater, does it run all the time or is it started when needed?
 
#7 ·
My two baths and utility room are all close together and the tankless unit is in the crawl space under the utility room. The water is all plumbed in Pex so I think I'm going to look at installing a Pex manifold and a pump so I can circulate the water back to heater inlet. This will light the burner and heat the water in the lines and manifold.
I think I'll have to put a 120 volt 6 gal Point of Use under the kitchen.
 
#8 ·
For your recirculating pump, I suggest looking up the maximum inlet temperature your tankless water heater can handle. I've noticed that a number of tankless water heater manuals specify a maximum inlet temperature below their maximum output temperature. So you may need to wire your pump to turn off if the water reaches a certain temperature.
 
#9 ·
I do not have any experience with tankless heaters, although I am looking at one to replace my tank, but I did see an episode of Ask This Old House on their website and they were installing a recirc pump on a system that had a tankless water heater. It was a relatively new episode. Might help you out.
 
#12 ·
yea you need the remote switch for recirculating pump for a on demand you can also if walls are open hard wire timer switches next to gfci outlet looks better, costs more though, and every time a guest comes over you are explaining what that switch is for if they are observant type :thumbup:
 
#19 ·
Okay, I see what they are saying, but I still don't see how that gives instant hot water if you have to press a button to get the pump going.

:huh:
You are right, it doesn't. You still wait for it but without the water running....saves a bit of water, but still takes the same amount of time.:thumbsup:
 
#21 ·
it is amazing to me the amount of people that confuse tankless as being the same as an instant hot
 
#22 ·
AFter shipping and tax, the on demand system costs about $500 for the STS-70T

If you have to wait 10-15 seconds at the shower for hot water, that means cold water is running down the drain for 10-15 seconds. Thats roughly 0.5 gallons of water.

My water bill for 1800 cubic feet or 13500 gallons was about $70. That comes to 0.52 cents per gallon or 0.26 cents per half gallon.(a quarter of a cent)

SO it would take 500/0.26 = 1923..... thats 1923 showers until this unit paid for itself just in water savings.
 
#26 ·
Your best bet is to add a small electric tank after the tankless unit. It sounds crazy, but makes a lot of sense. The tank will act as a buffer for the initial cold water from the tankless unit and also for the "cold water sandwich."

The storage losses on modern tanks are very low and the tank will be receiving hot water from the tankless unit. So, the tankless unit is doing almost all the heating. The tank just has to make up storage losses. It could also be used with a recirculating system.
 
#27 ·
Your best bet is to add a small electric tank after the tankless unit. It sounds crazy, but makes a lot of sense. The tank will act as a buffer for the initial cold water from the tankless unit and also for the "cold water sandwich."
A 1/2 gallon of cold water down the drain at shower time costs about 1/4 of a cent based on my figures in post #22.

Poster has to ask himself, Am I trying to save money on wasted water or am I trying to save 10-15 seconds with my shower??
 
#28 ·
hey thadsabb a.o.smith has a hybread that has both on demand with small water heater here is a link
http://www.hotwater.com/water-heaters/residential/hybrid/next-hybrid-gas/

and you guys still must not understand what the recurculating pump does for a on demand and if the you read the link and watched the video and still do not understand it i am not going to try here is another video for you

http://video.pbs.org/video/2195214406/

and the other thing the recurculater pump does is get rid of cold pockets and give you the instant hot water that most people think a tankless is
 
#32 ·
I'm not trying to be argumentative, but If you watched the video, you still have to WAIT for hot water. :huh:

It seems pretty simple to me that these videos are people who are trying to save water, because there is NO WAY that a recirculating pump can run 24/7 on a tankless water heater without a storage tank. NO WAY.
 
#31 ·
ohm corsair what to say but feel the resistance :jester: :thumbup::laughing:
some get it and some don't
 
#37 ·
#43 ·
ok so what part of that does not make sense? and here is just one scenario
my bathroom is 80' of pipe away from tankless instead of turning on faucet and waiting a minute or so letting water run down drain until it gets hot. i walk into bath and motion sensor or manual switch turns on recirculating pump and creates flow switch on heater to turn on running hot water up to faucet and putting cold water back into cold supply "instead of down the drain" then when hot water hits faucet bypass sensor recirculating pump turns off. then i finish my paper work and like a good boy i wash my hands and what is this instant hot water wow no waiting over a minute that is amazing. so like a curios little guy i am i have to look under sink to see if they got one of those instant hot boxes that seem to only hold up a few years and make the meter spin so fast and nothing no little leaking box,but what do i see a little pump using under a watt of electric.

no saving the water umm not a big deal pennies down the drain,but instant hot water in a i want it now world priceless
 
#45 ·
ok so what part of that does not make sense? and here is just one scenario
my bathroom is 80' of pipe away from tankless instead of turning on faucet and waiting a minute or so letting water run down drain until it gets hot. i walk into bath and motion sensor or manual switch turns on recirculating pump and creates flow switch on heater to turn on running hot water up to faucet and putting cold water back into cold supply "instead of down the drain" then when hot water hits faucet bypass sensor recirculating pump turns off. then i finish my paper work and like a good boy i wash my hands and what is this instant hot water wow no waiting over a minute that is amazing. so like a curios little guy i am i have to look under sink to see if they got one of those instant hot boxes that seem to only hold up a few years and make the meter spin so fast and nothing no little leaking box,but what do i see a little pump using under a watt of electric.

no saving the water umm not a big deal pennies down the drain,but instant hot water in a i want it now world priceless
...and you think that little pump is what is making the hot water?....and you think it's saving you money? When that little pump runs it tells the energy hog tankless to kick on, and that's much worse if it's electric rather than gas....and you still waited a minute for the water, just did something else while waiting.:laughing:
 
#44 ·
corsair in a month tell him how over rated they are by posting savings on gas and electric bill
 
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