On Demand Water Heater

Do you guys and gals ever watch Holmes on Homes? It’s a pretty entertaining show, but the other day he made a comment that I have been contemplating for a few days. He did some work on a house and had an on demand water heater installed…when he showed the house to the clients he stated that it had to be moving 2.5 gpm for it to kick on. That just got me thinking…what about the times you have a sink flowing slowly or something along that line. One of my favorite things to do after a long day is to take a hot shower adjusting the flow to just more than a trickle and just letting the hot water wash away the day.:mrgreen: Now that you got a visual, think about it, the comment of 2.5 gpm is a tough one to swallow.

In fact, I have an on demand unit (electric) and often have flow significantly less than 2.5 gpm.

So I guess what I’m looking for here is thoughts and opinions on this.

Thanks for indulging me:mrgreen:


I watched that show and had the same response. My showerhead is a low-flow at 1.6 gpm. Do I get hot water or not?

Different models have different flow rates. I’m seeing a lot of these in new construction, from .8 gpm for the small units serving just the master bathroom to 3.9 gpm units serving the kitchen dishwasher, sink, and nearby laundry room.

If he had one at 2.5 gpm, then that is indicating to me that he only had one unit serving the whole house, probably a 360,000 BTU unit.


Tankless not always best way to heat water TheStar.com - living - Tankless not always best way to heat water
March 22, 2008
**Steve Maxwell
**Special to the Star
Last month I wrote about installing a new tank-style water heater at my house, which made more than a few people wonder why I’d opt for such antiquated technology.
All the talk these days is about the energy savings offered by tankless water heaters and their compact size.
My choice seems even more bizarre when you realize that I like tankless heaters a lot.
I’m convinced this technology is ideal for many homes.
Trouble is, you need to understand a few issues before spending a lot of time considering a tankless system for your own place.
I know because I’ve wrestled with these issues several times, and they always turned out to be deal-breakers.
Tankless water heaters save energy by eliminating standby losses associated with traditional, tank-style heaters.
Even with excellent insulation around the tank, energy is inevitably lost to the surrounding air over the course of time, even when you don’t use any hot water.
Tankless systems, on the other hand, heat water only as you demand it by turning on the tap.
That’s why these units are sometimes called on-demand water heaters.
At the moment there are two kinds of tankless systems on the market: electric and gas-fired. Both can work well, but the logistics of installation sometimes poses problems that aren’t always easy to see ahead of time.
The big challenge faced by any kind of tankless water heater is the short time available for delivering thermal energy to the stream of cold water flowing through the unit.
To be useful in a typical household, a tankless heater must be able to raise the temperature of 20 litres of water by 50C in one minute.
That’s a huge energy transfer, and it’s the Achilles’ heel of electric tankless heaters.
As electricians have told me on several occasions, my 200-amp service isn’t large enough to handle the demands of a tankless electric heater along with all the other electrical loads connected to it.
But even if it had been, I’d be cautious about installing an electric tankless heater because of probable changes in the way electricity will be metered.
Right now, residential customers pay only for the total quantity of electricity used, with no special premium applied to peak demand.
But the huge current required by electric tankless heaters during operation is taxing on the grid, and something that the authorities will likely discourage in the future when smart metering systems are implemented.
Then there’s the issue of how much hot water an electric tankless system can actually provide.
For the various models I’ve investigated, water temperature would probably drop to lukewarm whenever more than a couple of hot water taps were turned on at the same time.
Who wants that?
The huge, short-term energy demands of a tankless heater make gas a much more practical energy source than electricity.
Trouble is, not all of us have convenient access to gas.
I don’t, and though I could’ve opted for a propane-fired tankless heater, that choice would have required the installation of an outdoor propane tank and supply lines.
I almost considered doing this, too, then I remembered how easy it is to fix a tank-style water heater, and how troubleshooting any kind of tankless system requires a specialized technician.
That did it for me.
All this said, I’m convinced that tankless water heaters are a great idea in many situations.
The technology behind gas-fired systems is mature and efficient.
Most units are about half the volume of a Blue Box and they operate quietly enough that they’re practical to install just about anywhere. Too bad my place isn’t one of them.


RR -

I think you are discussing the flow rate that the appliance will raise the temperature sufficiently at and he is discussing the minimum flow rate, which is the rate which turns the water heater on.

You are correct that the larger the model, the more water that can be heated (and the larger the rise). However, in every model I’ve looked at the specs for the minimum flow rate is in the area of half a gallon per minute.

Bosch, Titan, Takagi, etc all use a .5GPM minimum flow rate, at least all of their models that I’ve seen use that for the minimum flow rate.

Most models I’ve done research on tell you to use less then .5GPM continuous, then you have to at least start with the hot water flowing at that rate, and then you can turn down the volume of hot water to what you desire as long as you don’t turn it off.

Hope that helps!

– bz

Ah hah! That makes a ton of sense! 0.5gpm is a logical flow rate to start a system…2.5gpm seems a bit high.

Not really. Go try it. Get a gallon milk jug and see how long it takes you to fill it up.

I just tried it and, at 40 psi water pressure, got 1.75 gallons per minute, probably a little more since I lost some water during the two seconds it took to change from one gallon jug to the other.

Good article, Roy.


After the Enron fraud and electricity market manipulation of 1990-2001, I’m not sure I’ll ever have electrical appliances anymore if there’s a gas equivalent.

When we built our house 15+ years ago, I installed two gas tankless heaters in the basement. Since that time I have repaired each one time. In both cases the problem was a failed diaphram that operates the gas valve when sufficient differential pressure is generated by the water flow. I’m not at home right now and don’t recall the minimum flow required for my units. There is definitely more than a ‘drizzle’ required; however, I have never found it to be a problem when using water in a convential way.

On the other hand there are a few quirks that you have to put up with - the heaters do induce noticeable pressure drop in the hot water supply so the volume is lower at the point of use and the heater will indeed shut off if the flow is unintentially reduced below a certain point. In the case of my heaters, the turn-on and turn-off point seems to be about the same - newer heaters may be able to continue operation at lower flows once ingnited (as mentioned above). As a result of these ‘features’ we have had to learn to adjust shower water temperature, for example, a little differently. Basically, you just turn on the hot water to about mid way, then add or subtract cold to get the desired temperature. If the water is too hot you and you reduce the hot water, you risk the heater turning itself off. Sounds complicated, I know, but it is pretty easy to get used to and I really appreciate the entergy savings as well as the fact that we never run out of hot water.

Whenever you install a tankless, you have to follow these rules:

  1. Hire a plumber who actually has experience with them. He should be:
    a) a licensed plumber
    b) a licensed HVAC guy (for gas) or a licensed electrician (for electric)
    c) be factory (by the unit’s manufacturer) trained and certified.

  2. read the operating instructions.

  3. Have the unit properly sized for your house.

All of the “bad” installations I have seen were done by subs who did not have the proper qualifications (see above).

Also, make sure that the unit is installed with the proper isolation valves and that the plumbing is sized correctly (at least 3/4" input and output piping.

Also, if it is a gas unit, make sure that your gas meter is sized properly. They don’t use gas often (compared to tank units) but when they do, they use a whole lot.

Hope this helps;