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a7ecorsair 04-30-2012 11:16 AM

Tankless Water Heater
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?

md2lgyk 04-30-2012 12:58 PM

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.

a7ecorsair 04-30-2012 01:23 PM

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.

av-geek 04-30-2012 01:27 PM

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!

a7ecorsair 04-30-2012 02:24 PM

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?

bob22 04-30-2012 06:48 PM

I think the recirc pumps can be configured different ways: on demand (you push a button); when the water gets cold on the line (sensor activated); timed (like a light timer; circulates on a schedule).

a7ecorsair 05-01-2012 09:27 AM

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.

a_lost_shadow 05-01-2012 10:05 AM

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.

Centex2011 05-01-2012 10:10 AM

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.

jaydevries 05-01-2012 04:22 PM

try this it is a recirculate pump with a remote on so just hit button it clears cold out in like 5 to 10 seconds

a7ecorsair 05-01-2012 04:41 PM


Originally Posted by jaydevries (Post 912004)
try this it is a recirculate pump with a remote on so just hit button it clears cold out in like 5 to 10 seconds

This is along the line of what I was thinking, thanks for the link.:thumbup:

jaydevries 05-01-2012 05:01 PM

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:

Alan 05-01-2012 08:01 PM

You can't put a recirculating pump on a tankless water heater without having a storage tank.

jaydevries 05-01-2012 08:21 PM

allen that is false check link in post 10

Alan 05-01-2012 08:34 PM

I don't care about the button. Where is the water going?


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